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CURRENT TRANSFORMER
In electrical engineering, a current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry. A current transformer is defined as "as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections." This highlights the accuracy requirement of the current transformer but also important is the isolating function, which means no matter what the system voltage the secondary circuit need be insulated only for a low voltage. The current transformer works on the principle of variable flux. In the "ideal" current transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and opposite to the primary current. But, as in the voltage transformer, some of the primary current or the primary ampere-turns is utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to be "transformed" into the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into two-the current or ratio error and the phase error Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding. The alternating current flowing in the primary produces a magnetic field in the core, which then induces a current in the secondary winding circuit. A primary objective of current transformer design is to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current bears an accurate relationship to the primary current. The most common design of CT consists of a length of wire wrapped many times around a silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore consists of a single 'turn' of conductor, with a secondary of many tens or hundreds of turns. The primary winding may be a permanent part of the current transformer, with a heavy copper bar to carry current through the magnetic core. Window-type current transformers are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. When conductors passing through a CT are not centered in the circular (or oval) opening, slight inaccuracies may occur. Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical examples of low voltage single ratio metering current transformers are either ring type or plastic moulded case. High-voltage current transformers are mounted on porcelain bushings

to insulate them from ground. Some CT configurations slip around the bushing of a highvoltage transformer or circuit breaker, which automatically centers the conductor inside the CT window. The primary circuit is largely unaffected by the insertion of the CT. The rated secondary current is commonly standardized at 1 or 5 amperes. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. The load, or burden, of the CT should be of low resistance. If the voltage time integral area is higher than the core's design rating, the core goes into saturation towards the end of each cycle, distorting the waveform and affecting accuracy. Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amp. The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits, and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices.

Accuracy : The accuracy of a CT is directly related to a number of factors including:
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Burden Burden class/saturation class Rating factor

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Load External electromagnetic fields Temperature and Physical configuration. The selected tap, for multi-ratio CTs

For the IEC standard, accuracy classes for various types of measurement are set out in IEC 60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important especially in power measuring circuits, and each class has an allowable maximum phase error for a specified load impedance. Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults.

Rated primary current: The value of current which is to be transformed to a lower value. In CT parlance, the "load" of the CT refers to the primary current. Rated secondary current: The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondary current are 1 A or 5 A. In the case of transformer differential protection, secondary currents of 1/ root 3 A and 5/ root 3 A are also specified. Rated burden: The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards) Accuracy class: In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT. Composite error: The rms value of the difference between the instantaneous primary current and the instantaneous secondary current multiplied by the turns ratio, under steady state conditions. Accuracy limit factor: The value of primary current upto which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current.

Short time rating: The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds. Instrument security factor (factor of security): This typically takes a value of less than 5 or less than 10 though it could be much higher if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are therefore protected. In the case of double ratio CT's, FS is applicable for the lowest ratio only. Class PS/ X CT: In balance systems of protection, CT s with a high degree of similarity in their characteristics are required. These requirements are met by Class PS (X) CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turns ratio. Knee point voltage: That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current). Summation CT: When the currents in a number of feeders need not be individually metered but summated to a single meter or instrument, a summation current transformer can be used. The summation CT consists of two or more primary windings which are connected to the feeders to be summated, and a single secondary winding, which feeds a current proportional to the summated primary current. A typical ratio would be 5+5+5/ 5A, which means that three primary feeders of 5 are to be summated to a single 5A meter. Core balance CT (CBCT): The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished. Interposing CT's (ICT's) : Interposing CT's are used when the ratio of transformation is very high. It is also used to correct for phase displacement for differential protection of transformers. Safety precautions : Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance. This will produce a high voltage across the open secondary (into the range of several kilovolts in some cases), which may cause arcing. The high voltage produced will

compromise operator and equipment safety and permanently affect the accuracy of the transformer.

Advantage of live tank over dead tank CT:
Following are the advantages of Live Tank over dead Tank : a) The Primary conductor is short than (in live tank CT) than the Dead tank CT which gives better Rigidity and gives high short circuit current withstanding capability and reliability. b) Primary winding is uniformly distributed around Core, Hence CT is truly low reactance type, which has inherent better transient performance. c) Due to shorter length and the tank being live the major insulation is not over the high current carrying primary, which is the main source of heat (as in the case of Dead tank type design.) the insulation does not get heated up while dissipating the heat generated. This facilitates much superior thermal stability of insulation and longer life. OPERATION OF CT UNDER OPEN CIRCUITED SECONDARY: CT secondary is never opened while primary is live. Opening of secondary results in secondary ampere turns to become zero and the entire primary current provides extinction. The core reaches saturation at all levels of current wave except very near to zero crossing point in a very short period. As a result very high voltage peak will develop in the secondary sufficient to puncture the installation and break down the CT. Fuse must not therefore, be provided in the secondary circuit. Tests A number of routine and type tests have to be conducted on CT s before they can meet the standards specified. The tests can be classified as : a. Accuracy tests to determine whether the errors of the CT are within specified limits. b. Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, inter-turn insulation test at power frequency voltage, impulse tests with 1.2u/50 wave, and partial discharge tests (for voltage >=6.6kv) to determine whether the discharge is below the specified limits. c. Temperature rise tests. d. Short time current tests. e. Verification of terminal markings and polarity.

POTENTIAL TRANSFORMER
A potential transformer is aconventional transformer having primary and secondary windings. The primary windingis connected directly to the power circuit either between two phases or between one phaseand ground, depending on the rating of the transformer and on the requirements of theapplication. The standards define a voltage transformer as one in which "the secondary voltage is substantially proportional to the primary voltage and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections." This, in essence, means that the voltage transformer has to be as close as possible to the "ideal" transformer. In an "ideal" transformer, the secondary voltage vector is exactly opposite and equal to the primary voltage vector, when multiplied by the turns ratio. In a "practical" transformer, errors are introduced because some current is drawn for the magnetization of the core and because of drops in the primary and secondary windings due to leakage reactance and winding resistance. One can thus talk of a voltage error,which is the amount by which the voltage is less than the applied primary voltage ,and the phase error, which is the phase angle by which the reversed secondary voltage vector is displaced from the primary voltage vector. The ratio and phase-angle inaccuracies of any standard ASA accuracy classof potentialtransformer are so small that they may be neglected for protective-relaying purposes if theburden is within the "thermal" volt-ampere rating of the transformer. This thermalvoltampere rating corresponds to the full-load rating of a power transformer. It is higherthan the volt-ampere rating used to classify potential transformers as to accuracy formetering purposes. Based on the thermal volt-ampere rating, the equivalent-circuitimpedances of potential transformers are comparable to those of distributiontransformers. The "burden" is the total external volt-ampere load on the secondary at rated secondaryvoltage. Where several loads are connected in parallel, it is usually sufficiently accurateto add their individual volt-amperes arithmetically to determine the total voltampereburden. If a potential transformer has acceptable accuracy at its rated voltage, it is suitable over therange from zero to 110% of rated less voltage. Operation in excess of 10% overvoltage maycause increased errors and excessive heating. Where precise accuracy data are required, they can be obtained from ratio-correction-factor curves and phase-angle-correction curves supplied by the manufacturerDefinitions Typical terms used for specifying a voltage transformer (VT) a. Rated primary voltage: This is the rated voltage of the system whose voltage is required to be stepped down for measurement and protective purposes. b. Rated secondary voltage: This is the voltage at which the meters and protective devices connected to the secondary circuit of the voltage transformer operate.

c. Rated burden: This is the load in terms of volt-amperes (VA) posed by the devices in the secondary circuit on the VT. This includes the burden imposed by the connecting leads. The VT is required to be accurate at both the rated burden and 25% of the rated burden. d. Accuracy class required: The transformation errors that are permissible, including voltage (ratio) error and phase angle error. Phase error is specified in minutes. Typical accuracy classes are Class 0.5, Class 1 and Class 3. Both metering and protection classes of accuracy are specified. In a metering VT, the VT is required to be within the specified errors from 80% to 120% of the rated voltage. In a protection VT, the VT is required to be accurate from 5% upto the rated voltage factor times the rated voltage. e. Rated voltage factor: Depending on the system in which the VT is to be used, the rated voltage factors to be specified are different. The table below is adopted from Indian and International standards. Rated voltage Rated time factor 1.2 Continuous 1.2 1.5 1.2 1.9 1.2 1.9 Continuous for 30 seconds Continuous for 30 seconds Continuous for 8 hours Method of connecting primary winding in system Between phases in any network Between transformer star-point and earth in any network Between phase and earth in an effectively earthed neutral system Between phase and earth in a non-effectively earthed neutral system with automatic fault tripping Between phase and earth in an isolated neutral system without automatic fault tripping or in a resonant earthed system without automatic fault tripping

f. Temperature class of insulation: The permissible temperature rise over the specified ambient temperature. Typically, classes E, B and F. g. Residual voltage transformer (RVT): RVTs are used for residual earth fault protection and for discharging capacitor banks. The secondary residual voltage winding is connected in open delta. Under normal conditions of operation, there is no voltage output across the residual voltage winding. When there is an earth fault, a voltage is developed across the open delta winding which activates the relay. When using a three phase RVT, the primary neutral should be earthed, as otherwise third harmonic voltages will appear across the residual winding. 3 phase RVTs typically have 5 limb construction. h. Metering Units:11kV metering units consist of one 3 phase VT and 2 CT's connected together in a single housing. This can be used for three phase monitoring of energy parameters. It is used with trivector meters and energy meters.

Tests A number of routine and type tests have to be conducted on VT s before they can meet the standards specified above. The tests can be classified as: a. Accuracy tests to determine whether the errors of the VT are within specified limits b. Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, induced over-voltage test , impuse tests with 1.2u/50u wave, and partial discharge tests (for voltage>=6.6 kV) to determine whether the discharge is below the specified limits. c. Temperature rise tests d. Short circuit tests e. Verification of terminal markings and polarity

TYES OF POTENTIAL TRANSFORMER: Potential transformers are of two types: ELECTROMAGNETIC (WOUND) POTENTIAL TRANSFORMERS They work on the same principle as that of power transformers. The load to be transmitted is limited to only few VA depending upon application. The construction depends largely on the rated primary voltage. For voltages above 66 KV, electromagnetic potential transformers are generally arranged in cascade connection. The wound type potential transformer is conventionally used for systems up to or below 132 KV for economic reason. For high speed distance protection electromagnetic voltage transformers are preffered. CAPACITOR VOLTAGE TRANSFORMER A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a transformer to isolate and further step down the voltage for the instrumentation or protective relay. The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices. [1] The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of wound primary voltage transformers would be uneconomical. In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2.

The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. [2] This forms a carrier communication network throughout the transmission network The main purposes to use CVT’s in HV Networks are:  .Voltage Measuring: They accurately transform transmission voltages down to useable levels for revenue metering, protection and control purposes .Insulation: They guarantee the insulation between HV network and LV circuits ensuring safety condition to control room operators .HF Transmissions: They can be used for Power Line Carrier (PLC) coupling Transient Recovery Voltage: When installed in close proximity to HV/EHV Circuit Breakers, CVT’s own High Capacitance enhance C/B short line fault / TRV performances



 

Features:
The main features of CVT’s are given below: General

1. Available for HVS from 46 to 1200KV 2. Available in compliance with all World International Standards 3. Paper and/or PPR + Synthetic or Mineral Oil insulation 4. High range of Customization according Specific Customer requirements 5. Fully designed following the best/modern/experienced practices 6. Sealed Construction – Real Maintenance free for all service life 7. Easiest/fastest solutions for commissioning/put in service 8. Fully designed/produced acc Lean Production Concepts

Mechanical

Available with Porcelain or Composite Insulator Available with standard/heavy/very heavy creepage distance All external components are made by aluminum Reduced/optimized dimensions and weight High earthquake strength capability Suitable for ambient temperature -60/+70°C (extended range upon request) Electrical 1. Best capacitance/accuracy stabilities in all service conditions 2. Design solutions allow to reach High Rated Capacitance in reduced CVT’s dimensions 3. Suitable for HF transmissions 4. Integrated filter damps any kind of overvoltage due by ferroresonance phenomena 5. Best accuracy as transient performance – Suitable for ultra-rapid line protection devices

Optional:

Carrier Accessories inside/outside terminal box Fuses or MCBs inside terminal box Voltage Tap Ground Switch Puncture PIN on Capacitor Divider

CIRCUIT BREAKERS
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. So a circuit breaker is a equipment which can:   Make or break a circuit either manually or by remote control under normal conditions Break a circuit under fault condition

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source. Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs. The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys, and other highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms. Different techniques are used to extinguish the arc including:
   

Lengthening / deflection of the arc Intensive cooling (in jet chambers) Division into partial arcs Zero point quenching (Contacts open at the zero current time crossing of the AC waveform, effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency i.e. 100 times per second for 50 Hz and 120 times per second for 60 Hz AC) Connecting capacitors in parallel with contacts in DC circuits



Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit. SHORT CIRCUIT- CURRENT Circuit breakers are rated both by the normal current that they are expected to carry, and the maximum short-circuit current that they can safely interrupt. Under short-circuit conditions, a current many times greater than normal can exist . When electrical contacts open to interrupt a large current, there is a tendency for an arc to form between the opened contacts, which would allow the current to continue. This condition can create conductive ionized gases and molten or vaporized metal which can cause further continuation of the arc, or creation of additional short circuits, potentially resulting in the explosion of the circuit breaker and the equipment that it is installed in. Therefore, circuit breakers must incorporate various features to divide and extinguish the arc. In air-insulated and miniature breakers an arc chute structure consisting (often) of metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into the arc chute. Larger circuit breakers such as those used in electrical power distribution may use vacuum, an inert gas such as sulphur hexafluoride or have contacts immersed in oil to suppress the arc. The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a prospective short-circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to explode when reset.

Circuit Breaker Ratings
15 Amps – Use with 14 gauge wire for lights and light duty appliances 20 Amps – Use with 12 gauge wire for heavier appliances and kitchens and baths 30 Amps – Use with 10 gauge wire for heavy duty appliances or rated heaters 50 Amps – Use for rated appliances or to supply a sub-panel, use appropriate wire >50 Amps – Supply sub-panel or use as main breaker in main panel, use appropriate wire

Types of circuit breakers:
On the basis of voltage class the circuit breakers are classified in following three types: Low Voltage Circuit Breakers: These breakers are made for direct current (DC) applications and are commonly used in domestic, commercial, and industrial fields. They can be installed in multi-tiers in LV switchboards or switchgear cabinets. Low voltage circuit breakers are usually placed in draw-out enclosures that permit removal and interchange without dismantling the switchgear. Miniature circuit breakers (MCB) and molded case circuit breakers (MCCB) are some common types of low voltage circuit breakers Medium Voltage Circuit Breakers: Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 35 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protective relays instead of relying on built-in thermal or magnetic overcurrent sensors. Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:


Vacuum circuit breakers—With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers. Air circuit breakers—Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.





SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.

Medium-voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing the breaker to be removed without disturbing the power circuit connections, using a motor-operated or handcranked mechanism to separate the breaker from its enclosure High Voltage Circuit Breakers: These breakers help in protecting and controlling electrical power transmission networks. They are solenoid operated and are employed with current sensing protective relays that function through current transformers. Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoidoperated, with current sensing protective relays operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault. High-voltage breakers are broadly classified by the medium used to extinguish the arc.      Bulk oil Minimum oil Air blast Vacuum SF6

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc. High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability.

On the basis of construction the circuit breakers are classified as : Magnetic Circuit Breakers: Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. The core is restrained by a spring until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges

beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breake. Thermal Circuit Breakers: These breakers employ heat to break the circuit current flow and consist of a bimetallic strip, made of two types of materials welded together. At high heat levels, this strip bends at an angle that pulls the circuit breaker's lever down and breaks the connection between the circuit breaker's contact plate and the stationary contact plate. On the basis of medium used for arc extinction the circuit breakers are classified as: Oil Circuit Breaker: An oil circuit breaker uses oil as the main dielectric for the contacts. They are similar to air breakers, but arc quenching is achieved in an oil bath instead of in air. The oil in OCBs serves two purposes. It insulates between the phases and between the phases and the ground, and it provides the medium for the extinguishing of the arc. When electric arc is drawn under oil, the arc vaporizes the oil and creates a large bubble that surrounds the arc. The gas inside the bubble is around 80% hydrogen, which impairs ionization. The decomposition of oil into gas requires energy that comes from the heat generated by the arc. The oil surrounding the bubble conducts the heat away from the arc and thus also contributes to deionization of the arc. Main disadvantage of the oil circuit breakers is the flammability of the oil, and the maintenance necessary to keep the oil in good condition (i.e. changing and purifying the Air Blast Circuit Breaker: These type of breakers employ ‘air blast’ as the quenching medium. The contacts are opened by air blast produced by the opening of blast valve .The air blast cools the arc and sweeps away the arcing products to the atmosphere. This rapidly increases the dielectric strength of the medium between contacts and prevents from reestablishing the arc. Consequently the arc is extinguished and the flow of current is interrupted. Advantages An air blast circuit breaker has the following advantages over an oil circuit breaker: The risk of fire is eliminated The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations; the expense of regular oil is replacement is avoided The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very small .this reduces the size of device The arcing time is very small due to the rapid build up of dielectric strength between contacts. Therefore, the arc energy is only a fraction that in oil circuit breakers, thus resulting in less burning of contacts

Due to lesser arc energy, air blast circuit breakers are very suitable for conditions where frequent operation is required The energy supplied for arc extinction is obtained from high pressure air and is independent of the current to be interrupted. Disadvantages: Air has relatively inferior arc extinguishing properties Air blast circuit breakers are very sensitive to the variations in the rate of restriking voltage. Considerable maintenance is required for the compressor plant which supplies the air blast Air blast circuit breakers are finding wide applications in high voltage installations. Majority of circuit breakers for voltages beyond 110 kV are of this type. SF6 Circuit Breaker: In SF6 circuit breakers, sulfur hexafluoride (SF6) is used as the arc quenching medium. In an SF6 circuit-breaker, the current continues to flow after contact
separation through the arc whose plasma consists of ionized SF6 gas. For, as long as it is burning, the arc is subjected to a constant flow of gas which extracts heat from it. The arc is extinguished at a current zero, when the heat is extracted by the falling current. The continuing flow of gas finally deionises the contact gap and establishes the dielectric strength required to prevent a re-strike.

The direction of the gas flow, i.e., whether it is parallel to or across the axis of the arc, has a decisive influence on the efficiency of the arc interruption process. Research has shown that an axial flow of gas creates a turbulence which causes an intensive and continuous interaction between the gas and the plasma as the current approaches zero. Cross-gas-flow cooling of the arc is generally achieved in practice by making the arc move in the stationary gas. This interruption process can however, lead to arc instability and resulting great fluctuations in the interrupting capability of the circuit breaker. In order to achieve a flow of gas axially to the arc a pressure differential must be created along the arc. The first generation of the SF6 circuit breakers used the two-pressure principle of the air-blast circuit-breaker. Here a certain quantity of gas was kept stored at a high pressure and released into the arcing chamber. At the moment high pressure gas and the associated compressor was eliminated by the second generation design. Here the pressure differential was created by a piston attached to the moving contacts which compresses the gas in a small cylinder as the contact opens. A disadvantage is that this puffer system requires a relatively powerful operating mechanism. Neither of the two types of circuit breakers described was able to compete with the oil circuit breakers price wise. A major cost component of the puffer circuit-breaker is the operating mechanism; consequently developments followed which were aimed at reducing or eliminating this additional cost factor. These developments concentrated on employing the arc energy itself to create directly the pressure-differential needed. This research led to the development of the self-pressuring circuit-breaker in which the over – pressure is created by using the arc energy to heat the gas under controlled conditions. During the initial stages of development, an auxiliary piston was included in the interrupting mechanism, in order to ensure the satisfactory breaking of small currents. Subsequent improvements in this technology have eliminated this requirement and in the latest designs the operating mechanism must only provide the energy needed to move the contacts. Vacuum Circuit Breaker: TRANSFORMER A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive coupling. If a load is connected to the secondary, current will flow in the secondary winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp) and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus enables an alternating current (AC) voltage to be "stepped up" by making Nsgreater than Np, or "stepped down" by making Ns less than Np. In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception. . BASIC PRINCIPLE OF TRANSFORMER WORKING The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism) and second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils. If a load is connected to the secondary winding, the load current and voltage will be in the directions indicated, given the primary current and voltage in the directions indicated (each will be alternating current in practice). The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

where Vs is the instantaneous voltage, Ns is the number of turns in the secondary coil and Φ is the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicularly to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the

cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer,[30] the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for Vs and Vp gives the basic equation for stepping up or stepping down the voltage

Np/Ns is known as the turns ratio, and is the primary functional characteristic of any transformer. In the case of step-up transformers, this may sometimes be stated as the reciprocal,Ns/Np. Turns ratio is commonly expressed as an irreducible fraction or ratio: for example, a transformer with primary and secondary windings of, respectively, 100 and 150 turns is said to have a turns ratio of 2:3 rather than 0.667 or 100:150.

AUTOTRANSFORMER The primary and secondary windings of a two winding transformer have induced emf in them due to a common mutual flux and hence are in phase. The currents drawn by these two windings are out of phase by 180◦.. This prompted the use of a part of the primary as secondary. This is equivalent to fusing the secondary turns into primary turns. The fused section need to have a cross sectional area of the conductor to carry (I2−I1) ampere! This ingenious thought led to the invention of an auto transformer. Fig. shows the physical arrangement of an auto transformer. Total number of turns between A and C are T1. At point B a connection is taken. Section AB has T2 turns. As the volts per turn, which is proportional to the flux in the machine, is the same for the whole winding, V1 : V2 = T1 : T2 For simplifying analysis, the magnetizing current of the transformer is neglected.

When the secondary winding delivers a load current of I2 ampere the demagnetizing ampere turns is I2T2 . This will be countered by a current I1 flowing from the source through the T1 turns such that, I1T1 = I2T2 A current of I1 ampere flows through the winding between B and C . The current in the winding between A and B is (I2 − I1) ampere. The cross section of the wire to be selected for AB is proportional to this current assuming a constant current density for the whole winding. Thus some amount of material saving can be achieved compared to a two winding transformer. The magnetic circuit is assumed to be identical and hence there is no saving in

the same. To quantify the saving the total quantity of copper used in an auto transformer is expressed as a fraction of that used in a two winding transformer as, copper in auto transformer copper in two winding transformer (T1 − T2)I1 + T2(I2 − I1) T1I1 + T2I2

=

1 − 2T2I1 T1I1 + T2I2

But T1I1 = T2I2 ∴ The Ratio = 1 − 2T2I1 2T1I1 = 1 −T2/T1 This means that an auto transformer requires the use of lesser quantity of copper given by the ratio of turns. This ratio therefore denotes the savings in copper. As the space for the second winding need not be there, the window space can be less for an auto transformer, giving some saving in the lamination weight also. The larger the ratio of the voltages, smaller is the savings. As T2 approaches T1 the savings become significant. Thus auto transformers become ideal choice for close ratio transformations. The savings in material is obtained, however, at a price. The electrical isolation between primary and secondary has to be sacrificed.

PHOTO

If we are not looking at the savings in the material, even then going in for the auto transformer type of connection can be used with advantage, to obtain higher output. This can be illustrated as follows. Fig. 29 shows a regular two winding transformer of a voltage ratio V1 : V2, the volt ampere rating being V1I1 = V2I2 = S. If now the primary is connected across a supply of V1 volt and the secondary is connected in series addition manner with the primary winding, the output voltage becomes (V1 + V2) volt. The new output of this auto transformer will now be

I2(V1 + V2) = I2V2(1 +V1/V2) = S(1 +V1/V2) = V1(I1 + I2) = S(1 +I2/I1) Thus an increased rating can be obtained compared to a two winding transformer with the same material content. The windings can be connected in series opposition fashion also. Then the new output rating will be I2(V1 − V2) = I2V2(V1/V2− 1) = S(V1/V2− 1) …………(3) The differential connection is not used as it is not advantageous as the cumulative connection.

EQUIVALENT CIRCUIT

As mentioned earlier the magnetizing current can be neglected, for simplicity. Writing the Kirchoff’s equation to the primary and secondary of Fig, we have V1 = E1 + I1(r1 + jxl1) − (I2 − I1)(r2 + jxl2) …………….(4) Note that the resistance r1 and leakage reactance xl1 refer to that part of the winding Here only the primary current flows. Similarly on the load side we have, E2 = V2 + (I2 − I1)(r2 + jxl2) …………………………………(5) The voltage ratio V1 : V2 = E1 : E2 = T1 : T2 = a ; where T1 is the total turns of the Primary. Then E1 = aE2 and I2 = aI1 multiplying equation(4) by ’a’ and substituting in (3) we have V1 = aV2 + a(I2 − I1)(r2 + jxl2) + I1(r1 + jxl1) − (I2 − I1)(r2 + jxl2) = aV2 + I1(r1 + jxl1 + r2 + jxl2 − ar2 − ajxl2) + I2(ar2 + jaxl2 − r2 − jxl2) = aV2 + I1(r1 + jxl1 + r2 + jxl2 + a^2r2 + ja^2xl2 − ar2 − ajxl2 − ar2 − jaxl2) = aV2 + I1(r1 + r2(1 + a2− 2a) + jxl1 + xl2(1 + a^2− 2a)) = aV2 + I1(r1 + (a − 1)^2r2 + jxl1 + (a − 1)^2xl2)……………………. (6)

Equation (5) yields the equivalent circuit of Fig. 31 where Re = r1 + (a − 1)^2r2 and Xe = xl1 + (a − 1)^2xl2. The magnetization branch can now be hung across the mains for completeness. The above equivalent circuit can now be compared with the approximate equivalent circuit of a two winding case Re = r1 + a^2r2 and Xe = xl1 + a^2xl2. Thus in the case of an auto transformer total value of the short circuit impedance is lower and so also the percentage resistance and reactance. Thus the full load regulation is lower. Having a smaller value of short circuit impedance is sometimes considered to be a disadvantage. That is because the short circuit currents become very large in those cases. The efficiency is higher in auto transformers compared to their two winding counter part at the same load. The phasor diagram of operation for the auto transformer drawing a load current at a lagging power factor angle of θ2 is shown in Fig. 32. The magnetizing current is omitted here again for simplicity. From the foregoing study it is seen that there are several advantages in going in for the autotransformer type of arrangement. The voltage/current transformation and impedance conversion aspects of a two winding transformer are retained but with lesser material (and hence lesser weight) used. The losses are reduced increasing the efficiency. Reactance is reduced resulting in better regulation characteristics. All these benefits are enhanced as the voltage ratio approaches unity. The price that is required to be paid is loss of electrical isolation and a larger short circuit current (and larger short circuit forces on the winding).

1. Over head contactor:  Uses ACSR type conductors(MOOSE type)

Aluminium Conductor Steel Reinforced (or ACSR) cable is a specific type of highcapacity, high-strength stranded cable typically used in overhead power lines. The outer strands are aluminium, chosen for its excellent conductivity, low weight and low cost. The centre strand is of steel for the strength required to support the weight without stretching the aluminium due to its ductility. This gives the cable an overall high tensile strength. ACSR cables are available in several specific sizes, with multiple centre steel wires and correspondingly larger quantities of aluminium conductors. For example, an ACSR cable with 72 aluminium conductors that requires a core of 7 steel conductors will be called 72/7 ACSR cable.

The higher resistance of the steel core is of no consequence to the transmission of electricity since it is located far below the skin depth where essentially no AC current flows.

2. Lightning arrester (LA): TYPES OF LIGHTNING STROKES 1. Direct stroke 2. Indirect stroke

1. Direct stroke – In the direct stroke, the lightning discharge (i.e. current path) is directly from the cloud to the subject equipment e.g. an overhead line. From the line, the current path may be over the insulators down the pole to the ground. The over voltages set up due to the stroke may be large enough to flash over this path directly to the ground. The direct strokes can be of two types: (i) Stroke A and (ii) stroke B. (i) In stroke A, the lightning discharge is from the cloud to the subject equipment i.e. an over- head line in this case as shown in Fig 4 (i).The cloud will induce a charge of opposite sign on the tall object (eg. an overhead line in this case). When the potential between the clouds exceeds the breakdown value of air, the lightning discharge occurs between the cloud and the line.

(ii) In stroke B, the lightning discharge occurs on the overhead line as a result of stroIj4 between the clouds as shown in Fig. 4 (ii). There are three clouds P, Q and R having positive, negative and positive charges respectively. The charge on the cloud Q is bound by the cloud R. If the cloud P shifts too near the cloud Q then lightning discharge will occur between them and charges on both these clouds disappear quickly. The result is that charge on cloud R suddenly becomes free and it then discharges rapidly to earth ignoring tall objects. 2. Indirect stroke – Indirect strokes result from the electro statically induced charges on the conductors due to the presence of charged clouds. This is illustrated in fig 5. A positively cloud is above the line and induces a negative charge on the line by electrostatic induction. This negative charge will be only on that portion of the line right under the cloud and the portions of the line away from it will be positively charged as shown in Fig 5. The induced positive charge leaks slowly to earth via the insulators. When the cloud discharges to earth or to another cloud, the negative charge on the wire is isolated as it cannot flow quickly to earth over the insulators. The result is that a negative charges rush along the line is both directions in the form of travelling waves. Majority of surges in transmission lines are caused by indirect lightning strokes.

A lightning arrester is a device used on electrical power systems to protect the insulation on the system from the damaging effect of lightning. Metal oxide varistors (MOVs) have been used for power system protection since the mid 1970s. The

typical lightning arrester also known as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrester, the current from the surge is diverted around the protected insulation in most cases to earth. An LA provides a low impedance path for lightning to the ground and simultaneously a high resistance path for transmission current, thus restricting any loss of useful power. High impedance, 50Hz (low leakage current 0.6-0.7a) Low impedance, for lightning, shunts it to ground

4. Wave trap:
A transmission line carries 2 types of signals: 1. Power Signals: high voltage(kV’s or MV’s), low frequency ( 50-100 Hz) 2. Communication Signals : Low voltage, high frequency ( kHz or MHz) It thus becomes necessary to differentiate between the two signals in order to have effective transmission of power signals. A wave trap does the necessary job by filtering the communication signals. A wave trap is also used for security purposes. A signal is sent from the generating station in the form of electrical energy (using a transducer). The abnormality is detected by the wave trap. The secondary side of the transformer (33kV) is tripped which acts as a no load condition for the transformer and thus minimum power is drawn.

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