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Engineering Encyclopedia
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Sizing Power And Distribution Transformers

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: EEX10402

For additional information on this subject, contact W. A. Roussel on 874-1320

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Electrical Sizing Power and Distribution Transformers

CONTENTS CALCULATING POWER AND DISTRIBUTION TRANSFORMER RATIOS TRANSFORMER MODEL Primary and Secondary Windings Primary and Secondary Voltages/Currents TRANSFORMER RATIOS Turns Ratio Voltage Ratio Ratio of Currents Ratio Relationships CALCULATING POWER AND DISTRIBUTION TRANSFORMER EFFICIENCY AND VOLTAGE REGULATION TRANSFORMER EFFICIENCY Copper or I2R Losses Iron or Magnetic Core Losses Power Factor and Loading TRANSFORMER VOLTAGE REGULATION Impedance Voltages POWER AND DISTRIBUTION TRANSFORMER COMPONENTS CORES Laminations Types Of Core Construction CONCENTRIC WINDINGS Rectangular Concentric Round Concentric INSULATION Liquid-Filled Transformers

PAGES

2 2 3 3 5 5 5 5 6 8 8 9 9 10 11 12 14 15 15 16 17 20 21 22 22 23

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Dry-Type Transformers ENCLOSURES Dry-Type Transformers Liquid-Filled Transformers Conservator Tank-Type BUSHINGS Bulk-Type Bushings (Less Than 15 kV) Condenser-Type Bushings (Greater Than 15 kV) COOLING SYSTEMS Cooling Circuit Cooling Methods Cooling Classes TAP CHANGERS No-Load Tap Changer (NLTC) Load Tap Changers (LTC) AUXILIARY COMPONENTS Transformer Nameplate Winding Temperature Indicator Liquid Temperature Indicator Liquid Level Indicator Pressure/Vacuum Indicator Pressure Relief Device Sudden Pressure Relay Fill and Drain Valves Control Cabinets POWER AND DISTRIBUTION TRANSFORMER CLASSIFICATION: CONSTRUCTION AND APPLICATION CONSTRUCTION CLASSIFICATIONS Dry-Type (Conventional) Dry-Type (Cast Coil) Oil-Filled
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Non-Flammable Insulating Liquid-Filled Gas-Filled APPLICATION CLASSIFICATIONS Saudi Aramco Classifications Generator Power Transformer Substation Power Transformers (Sub-Transmission Circuits) Substation Power Transformers (Primary-Feeder Circuits) Distribution Transformers THREE-PHASE POWER AND DISTRIBUTION TRANSFORMER CONNECTIONS ANSI/NEMA LABELING CONVENTIONS TYPES OF CONNECTIONS Delta Connection Wye Connection TYPICAL CONNECTIONS Delta-Delta (D-D) Wye-Wye (Y-Y) Delta-Wye (D-Y) Wye-Delta (Y-D) TRANSFORMER POLARITY Single-Phase Transformers Three-Phase Transformers CALCULATING BASIC POWER AND DISTRIBUTION TRANSFORMER RATINGS SELECTION FACTORS Voltages Loads Ambient Temperature Standard Sizes Cooling Classes
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Temperature Rise SECONDARY UNIT SUBSTATIONS Components Layout and One-Line Diagrams Single-Ended Substations Double-Ended Substations TRANSFORMER LET-THROUGH CURRENT Impedance Transformer Categories PARALLELED TRANSFORMERS Increased Fault Levels Circulating Currents Limiting kVA WORK AID 1: PROCEDURES FOR CALCULATING POWER AND DISTRIBUTION TRANSFORMER RATIOS WORK AID 2: PROCEDURES FOR CALCULATING POWER AND DISTRIBUTION TRANSFORMER EFFICIENCY AND VOLTAGE REGULATION WORK AID 2A: APPLICABLE PROCEDURES FOR CALCULATING POWER TRANSFORMER EFFICIENCY WORK AID 2B: APPLICABLE PROCEDURES FOR CALCULATING POWER TRANSFORMER VOLTAGE REGULATION WORK AID 3: RESOURCES USED TO CALCULATE BASIC RATINGS OF POWER AND DISTRIBUTION TRANSFORMERS WORK AID 3A: ANSI/IEEE STANDARD C57 (DISTRIBUTION, POWER, AND REGULATING TRANSFORMERS) WORK AID 3B: SAES-P-121 (TRANSFORMERS, REACTORS, VOLTAGE REGULATORS) WORK AID 3C: APPLICABLE PROCEDURES

87 88 89 89 89 89 93 93 94 95 95 95 97 100

101 101 101 102 102 115 119

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GLOSSARY AA
ADDITIVE POLARITY

123 123 123 123 123 123 123 123 123 123 (CT) 123 123 123 123 124 124 124 124 124 124 124 124 124 124 124 125 125 125 (LTC) 125 125 125

AFA ANSI BIL
BUSHING COPPER LOSSES CORE

CSD
CURRENT TRANSFORMER DIELECTRIC STRENGTH DISTRIBUTION TRANSFORMER DISTRIBUTION TRANSFORMER INSTALLATION DRY-TYPE TRANSFORMER EDDY CURRENT LOSS EFFICIENCY ELECTROMAGNETIC INDUCTION EXCITATION CURRENT

FA
HYSTERESIS LOSS

I2R LOSSES IEEE
IMPEDANCE VOLTAGE INERT GAS INSTRUMENT TRANSFORMER IRON LOSSES LOAD LOSSES LOAD TAP CHANGER

MRD
NO-LOAD LOSSES

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NO-LOAD TAP CHANGER

(NLTC)

125 125 125 125 125 126 126 126 126 126 126 126 126 126 126 126 127 127 127 127 127 127 127 127 127 127 128 128 128 128 128

OA
PAD-MOUNTED TRANSFORMER POLE-MOUNTED TRANSFORMER POWER TRANSFORMER PRIMARY RATED CURRENT PRIMARY RATED VOLTAGE PRIMARY WINDING PRINCIPAL TAP RATED K VA TAP RATED POWER (SINGLE-PHASE TRANSFORMER ) RATED POWER (THREE-PHASE TRANSFORMER ) REDUCED K VA TAP REGULATION RELIABILITY (SUBSTATION )

SAES
SECONDARY FULL -LOAD VOLTAGE SECONDARY RATED CURRENT SECONDARY RATED VOLTAGE SECONDARY WINDING SHORT CIRCUIT IMPEDANCE

SPR
STEP-DOWN TRANSFORMER STEP-UP TRANSFORMER SUBSTATION SUBSTATION (INDUSTRIAL ) SUBSTATION (RESIDENTIAL ) SUBTRACTIVE POLARITY TAP TAP CHANGER TEMPERATURE RISE

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TURNS RATIO UNDERGROUND -TYPE TRANSFORMER VOLTAGE RATIO VOLTAGE TRANSFORMER

128 128 128 (VT) 128

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Figure 1. Power Distribution System

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CALCULATING POWER AND DISTRIBUTION TRANSFORMER RATIOS Transformer Model A transformer operates on the principle of electromagnetic induction between two or more inductively coupled coils. Figure 2 shows the three basic parts of a simple transformer: the primary winding, the secondary winding, and the core. Both windings are wound around the core. The primary winding receives its energy from the alternating current source, which creates an alternating magnetic flux (φm) in the magnetic core. The core is designed to focus the alternating flux so that the flux passes through the secondary winding. This alternating flux then produces a voltage on the secondary winding. Energy is transferred from the primary winding to the secondary winding by electromagnetic induction. The amount of energy transferred depends on the design of the transformer. In a properly designed transformer, almost all of the energy received from the supply by the primary winding is transferred to the secondary winding. The output voltage depends on the relationship between the number of turns on the primary (Np) to the number of turns on the secondary (Ns).

Figure 2. Simple Transformer Model

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Primary and Secondary Windings Power can flow through any transformer with equal effectiveness from the low-voltage to high-voltage windings or from the high-voltage to low-voltage windings. The winding which nominally receives the input power is called the primary winding. The power output windings are called the secondary or tertiary windings. Note: The terms primary winding and high-voltage winding are not equivalent. Example A: Referring to Figure 3, is the low-voltage winding of the generator-transformer (T1) the primary winding or the secondary winding? Is the high voltage winding of the utility transformer (T2) the primary winding or the secondary winding?

Figure 3. Example A One-Line Diagram Answer: The low voltage winding of T1 is called the generator-transformer’s primary winding and the high voltage winding of T2 is called the source transformer’s primary winding.

Primary and Secondary Voltages/Currents All electrical equipment must have voltage and current ratings, either directly or indirectly, labeled on the device itself. These voltage or current ratings show the maximum voltages and currents that the electrical equipment can safely withstand (voltage) or carry (current) without overheating or damaging the insulation.

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Transformers are rated in voltamperes (VA) or kilovolt-amperes (kVA), which means that definite voltage and current ratings (limits) have been assigned by the manufacturer, both to the primary and secondary windings of the transformer. This VA or kVA rating is intended to indicate the maximum load that can be connected to its secondary circuit, which, as explained above, could be either its high or low voltage winding. The same kVA load rating also applies to the primary circuit, because it must take from the source the same power that the secondary circuit delivers to the load. The fact that the primary side kVA rating equals the secondary side kVA rating is called the conservation of power relationship, that is, power in (kVAin) equals power out (kVAout). Expressed as a formula, this relationship leads to the following equations: • Single-phase transformers: - VAin = VAout - Ep x Ip = Es x Is or kVp x Ip = kVs x Is kVAin = kVAout

- Note: All voltage and current quantities are phase quantities. • Three-phase transformers: - VAin = VAout 3 x Ep x Ip = 3 x Es x Is or kVAin = kVAout 3 x kVp x Ip = 3 x kVs x Is

- Note: All voltage and current quantities are line quantities. Example B: Answer: What are the current ratings of the primary and secondary windings of threephase transformer T2, as shown in Figure 3? 1. kVAin = kVAout = 25 MVA = 25,000 kVA 2. Ip = 25000/( 3 x 69) = 209.2 A 3. Is = 25000/( 3 x 13.8) = 1045.9 A

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Transformer Ratios The simple transformer circuit diagram that is shown in Figure 4 will be used in this Information Sheet to help explain the ratios of a transformer, as well as the relationships between the transformer ratios. Note: Work Aid 1 has been developed to teach the Participant procedures to calculate power and distribution transformer ratios.

Figure 4. Simple Transformer Circuit Diagram Turns Ratio The turns ratio of a transformer, which is identified by the symbol “a”, is defined as the ratio of the number of turns on the primary winding (Np) to the number of turns on the secondary winding (Ns). Voltage Ratio The voltage ratio of a transformer (Ep/Es), which also equals the turns ratio, is defined as the ratio of the primary voltage (Ep) to the secondary voltage (Es). Ratio of Currents The ratio of the currents (Ip/Is) is equal to the inverse of the turns ratio (1/a).

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Ratio Relationships
Volts-Per-Turn - The turns ratio of a transformer is directly proportional to the voltage ratio of a

transformer, as shown in the following formula: • • Turns ratio = Np/Ns = voltage ratio = Ep/Es = a where: Np = the number of turns in the high voltage winding Ns = the number of turns in the low voltage winding Ep = the voltage generated within the high voltage winding Es = the voltage generated within the low voltage winding Rewriting the above formula relationships leads to the following formula, which states that the volts-per-turn ratio on the primary side of the transformer equals the volts-per-turn ratio on the secondary side of the transformer. • volts-per-turn (primary) = volts-per-turn (secondary) Ep/Np = Es/Ns
Ampere-Turns - The turns ratio of a transformer is inversely proportional to the current ratio of

a transformer, as shown in the following formula: • • Turns ratio = Np/Ns = the reciprocal of the ratio of the currents = Is/Ip where: Np = the number of turns in the high voltage winding Ns = the number of turns in the low voltage winding Ip = the current flowing in the high voltage winding Is = the current flowing in the low voltage winding Rewriting the above formula relationships leads to the following formula, which states that the number of ampere-turns on the primary side of the transformer equals the number of ampere-turns on the secondary side of the transformer. • ampere-turns (primary) = ampere-turns (secondary) Ip x Np = Is x Ns
Combined Relationships - Combining the volts-per-turn relationship with the ampere-turns

relationship, it is easily seen that the primary and secondary current relationships are exactly

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the opposite of the primary and secondary voltage relationships. The following formula summarizes the combined relationships: • Np/Ns = Ep/Es = Is/Ip = a What is the current rating of each winding of a 100 kVA, 2400/240 volt, singlephase distribution transformer? 1. Ip = 100/2.4 = 41.67 A 2. Is = 100/.24 = 416.7 A Example D: Referring to Example C, and assuming that there are 60 turns on the secondary winding of the transformer, how many turns are on the primary winding of the transformer. 1. Ep/Es = Np/Ns Np = (Ep/Es) x Ns = (2400/240) x 60 = 600 turns Example E: Answer: Referring to Examples C and D, what is the transformer’s voltage ratio, turns ratio, and the ratio of currents? 1. Ep/Es = Np/Ns = 2400/240 = 600/60 = 10:1 2. Ip/Is = 41.67/416.7 = 1:10 = 1/a

Example C: Answer:

Answer:

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CALCULATING POWER AND DISTRIBUTION TRANSFORMER EFFICIENCY AND VOLTAGE REGULATION Note: Work Aid 2 has been developed to teach the Participant procedures to calculate power and distribution transformer efficiency and voltage regulation. Transformer Efficiency In a perfect transformer, the power measured on both the primary and secondary windings would be exactly equal. Unfortunately, there is no such thing as a perfect transformer. In a practical transformer, the power measured in the primary and secondary windings will be very close, but never equal. There will always be a small power loss, which means that not all of the energy is transferred from the primary winding to the secondary winding. This loss of energy is a measure of the transformer’s efficiency. Transformer efficiency is the ratio of real output power (Pout) to real input power (Pin), expressed as a percentage. The real output power is simply the power rating of the load, whereas the input power is the sum of the output power plus the copper and iron losses of the transformer. Figure 5 shows a typical test procedure for determining the losses of a transformer. The short circuit test, which is often called the full-load test, determines the copper losses of the transformer. The open circuit test, which is often called the no-load test, determines the iron or core losses of the transformer.

Figure 5. Loss Measurements for Transformers

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Copper or I2R Losses Copper losses are caused by the resistance of the primary and secondary windings (conductors) of the transformer. These copper losses are directly proportional to the magnitude of the load current squared (I2) multiplied by the winding resistance (R). Because the copper losses are directly proportional to the load current, they are often called load losses. Referring to Figure 5, the copper losses are measured by closing switch SW (short circuited secondary) and slowly increasing the applied primary voltage until rated current is flowing in the windings. Under these “full-load” conditions, the readings on wattmeter WM2 are subtracted from the readings on wattmeter WM1. This difference is the copper or I2R losses of the transformer. Iron or Magnetic Core Losses Iron or magnetic core losses are another type of losses that occur in a transformer. These core losses occur regardless whether there is or is not load on the transformer. Referring to Figure 5, the iron losses are measured by opening switch SW (opened secondary) and then applying rated voltage on the primary winding. Although there is no load current flowing, there is a small amount of current flowing in the energized primary winding that is caused by the losses in the core of the transformer. Wattmeter WM1 is read, which is the iron losses of the transformer. The iron losses of a transformer consist of two distinguishable losses, which are called eddy current losses and hysterisis losses.
Eddy Current losses are caused by the currents that circulate through the mass of the magnetic

steel of the core. Because the magnetic core is laminated and oriented perpendicular to the path of magnetic flux, the eddy currents are confined within small volumes. Lamination of the core effectively reduces the eddy current losses compared to the losses in a “solid” core.
Hysterisis losses represent the extra power that is required to overcome the small magnetism

that resides after each reversal of the magnetic field (residual magnetism). Hysteresis loss is minimized by the use of silicon steel or amorphous steel as the material for a magnetic core.

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Power Factor and Loading The efficiency of a transformer changes as a function of loading and power factor. Figure 6 is plot of 5 different efficiency curves for 5 different conditions of lagging power factor and it represents the performance of a typical power transformer. Note that the maximum efficiency is achieved at about half-load. This maximum efficiency at half-load is established by adjusting a balance between specific design variables related to the magnetic core and conductors. Efficiency variations are usually very small between different power transformers, and most power transformers are typically in the 98% efficiency range.

Figure 6. Transformer Efficiency

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Example F:

Given the following information, what is the efficiency of a 500 kVA transformer at 100% loading. Given Information: • • • Rated power - 400 kW @ 80% p.f. or 500 kVA Copper losses - 5500 watts Iron losses - 2100 watts

Answer:

1. See the given information. 2. Pout = 400,000 watts (given)

3. Pin = Pout + Plosses = 400,000 + 5500 + 2100 = 407,600 watts 4. % efficiency = (Pout/Pin) x 100 = (400/407.6) x 100 ≈ 98.1% The following items should be noted from Figure 6. • • • • • At no-load, the losses are essentially iron losses, which means that the efficiency is zero since power out is zero watts. Efficiency rises quickly with a small application of load. Maximum efficiency usually occurs at approximately one-half load. For the same value of load current, efficiency decreases slightly with decreasing values of power factor. Maximum efficiency occurs at the same load point regardless of the power factor.

Transformer Voltage Regulation The change in terminal-to-terminal voltages between no-load condition and underload condition is called voltage regulation. The amount of voltage regulation is a function of winding impedances, the magnitude of load currents, and the power factor associated with the load circuit. Full-load voltage regulation is usually stated as a percent as follows:

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% Full-Load Voltage Regulation = [(Eno-load - Efull-load)/Eno-load] x 100 Note: The % voltage regulation is approximately equal to the % impedance (%Z). Impedance The impedance of a transformer is a measured parameter that indicates the transformer’s ability to support a given secondary load with a prescribed primary voltage drop. The measured impedance includes the effects of the winding resistance, inductance, reactance, core losses, and various other parameters. a transformer in ohms. Transformer impedance is measured by short circuiting the secondary winding and applying a low voltage to the primary winding until rated current is circulating in the primary winding. The impedance is then calculated using the following formula: • ZΩ = Vmeasured(primary)/Irated(primary)
Ohmic Measurements - Figure 7 shows the test connection for measuring the impedance (ZΩ) of

Percent Measurements - The percent impedance (%Z) shown on a transformer nameplate is the

impedance voltage drop of the transformer that is based on the self-cooled (OA) rating of the transformer. Figure 7 is also the same test connection that is used for measuring the percent impedance of a transformer. The manufacturer calculates the percent impedance using the following formula: • %Z = [Vmeasured(primary)/Vrated(primary)] x 100

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Figure 7. Test Circuit for Measuring Transformer Impedance (Z)

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Voltages
No-Load Voltage is the transformer’s rated voltage that is stamped on the nameplate by the

manufacturer. As the name implies, the no-load voltage of a transformer is the measured (rated) voltage of the transformer without any load connected to the secondary terminals.
Full-Load Voltage - As a transformer is loaded, its secondary voltage decreases because of the

internal impedance (Z) of the transformer. Under full-load (rated current flowing) conditions, the full-load voltage is measured, as illustrated in Figure 8.

Figure 8. Test Circuit for Measuring Voltage Regulation Example G: Answer: Calculate the approximate full-load voltage (Efull-load) for a 480 volt transformer that has a percent impedance equal to 5%. 1. 2. 3. 4. % Voltage Regulation ≈ % Z = 5% % Voltage Regulation = [(Eno-load - Efull-load)/(Eno-load)] x 100 (5/100) = [(480 - Efull-load)/(480)] Efull-load = 480 - (480)(.05) = 456 V

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POWER AND DISTRIBUTION TRANSFORMER COMPONENTS The ordinary power transformer consists of two windings that are wound on a common core and that are insulated from each other. The low voltage winding is usually wound around and next to the core; the high voltage winding is wound over the low voltage winding. The core provides a path for the magnetic flux. In addition to the windings and core, there are several other parts that make up a typical power transformer. These parts include the insulation enclosure, bushings, cooling systems, tap changers, and auxiliary components. Cores The core of a transformer is made from a special type of iron. Iron cores are used to reduce the reluctance of the flux path so that very little current is required to induce flux. The core cannot be made from a solid block of iron because a solid core acts as a short circuited turn. A solid core would cause a very high circulating current in the core, which would result in very high losses. These circulating currents are called eddy currents. To reduce the level of eddy currents, transformer cores are made from placing laminations next to each other. Each lamination is insulated from the other. Figures 9 and 10 compare the eddy current effects between a solid piece of iron and a core made from laminations.

Figure 9. Eddy Currents in a Solid Core

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Figure 10. Eddy Currents in a Laminated Core Laminations A lamination is a thin sheet of core iron material. Transformer designers have determined that the most economical thickness for core laminations is between 0.010 and 0.020 inches. These laminations are then assembled into a core by placing several laminations together. The lamination material is usually a silicon-iron alloy sheet. This material is closely controlled during the manufacturing process. It is made of approximately 3 percent silicon and 97 percent iron. The silicon reduces the viscous resistance to magnetization, or hysteresis, and prevents increased loss with age and environment. The laminated material is cold rolled and annealed to orient the grains or iron crystals to obtain very high permeability and low hysteresis to flux in the direction of rolling. The laminations are then treated to develop a chemical coating to insulate the laminations from each other. This chemical insulation prevents the laminations from shorting to each other. If the laminations were not insulated, the whole core would act like one large conductor, and would result in very high power losses in the core.

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The laminations are then stacked in one of several configurations. Figure 11 shows several basic types of core construction or lamination configurations. The three-legged core is the most common type of core that is found in large three-phase power transformers. Each leg of the transformer has a primary and secondary winding placed around it.

Figure 11. Typical Transformer Core Designs Types Of Core Construction There are two basic types of core construction. The two types of core construction are called core-form type and shell-form type. Figure 12 shows the configuration for both types of cores.

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Figure 12. Core Types
Core-Form Construction Type - The core-form type of construction has the windings constructed

around the laminated core. Most manufacturers use this type of design. In power transformers, the high voltage winding is placed or wound over the low voltage winding, as shown in Figure 13.

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Figure 13. Core-Form Type Design
Shell-Form Construction Type - Figure 14 shows the configuration of a shell-form type core

construction. The shell type design has the laminated core constructed around the windings. The windings in this design are placed vertically or stacked over each other.

Figure 14. Shell-Form Type Design

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Concentric Windings There are many types of transformer windings. For a given range of capacity and voltage ratings, each has its own advantages and disadvantages. The windings are designed to place the required number of turns into a minimum amount of space. The wire must be large enough to carry the current without overheating, and there also must be space for insulation and cooling paths. The windings are made from either copper or aluminum wire. For large power transformers, the windings are made from insulated rectangular wire. The ultimate goal in deciding which winding to use is to pick the one with the minimum cost that will meet all of the requirements for capacity, voltage, service ability, size, and efficiency. The transformer designer determines which type of winding to use. Some of the parameters the winding designer considers include: • Adequate effective dielectric strength against • • • operating voltages system faults switching surges lightning surges

Adequate coil ventilation (effective cooling) Adequate mechanical strength Minimum cost

Concentric windings are the most common type of power transformer windings. In concentric windings, the high voltage winding is placed or wound over the low voltage winding to obtain good coupling between the windings. The low voltage winding may be in two sections with the high voltage winding sandwiched between the two, which improves the leakage reactance. Figure 15 shows several of the types of concentric windings found in power transformers.

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Figure 15. Typical Winding Designs for Power Transformers Rectangular Concentric Because very little space is wasted, rectangular concentric windings, which have a rectangular core section with the coils wound in an approximately rectangular form, are the most economical winding construction. This rectangular concentric type of winding is limited to a finite ampere density, because high currents tend to force the windings into a circular form due to the intense magnetic field. This type of winding is usually limited to distribution transformers that are rated up to 15,000 volts.

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Round Concentric Round concentric windings are used when the mechanical and magnetic forces due to short circuit currents become too large for a rectangular winding. There are two common round concentric forms that are generally used. One is the cylindrical or layer coil and the other is the continuous wound disk-type coil. The cylindrical layer type is constructed of layers of wire wound in a cylindrical form, while the disk type is constructed of many individual wound pancakes that are stacked one upon the other. The usual configuration is to alternately stack high and low voltage disks. Round concentric winding types of design are common in shell-type cores. Insulation Insulation is one of the most important, if not the most important, component in a power transformer. Within the transformer assembly are a number of critical areas that must be adequately insulated to assure that the transformer will function properly and also to provide a satisfactory service life. These critical areas include the turn-to-turn, high-voltage to lowvoltage, low-voltage-to-core, phase-to-phase, and core-to-ground (case) insulation. This insulation must be properly selected and installed, otherwise the transformer will have a very short service life. Figure 16 illustrates the main insulation system of a three-phase, core-form type transformer.

Figure 16. Core-Form Type Transformer Insulation System

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Liquid-Filled Transformers Figure 17 shows an expanded view of the insulation arrangement of a liquid-filled power transformer.

Figure 17. Insulation Arrangement of a Power Transformer

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Insulating Oil is an equally important part of the transformer’s insulation system. Although

there are synthetic types of oil, the most commonly used type of insulating oil is mineral oil, which is a product of nature. Mineral oil is used principally because of its extremely low cost compared to any other synthetic insulating liquid. It also has the advantage, when used to impregnate paper insulation, of being extremely effective in reducing electrical stress and increasing dielectric strength. Transformer insulating oil is a refined mineral oil that is obtained from the fractional distillation of crude petroleum. It is principally a mixture of hydrocarbons, that when new, is free from moisture, inorganic acid, alkali, sulfur, asphalt, tar, vegetable, and animal oils. A good grade of transformer oil has high dielectric strength, low viscosity, is free of inorganic acids, has good resistance to emulsification, resists sludging, has a low pour point, and a high flash point. Enemies of transformer oil include oxidation, contamination, and excessive temperature. Oxidation is the most common cause of oil and insulation deterioration. Faulty gaskets and poor welds permit a unit to breathe air. The oxygen in the air reacts with the oil to form acids, water, and eventually oil sludge. Once sludging has started, a clay treatment is required to remove the deterioration products. With regard to contamination, moisture is typically the chief cause. The main source of moisture is moist air that is breathed into the transformer through faulty gaskets, or during transformer assembly and inspection with the manhole cover removed. Other solid contaminates in oil can come from solid insulation, metal parts, or airborne contamination that is breathed into the transformer. Excessive heat accelerates oxidation and it affects the decomposition of the oil. Heat also degrades the other insulating materials in a transformer, generating carbon dioxide and water, which in turn contributes to the deterioration of the oil. Additionally, minor faults such as corona discharges, sparking, and local overheating under the oil level, cause the oil to break down, and thus produce combustible gases that are harmful to the operation of the transformer.
Solid Insulation - Most of the solid insulation found in a liquid-filled transformer is made from

cellulose. Cellulose is an organic compound that exists in nature as various types of fibers. Some of the fibers that are used to make good dielectric paper insulation include the following: • • • Wood Fiber.............................Kraft paper & Kraftboard Wood & Cotton Fibers............Pressboard Manila Rope...........................Manila Paper

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Paper has excellent dielectric strength when it is dry. Typically the dielectric strength of paper is 1000 volts/mil (1 mil = 0.001 inch). However, dry paper will absorb water very quickly, and, once the paper has absorbed water, its dielectric strength decreases rapidly. To prevent the paper from absorbing water, it is impregnated with a resin, varnish, or insulating liquid. The impregnation treatment fills the spaces between the fibers and it deters the absorption of water. Paper insulation is also affected by temperature. If the insulation is operated at a temperature higher than its rating, then it will deteriorate rapidly, leading to a premature failure of the insulation, thus reducing the service life of the transformer. ANSI/IEEE Standard C57.12 rated temperatures for liquid-filled transformers are listed in Figure 18.

Ave. Winding Temp. Rise (0C) 55 65

Hottest-Spot Temp. Rise (0C) 65 80

Hottest-Spot Temperature (0C) 95 110

Figure 18. ANSI/IEEE Standard C57.12 Liquid-Filled Transformer Temperature Ratings As a rule of thumb, engineers design transformers to have a minimum service life of ten years, when operated 24 hours a day at their rated temperature. The ANSI/IEEE Transformer Committee takes a more conservative approach to the estimated life of a transformer and places a normal life expectancy of 65,000 hours (approximately 7.5 years). However, owing to a multitude of variables that occur during normal operation, it is impossible to accurately predict the true length of service life. In a similar manner, just as operating at elevated temperatures will reduce the service life of a transformer, the opposite is also true. That is, operation of a transformer below its rated temperatures can be expected to increase the service life. As a result, in real life, transformers normally exceed their design life expectancy by a large margin because they are very seldom operated 24 hours a day at maximum temperature.

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Dry-Type Transformers Dry-type transformers typically operate at higher temperatures than liquid-filled transformers, and, therefore, use insulating materials that are rated for higher temperatures. Furthermore, the temperatures of the different parts of the dry-type transformer vary much more widely than those of liquid-filled, where all of the insulation is at least as hot as the liquid. Figure 19 lists the ANSI/IEEE Standard C57.12 rated temperatures for dry-type transformers. Ave. Winding Temp. Rise by Resistance (0C) 60 80 115 130 150 Insulation System Temperature (0C) 130 150 185 200 220

Figure 19. ANSI/IEEE Standard C57.12 Dry-Type Transformer Temperature Ratings The materials that are used in dry-type transformers deteriorate differently than that of the paper insulating materials used in liquid-filled transformers. Without the benefit of being totally immersed in an insulating liquid (e.g., oil), dry-type insulations are much more dependent on the integrity of their bonding resins for dielectric strength. In addition, during normal operation, the insulations suffer from greater exposure to temperature and humidity. As a result, a number of factors combine with time and temperature to increase the rate of material degradation. The primary factors include the following: • • • • • Electric stress and associated effects Vibration or varying mechanical stress Repeated expansions and contractions Exposure to moisture, contaminating environments, and radiation Incompatible materials

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Enclosures Enclosures are necessary to contain and protect the critical components of the transformer, as well as to provide protection for personnel from electrical hazards. Specifically, the enclosure serves to: • • • Protect personnel from energized parts. Protect the windings (and insulating fluid) from the deteriorating effects of dirt, air, moisture, and other environmental contaminates. Prevent mechanical damage to the internal parts of the transformer.

There are basically two groups of enclosures, one for dry-type transformers and one for liquid-filled transformers. For each group, there are a variety of designs (types) based on the methods that are used for cooling and protecting the transformer’s internal components. Dry-Type Transformers Types of enclosures for dry-type transformers include ventilated cases, non-ventilated cases, and gas-filled sealed cases. ANSI/IEEE Standard C57.12.55-1987 groups transformers into 4 categories: Category A, B, C, and Sealed Enclosures. These enclosures are typically constructed using a heavy gauge sheet metal in accordance with thickness dimensions given in the ANSI/IEEE standards. For the ventilated and non-ventilated cases, removable panels provide access to the core, coils, and taps to allow inspection, cleaning, and testing. Often, these cases are designed to allow for the complete removal of the case structure to reduce size and weight for rigging into tight locations. To support rigging of the transformer, features that are normally included in the enclosure design are provisions for jacking, rolling, and lifting. ANSI/IEEE standards address the construction of dry-type transformer enclosures in considerable detail. Following is a summary outline of the requirements given in ANSI/IEEE C57.12.55-1987 and ANSI/IEEE C57.12.52-1981: • Category “A” Indoor or Outdoor Enclosures - This type of enclosure is intended (1) for ground level use in installations subject to deliberate, unauthorized acts by members of the unsupervised general public, and (2) primarily to provide a degree of protection against contact with the enclosed equipment.

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Category “B” Indoor or Outdoor Enclosures - This type of enclosure is intended (1) for use in installations not subject to deliberate, unauthorized acts by members of the unsupervised general public, and (2) primarily to provide a degree of protection to unauthorized and untrained personnel against incidental contact with the enclosed equipment. Category “C” Indoor or Outdoor Enclosures - This type of enclosure is intended (1) for use in installations in secured areas generally inaccessible to unauthorized and untrained persons and (2) primarily to provide a degree of protection against contact with the enclosed equipment. Sealed Enclosures - Sealed tanks are hermetically sealed and have welded covers. Tanks insulated with dielectric gases other than nitrogen or dry air are designed and constructed for a pressure of 15 psig (103 kPa gauge) and a vacuum equivalent to at least 2.7 kPa absolute pressure. Tanks insulated with nitrogen or dry air are designed for pressures or vacuum of + 7 psig (+ 48.3 kPa gauge).





Liquid-Filled Transformers The most common types of enclosures for liquid-filled transformers include sealed units (tanks), inert air systems, and conservator tank-types, which are often called constant oil pressure systems. These enclosures (tanks) are designed to provide dependable liquid and gas seals and withstand the stresses of processing, shipment, installation, and internal operating pressures. The tanks are typically constructed using 3/8-inch to 1/2-inch steel plates with reinforced sidewall bracing as required.
Sealed Units (Tanks) - The main enemies of transformer oil are heat, oxygen, and moisture.

When present, these elements cause oxidation and deterioration of the oil. In the sealed tank transformer (Figure 20), oxygen and moisture are prevented from entering the tank by either welding the tank or carefully sealing it so that it remains tight in operation. The tank is designed to withstand internal operating pressures without leaking oil or gas from the gas space. Sufficient gas space is provided above the liquid to allow for its expansion and contraction with temperature. Initially, the gas space above the oil is purged and then filled with nitrogen to a pressure of 3 psig @ 250C oil temperature. Following this, the gas pressure is allowed to vary (within limits) as the temperature fluctuates.

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Transformers of this sealed unit type design are provided with a pressure-vacuum bleeder valve assembly to correct any major pressure abnormalities. The assembly is composed of three parts: a relief valve, a pressure-vacuum gauge, and a gas sampling valve. The pressurevacuum bleeder valve is adjusted at the factory to bleed pressure or vacuum if either exceeds a predetermined limit (typically 6.5 psig). Adjusting screws located on the bleeder valve allow resetting these limits in the field. The sealed unit (tank) enclosure has the advantage of requiring only a minimum of attention, because the only concern is to ensure that no leaks occur that will allow the transformer to breath. This type of sealed unit transformer is particularly useful in remote locations where periodic inspections are limited.

Figure 20. Sealed Unit (Tank) Enclosure

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Inert Air Type System - The limitations of the sealed tank design are that it may leak and draw

air and moisture into the transformer, and that it has no way to remove any moisture given up by the insulation. The inert air type system (Figure 21) overcomes these limitations by maintaining a cushion of low pressure dry nitrogen in the gas space above the transformer oil at all times. For a typical inert air type system, a “bottle”, or cylinder, of compressed (2000 psi) dry nitrogen is connected through a multi-stage regulator to the gas space of the transformer. When the pressure in the gas space falls below a predetermined value (0.5 psig) the final stage of the regulator (also set at 0.5 psig) admits added nitrogen to maintain the pressure at 0.5 psig minimum regardless of temperature. Note: Initial filling of the gas space is typically 3 psig @ 250C oil temperature.

Figure 21. Inert Air Type System

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When high ambient temperatures and electrical load cause the internal pressure to increase to an undesirable high value, the excess pressure is vented from the transformer. A primary relief valve that is installed on the system is normally set to vent at 6.5 psig. This valve recloses as soon as the pressure falls slightly below 6.5 psig (approximately 0.75 psig) to prevent unnecessary loss of gas. A secondary or backup relief valve, incorporated in the system, is set to prevent the transformer from exceeding 8 psig. A compound pressure gauge is connected to the system and used to indicate the pressure in the transformer gas space. The gauge typically has a range of -10 to +10 psig and is normally equipped with two alarm microswitches. One alarm switch is set to operate at abnormally high pressure and the other set to operate should a vacuum occur. Normally these switches are set to operate at 8.5 psig on the pressure side, and -1.5 psig on the vacuum side. It should be noted that in accordance with the operating logic for this system, the vacuum alarm switch will never operate except in the case when the nitrogen cylinder is allowed to become empty. In a like manner, the pressure alarm switch will not operate unless the relief valve fails to perform its function, or the pressure builds up faster than it can be relieved by the relief valve due to a fault in the transformer. A transformer designed with an inert air system requires very little maintenance. Since the gas space is always under a positive pressure of nitrogen, there is little chance for oxygen or moisture to come in contact with the oil, thus oil life is extended with a reduced need for ongoing maintenance. It is of course important that the tank be tightly sealed to keep the use of nitrogen at the lowest possible level. The disadvantages of this type system are the initial added cost of the equipment needed to control and regulate the nitrogen, and the expense of replacing nitrogen bottles at regular intervals. Conservator Tank-Type The conservator tank-type enclosure system (Figure 22), which is commonly called a constant oil pressure system, is a design that provides for the transformer to be filled with oil to the very top of the tank, as opposed to having a gas space in the tank. Oil expansion or contraction takes place in a reservoir mounted above the transformer tank, similar to a regular expansion tank transformer. An air cell is placed inside the reservoir to prevent the oil from coming in contact with the air. The cell interior containing air is vented to the atmosphere. As the oil volume in the reservoir changes, the cell expands or contract to make up the remaining space. Atmospheric pressure is maintained at the oil surface through the flexible wall of the air cell. Thus, the pressure on the oil surface does not change with oil volume, which is in contrast to the pressure changes that result in most gas sealed systems.

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Figure 22. Conservator Tank-Type System

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A variation in design for many conservator tank-type transformers is the use of a rubber diaphragm in place of the expandable air cell. For this type design, the rubber diaphragm is attached to the reservoir tank wall and the expansion and contraction of the oil raises and lowers the diaphragm. As long as the diaphragm remains tight, it allows the pressure on the oil to remain constant while protecting it from air and moisture. Two primary advantages of the conservator tank-type enclosure (constant oil pressure system) are the following: • • The air cell in the reservoir tank provides an effective barrier to prevent atmospheric contamination of the oil by oxygen, moisture, or other airborne contaminants. The benefit of maintaining atmospheric pressure on the oil at all times prevents supersaturation of the oil with gas, as may be encountered in transformers with positive pressure on the oil. If oil supersaturated with nitrogen is cooled or the pressure reduced on the oil, the gas begins to leave the oil. Bubbles of gas leaving transformer oil can be a serious hazard to the electrical integrity of a transformer.

Bushings Leads from the windings are brought outside of the transformer enclosure or tank for making the necessary electrical connections. In bringing the leads through the tank wall or cover, the insulation of the electrical system must be maintained. For very low voltages, the method that is used can be as simple as passing insulated cables through an insulating collar. However, as the operating voltage increases, the method of bushing construction becomes necessarily more complex. The two types of bushing construction that are used by most manufacturers are bulk-type bushings and condenser-type bushings. In most cases, bushings are designed and manufactured in accordance with the electrical and mechanical characteristics that are specified in ANSI Standard C76.1. Bulk-Type Bushings (Less Than 15 kV) There are two basic types of bulk-type bushings: porcelain and resin. The porcelain bulk-type bushing (Figure 23a) consists of a conductor surrounded by a rigid insulator that is used to isolate the conductor electrically, but to anchor it mechanically. The typical construction for this type bushing uses a wet processed porcelain insulator. A metal flange is attached to the insulator by rolling a sleeved portion of the flange into a groove on the porcelain.

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A silicone gasket that is placed between the sleeve and the porcelain before rolling eliminates mechanical stress on the porcelain and it provides a gas tight seal. A conducting rod or lead is inserted in the porcelain and the assembly is sealed with gaskets. This type of bushing is generally used at 15 kV or below, but is available for use up to 25 kV. The cast-resin bulk-type bushing (Figure 23b), in its simplest form, consists of a conducting stud cast into a resin body. The resin body may have (1) a cast flange or resin for bolting in place, (2) a cast in metal ring for welding on applications, or (3) a cast in metal flange for bolting applications. For some applications, multiple bar conductors are cast into a single block that provides a single unit for three-phase connections, which gives the advantage of reducing the number of tank openings required for the bushing installation. Multiple conductor cast-resin bushings are also used for special high current applications.

Figure 23. Bulk-Type Bushings

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Condenser-Type Bushings (Greater Than 15 kV) Condenser-type bushings are generally used on transformers that are rated above 15 kV. This type bushing uses the condenser principle for its construction. Specifically, the bushing is constructed by wrapping insulation around a conducting stud. At carefully determined spacings from the stud, properly dimensioned metal foil layers are placed and wound into the insulation layers. The carefully placed layers of insulation and metal foil create the effect of a string of capacitors connected in series, across which voltage divides evenly. Using this method to assemble the insulation for the bushing provides the desired result of evenly dielectrically stressing the insulation. Several types of condenser-type bushings are used in constructing transformers. The most commonly used type is the oil-impregnated condenser bushing (Figure 24). This type of bushing is constructed by winding Kraft paper over a central tube or stud with metal foil wound into the paper at specified diameters to form a condenser. Using vacuum and heat, moisture is removed from the wrapped insulation. The condenser portion of the bushing is then impregnated with transformer oil. The impregnated condenser winding is completely encased in a chamber consisting of upper and lower porcelain. The porcelain housing is flushed and filled with oil under vacuum. An expansion bowl is provided at the top of the bushing to allow the oil to expand. A sight glass or oil level gauge placed at the top indicates the oil level in the bushing. A mounting flange is located at the middle of the bushing with gaskets placed between it and the upper and lower porcelains. For higher voltage ratings, this flange is fitted with a voltage tap receptacle. Gaskets at the top and bottom of the bushing seal the oil chamber. A special spring arrangement is used at the top to clamp and to seal the entire assembly against the bottom support, but still allow movement of the stud due to dimensional changes that occur with temperature. Fluted grooves, or protrusions, which are normally called “skirts”, are designed into the outside surface of the porcelain casing. The addition of skirts to the outside surface increases the surface “creep” distance between the high voltage terminal and ground, which reduces the possibility of flashover due to rain and dirt. Saudi Aramco standards require a creepage distance of 40 millimeters per system lineto-line kilovolts. This creepage distance is typically longer than the standard bushing for any voltage class. This longer creepage distance will most likely result in the bushing having a higher insulation class than that which is normally applied.

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Figure 24. Condenser-Type Bushing (Oil Impregnated)

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Figure 25 is another example of a condenser-type bushing that is used on large medium and high voltage transformers rated above 25 kV.

Figure 25. Typical 60 kV Condenser-Type Bushing

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Cooling Systems Energy losses in a transformer appear as heat in the core and coils. This heat must be dissipated without allowing the windings to reach a temperature that will cause deterioration of the insulation. Cooling Circuit The cooling of a transformer is required to conserve the life of the insulation. The life of the insulation is a function of temperature and time. The heat generated in the core and coils must be transferred through the insulation to the surrounding air or cooling fluid and then through the enclosure to the outside environment. This path is similar to an electrical circuit with resistances in series; each step impedes the flow of heat. This circuit relationship is shown in Figure 26.

Figure 26. Transformer Cooling Circuit

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The more efficient (lower resistance) each step, the more heat that can be transferred from the core and coils. This heat transfer allows the core and coils to be cooler. When designing a cooling system, the designer must look at a number of parameters that affect this heat transfer. Following are some of the more important parameters: • • • • • • • The insulation must be able to provide good dielectric and mechanical strength, but also be thin enough to allow for the fast transfer of heat. The cooling medium (air or oil) must move past the heat source so that the heat may be quickly removed from the source. The enclosure must be mechanically strong, but also thin enough to rapidly transfer heat through it. External devices such as fins and radiators may be added to speed up the transfer of heat to the environment. Pumps may be added to the external circuit to enhance flow in a liquid-filled transformer. Fans may be attached to the radiators to cool the fluid, which will increase the flow of fluid through the radiators. The pumps and fans may be controlled with a feedback circuit consisting of thermal sensitive elements in the oil, air, and windings.

Cooling Methods The methods used by manufacturers to cool transformers vary depending on transformer type, size, and application. The important principle is that the cooling medium efficiently transfer the heat from the core and coils to the outside air. The cooling medium for dry-type transformers is air and in some cases gas. In liquid-filled transformers, the insulating dielectric fluid is used to transfer the heat. Figure 27 shows the flow of oil in a self-cooled (Class OA) transformer and Figure 28 shows the various cooling methods for 5 types of cooling classes.

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Figure 27. Self-Cooled (OA) Transformer

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Figure 28. Various Transformer Cooling Methods

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Cooling for Dry-Type Transformers - For dry-type transformers, the non-ventilated and gas-filled

sealed designs depend on natural conduction, convection, and radiation to transfer heat to the enclosure and then to the outside air. For some non-ventilated designs, heat transfer efficiency is improved by adding cooling fins to the outside surface of the enclosure. Ventilated dry-type transformers add panels with grilles to provide an additional degree of cooling through the natural circulation of air. Beyond this, forced-air cooling can be added to ventilated designs with the addition of one or more banks of fans. The fans move the air past the windings, thus accelerating the rate of heat transfer. For some designs, the fans are controlled by thermal sensors located in the hottest spots of the core and coil assembly.
Cooling for Liquid-Filled Transformers - In liquid-filled transformers, heat from the core and

coils is transferred through the fluid. For small-sized transformers the tank surface is usually adequate to dissipate the heat, especially when the surface area is increased by making the tank taller than actually needed for enclosure of the transformer. For medium-sized transformers, the rate of heat dissipation can be improved by adding cooling tubes (radiators) to the tank (Figure 28c). These radiators are typically flattened external vertical tubes welded into horizontal headers that are, in turn, welded into the tank wall. The increased rate of cooling results from the increased surface area that is exposed to the air. The action that occurs inside the cooling tubes is referred to as thermosiphon flow. The oil next to the windings heats up, and thus rises to the top of the tank where it moves into the upper cooling header and tubes. In the tubes, the oil cools and sinks to the bottom, where it returns to the tank through the bottom header, ready to begin the cycle again. For largersized transformers, or units with shipping or installation limitations, detachable radiators are used. Flanged valves welded to the transformer tank are used to connect this type of detachable radiator during installation. When additional cooling is needed to allow a transformer to operate at its nameplate rating and temperature rise, fans can be used in conjunction with the radiators to provide forced air cooling. For some designs, the fans are mounted on top of the radiators to blow air downward over the tubes for maximum air flow over the areas of highest temperature. For other designs, typically those with removable radiators, the fans are mounted on the side of the radiators (Figure 28d) to force a high volume of air through ducts formed by the rows of tubes. The typical fan assembly consists of a fractional horsepower motor (either three-phase or single-phase) that uses a non-metallic fan blade (e.g., polyester). The motor is then mounted on a steel wireform bracket, which also serves as a guard for the fan blade.

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When the normal rate of movement of the oil by the thermosiphon action is insufficient for the cooling required, oil pumps (Figure 28e) can be used in conjunction with the radiators and fans to provide a self-cooled/forced-air, forced-oil cooled system. For this design, the oil is collected at the bottom of the radiators and it is forced by the pumps at an accelerated rate past the core and coils, which increases the thermal capability of the transformer. The pump is typically close coupled with its motor and it is enclosed in a single housing that utilizes the transformer oil for its lubrication. An oil flow indicator is provided to visually confirm that the pump is operating. Operation of the pump is typically controlled by one or more of the thermal sensors that are located in the transformer. Another method that is used to cool large transformers is the use of an oil-to-water heat exchanger. This exchanger type of cooling system is both efficient and compact, but it is only practical for transformers that have a suitable source of cooling water available for use. For this exchanger system, the oil is pumped from the transformer through the shell of the heat exchanger, while cooling water is circulated through the cooling tubes at the center of the unit. Heat from the oil is transferred through the cooling tubes and carried away by the discharged water. A double wall construction between the water and oil chambers prevents the possibility of contaminating the oil. Cooling Classes To provide a degree of standardization for transformer cooling systems, ANSI/IEEE has identified and published cooling classes for liquid-immersed and dry-type transformers (Figure 29). In selecting the type of cooling to be used for a transformer, the designer applies the principle that the minimum amount of cooling required by a transformer is the amount that is needed to allow its operation at rated conditions and at rated temperature rise. In accordance with ANSI/IEEE Standard C57.12.00-1987, the rated temperature rise for liquid filled transformers is 550C and 650C with hottest-spot temperature rise of 650C and 800C. Per ANSI/IEEE C57.12.01-1989, dry-type transformers have rated temperature rises of 600C to 1500C with hottest-spot insulation system temperatures of 1300C through 2200C, respectively. The designer first determines if self-cooling will be sufficient to allow the transformer to operate at its rating without overheating. When self-cooling is insufficient, additional cooling is added that uses the available methods of radiators, fans and/or pumps. Ultimately, the use of additional cooling methods provides the transformer with an increased kVA capability.

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Transformer Type Liquid Liquid Liquid

Class Code

Method of Cooling

OA OA/FA OA/FA/FA

Liquid-immersed, self-cooled Liquid-immersed, self-cooled/forced-air cooled Liquid-immersed, self-cooled/forced-air cooled/forced-air-cooled Liquid-immersed, self-cooled/forced-air cooled/forced-liquid-cooled/forced-air-forced-liquidcooled

Liquid

OA/FA/FO A

Liquid Liquid Liquid

OW OW/A FOW

Liquid-immersed, water-cooled Liquid-mmersed, water-cooled/self-cooled Liquid-immersed, forced-liquid-cooled with water coolers

Dry Dry Dry Dry Dry

AA AFA AA/FA ANV GA

Ventilated self-cooled Ventilated forced-air-cooled Ventilated self-cooled/forced-air-cooled Nonventilated self-cooled Sealed self-cooled

Figure 29. ANSI/IEEE C57 Cooling Class Codes With reference to Figure 29, the cooling classes most commonly used for liquid or oil-filled transformers are OA and OA/FA. The most common classes usually applied to dry-type

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transformers are AA and AA/FA. Note: The transformer nameplate lists the kVA rating(s), temperature rise, and cooling class. For example, the nameplate kVA and cooling class for a liquid immersed transformer might be listed as follows: • 2500/3125 kVA OA/FA 550C

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The first number listed for the kVA rating (2500) corresponds to the first cooling class (OA), and likewise, the second kVA rating (3125) corresponds to the second cooling class (FA). The first rating means that the transformer has a rating of 2500 kVA when the fans are turned off and there is no forced cooling. This rating is known as the self-cooled rating (OA). When the fans, which are mounted on the cooling radiators, are turned on and operating, the transformer has the higher (second) rating of 3125 kVA. This rating is called the forced-aircooled rating (FA). In both cases, the temperature rise is restricted to 550C. Tap Changers Often it is desirable to change, by a relatively small amount, the voltage ratio of a transformer. This voltage ratio change is done to compensate for the voltage drop in the supply source or to accommodate a change in the load. Most transformers have a device or method to change the voltage rating of one or both of the transformer windings. To change the voltage relationship, the ratio of the turns between the winding must be changed. Changing of the turns ratio is exactly what a tap changer does; it adds or subtracts turns in a winding. The simplest type of tap changer is one that brings taps from one or both windings to a terminal board where jumpers can be manually reconnected. Dry-type transformers will normally have this jumper board located inside of the winding enclosure or in an adjacent compartment. In small liquid-filled transformers, the tap connection board is usually located just below the surface of the oil, which requires removal of the tank cover and manual changing of the jumpers. For larger liquid-filled transformers, the manually connected jumpers are replaced with a rotating switch that is manually operated from outside of the tank (Figure 30). For all of these types of tap changers, the transformer must be de-energized before the connections or taps can be changed. These types of tap changers are know as NLTC or no-load tap changers. They are also called DETC or de-energized tap changers; both terms are commonly used. A second type of tap changer is one that allows the taps to be changed while the transformer is still energized and under load. Typically this type is applied to voltage regulator applications where the maintenance of voltage is of paramount importance. This type of tap changer is known as a load tap changer or an LTC.

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Figure 30. NLTC Set at Position #5 No-Load Tap Changer (NLTC) The no-load tap changer (NLTC), as its name implies, requires that the transformer be deenergized (no-load) before changing the taps. It is typically used for applications that require only infrequent tap changes or adjustments, and where the transformer can be disconnected from the line. The NLTC must never be operated while the transformer is energized, since it has no capability of interrupting load current. An NLTC adds or subtracts turns in the high voltage winding of the transformer. The standard arrangement includes five tap positions, with the middle position being the rated voltage position. There are two tap positions above the rated voltage position, and two below. Each tap selection represents a terminal-to-terminal voltage which is normally different by an amount equal to 2.5% of the rated voltage. The nameplate voltage associated with each tap selection represents the voltage which is needed at the high voltage terminals to produce rated voltage at the low voltage winding terminals, whenever that tap is selected, and with a condition of no-load current. The five positions thus provide a range of +5% and -5% from rated voltage.

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Figure 31 (item 12) shows the voltage ratings for the 5 different positions of the NLTC. Position 3 (68,800 V) is the nominal tap, positions 2 (70,600 V) and 1 (72,400 V) are the +2.5% and +5% taps, respectively, and positions 4 (67,000 V and 5 (65,200 V) are the -2.5% and -5% taps, respectively. The last column of item 12 indicates which terminals of the winding (item 11) are connected in each of the different tap positions. For example, in tap position 5, terminal 4 is connected to terminal 5, which means that the entire winding is in use. In tap position 5, terminal 2 is connected to terminal 7, which means that the windings between the 2 terminals have been effectively removed (shorted) from the transformer. Again, with reference to the nameplate shown in Figure 31, it should be noted that the turns ratio of a transformer is also equal to the voltage ratio. Thus, as the tap position is changed and turns are by-passed or added in, the turns ratio can be determined from the voltage ratio given on the nameplate. For a delta-wye transformer, the line-to-line voltage rating of the delta winding is divided by the line-to-neutral voltage rating of the wye winding. Example H: Answer: What is the turns ratio of the transformer described in Figure 31 for each of the different tap positions? 1. Np/Ns = Ep/Es 2. Tap Position 1: Np/Ns = 72400/7560 = 9.577/1 3. Tap Position 2: Np/Ns = 70600/7560 = 9.339/1 4. Tap Position 3: Np/Ns = 68800/7560 = 9.100/1 5. Tap Position 4: Np/Ns = 67000/7560 = 8.862/1 6. Tap Position 5: Np/Ns = 65200/7560 = 8.624/1 In accordance with ANSI standards, all taps for a transformer, unless otherwise marked on the nameplate, are considered to be full-rated kVA taps.

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Figure 31. Typical Transformer Nameplate

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Example I:

What is the kVA rating of the transformer described in Figure 31 for each of the different tap positions? 1. kVA = 3 x kV x I 3 x 72.4 x 12.0 ≈ 1504.8 kVA 3 x 70.6 x 12.3 ≈ 1504.1 kVA 3 x 68.8 x 12.6 ≈ 1501.5 kVA 3 x 67.0 x 12.9 ≈ 1497.0 kVA 3 x 65.2 x 13.3 ≈ 1502.0 kVA

Answer:

2. Tap Position 1: kVA = 3. Tap Position 2: kVA = 4. Tap Position 3: kVA = 5. Tap Position 4: kVA = 6. Tap Position 5: kVA =

The location and placement of taps within a winding are constructed with care to avoid potential physical and electrical problems. Normally, taps are not placed in the end of a winding because this would make the winding physically and electrically unsymmetrical, causing increases in the stray losses and the mechanical forces. Taps placed in the end of a winding would also be directly exposed to impulse voltages coming from the line. As a result, taps are normally placed in the center of a divided winding. This centered tap placement provides a more symmetrical construction for the winding, which is better able to balance out mechanical forces, while at the same time providing the taps with added protection against impulse voltages. With reference to Figure 31 (item 11), it is noted that for this example that the taps have been placed at the center of the winding. Load Tap Changers (LTC) One problem with the no-load tap changer (NLTC) is that the transformer must be deenergized to change the taps, which means that the power must be turned off, and subsequently disconnecting service and shutting down the operation of equipment. In many cases the user does not want, or cannot permit the power to be shut down. Typical examples are large users of power that have transformers that feed industrial plants, or large distribution systems that provide power to critical services (i.e., hospitals, schools, etc.). For cases where power must be maintained while adjustments are made to the voltage level, a load tap changer (LTC) must be used. The most important advantage of a load tap changer (LTC) is that it allows changing of a transformer’s turns ratio (adjusting voltage level) without shutting off the power. Another difference of the LTC, when compared to the NLTC, is that it is located in the low voltage winding of the transformer and it accomplishes voltage adjustments by changing the number of turns in the low voltage winding.

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Normally, load tap changers have 16 steps or taps above and 16 steps below the normal rated voltage level, resulting in a total of 33 tap positions. The preferred step change for each tap is 0.625% of normal rated voltage, which provides an adjustable voltage range of 10% above and 10% below the normal rated voltage (16 x 0.625% = 10%). In accordance with ANSI/IEEE C57.12.10-1988, liquid-filled transformers supplied with a load tap changer are required to have a tap position indicator similar to the design shown in Figure 32. The indicator must be fitted with hands to indicate the maximum and minimum positions used, and it must be located so that it can be read while operating the tap changer by hand. The face of the indicator must be marked to show the normal rated voltage (N neutral) position, the 16 steps of the raise range marked with an “R” and the 16 steps of the lower range marked with an “L”.

Figure 32. Position Indicator for LTC

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Auxiliary Components Transformer Nameplate In accordance with ANSI standards, a nameplate must be provided with each transformer. For both liquid-filled and dry-type transformers, the nameplate is required to be made of a corrosion-resistant, durable metal and it must provide (list) the ratings and other essential operating data for the transformer. ANSI/IEEE C57.12.00-1987 specifies that the data that must be included on the nameplate for liquid-filled transformers, and ANSI/IEEE C57.12.01-1989 specifies the information that must be included on the nameplate for dry-type transformers. With only a very few exceptions, the information required is the same for both types of transformers. As an example of the information provided on a typical nameplate, Figure 33 provides an itemized description of the nameplate that is illustrated in Figure 31.

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Item No. Item Item Description

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1

Serial Number

Identifies the specific transformer.

2

Voltage

Rated terminal-to-terminal voltage for the primary and secondary windings.

Frequency at which the transformer is designed to operate. 3 Frequency Method used to dissipate the heat generated during operation. 4 Cooling Class The kVA capacity the transformer can transmit without exceeding the stated temperature rise. 5 kVA Average winding rise above ambient temperature at rated kVA.

6

Temperature Rise

Instruction book that applies to the specific transformer.

Liquid volume of oil in the transformer. 7 Instruction Book Full wave BIL (basic insulation level) in kilovolts of the line and neutral terminals. 8 Gallons of Fluid Percent impedance measured by test. 9 Impulse Levels Winding connection diagram to show the relative location of bushings and internal terminals. 10 Impedance Chart showing the voltage, current, and connection of each tap changer position. 11 Winding Connection Diagram Saudi Aramco DeskTop Standards Contains valuable information about the operation and maintenance of the transformer. 54

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Figure 33. Transformer Nameplate Data

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Winding Temperature Indicator Heat is a natural enemy to winding insulation. As a result, designers determine and specify the maximum temperature that windings may operate at and still achieve normal expected service life. For liquid-filled transformers, the rated maximum hottest-spot winding temperatures are 950C and 1100C. For dry-type transformers, the maximum hottest spot temperatures range from 1300C to 2200C. The winding temperature indicator is a device that monitors a winding’s hottest-spot temperature and, as needed, sounds an alarm or turns on additional cooling equipment. The method varies depending on whether the transformer is a dry-type or liquid-filled transformer. For dry-type transformers, a thermal sensor is appropriately insulated and is directly placed at the estimated hottest-spot location in the low voltage windings. The sensor is connected to a dial type indicator that is mounted to the outside of the enclosure where it is readily visible to maintenance personnel. In addition to the (black) temperature indicating pointer, the dial typically has a second resettable (red) pointer that is used to indicate the maximum temperature attained since the last time it was reset. The temperature scale that is marked on the dial is well above the hottest-spot temperature rating that could possibly occur. Winding temperature indicators for dry-type transformers may have scales that extend to 2700C, depending on the temperature rating of the transformer. Alarm and/or control switches may be included in the dial assembly to sound alarms or to switch-on bands of cooling fans. For liquid-filled transformers, it is not practical to pass a sensor through the tank wall and to attach it to the windings. For this reason, a different method is used to monitor the winding hottest-spot temperature. Typically, the temperature is measured by placing a thermal sensor (i.e., thermometer bulb, bimetallic element) in a metal thermometer well, together with a heating coil. The thermal sensor is connected to a dial indicator that is located on the tank wall at eye level for convenient reading. The meter thermometer well is located in the hottest oil near the top of the transformer. Per ANSI/IEEE Standard C57.12, the thermometer well is required to be placed in the tank at least one inch below the liquid level at minimum operating temperature (-200C). Current, proportional to the transformer load current, is supplied to the heating coil through a current transformer (CT), which is typically one of the bushing CTs. The increment of temperature provided by the heater adds to the temperature sensed by the thermal element from the hot oil, to give a dial indicator reading that is equal to the winding hottest-spot temperature. In this manner, the device is able to indicate the winding hottest-spot temperature for any constant load.

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The temperature scale that is marked on the indicator is above the transformer’s hottest-spot temperature rating based on the transformer’s application and rating. Typical indicators for liquid-filled transformers have scales that extend to 1200C and 1600C, although some indicators have higher scales. Similar to the indicators used for dry-type transformers, indicators for liquid-filled transformers have resettable maximum indicator pointers and they are also fitted with control and alarm switches. Access to the control switch terminals is normally made through a disconnect plug in the bottom of the indicator case. A typical indicator might have as many as three or four adjustable control switches that come preset from the factory. Normal factory settings for the switches might include the setting of switch #1 to close at 700C for turning on a first bank of cooling fans. Switch #2 might be set at 750C to turn on a second bank of fans or a first oil pump. Switch #3 might be used to trigger an alarm and is set to close at 1170C. If a fourth switch were available, it might be set at 1250C and it is used to open a selected circuit breaker to shed some of the transformer’s load. As necessary, control switches may be field set by removing the indicator’s glass cover and then change the adjusting screws (Figure 34).

Figure 34. Winding Temperature Indicator

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Liquid Temperature Indicator The liquid temperature indicator (Figure 35) is a device that measures the transformer’s top liquid temperature. It is typically constructed of a stem type bimetallic element that is housed in a weatherproof casing. The stem fits closely into a thin walled well that screws into the side of the tank. Use of the metal thermometer allows the thermal unit to be removed for inspection or calibration without loss of liquid, and with no need to lower the liquid level. In accordance with ANSI/IEEE Standard C57.12, the thermometer well is required to be placed in the tank at least 1-inch below the liquid level at minimum operating temperature (e.g., -200C). ANSI/IEEE Standard C57.12 requires that the indicator face be mounted vertically when located at heights of 96 inches and below, and angled down at 300 when mounted higher. Per standards, the indicator is required to have a dark-face dial with light markings, a light colored indicating hand, and an orange-red, resettable maximum indicating hand. The standards additionally require that the dial indicator have a diameter of 4-1/2” + 1”, with a temperature range of 00C to 1200C, and have the words “Liquid Temperature” on the dial face or on an adjacent nameplate. Liquid temperature indicators typically come with two built-in control micro-switches set to operate at different temperature levels. The lower temperature switch is set to control the fan circuit and to turn on the cooling fans when the transformer’s temperature comes within range of the switch setting (approximately 600C). The higher temperature switch is designed to operate an alarm and give warning if, for any reason, the fans do not limit the temperature to the proper range. The alarm switch is usually set to close at approximately 900C. However, it should be noted that the function of controlling the cooling fans is normally handled by the winding temperature indicator. The liquid temperature indicator is connected and used to control the cooling fans only when there is no winding temperature indicator installed on the transformer. To determine the nominal expected temperatures that might be read from the indicator, consider that liquid filled transformers are rated to have either a 550C or a 650C average winding temperature rise above ambient, when measured by resistance. Using a rated 650C rise transformer in a 300C ambient temperature means that the transformer winding should not exceed 950C when operated at nameplate-rated kVA.

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Because all of the heat generated by the windings is transferred to the liquid, the liquid will gradually approach the temperature of the windings, but with a time lag. As a result, when the transformer is operated within its rating, the liquid temperature should never exceed 950C. In reality, for transformers operated at rated kVA, the liquid temperature indicator never quite reaches this temperature because it is located at a point below the surface where the liquid is slightly cooler. Note: Liquid temperature indicator protection is usually set at the factory.

Figure 35. Liquid Temperature Indicator

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Liquid Level Indicator The liquid level indicator is a self contained float type device that indicates the level of the liquid inside of the transformer. The construction is a two part assembly. One part is a sealed body that is attached to the tank wall with an actuating magnet shaft and float arm located inside the transformer. The other part is the bezel (Figure 36), or outer assembly, containing the calibrated dial and indicating needle. The indicating needle is directly connected to a second magnet that is positively displaced by the rotation of the magnet and the float arm assembly that is inside the tank. In accordance with ANSI/IEEE Standard C57.12, the dial is marked to show the 250C liquid level. It is also required to show, by a permanent marking on the tank or an indication on the nameplate, the distance from the liquid level to the highest point of the hand-hole or manhole flange surface. For the nameplate shown in Figure 31, it specifically states that “the 250C liquid level is 13.5 inches below top of highest manhole flange. Oil level changes .55 inches for each 100C.” The 250C mark also indicates the normal level for the liquid when it is at a temperature of 250C. As the liquid warms during transformer operation, its volume will expand (the volume of oil expands approximately 5% when heated from 250C to 850C). Because the surface area for the liquid remains the same, the change will take place in the height or level of the liquid. In a similar manner, as the liquid cools, its volume decreases, and its level goes down. The maximum or “HI” level mark indicates the correct level for the liquid when it is operating at its maximum rated temperature (850C). The minimum or “LO” level mark indicates the correct level for the liquid at its minimum rated operating temperature (-200C).

Figure 36. Liquid Level Indicator

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Pressure/Vacuum Indicator The pressure/vacuum indicator (Figure 37) indicates whether the gas space in a transformer tank is under positive or negative pressure. Most liquid-filled transformers are sealed to prevent the entrance of oxygen, moisture, and other harmful contaminates. The gas space above the liquid is then normally filled with nitrogen to a pressure of 3 psig at a 250C liquid temperature. However, after the initial filling, the pressure will vary depending on barometric pressure and the temperature inside the transformer. If the transformer is de-energized or operated under light load in low ambient temperatures, the pressure might turn negative. A positive pressure (minimum 0.5 psig) should always be maintained to prevent the breathing in of moist air through potential leaks in the seals. In accordance with ANSI/IEEE Standard C57.12, the pressure/vacuum indicator is required to be a dial type gauge with a diameter of 3-1/2” + 1/4”, have a dark-face with light colored markings and pointer, and have a scale with a range of + 10 psig. This gauge type of indicator will very often have two alarm microswitches set to operate at approximately 8.5 psig on the pressure side, and -1.5 psig on the vacuum side. On oil-filled transformers, the pressure/vacuum indicator is normally furnished in combination with a pressure regulator that will automatically bleed off excess pressure, or add make-up gas.

Figure 37. Pressure/Vacuum Indicator

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Pressure Relief Device Typically, transformers rated above 2500 kVA are furnished with a pressure relief device (Figure 38), sometime referred to as a mechanical relief device (MRD), mounted on the transformer cover. The device consists of a self-resetting, spring-loaded diaphragm and mechanical operation indicator (semaphore). Should the tank pressure increase above 10 psig, the gas pressure will lift the diaphragm and vent the excess pressure. Immediately after the pressure returns to normal, the diaphragm will reset and reseal the transformer. The mechanical operation indicator is a lightweight plastic semaphore that rests on the diaphragm. When pressure increases and the diaphragm rises, it lifts the semaphore into view and indicates that the relief device has operated. Although the relief device resets and reseals automatically after each operation, the semaphore remains in the tripped (visible) position until manually reset. For some transformer installations, a vent hood is bolted over the pressure relief device with an exhaust pipe connecting it to an outside area. The purpose of the vent hood is to allow any toxic and/or combustible gases, released by the relief device, to be carried outside of a vault or building. As a precaution, personnel should not enter a vault or any confined area in which a transformer relief device has been know to operate or in which a transformer has failed until the area has been thoroughly ventilated. Gases released from a transformer can be toxic and life threatening. When a transformer is to be pressure tested for leaks at a pressure greater than 8 psig, the relief device must be replaced with a blind flange. On the other hand, the typical relief device will withstand full vacuum and need not be removed from the transformer tank during any vacuum treatment. Should disassembly of the relief device be necessary, caution must be taken to guard against injury to personnel. The pressure releasing springs are under tension, and if this tension is not carefully released, the force from the springs could cause the cover to blow off. Similar to the other indicators and devices attached to a transformer, the pressure relief device is sometimes supplied with alarm contacts for use in triggering remote indicators to alert maintenance personnel that the pressure device has operated.

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Figure 38. Pressure Relief Device or Mechanical Relief Device Sudden Pressure Relay The sudden pressure relay (SPR), as shown in Figure 39, is a device designed to respond to a sudden increase in gas pressure in a power transformer, which is typically caused by internal arcing. An SPR normally consists of three main parts: a pressure sensing bellows, a microswitch, and a pressure equalizing orifice. The relay is typically enclosed in a sealed case and is mounted on top of the transformer with its pressure sensing element in direct contact with the gas cushion of the transformer. When an arcing internal fault in the transformer produces an abnormal rise in gas pressure, the bellows expands, causing the microswitch to operate, which in turn signals a circuit breaker to trip and clear the fault. Under fault conditions, the rate of rise of gas pressure in a transformer is proportional to the arc power, and inversely proportional to the volume of the gas space. The SPR operates on the difference between the pressure in the gas space of the transformer and the pressure that is inside the relay. The SPR is constructed to be very sensitive to rapid pressure change and it will normally operate within 3 to 4 cycles (0.049 to 0.066 seconds) in response to a pressure rise of 5.5 psi per second. At high rates of rise, 30 to 40 psi per second, the SPR will operate in as little as 1/2-cycle (0.008 seconds).

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For normal pressure changes resulting from changing ambient temperatures or load conditions, the equalizing orifice allows the pressure within the relay to equalize with the pressure in the transformer, which prevents any operation of the relay due to slow pressure changes. Note: An SPR requires no settings and upon operation it will trip a lockout relay, and the lockout then trips all breakers.

Figure 39. Sudden Pressure Relay (SPR) Fill and Drain Valves A liquid-filled transformer typically has two valves for handling the fluid. One valve is located at the top of the tank, slightly below the normal oil level, and it is referred to as the upper fill (and filter) valve. The other valve is positioned at the bottom of the tank for draining (and filtering) the fluid, and it is typically called the drain valve. In accordance with ANSI/IEEE C57.12, the lower combination drain and filter valve must be located on the side of the tank and it must provide for drainage of the liquid to within 1-inch of the bottom of the tank. The drain valve must have a built-in 3/8-inch sampling device located in its side between the main valve seat and the pipe plug. The sampling device must be supplied with a 5/16”-32 male thread for the user’s connection and it must have a thread protecting screw-on cap.

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The size of the drain valve must be 1-inch for transformers through 2500 kVA and 2 inches for larger kVA ratings. Transformers rated through 2500 kVA must have a 1-inch upper fill (filter) plug or cap located above the maximum liquid level. Above 2500 kVA, transformers must have a 1-inch fill (filter) valve located below the 250C liquid level. Control Cabinets Many transformers have a weatherproof control cabinet to provide for termination and remote wiring of the various devices mounted on, or in, the transformer. The devices that are terminated in or wired through the control cabinet include internal current transformers, automatic and manual controls for cooling fans and pumps, wiring to and from indicator microswitches, controls for load tap changing equipment, wiring for pressure relief and sudden pressure relays, and main and auxiliary power sources for operation of cooling equipment. Depending on the design and requirements of the transformer, there may be none, one, or more control cabinets.

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POWER AND DISTRIBUTION CONSTRUCTION AND APPLICATION

TRANSFORMER

CLASSIFICATION:

Most transformers used in an industrial plant are of three-phase construction. A three-phase transformer costs less, has higher efficiency, and requires less space than three, single-phase transformers. The key to dependable and economic power distribution is the selection of the proper transformer type for each application. Power transformers are used in electrical distribution systems to transfer large amounts of power. Power transformers are different from other types of transformers, for example, electronic circuit transformers or instrument transformers. There are many types of power transformers. The type of transformer to use depends on the size required, type of application, and installation requirements. Transformers are classified by the type of construction and by application. Construction Classifications The four construction classifications of transformers are the following: • • • • Dry-type (conventional and cast coil) Oil-filled Non-flammable insulating liquid-filled Gas-filled

Dry-Type (Conventional) Dry-type (conventional or standard) transformers are used for indoor applications at low and medium voltage levels. The installation location must be dry and well ventilated so that cooling air may move or be forced across the windings and core. Dry-type transformers are applied at load centers and other locations where economics are an issue, because they are less expensive than fluid-immersed units, they are air-cooled and air-insulated for use in buildings and industrial locations near the intended load, they are free of explosive hazards, and they contain few or no organic materials. Class H insulation must be specified for drytype transformers.

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Dry-Type (Cast Coil) In a cast coil transformer, the windings are completely covered by an epoxy resin. The epoxy resin provides enhanced protection of the windings from atmospheric contaminants. For most indoor applications the cast coil transformer is preferred over the standard dry-type transformer. The cast coil transformer has the following advantages over the conventional or standard dry type transformer: • • • • • Unaffected by humidity Easier to replace the windings Less likely to trap dirt and sand Less noisy Better insulation system

Oil-Filled Oil-filled transformers have the lowest cost of any other type of transformer, but their use is primarily for outdoor applications. The mineral oil used in oil-filled transformers is flammable, and, therefore, it cannot be used indoors except in specially designed transformer vaults. Oil-filled transformers are suitable for use in dirty and dusty areas and adverse weather conditions. Non-Flammable Insulating Liquid-Filled Non-flammable insulating liquid-filled transformers use silicone as an insulating fluid. Silicone-filled transformers can be used indoors and are best suited for indoor applications where there is a very dirty, dusty, or corrosive environment. Gas-Filled Gas-filled transformers are used where liquid-filled transformers cannot be used because of the potential for explosion (i.e., hazardous areas). The gas used as the insulating medium is fluorocarbon, which is a nonflammable and nonexplosive gas. Gas-filled transformers can be used indoors or outdoors. Gas-filled transformers are more expensive than most other types of transformers. Application Classifications While there are specific ANSI/IEEE definitions for power and distribution transformers, the differences are hard to apply.

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Saudi Aramco Classifications
Power - Saudi Aramco standards specify that power transformers are classified by application

as follows: • • Transformers in plant areas that supply motor control centers or switchgear are designated as power transformers. Transformers that supply residential area distribution systems at 2.4 kV, 4.16 kV, or 13.8 kV are power transformers.

Distribution - Saudi Aramco standards specify that transformers that supply residential areas

directly at 480 volts or less are designated as distribution transformers. Overhead or pad mounted transformer types will normally fall in this distribution transformer category. Overhead transformers are limited to 500 kVA and below and they are mounted on poles above the ground. Pad mounted transformers are rated 500 kVA and below and are used in residential areas where underground service distribution is installed. Generator Power Transformer Generator power transformers are classified by application as follows: • • • • There are no voltage regulating windings on generator power transformers because regulation is usually accomplished by the generator field circuit. There is uniform load because the load is maintained at maximum most of the time. There is little need for efficiency and quiet operation because power is cheap and other equipment is much noisier. The maintenance requirements are less stringent because skilled personnel are usually available at all times.

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Substation Power Transformers (Sub-Transmission Circuits) Substation transformers (sub-transmission circuits) are classified by application as follows: • • • • Taps and regulating schemes are almost always required. Loads vary over a wide range both daily and seasonally. Losses are expensive so that high efficiency is desirable. Less maintenance and supervision is available, so the transformers must be better designed for unsupervised operation.

Substation Power Transformers (Primary-Feeder Circuits) Substation transformers (primary-feeder circuits) are classified by application as follows: • • • • • Load varies over wider limits so that transformer impedance should be low. Losses are very expensive, and very high efficiencies are required. Sometimes located in remote areas without constant supervision. Load tap changers and regulation are necessary to accommodate wide load variations. Quiet operation is very desirable because the units may be in residential areas.

Distribution Transformers Distribution transformers are classified by application as follows: • • • The load varies widely and the unit is often overloaded. Very little supervision is possible. The transformer must operate reliably with a minimum of maintenance. The cost of supplying losses is the highest. A low exciting current is important.

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Voltage regulation is very important. Taps in the primary are usually used to set the primary system voltage. Separate voltage regulation equipment is usually provided to allow wide ranges of operational load. It is usually impossible to avoid excessive voltages in distribution circuits, therefore the transformer must be designed to withstand wide voltage swings. The units are sometimes subject to mechanical abuse. Because many transformers are required for each system, they must be constructed with the cost of installation and replacement in mind.

• • •

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THREE-PHASE POWER AND DISTRIBUTION TRANSFORMER CONNECTIONS ANSI/NEMA Labeling Conventions ANSI/IEEE Standard C57.12.70 is the standard for transformer terminal markings (labeling) and connections. The highest voltage winding is designated H. The other windings (i.e., low voltage) are designated X, Y, Z in order of decreasing voltage. If two or more of the windings have the same voltage but different load-current ratings, the winding with the larger load-current rating is assigned the prior letter designation. If windings of the same voltage have the same loadcurrent rating, the letter assignment is arbitrary. External terminals are distinguished by marking each terminal with a capital letter followed by a subscript number (Figure 40a). The terminals are numbered 1, 2, 3, 4, 5, etc. with the lowest number and highest number being the full winding and intermediate numbers being the fractions of windings or winding taps. If the terminals of a low voltage winding are split in two portions, with each having two terminals, they are labeled (marked) X1, X2, X3, X4, as illustrated in Figure 40b.

Figure 40. ANSI/NEMA Labeling (Terminal Marking) Convention

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Figure 41 also shows the standard labeling (terminal marking) of a three-phase transformer. If a neutral point is brought out of the transformer, it is labeled X0 or Ho. When facing the high voltage side of a three-phase transformer, the high voltage lead H1 is brought out on the right side. The remaining high side leads are brought out and numbered in sequence, from right to left, H2, H3, etc., (Figure 41). When facing the low voltage side of a three-phase transformer, the low side lead X1 or X0 if the neutral point is available, is brought out on the left side. The remaining low side leads are brought out and numbered in sequence, from left to right, X2, X3, etc., or X1, X2, X3 if the neutral is unavailable (Figure 41).

Figure 41. Labeling Convention (Top View)

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Types of Connections A three-phase transformer connection can be made with three, single-phase transformers or with multiple windings installed on a common magnetic core of a single, three-phase transformer. The two most common three-phase transformer connections are called delta (∆) and wye (Y). Delta Connection Figure 42 shows a delta-connected winding on a three-phase transformer. The winding’s terminals are connected end-to-end so that each winding will “see” full line-to-line voltage. Figure 43 shows the phasor voltage and current relationships for the same delta-connected transformer that is shown in Figure 42. Wye Connection Figure 44 shows a wye-connected winding on a three-phase transformer. One end of each of the windings is connected to a common point that is called the neutral point. This neutral point, in most applications, is connected to ground, at a point external to the transformer. Figure 45 shows the phasor voltage and current relationships for the same wye-connected transformer that is shown in Figure 44.

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Figure 42. Delta-Connected Transformer

Figure 43. Delta Connection Phasor Relationships

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Figure 44. Wye-Connected Transformer

Figure 45. Wye Connection Phasor Relationships

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Typical Connections To parallel three-phase distribution banks successfully, four conditions must be met: voltage ratings must be identical, tap settings must be identical, frequency ratings must be identical, and the percent impedance of the transformers must be nearly equal. In addition, the angular displacement of the banks must be the same. The angular displacements shown in the next four figures (Figures 46 through 49) must be the same. The angular displacement is the angle between a line drawn from neutral to H1 and a line drawn from the neutral to X1 measured in a clockwise direction from H1 to X1. The dash lines in the figures show the angular displacement between the high and low voltage windings. Note: The angular displacements described in the following four figures are NEMA standards for three-phase transformers. Delta-Delta ( - ) The delta-delta connection (Figure 46) is often used to supply large three-phase motor loads.

Figure 46. Delta-Delta ( - ) Connection

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Wye-Wye (Y-Y) The wye-wye connection (Figure 47) is typically used to supply 208 volts and 120 volts or 480 volts and 277 volts on systems where both the line-to-line and line-to-neutral values can be taken from all three phases. The wye-wye connection also permits balancing single-phase loads among the three phases.

Figure 47. Wye-Wye (Y-Y) Connection

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Delta-Wye ( -Y) The delta-wye connection (Figure 48) is typically used to supply 208 volts and 120 volts or 480 volts and 277 volts on systems where both the line-to-line and line-to-neutral values can be taken from all three phases. The delta-wye connection also permits balancing single-phase loads among the three phases. The delta-wye connection is the most common transformer connection that is used in Saudi Aramco power distribution systems.

Figure 48. Delta-Wye ( -Y) Connection

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Wye-Delta (Y- ) Similar to the delta-delta connection, the wye-delta (Figure 49) connection is used to supply large three-phase motor loads.

Figure 49. Wye-Delta (Y- ) Connection Transformer Polarity Single-Phase Transformers The relationships between the instantaneous voltage polarities of two transformer windings that envelop the same segment of magnetic core are defined as either additive or subtractive polarity. Transformer windings are additive polarity if the potential at the low-voltage winding terminal, which is physically nearest the H1 terminal, arrives at a negative peak at the same instant as the positive peak for the H1 terminal. The windings have subtractive polarity if the H1 terminal and its adjacent low-voltage terminal reach a positive peak at the same instant of time. Whichever low voltage winding terminal arrives at a positive peak simultaneously with H1 is labeled the X1 terminal. Figure 50 shows the correct terminal markings for a two-winding, single-phase transformer.

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Figure 50. Single-Phase Transformer Polarity Figure 51 is a schematic representation of a test circuit that is used to determine the polarity of transformer windings. A temporary test jumper is installed between the H1 terminal and the closest low voltage terminal. If the voltage measured between terminal H2 and the other low voltage terminal is greater than the ac source voltage, the implication is that the low voltage winding potential is adding voltage to the source voltage and the windings are additive polarity. If the low voltage winding potential subtracts from the source voltage, the windings are subtractive polarity.

Figure 51. Test Circuit to Determine Transformer Polarity

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In general, transformers which have different winding polarities should not have their windings connected in parallel unless a modification in winding interconnections is incorporated to account for the voltage phase relationships. But polarity also implies differences in the physical arrangement of windings within the transformer. Figure 52 is a simplified diagram of the core and coil arrangement of a two-winding transformer. It implies that with additive windings (Figure 52a), the top-to-bottom progression of the low-voltage winding is different than in the subtractive windings. Although the summation of magnetic flux within the magnetic core is the same, linkages of flux between adjacent turns is different, which affects the transformers ability to withstand the forces of a through fault, its impedance, and efficiency. Because of these considerations, almost all large transformers have subtractive polarity windings (Figure 52b).

Figure 52. Physical Arrangements of Windings for Polarity

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Three-Phase Transformers Earlier we discussed polarity for single-phase transformers. Polarity on three-phase transformers is not as simple because of phase rotation, lead markings, and the types of internal connections. To determine if a three-phase transformer has subtractive or additive polarity, the voltage vector diagram that is associated with the particular transformer must be correctly interpreted. Large power transformers are subtractive polarity by standard. The vector diagram is stamped on the nameplate of the transformer or it can be found on drawings that are supplied with the transformer. With a delta-delta or a wye-wye connection, the corresponding line voltages may have either a 00 or 1800 displacement. If the phase shift is 00, the transformer is subtractive polarity. If the phase shift is 1800, the transformer is additive polarity. In Figure 53a, the X1-X3 voltage vector is in parallel and in phase with the H1-H3 voltage vector. The same holds true for the X1-X2 and the H1-H2 voltage vectors, as well as the X2-X3 and the H2-H3 voltage vectors. There is no phase shift or angular displacement, and therefore this connection is subtractive polarity. In Figure 53b, the X3-X1 voltage vector is parallel, but there is a 1800 displacement with the H1-H3 voltage vector. The X3-X2 voltage vector is parallel with the H2-H3 voltage vector, but it is displaced by 1800. The same holds true for the third voltage vector. In this case (Figure 53b), the connection is additive polarity. Figures 53c and 53d show the same relationships for a wye-wye connected transformer. With a wye-wye connection, compare the phase-to-neutral vectors. In Figure 53c, the X1-X0 voltage vector is in phase and it is parallel with the H1-H0 voltage vector. Therefore it is subtractive polarity. Figure 53d shows the vectors that are displaced by 1800. This connection (Figure 53d) is additive polarity. Figure 53e shows a delta-wye transformer connection with subtractive polarity. Remember, a delta-wye transformer causes a 30 degree phase shift between the primary and secondary windings. The X1-X0 voltage vector is parallel, and there is no displacement when compared to the H1-H3 voltage vector. The same holds true for the other voltage vectors. In Figure 53f, the X0-X1 voltage vector is displaced by 1800 when compared to the H1-H3 voltage vector. This connection (Figure 53f) is additive polarity. Note: The X0 voltage vector is sometimes labeled N.

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Figure 53. Voltage Polarity Diagrams for Three-Phase Transformers

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CALCULATING BASIC POWER AND DISTRIBUTION TRANSFORMER RATINGS Note: Work Aid 3 has been developed to teach the Participant procedures to calculate basic power and distribution transformer ratings. Selection Factors The following factors should be considered for calculating the basic power and distribution transformer ratings: • • • • • • Voltage Loads Ambient Temperature Standard Sizes Cooling Classes Temperature Rise

Voltages There are three classes of voltage: low, medium, and high voltage.
Low Voltage, Three-Wire - ANSI C84.1-1989 lists four voltages in this class (120/240, 240, 480,

and 600 V); SAES-P-100 specifies only 120/240 or 480 V. The 120/240 V, single-phase, three-wire, nominal system is primarily used for residential areas and light industrial loads. The 480 volt, three-wire, nominal system is primarily used for supplying motor loads.
Low Voltage, Four-Wire - ANSI Standard C84.1-1989 lists three voltages in this class

(208Y/120, 240/120, and 480Y/277); SAES-P-100 specifies only two (208Y/120 and 480Y/277 V). The 208Y/120 V system is typically used in commercial or very light industrial applications, whereas the 480Y/277 V is used in most industrial applications and very large commercial applications (e.g., large office complexes, commissaries, etc.).
Medium Voltage, Three-Wire- ANSI Standard C84.1-1989 lists 9 nominal voltages in this class;

SAES-P-100 specifies only three (4.16, 13.8, and 69 kV). As mentioned previously, although three other voltages (2.4, 6.9, and 34.5 kV) in this class exist on Saudi Aramco installations, they are not permitted to be extended without approval of CSD. Voltages in this class are used in Saudi Aramco to distribute large blocks of power and as a utilization voltage for large motors (isolated neutral).
High Voltage, Three-Wire - ANSI Standard C84.1-1989 lists nine voltages in the class; SAES-P100 specifies only 115 and 230 kV nominal system voltages within this class to transmit large blocks of power over long distances. Saudi Aramco DeskTop Standards 84

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Loads
Normal Load- SAES-P-121 specifies that the minimum self-cooled kVA rating of each OA/FA

transformer shall be equal to the maximum normal operating load plus projected future load.
Emergency Load- Transformers may be operated under emergency conditions at ratings above

normal load ratings. However, there will be some sacrifice of life expectancy. ANSI/IEEE Standard C57.91 and C57.92 provide methods for determining the life expectancy of power transformers when they are operated at loads above their listed ratings.
Future Growth Load - SAES-P-121 specifies that for self-cooled transformers (OA or AA) only,

a 10% load growth factor should be added to the calculated load (normal maximum operating load plus projected future load).
Projected Future Load is a known load that will be added in the future. Projected future load

should not be confused with the 10% load growth factor that was discussed in the immediate previous paragraph. Ambient Temperature The temperature rise ratings of transformers are all based on an ambient temperature of 300C averaged over a 24-hour period, and the temperature not to exceed 400C at any time. If the transformer is operated at rated load and at temperatures greater than an average ambient temperature of 300C, some decrease in life expectancy will occur. To avoid this decrease in life, ANSI/IEEE Standard C57 requires that the transformer be derated 1.5% for each 10C that the average ambient temperature exceeds 300C for liquid-filled power transformers and 0.57% and 0.34% for each 10C that the average ambient temperature exceeds 300C for 1500C and 2200C dry-type transformers, respectively. SAES-P-121 requires that power transformer ratings be based on an average ambient temperatures of 400C. The standard specifies derating factors as follows: • • • Liquid-filled transformers - 15% Dry-type 1500C transformers - 6% Dry-type 2200C transformers - 4%

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Standard Sizes After an initial calculation to determine the kVA load requirements, the next standard (ANSI C57) size transformer is selected for a particular application. Work Aid 3A contains the complete lists of standard size liquid-filled and dry-type single-phase and three-phase transformers. Cooling Classes SAES-P-121 requires that forced-air cooling fans and controls be specified on all transformers that are rated 2500 kVA and larger. Figure 54 lists the cooling classes for liquid-filled transformers and Figure 55 lists the cooling classes for dry-type transformers. CLASS CODE OA OA/FA OA/FA/FA METHOD OF COOLING Liquid-immersed, self-cooled Liquid-immersed, self-cooled/forced-air-cooled Liquid-immersed, self-cooled/forced-air-cooled/forced-aircooled Liquid-immersed, self-cooled/forced-air-forced-liquid-cooled

OA/FA/FOA

Figure 54. Cooling Classes for Liquid-Filled Transformers

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CLASS CODE AA AFA AA/FA ANV GA

METHOD OF COOLING Ventilated, self-cooled Ventilated, forced-air-cooled Ventilated, self-cooled/forced-air-cooled Non-ventilated, self-cooled Sealed, self-cooled

Figure 55. Cooling Classes for Dry-Type Transformers Work Aid 3A contains the complete lists of standard forced-air (FA) ratings for both liquidfilled and dry-type transformers. Temperature Rise The rated kVA of a transformer is the kVA output that can be delivered for a specified time, at rated secondary voltage and rated frequency, without exceeding the specified temperature rise under prescribed conditions. For liquid-filled power transformers, the specified time is continuous operation. The rated secondary voltage and frequency are as stated on the transformer nameplate. The specified temperature rise is the average rise of temperature in the windings, which is either 550C or 650C and is stated on the nameplate. The corresponding hottest spot winding temperatures are 650C and 800C and are not stated on the nameplate. Sixty-five degrees average rise is the “preferred” rating specified for modern transformer designs. Some transformers have a dual temperature rise rating of 550/650C, with a corresponding dual kVA rating specified on the nameplate. Prescribed conditions are an ambient temperature not to exceed 300C averaged over a 24 hour period and not to exceed 400C at any time. The kVA ratings of outdoor transformers account for the warming effects of full sunlight during daytime hours.

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For dry type transformers there are 5 classes of insulation: Class 1300C with a 600C average rise, Class 1500C with an 800C average rise, Class 1850C with a 1150C rise, Class 2000C with a 1300C average rise, and Class 2200C with a 1500C average rise. The kVA ratings for drytype transformers have the same ambient temperature basis as for liquid-filled transformers. SAES-P-121 specifies that all Saudi Aramco liquid-filled transformers shall be 650C rise transformers and that all dry-type transformers shall be Class 1500C or 2200C transformers. Secondary Unit Substations A secondary unit substation, sometimes called a power center, is a close-coupled assembly consisting of three-phase power transformers, enclosed high voltage incoming line sections, and enclosed secondary low voltage outgoing sections encompassing the following electrical ratings: • • • Transformer kVA: 112.5 thru 2500 kVA (self-cooled rating), liquid-filled, dry-type, or cast coil Primary Voltages: 2.4 kV thru 34.5 kV Secondary Voltages: 208, 240, 480, or 600 Volt (maximum)

As a result of locating power transformers and their close-coupled secondary switchboards as close as possible to the areas of load concentration, the secondary distribution cables or busways are kept to minimum lengths. This concept has obvious advantages such as: • • • • • • • Reduced power losses. Improved voltage regulation. Improved service continuity. Reduced exposure to low voltage faults. Increased flexibility. Minimum installed cost. Efficient space utilization.

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Components The major components of a unit substation are: primary switchgear, for example, power fuses, medium voltage circuit breakers, or interrupters, the transformer (liquid-filled or drytype), and the secondary distribution section (main and feeder low voltage power circuit breakers). Layout and One-Line Diagrams The one-line diagram of a secondary unit substation is very useful in showing, by means of standard graphical symbols and nomenclature, the overall layout of the substation. Figure 56a illustrates a one-line diagram of a single-ended unit substation and Figure 57a illustrates a one-line diagram of a double-ended unit substation. Layout drawings of unit substations physically describe the location or assembly of the unit substation equipment in a room or in prefabricated packaged assemblies. Layout drawings require that the installation crews install the electrical equipment exactly in accordance with the design, or in the case of prepackaged assemblies, they provide the manufacturer with an assembly diagram that is in accordance with the “customer’s” requirements. Figure 56b illustrates the layout diagram of a single-ended unit substation and Figure 57b illustrates the layout diagram of a double-ended unit substation. Single-Ended Substations Figure 56 shows the one-line diagram and physical layout of a secondary unit substation that uses a radial system arrangement. This type of radial substation arrangement is called a single-ended substation because there is only one incoming line section at the one end (west) of the unit (prepackaged) assembly. Double-Ended Substations Figure 57 shows the one-line diagram and physical layout of a secondary unit substation that uses a secondary selective system arrangement. This type of arrangement, the secondary selective system, overcomes the major disadvantage of the radial system in that it provides duplicate paths of supply to the secondary bus of each load center. This selective system has two stepdown transformers, each with its own incoming primary feeder. The secondary bus associated with each transformer is connected through a tie breaker. Normally, the system is operated with the tie connection open, that is, as two separate radial systems operating independently of each other.

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With the loss of one of the primary feeders and/or transformers, the main secondary breaker for that circuit can be opened and the tie breaker closed, allowing the one remaining primary feeder and transformer to energize all of the secondary bus. The service to one-half of the load is momentarily interrupted during this transition period. SAES-P-121 specifies that the forced-cooled (FA) site rating of each transformer that serves a double-ended unit substation shall be capable of feeding the entire load of both buses with the normally-open (N.O.) tie breaker closed. Saudi Aramco operating procedures also permit closing of the tie-breaker before opening one of the two main breakers. This procedure avoids the momentary interruption during the transition period that is normally associated with a Kirk Key type operating system.

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Figure 56. Single-Ended Unit Substation

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Figure 57. Double-Ended Unit Substation

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Transformer Let-Through Current The selection of kVA and impedance ratings of unit substation transformers is very critical to the levels of available fault current on the secondary main bus side of the substation. This available fault current on the secondary side of the transformer, assuming that there are no other fault current sources on the secondary side, is called the transformer let-through current. Note: This section, as well as the next section on Paralleled Transformers, is presented for informational purposes only. Procedures to calculate the transformer let-through current and the paralleled transformer parameters are beyond the time constraint scope of this Module, however, both topics are thoroughly covered in the advanced course on transformers, Course EEX 202. Impedance The impedance (%Z) of the transformer, which is based on the self-cooled (OA) rating of the transformer, is the main factor in limiting the magnitude of the fault current available on the low voltage section of the system, especially at points close to the substation secondary bus. The lower the impedance of the transformer, the higher the level of available fault current. The higher the impedance of the transformer, the lower the level of available fault current. See Example J. Example J: Referring to Figure 56, what is the transformer let-through current (ILT) at the secondary bus? What is ILT if the impedance of the transformer is specified at 5% versus 5.75%? What is ILT if the impedance of the transformer is specified at 7% versus 5.75%? 1. ILT = IFLA-sec/Z = kVA/[( 3 x kVsec) x Z] • where Z is expressed as a decimal

Answer:

2. ILT @ 5.75% = 2500/[( 3 x .48) x .0575] = 3007/0.0575 = 52.3 kA 3. ILT @ 5% = 3007/0.05 = 60.1 kA 4. ILT @ 7% = 3007/0.07 = 43.0 kA

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Because of the importance of the transformer impedance in limiting the available fault current, unit substation transformers are designed on purpose to have impedances values of at least 5.0%, with a typical value being 5.75%. This higher impedance value is obtained by increasing the leakage reactance of the transformer windings, rather than increasing the winding resistance. Higher-resistance windings would increase the heat losses in the transformer, which is not desirable. Transformer Categories ANSI/IEEE Standard C57.12.00 recognizes four categories for the rating of transformers, as listed in Figure 58. Single-Phase kVA Ratings I II III IV 5 to 500 501 to 1667 1668 to 10,000 above 10,000 Three-Phase kVA Ratings 15 to 500 501 to 5000 5000 to 30,000 above 30,000

Category

Figure 58. Transformer Categories (Liquid-Filled) ANSI/IEEE Standard C57.12.01 lists the same categories of dry-type transformers as liquidfilled transformers, except that Category IV does not exist for dry-type transformers. For Category I distribution transformers, the duration of the short circuit is determined by the following formula: • • t = 1250/ISC2 where ISC = (IFLA-sec)/ZT and ZT is expressed as a decimal

For Category II, III, and IV transformers, the duration of the short circuit is limited to 2 seconds. For Category II transformers the short circuit current is calculated based on the transformer impedance (ZT) only. For Category III and IV transformers the short circuit current is calculated based on the transformer impedance (ZT) plus the system impedance (ZS).

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Paralleled Transformers Saudi Aramco operating procedures do not normally permit paralleling of transformer secondaries. However, operating procedures do permit, for secondary selective systems, momentarily paralleling of transformers. Should a feeder fault occur at the instant the transfer (closing the normally-open (N.O.) tie breaker) occurs, the short circuit duty at the secondary bus would be double the normal short circuit duty. This section of the Information Sheet will briefly explain the following factors that should be considered when paralleling transformers: • • • Increased Fault Levels Circulating Currents Limiting kVA

Increased Fault Levels The paralleling of the secondaries of transformers increases the short circuit current available, and, therefore necessitates higher interrupting capacity and more expensive secondary switchgear (bus, breakers, etc.). Example K: Answer: Referring to Figure 57, what is the total transformer let-through current at the secondary bus? 1. IT = ILT(T1) + ILT(T2) = 2ILT(T1) = 2 [1500/( 3 x .48 x 0.055) = 2 (32803) = 65.6 kA Circulating Currents Figure 59 shows two transformers connected in parallel. The transformers are connected in parallel by connecting the similarly marked terminals together. For example, terminal X1 of T1 to terminal X1 of T2, terminal X2 of T1 to X2 of T2, etc.

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Figure 59. Paralleled Transformers The ideal conditions for connecting transformers in parallel are the following: • • • • • Both transformers must have the same turns ratio and voltage ratings. Both transformers must have equal percent impedance. Both transformers must have equal ratios of resistance to reactance. Both transformers must have the same phase shift and polarity. All connections between the transformers are in phase and have the same polarity.

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If the above listed ideal conditions do not exist, circulating currents will flow between and through the secondary windings of both transformers. The magnitude of the circulating current that flows depends on which of the above ideal conditions do not exist. Let’s consider each of these ideal conditions to determine what occurs if they do not exist. If the turns ratios are not equal, a circulating current flows even if there is no load on the transformers. This circulating current flows because the voltage is different on the secondary side of the transformer. If transformer T1 has a higher turns ratio than transformer T2, then T1 will have a lower secondary voltage than T2. If the transformers do not have equal impedances, they will not equally share the load. Current will divide between the two transformers, but not necessarily equally. If transformer T1 has a higher impedance than transformer T2, then more current will flow through T2 than T1. This is the same principle as the principle of current dividing between two paralleled impedances. If unequal impedances exist, one transformer can easily overload. Phase shift and polarity was explained in an earlier (previous) section of this Module. Paralleling two transformers with different phase shifts (e.g., ∆-∆ to ∆-Y) or different polarities (e.g., subtractive to additive) will cause large secondary circulating currents to flow. Limiting kVA In daily practical situations, these ideal paralleling conditions do not exist. To parallel two transformers, they must have the same polarity and phase shift. Voltages and impedances do not have to be identically equal, but their values need to be as close to each other as possible. Two dissimilar transformers may be operated in parallel, but the following two conditions must be met: • • The circulating current should not exceed ten percent of the full load rated current of either transformer. The total load should be limited to a value such that rated current is not exceeded in either transformer. This total load limit is called the limiting kVA or the maximum kVA load of two transformers that are being operated in parallel. See Example L. Referring to Figure 59, what is the circulating current magnitude that flows between the paralleled transformers, and what is the limiting kVA of the two transformers that are being operated in parallel?

Example L:

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Answer:

1. Icirc = (kV1 - kV2)/[(Z1kV1)/(100I1) + (Z2kV2)/(100I2)] • where: • • • kV1 and kV2 are the kV secondary voltages for transformers T1 and T2 at no-load. Z1 and Z2 are the percent impedances for transformers T1 and T2. I1 and I2 are the full load current for transformers T1 and T2.

2. I1 = kVA1/( 3 x kV1) = 12000/( 3 x 14.0) = 494.9 A I2 = kVA2/( 3 x kV2) = 10000/( 3 x 13.9) = 415.4 A 3. Icirc = (14.0 - 13.9)/[(9 x 14.0)/(100 x 494.9) + (8 x 13.9)/(100 x 415.4)] = 19.15 A 4. The circulating current must be less than 10% of the smaller of I1 or I2. Icirc <(0.10 x I1)<(0.10 x 494.9) = 49.5 A Icirc <(0.10 x I2)<(0.10 x 415.4) = 41.5 A 5. Conclusion: The circulating current is less than 41.5 A and therefore it is acceptable. 6. kVAlimit = Zmin x [(kVA1/Z1) + (kVA2/Z2)] x 0.9 = 8 x [(12000/9) + (1000/8)] x 0.9 = 18600 kVA = 18.6 MVA 7. Conclusion: Total power supplied to the secondary bus cannot exceed 18.6 MVA. If more than 18.6 MVA of load is connected, transformer T2 will overload resulting in possible damage.

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In closing out this discussion on paralleling of transformers, the following listed additional precautions must be considered before paralleling transformers. • • Much higher fault (short circuit) current magnitudes are available at the secondary bus. If paralleling two transformers with load tap changers, the automatic sensing relay which controls the tap position of the transformers must be set up in a master/slave configuration. Both load tap changers must “stay in step” (stay on the same taps). The master/slave configuration system should allow one transformer’s sensing relay to control the tap changer on both transformers. If paralleling transformers that have a high secondary current, consider the impedance of the cables or bus that is used to parallel the secondary windings. Small changes in impedance may have a large affect on load sharing.



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WORK AID 1:

PROCEDURES FOR CALCULATING DISTRIBUTION TRANSFORMER RATIOS

POWER

AND

Step 1. Determine the turns ratio. • Np/Ns = Ep/Es = a (transformer turns ratio)

Step 2. Determine the volts-per-turn ratios. • volts-per-turn (primary) = volts-per-turn (secondary) Ep/Np = Es/Ns • • Ep = (Np/Ns) x Es Np = (Ep/Es) x Ns Es = (Ns/Np) x Ep Ns = (Es/Ep) x Np

Step 3. Determine the ampere-turns relationships. • ampere-turns (primary) = ampere-turns (secondary) Ip x Np = Is X Ns • • Ip = (Ns/Np) x Is Np = (Is/Ip) x Np Is = (Np/Ns) x Ip Ns = (Ip/Is) x Np

Step 4. Determine the voltampere relationships. • • • VAin = VAout Ep x Ip = Es x Is (for single-phase transformers) 3 x Ep x Ip = or • • • kVAin = kVAout kVp x Ip = kVs x Is (for single-phase transformers) 3 x kV x I = p p 3 x kV x I (for three-phase transformers) s s 3 x Es x Is (for three-phase transformers)

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WORK AID 2:

PROCEDURES FOR CALCULATING POWER DISTRIBUTION TRANSFORMER EFFICIENCY VOLTAGE REGULATION Applicable Efficiency Procedures for Calculating Power

AND AND

Work Aid 2A:

Transformer

Note: Efficiency is calculated by dividing the output real power by the input real power. The efficiency will vary depending on the amount of load because the copper losses of a transformer vary with load. Step 1. Step 2. Determine the transformer kVA, total copper losses, and total iron losses. Calculate the output power of the transformer at the given power factor. • Step 3. Pout = kVA x 1000 x power factor (p.f.) in watts

Calculate the required input power. • Pin = Pout + copper losses + iron losses

Step 4.

Calculate the percent efficiency. • % efficiency = (Pout/Pin) x 100 Applicable Procedures Voltage Regulation for Calculating Power Transformer

Work Aid 2B:

Note: As a transformer becomes loaded, the voltage at the secondary terminals of the transformer decreases. This decrease in voltage is caused by the voltage drop across the internal impedance of the transformer. The higher the transformer impedance, the higher the voltage drop. Step 1. Step 2. Determine the voltage (Efull-load) of the transformer at the secondary terminals under full-load conditions. Calculate the percent voltage regulation. • • % Voltage Regulation = [(Eno-load - Efull-load)/(Eno-load)] x 100 where Eno-load = the transformer’s rated secondary voltage

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WORK AID 3: Work Aid 3A:

RESOURCES USED TO CALCULATE BASIC POWER AND DISTRIBUTION TRANSFORMERS ANSI/IEEE Standard Regulating Transformers) C57 (Distribution,

RATINGS Power,

OF and

1. C57.12.00-1987 specifies the following service conditions for liquid-immersed distribution and power transformers: a. Temperature should not exceed an average ambient temperature of 300C. b. Altitude should not exceed 3300 ft (1000 meters). c. Voltage should be approximately sinusoidal. d. Load current should be approximately sinusoidal and a harmonic content should not exceed 0.05 per unit. e. Frequency should be at least 95% of the rated value (e.g., 60 Hz). 2. C57.12.00-1987, Table 2, lists the following preferred continuous kVA ratings for liquidimmersed power and distribution transformers (Figure 62).

Single-Phase Transformers kVA kVA kVA

Three-Phase Transformers kVA kVA kVA

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3 5 10 15 25 37.5 50 75 100

167 250 333 500 833 1250 1667 2500 3333

5000 6667 8333 10,000 12,500 16,667 20,000 25,000 33,333

15 30 45 75 112.5 150 225 300 500 750

1000 1500 2000 2500 3750 5000 7500 10,000 12,000

15,000 20,000 25,000 30,000 37,500 50,000 60,000 75,000 100,000

Figure 62. Standard Transformer kVA Ratings (Liquid-Filled)

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3. C57.12.00-1987, Table 4, lists the following basic dielectric insulation levels (BIL) for liquid-immersed transformers (Figure 63).

Nominal System Voltage kV rms Transformer Category

BIL Full Wave kV crest

Chopped Wave

Low Frequency Test Label

Minimum Voltage kV Crest

Minimum Time to Flashover sec 1.0

kV rms

0.48 and below 2.4 2.4 4.16 4.16 13.8 13.8 34.5/19.9 34.5/19.9 69 115 230

Power and Distribution Distribution Power Distribution Power Distribution Power Distribution Power Power Power Power

30

36

10

45 60 60 75 95 110 150 200 350 450 825

54 69 69 88 110 130 175 230 400 520 950

1.5 1.5 1.5 1.6 1.8 2.0 3.0 3.0 3.0 3.0 3.0

15 15 19 19 34 34 50 70 140 185 360

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Figure 63. Transformer BIL Ratings

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4. C57.12.00-1987, specifies that the angular placement for three-phase transformers shall be in accordance with Figure 64.

Figure 64. Phase Relation of Terminal Designations

Category

Single-Phase kVA Ratings

Three-Phase kVA Ratings 15 to 500 501 to 5000 5000 to 30,000 above 30,000

I II III IV

5 to 500 501 to 1667 1668 to 10,000 above 10,000

Figure 65. Transformer Categories (Liquid-Filled)

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6. C57.12.00-1987 specifies that power transformers that are rated 200 kVA and below, and that have high voltage ratings 8660 volts and below, shall have additive polarity. All other single-phase transformers shall have subtractive polarity. 7. C57.12.00-1987 specifies that the average winding temperature rise above ambient temperature shall not exceed 650C when measured by resistance and that the winding hottest-spot temperature rise shall not exceed 800C. Note: Transformer ratings are based on a 24-hour average ambient of 300C. 8. C57.92-1981, Table 1, lists the following derating factors for 550C or 650C rise transformers that are being operated at ambient temperatures above or below 300C (Figure 66). Note: Figure 66 is valid for temperatures that range from 00C to 500C.
% of Rating Decrease Load for each C Type of Cooling Self-cooled Water-cooled Forced-air-cooled Forced-oil-cooled -- OA -- OW -- OA/FA, OA/FA/FA -- FOA, FOW and OA/FOA/FOA Higher Temperature 1.5 1.5 1.0 1.0
0

Increase Load for each C Lower Temperature 1.0 1.0 0.75 0.75
0

Figure 66. Transformer Loading on Basis of Temperature for Liquid-Filled Transformers

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9. C57.12.10, Table 12, lists the following OA/FA ratings for power transformers that are rated 750 - 12,500 kVA (Figure 67).
Three-Phase (kVA) Single-Phase (kVA) OA 833 1 250 1 667 2 500 3 333 5 000 6 667 8 333 FA 958 1 437 1 917 3 125 4 167 6 250 8 333 10 417 Without Load Tap Changing OA 750 1 000 1 500 2 000 2 500 3 750 5 000 7 500 10 000 FA 862 1 150 1 725 2 300 3 125 4 687 6 250 9 375 12 500 Three-Phase (kVA) With Load Tap Changing OA 3 750 5 000 7 500 10 000 FA 4 687 6 250 9 375 12 500

Figure 67. OA/FA Ratings (750 - 12,500 kVA) 10. C57.12.10, Table 13, lists the following OA/FA/FA ratings for power transformers that are rated 18 MVA and larger (Figure 68).

OA

First-Stage

Second-Stage

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18 000 21 000 24 000 27 000 40 000 45 000

24 000 28 000 32 000 36 000 53 333 60 000

30 000 35 000 40 000 45 000 66 667 75 000

Figure 68. OA/FA/FA Ratings (Greater Than 12.5 MVA) 11. C57.12.01-1989 specifies the same usual service conditions for dry-type transformers as for liquid-filled transformers. See paragraph 1 of this Work Aid. 12. C57.12.01-1981, Table 2, lists the following continuous kVA ratings for dry-type power and distribution transformers (Figure 69).

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Single-Phase Transformers kVA 1 3 5 10 15 25 37.5 50 75 100 kVA 167 250 333 500 833 1250 1667 2500 3333 kVA 5000 6667 8333 10,000 12,500 16,667 20,000 25,000 33,333

Three-Phase Transformers kVA 15 30 45 75 112.5 150 225 kVA 300 500 750 1000 1500 2000 2500 kVA 3750 5000 7500 10,000 12,000 15,000 20,000

Figure 69. Standard Transformer kVA Ratings (Dry-Type) 13. C57.12.01, Table 4A, lists the following limits of temperature and temperature rise ratings for dry-type transformers (Figure 70). Average Winding Temperature Rise by Resistance (0C) Insulation System Temperature (0C)

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60 80 115 130 150

130 150 185 200 220

Figure 70. Limits of Temperature and Temperature Rise Ratings for Dry-Type Transformers

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14. C57.12.01-1989 lists the same categories of dry-type transformers as liquid-filled transformers, except that Category IV does not exist for dry-type transformers. See paragraph 5, Figure 65, of this Work Aid. 15. C57.12.01-1981, Table 3B, list the following BIL ratings for dry-type transformers (Figure 71).

Nominal System Voltage

BIL

Low Frequency Voltage Insulation Level Full Wave 1.2 x 50 s Crest

Impulse Levels Chopped Wave Minimum Time To Flashover

kV kV rms 0.48 and below 2.4 4.16 13.8 34.5 20 30 60 150 10 12 19 50 20 30 60 150 20 30 60 150 1.0 1.0 1.5 2.25 10 kV crest 4 kV crest 10 kV crest 10 s 1.0

Figure 71. BIL Ratings for Dry-Type Transformers 16. C57.12.51-1981, Table 6, lists the AA/FA ratings for dry-type transformers (Figure 72).

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Self-Cooled (AA) Ratings (kVA) 750 1000 1500 2000 2500 3750 5000 7500

Forced-Air-Cooled (AA/FA) Ratings (kVA) 1000 1333 2000 2667 3333 5000 6667 10000

Figure 72. AA/FA Ratings for Dry-Type Transformers 17. C57.96-1989, Table 1, lists the following derating factors for dry-type transformers that are being operated at temperatures above or below 300C (Figure 73). Note: Figure 73 is valid for temperatures ranging from 00C to 500C.
Percent of Rated kVA/0C Maximum Rated Hottest-Spot Type of Unit Ventilated Self-Cooled Temperature 0C 150 185 220 Hottest-Spot Temperature 30 C Ambient 140 175 210
0

Increase for Average Ambient Less than 300C or Decrease for Average Ambient Greater Than 30 C (0.57) (0.43) (0.35)
0

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Sealed Self-Cooled

150 185 220

140 175 210

(0.65) (0.49) (0.40)

Figure 73. Transformer Loading on Basis of Temperature for Dry-Type Transformers

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Work Aid 3B:

SAES-P-121 (Transformers, Reactors, Voltage Regulators)

1. Section 4.6 specifies that transformers shall be provided with a HV tap changer for either de-energized operation or load tap operation. 2. Section 4.7 specifies that all step-down transformers that are rated 10 MVA OA and above, for residential or mixed commercial loads, shall have a HV load tap changer with automatic voltage control. 3. Section 4.10 specifies that all load tap changers shall be equipped for remote operation. 4. Section 5.1 specifies that transformers shall be supplied with ANSI standard preferred kVA ratings at usual service conditions, unless specified otherwise on AMSS Data Schedule-1. 5. Section 5.2 specifies that the minimum OA self-cooled kVA rating of each OA/FA transformer shall be equal to the maximum operating load plus projected future load. 6. Section 5.3 specifies that for transformers that are self-cooled only, a 10 percent load growth factor shall be added to the calculated load (maximum operating load plus projected future load). 7. Section 5.4 specifies that the forced-cooled FA site rating of each transformer serving a double-ended substation shall be capable of feeding the entire load of both buses with the bus tie breaker closed. 8. Section 5.5 specifies that forced-air cooling fans and controls shall be provided on all transformers that are rated 2500 kVA or larger. On transformers smaller than 2500 kVA, forced-air cooling shall not be provided. 9. Section 5.6 specifies that two stages of forced cooling shall be allowed for transformers with OA ratings of 90 MVA or larger. The forced cooling may be forced-air (FA) and/or forced-oil-air (FOA). 10. Section 5.9 specifies that Table 1 (Figure 74) lists the maximum allowable percentage deratings for various site installations and transformer types and sizes.

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OIL-IMMERSED TRANSFORMER (650C RISE)

DRY-TYPE LT 225 kVA GE 225 kVA

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LOCATION AND AMBIENT TEMP. LT 255 kVA GE 225 kVA 1500C 2200C 150 0C 2200C

OUTDOOR SOLAR EXPOSED 45 oC AVG, 55 oC MAX 22.5% 15% 12% 8% 9% 6%

OUTDOOR NOT-EXPOSED 40 oC AVG, 50 oC MAX 15% 15% 6% 4% 6% 4%

INDOOR WELL-VENTILATED 40 oC AVG, 50 oC MAX

NOT ALLOWED INDOORS

NOT ALLOWED INDOORS NOT ALLOWED INDOORS NOT ALLOWED INDOORS

6%

4%

6%

4%

INDOOR-UNMANNED AIR-CONDITIONED 35 oC AVG, 40 oC MAX

NOT ALLOWED INDOORS

3%

2%

3%

2%

INDOOR-UNMANNED AIR-CONDITIONED 30 oC AVG, 40 oC MAX Note: kVA Rating Symbols:

NOT ALLOWED INDOORS LT = Less than; GE = Greater than or equal to. 0% 0% 0% 0%

Figure 74. Transformer kVA Derating Factors
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11. Section 5.12 specifies that when transformers are operated in parallel, the total circulating current shall not exceed 10 percent of the rated current of the lowest kVA rated transformer.

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Work Aid 3C:

Applicable Procedures

Step 1. Select a liquid-filled or a dry-type power or distribution transformer based on given information. • • Power or Distribution Liquid-Filled or Dry-Type -

Step 2. Select the primary and secondary voltage ratings from Figure 75 and the given information. For three-phase transformers, assume 3φ, 3-wire, delta-connected primaries and 3φ, 4-wire, wye-connected secondaries. Voltage 120/240 V 480 V 208Y/120 V 480Y/277 V 2400 V* 4160 V 6900 V* 13,800 V 34,500 V* 69,000 V 115,000 V 230,000 V *Note: Requires CSD approval to extend. 3φ 3φ No. of Phases 1φ 3φ 3φ 3φ 3φ 3φ 3φ 3φ 3φ No. of Wires 3 3 4 4 3 3 3 3 3 3 3 3

• •

Primary Secondary 119

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Figure 75. SAES-P-100 Standard Saudi Aramco Voltages Step 3. Select the BIL for the transformer from Work Aid 3A, Figure 63, for liquid-filled transformers, or from Figure 71, for dry-type transformers. • • BIL (primary winding) BIL (secondary winding) -

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Step 4. Calculate the required site kVA. a. Calculate the load kW for each load or combination of loads at rated utilization voltage. Note: Include any projected (known) future loads. • • b. kWload = 3 x kV x I x p.f. or

kWload = kVAload x p.f.

Calculate the load kVAR for each load or combination of loads at rated utilization voltage. Note: Include any projected (known) future loads. • • kVARload = 3 x kV x I x sin (cos-1 p.f.) or

kVARload = kWload x tan (cos-1 p.f.)

c.

Calculate site kVA. • kVA(site) = (kW2total + kVAR2total)1/2

d.

Calculate the adjusted site kVA after derating for temperature. Note: Use a design ambient temperature of 400C. • • kVA (adjusted) = kVA (site)/0.85 for liquid-filled transformers (Work Aid 3B, Figure 74)

kVA (adjusted) = kVA (site)/0.94 for dry-type transformers with 1500C insulation (Work Aid 3B, Figure 74) kVA (adjusted) = kVA (site)/0.96 for dry-type transformers with 2200C insulation (Work Aid 3B, Figure 74)



Step 5. Select the next standard size kVA-rated transformer from Work Aid 3A, Figure 62, for liquid-filled transformers, or from Figure 69, for dry-type transformers. Note: For self-cooled transformers (OA or AA only) add 10% for growth. See paragraph 6 of Work Aid 3B. • kVA rating -

Step 6. Determine the cooling class kVA ratings from Work Aid 3A, Figure 67, for liquidfilled transformers, or from Figure 72, for dry-type transformers.

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Cooling class kVA ratings -

Notes: a. The forced-cooled kVA ratings of each transformer serving a doubleended substation shall be capable of feeding the entire load of both buses with the bus tie breaker closed. See paragraph 7 of Work Aid 3B. b. Forced-air cooling shall be provided on all transformers that are rated 2500 kVA and larger. See paragraph 8 of Work Aid 3B. Step 7. Summary of Selection Procedures: • • • • • • • • • • Power or Distribution Transformer Liquid-Filled or Dry-Type Transformer Primary Voltage Secondary Voltage BIL Ratings Load kW and kVAR Calculations Site kVA Adjusted Site kVA Transformer kVA Ratings (Standard Sizes) Transformer Cooling Class kVA Ratings

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GLOSSARY AA additive polarity Ventilated, self-cooled rating of a dry-type transformer. The polarity designation of a transformer that has the primary and secondary terminals of corresponding polarity directly opposite of each other. Ventilated, forced-air-cooled rating of a dry-type transformer. American National Standards Institute Basic Impulse Level A device that permits a conductor to pass through an enclosure, yet to be isolated from the enclosure. Typically the conductor is insulated from the enclosure by a porcelain or epoxy cylinder. Transformer power losses that are caused by the resistance of the windings. Copper losses are also called I2R losses. A permeable metal frame around which transformer windings are wound. Usually made from silicon steel. Consulting Services Division (Saudi Aramco) transformer An instrument transformer that is intended to have its primary winding connected in shunt with a power supply circuit, the voltage of which is to be measured or controlled. The insulation quality of a material or oil that is measured in kilovolts (kV). A small transformer that is used to supply low voltage residential loads. Pole-mount and pad-mount transformers fall into this category. A distribution transformer usually has a kVA rating less than 500 kVA. Installations that contain transformers 500 kVA and less are designated distribution transformer installations.

AFA ANSI BIL bushing

copper losses core CSD current (CT)

dielectric strength distribution transformer

distribution transformer installation

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dry-type transformer

A transformer in which the windings are immersed in air or some other dry gas, e.g., nitrogen. For Saudi Aramco purposes, a dry-type transformer in which neither of the windings is cast in resin is called a conventional dry-type transformer. One component of transformer iron losses. Eddy current loss is the loss caused by the currents that are induced in the iron core of a transformer. The efficiency of a transformer is the ratio of real power output to real power input, and it is usually expressed in percent. The process by which an electromotive force is produced in a conductor when there is relative motion between the conductor and a magnetic field. Excitation current is the current that flows in any winding used to excite the transformer when all other windings are opened circuited, and it is usually expressed in percent of the rated current of the winding in which it is measured. Forced-air-cooled rating of a liquid-filled transformer. One component of transformer iron losses. Hysteresis loss is the amount of energy required to overcome residual magnetism of the transformer core. Transformer power losses that are caused by resistance of 2 the windings. I R losses are also called copper losses. Institute of Electrical and Electronics Engineers The voltage at rated frequency that is applied to the line terminals of one of the windings of a two winding transformer to cause the rated current to flow through these terminals when the terminals of the other winding are short circuited. The applied voltage is measured while the windings are at the same specified winding temperature. A gas that does not chemically react with other substances.

eddy current loss

efficiency

electromagnetic induction excitation current

FA hysteresis loss

I2R losses IEEE impedance voltage

inert gas

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instrument transformer

A transformer that is intended to reproduce in its secondary circuit, in a definite and known proportion suitable for utilization in measurement, control of protective devices, the voltage or current of its primary circuit with its phase relationships substantially preserved. The combination of eddy current and hysteresis losses. The load or copper losses of a transformer are those losses incidental to the carrying of load. changer A tap changer designed so that the transformer’s turns ratio can be changed without interrupting the power source. Mechanical Relief Device No-load losses are the losses in a transformer that is excited at rated voltage and frequency but which is not supplying load. Note: No-load losses include core loss, dielectric loss, and the loss in the windings due to exciting current.

iron losses load losses load tap (LTC) MRD no-load losses

no-load tap changer A tap changer that can only be operated when the (NLTC) transformer is completely de-energized from the power source. OA pad-mounted transformer Self-cooled rating of a liquid-filled transformer. An outdoor distribution-type transformer that is used as part of an underground distribution system, with enclosed compartments for high voltage and low voltage cables entering from below, and that is mounted on a foundation pad. A distribution-type transformer that is suitable for mounting on a pole or platform on overhead installations up to 69 kV, with ratings of 1 kVA to 167 kVA single-phase and 15 kVA to 500 kVA three-phase. Sometimes called an overhead transformer. A large transformer that is used to deliver large amounts of power to industrial loads or to transmission and distribution systems. A power transformer usually has a kVA rating greater than 500 kVA.

pole-mounted transformer

power transformer

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primary rated current The primary rated current should be equal to the secondary current divided by the turns ratio. Note: This definition of primary rated current ignores magnetizing current. primary voltage rated The primary rated voltage designated to be applied to the input terminals should be equal to the secondary rated voltage times the turns ratio. Note: Primary rated voltage is not necessarily equal to nominal system voltage. The primary winding is the winding on the energy input side of the transformer. The primary winding is usually the high voltage winding on a power transformer. The tap to which operating and performance characteristics of a transformer are referred. A tap through which the transformer can deliver its rated kVA output without exceeding the specified temperature rise. Sometimes called the fully-rated tap.

primary winding

principal tap rated kVA tap

rated power (single- Rated power is equal to the product of rated voltage and phase transformer) rated current of the same winding of a transformer. rated power (three- Rated power of a three-phase winding is equal to the phase transformer) product of rated voltage, rated current, and the 3 . Both windings of a two-winding transformer have the same rated power, which by definition is the rated power of the transformer. reduced kVA tap A tap through which the transformer can deliver only an output less than rated kVA without exceeding the specified temperature rise. The full-load regulation of a transformer is the arithmetic difference between the secondary no-load and full-load voltages of a winding, divided by the secondary no-load voltage, with rated voltage applied to the primary winding at rated frequency and with the winding at specified temperature. The reliability of a substation is defined as its ability to serve the intended function without failure, although the complete elimination of failures is impossible to achieve. Saudi Aramco Engineering Standard
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regulation

reliability (substation) SAES
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secondary voltage

full-load The secondary full-load voltage should be equal to the fullload rms voltage available at the output terminals of the transformer, at rated frequency and current, and the required power factor, at principal tap. rated The secondary rated current shall be equal to the full load rms current deliverable at the output terminals of the transformer, at principal tap, without exceeding the specified temperature rise. rated The secondary rated voltage should be the equivalent noload voltage corresponding to the full-load condition. The secondary winding is the winding on the energy output side of the transformer. The secondary winding is usually the low voltage winding on a power transformer.

secondary current

secondary voltage

secondary winding

short impedance

circuit The short circuit equivalent star-connection impedance of a two winding transformer, expressed in ohms per phase, measured between the terminals of a winding when the other winding is short circuited. The impedance is based on rated frequency and the specified winding temperature. Sudden Pressure Relay A transformer in which the energy transfer is from a high voltage circuit to a low voltage circuit. A transformer in which the energy transfer is from a low voltage circuit to a high voltage circuit. Installations that contain power transformers (501 kVA and above) are designated substations.

SPR step-down transformer step-up transformer substation

substation (industrial)

A substation is defined as an industrial substation if its main function is to deliver power to one or more major industrial plants. Substations serving a hospital load or any other critical load facility and substations supplying industrial facilities are considered industrial substations. An industrial substation facility includes the transformer switching devices, the transformer(s), the secondary or low voltage bus, circuit breakers and load feeder cables together with all controls, metering, relaying, and auxiliary equipment.

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substation (residential) subtractive polarity

A substation is defined as a residential substation if its main function is to deliver power to residential areas. The polarity designation of a transformer that has primary and secondary terminals of corresponding polarity diagonally opposite of each other. A tap is a connection brought out of a winding at some point between its extremities to permit changing the voltage or current ratio of a transformer or impedance of a reactor. A tap changer is a switching device that is used to change the voltage ratio of a transformer by means of taps brought out from between the extremities of the windings. Tap changer operating can be arranged for either de-energized operation, where the transformer must be disconnected from its supply before the tap switch or link can be moved; or loaded operation, where the load tap changer (LTC) is designed to operate, in conjunction with a transition impedance, while the transformer is carrying load. The increase in operating temperature that is above an ambient temperature of a winding or insulating fluid.

tap

tap changer

temperature rise

turns ratio

The turns ratio of a transformer is the ratio of the effective number of turns in the high voltage winding to that in the low voltage winding. Note: In the case of a transformer having taps for changing its voltage ratio, the turns ratio is based on the principal tap. A distribution-type transformer that is designed for location in an underground enclosure. The voltage ratio of a transformer is the ratio of the rated voltage of the high voltage winding to the rated voltage of the low voltage winding, and is equal to the turns ratio.

underground-type transformer voltage ratio

voltage (VT)

transformer An instrument transformer that is intended to have its primary winding connected in shunt with a power supply circuit, the voltage of which is to be measured or controlled.

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