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52
GENERATOR SYNCHRONISATION AND ROUTINE & EMERGENCY CONDITION CHECKUP OF ELECTRICALSYSTEM.
Synchronization : Matching of parameters (voltage, frequency, phase rotation & phase angle) of electrical machine with Grid Parameters and connecting the electrical machine to the Grid. Two types of synchronizations are discussed : (a) Generator Synchronization. (b) Feeder Synchronization
Generator synchronization can be done in two methods: (a) Manual mode (b) Auto mode Feeder synchronization can be done in two methods: (a) In circuit method (b) Bypass method Gen. synchronization procedure : Turbine speed reaches to 3000 rpm . Next UCR Electrical Control Engineer says G.C.R. to close 29A or 29B ( according to MB-1 & MB-2) and 29C isolator of the Generator Transformer Bay. Next clearance for synchronization has to be obtained from shift charge engineer. First of all we have to check whether AVR is in ‘Auto’ mode or in ‘Manual’ mode. If AVR is in ‘manual’ mode then after closing field Bkr. we have to build up voltage manually and if AVR is in ‘Auto mode’ then after closing field Bkr. voltage build up will be automatically up to 95% of the rated value . Next GCR is asked to switch on synchronization permissive switch from GCR. Now in case of manual synchronization, synchronization mode key is placed at check position. Synchroscope switch is made on. Then after matching incoming and running voltage , frequency, phase rotation & phase angle we have to close generator breaker manually at the instant when “Synchronization in limit” red lamp glows. After synchronization synchroscope switch and synchronization mode key is made off and GCR is asked to switch off synchronization permissive switch. In case of ‘Auto synchronization, Synchroscope switch is made on & synchronizing mode key is placed in ‘Auto’ position. Auto synchronizer will match the incoming and running voltage, frequency phase rotation and phase angle automatically and next generator breaker will be closed automatically. After ‘Auto’ synchronization we have to switch off Synchroscope switch and synchronizing mode key GCR is asked to switch off synchronization permissive switch. Details of generator synchronization: The conditions are that the magnitude, frequency, phase rotation and phase angle of the voltage on the generator side of the breaker match those on the system side of the breaker. Prior to synchronization matching voltage magnitudes required adjustment of the generator excitation level such that the generator terminal voltage matches the system voltage. Matching the frequency requires that the turbine speed be precisely adjusted such that the generator output frequency is the same as or slightly higher than the system frequency. Matching phase rotation means that the order in which the three phase voltages reach their peaks in the same, which requires proper connections and checks prior to initial start up . Matching phase angle means that the voltage in each phase on the generator and system side of the breaker are in phase or reach their peaks at precisely the same time. Synchronization can be done manually by operation engineer or automatically by an automatic synchronizer. Manual synchronization: A synchronizing panel will ordinarily contain two volt meters, frequency meters and a Synchroscope and synchronizing lights . One voltmeter and frequency meter monitor the generator to be synchronized and the other

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monitor the system side. The Synchroscope pointer indicates the phase angle between the voltages on either side of the breaker. The straight up or 12’0 clock position indicator that the voltages are in phase. One full rotation represents movement of 360°C of phase angle. The rotational speed of the Synchroscope pointer is the difference in frequency between the generator and the system with clock wise rotation meaning that the generator frequency is higher than the system frequency. Synchronizing lights usually serve as back up to Synchroscope. They are connected to indicate the voltage appearing cross the C.B. contacts. When the pointer is rotating it shows that the two frequencies are not exactly the same. Synchronization with the pointer rotating slowly ( no more than about once per 15S) clockwise ensures that when the breakers is closed, a small amount of power flows out from the connected generator and therefore does not tend to pick up its reverse power relay. The breaker is typically closed when the Synchroscope is rotating slowly in the clockwise direction and the Synchroscope pointer is as close as possible to or slightly before 12’0 clock. It should be possible to close the Bkr. to within 5° of exact matching to achieve a smooth synchronization. Synchronizing out of phase can subject the generator to a shock which could be more severe than a terminal short circuit and could result in damage to generator end winding and rotor shaft system. Automatic Synchronization: The simplest automatic synchronizer is called synch.-check relays, which simply check the two voltages and close a contact when the voltages are within certain limits for a certain length of time. On small machines. Synch-check relays may serve to close the C.Bs. On large machines, they are primarily back ups in manual synchronizing schemes to block closing of C.B. too far out of synchronization. Highly accurate and reliable automatic synchronizers are available with adjustable ranges to monitor both synchronization and the voltage levels of the machines being synchronized. These devices automatically send out signals to (1) the prime mover control to adjust the speed to match the system frequency & (2) the voltage regulator to adjust the Generator terminal voltage to match the system voltage, and then (3) they give a closing signal to the C.B. when all parameters, including the phase angle, are correct. Dead bus relays, sometimes included in the synchronizing relay itself and sometimes provided as a separate relay, allow connecting a machine to a dead bus where the synchronizing relay itself would not give a close signal. Feeder synchronization: Here synchronization is done with the help of a synchronizing trolley from outside by matching voltage, frequency phase rotation and phase angle of incoming & running. It is the common practice that the remote end from generating station will charge the feeder first and generating station will synchronize the feeder later. This is done because if there be any fault in the feeder then it will trip due to its protection and the fault will not feed to the generating station. Two methods are there to synchronize a feeder. (a) In circuit method. (b) Bypass method. In circuit method: When line is charged from the remote end and we are going to synchronize it then we apply this method. After telephonic confirmation of line charging from remote end and after observing the voltage level of the corresponding feeder from the trend menu of DAS, we connect the connector of synchronizing trolley to the socket holder placed at lower portion of feeder panel. Next we make on the synchronizing switch at the feeder panel. At the synchronizing trolley, ‘IN/OUT switch is placed at ‘IN’ position & IN circuit /Bypass switch is placed at “IN circuit” position. After observing check synchro-relay lamp is glowing and incoming and running voltage and frequency is matching then closing pulse is given at circuit breaker. None we switch off the synchronizing switch and open the synchrotrolley connector from the socket. Next we check the balanced loading of the feeder through analog ammeter. Now, in circuit method of synchronization is complete. Bypass method: When line is not charged from the remote end (in case of dead line) & we are going to charge the line first then bypass method of synchronization is taken into account. Here also at first we connect the connector of the

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synchronizing trolley to the socket holder. Next we make on the synchronizing switch at feeder panel. At the synchro-trolley, IN/OUT switch is placed at “OUT” position & IN circuit /Bypass switch is placed at “bypass” position. Now a closing pulse is given at circuit breaker. Now we switch off the synchronizing switch and open the synchronizing trolley connector from the socket. Now dead feeder synchronization is complete.

ROUTINE CHECK UP OF ELECTRICAL SYSTEMS
As a routine check up, we do the followings mainly in order to maintain electrical equipments properly and by this checking we can avoid long break downs and unnecessary hazards: 1) At Grid Control Room, hourly checking of feeder parameters (like voltage, current, power flow) & Bus parameters (like voltage, frequency) is done and a regular log sheet is maintained for that. If there be any abnormalities of the parameters, than we contact with CLD for system normalization. Switchyard inspection is done regularly in the evening shift as a routine check up. Through this we can check if there be any SF6 gas pressure drop at circuit breaker or any oil leakage from the circuit breaker or any flashing occurs at the isolator contacts and different joints. We also check for any oil leakage in transformers. Reactors, CTS. & also check the silicagel condition at the breather of transformer. Bus change over of the 400KV /220KV feeders is checked from grid control room by making parallel operation of the Bus side isolators. This routine checkup is done at least once in a week. At unit control room, Generator capability curve and unit Bus voltage are checked at a regular interval and necessary action is taken as per situation. Stroboscopic method of fuse checking is done at diode wheels mounted on the Generator rotor shaft. These are 30 diodes & 30 fuses (400 Amp. each) per diode wheel. Hence, 30 diodes & 30 fuses for positive diode wheel and 30 diodes and 30 fuses for negative diode wheel. By moving the wheels slowly in the forward or reverse direction by switches at the brushless exciter cover and observing through the small glass windows, we can check the fuses whether blown out or not.

2)

3) 4)

5)

6)

2 nos. then we have to switch off field forcing . 3 nos. than we have to shut down the generator. ST-3, ST-1, ST-2, UATs & Generator Transformer cooling fans and pumps are manually operated once in a week to check any abnormality with fan operation so that if transformer oil / winding temperature goes high then the corresponding cooling fans and pumps come into service automatically. Hydrogen gas pressure and hydrogen purity in Generator casing, cold gas temperature before and after cooler are important hourly checking parameters. Generator stator winding stator core and rotor winding temperatures are checked hourly through CRT at unit control room. Generator Bearing vibrations and exciter end bearing vibration should be checked hourly.

No. of fuses blown per bridge arm

7)

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Battery charger at GCR side & UCR side are checked regularly once in the shift. D.G. sets are checked twice in a week so that in case of failure of emergency bus it comes into service immediately. Auto position, so that in case of any failure of one incomer, Bus coupler breaker comes into service automatically.

10) In every shift at the beginning, it is checked whether the Auto/ Manual switch of emergency buses are in

11) We have to check in every shift whether six nos. of fans are in running condition inside the TG set acoustic
covering. In case of any failure of the fans , we have to check the control supply from outside of the covering

EMERGENCY OPERATIONS IN ELECTRICAL SYSTEM
1) If we notice any “SF6 Gas press low” or “C.B. oil press low” annunciation came at any feeder panel facia at GCR then we immediately run to switchyard and check the corresponding gas pressure and oil pressure of the feeder breaker. If any leakage is there, the pressure goes down and we immediately divert the feeder through Bus–tie breaker, otherwise if SF6 gas pressure or oil pressure lockout comes then we can not operate the circuit breaker from remote (i.e. from GCR). 2) If D.G. does not take start in Auto in case of total power failure or in case of failure of emergency Bus, and then we have to start D.G. from locally and have to normalize emergency bus immediately. 3) In case of total power failure, we have to minimize unnecessary battery drains. D.C. lamps should glow only at the important areas. 4) In case of total power failure, we have to bring power through any radial feeder as quickly as possible and have to normalize switchyard Bus and Station buses so that we can go for synchronization of at least one unit immediately. 5) In case of low hydrogen pressure in generator casing, we have to charge fresh hydrogen, if consumption is high then have to investigate for leaking points. If the pressure persist to drop then reduce load on generator and if pressure can not be maintained then trip the set. In case of low hydrogen purity, we have to purge some hydrogen from casing and charge fresh hydrogen. 6) In case of generator seal oil differential high / low, we have to check DPRV is functioning normally. If not, change to the bypass line and get DPRV attended. 7) In case of generator rotor temperature high, we have to check immediately the gas cooler water flow and cooler inlet / outlet temperature. Also we have to check and maintain hydrogen pressure and have to reduce excitation and load. 8) In case of loss of excitation, we have to check if unit has tripped and generator breaker has opened, if not then we have to do it manually. 9) In case of any trouble in generator transformer cooling system, we have to check load on machine and ambient temperature. Also we have to confirm whether all cooling fans are in service. We have to check transformer oil quality and cooling oil pumps operation is normal. Capability curve of synchronous Generator The capability curve of synchronous generator defines the bounds within which it can operate safely. Various bounds imposed on the machines are: 1. MVA loading can not exceed the generator rating. This limit is imposed by stator heating. 2. MW loading can not exceed turbine rating which is given by MVA (rating) X PF (rating). The name plate rating of synchronous Generator specifies this Power factor. 3. Generator must operate a safe margin away from steady state stability limit (σ = 90°). This is the maximum allowable value of σ. 4. The maximum field current can not exceed a specified value imposed by rotor heating.

The phasor diagram of synchronous generator is drawn and armature resistance is neglected.

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Now OMN is the complex power triangle ( in 3-phase values ) Where OM=S(VA); NM = P(W); ON = Q (VAR). Ø is positive for longing P.F., Ø is the angle of OM from the P – axis. Constant S operation will lie on a circle centered at O & radius OM constant P operation will lie on a line parallel to Q axis. Constant excitation (Ef) operation will lie on a circle centered at O´ & constant PF operation will lie on a radial line through O. Now with specified upper limits of S, P, If (field current ), the boundaries of the capability curve is drawn . The limit on the left side is specified by δ max , the safe operating value in respect of transient stability. It is seen that when M/C is in ‘Auto’ channel and load angle limiter in service then safe angle limit is 70° approximately. In figure it is shown as δ max.

The above figure is to give a general idea about capability curve and this is not as per measurement. 10% power margin is shown in the figure & this is the load angle stability limit on hand. This is given because for the same excitation when generator is at a point on load angle practical limit, a 10% load increase will keep the M/C out of this practical limit but if the M/C is on 10% power margin line then a 10% load increase will keep the M/C inside load angle practical limit.

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53
EXCITATION SYSTEM
Basic concept of Voltage Generation :
EMF generated due to change of flux. Change of flux occurs due to rotation of magnetic pole / field by turbine as prime mover. This magnetic field may be a permanent magnet or an electro magnet type. The electro magnet may be developed by application of DC voltage across the field terminal of Generator. The number of pole of the magnetic field depends upon the speed of the Generator and frequency of voltage in the following relation.

The terminal voltage or emf generated, depending upon on the magnetic field strength, which in turn depends on the value of the applied DC voltage at field terminal of the Generator. Generator delivered Power according to demand of the system with a limitation to the capability diagram (as shown below) of the installed capacity: Active Power (MW) will be delivered up to the Turbine limitation Reactive Power (MVAR) will be delivered as per system demand with a certain limitation both in under excited (Leading MVAR) and over excited (Lagging MVAR) zone

CAPABILITY DIAGRAM
Generally Active Power depends on turbine i.e. steam input. Reactive Power depends on Excitation System DC Voltage. As the load (both active & reactive) depend on system demand, so power will be delivered to system requirement both for Active and Reactive Power to maintain the voltage and frequency constant for theoretical concept of infinite bus bar. Our main concern to the topic on Reactive Power Control as per requirement of Reactive load. For a constant excitation, the Generator will deliver the required reactive load by compromising the terminal voltage, which is not unwanted for an infinite bus bar system. To main terminal voltage constant we have to apply the variable DC excitation voltage as per system reactive load demand in nominal steady state operation. The variable DC voltage generation and application to Generator field winding may be of different type: Indirection Excitation System: An exciter M/C is employed for feeding current to the field of Synchronous M/C (Generator) . In the early stage of small capacity Generator, Exciter machine was DC type. The field winding of the exciter controlled by amplidyne through BUCK field (for Gen voltage feedback.) and BOOST field (from pilot exciter as set point) to produce error signal for amplidyne control winding as shown in the fig-1A. Later on this amplidyne for controlling the field winding of exciter replace by additional field winding of the dc exciter where a variable dc applied through thyristor control as shown in Fig-1B. ( See Fig-1A & 1B ) In gradual development AC Exciter is replaced by DC exciter. For AC Exciter, un-controlled rectifier for converting AC to DC is used. This converter may be- stationary type applied through slip ring assembly of the Generator or rotating type fixed inside rotating shaft forming brush less excitation system as shown in Fig-2A , 2B & 2C respectively. (See Fig-2A , 2B & 2C ) In indirect excitation system the overall performance of the system is reduced because of the time constant of exciter machine. Remedial action for this is taken by overriding the exciter so that of fast response of greater than -2 is obtained. To reduce the time, required for building up the current in the exciter field winding, the Automatic Voltage regulation calling for maximum excitation of the Generator, the thyristor initially supplies a higher voltage to field

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winding of the main exciter than actually required for, upto to the ceiling excitation condition and this improve the exciter response. Redundancy in both Power and regulation circuit Auto and Manual channel have been provided with separate thyristor bridge. For full redundancy there may be two-auto channel of independent nature in place of manual channel this is generally used in Atomic Power Station. Direct exciter system : In this system Generator output terminal voltage is used for excitation through step-down Excitation Transformer. This three-phase ac voltage rectified by controlled rectifier (Thyristor) and applied directly to the Generator field winding through Slip Ring system. With conventional Electronic based control for regulation. Or Microprocessor based control for regulation (DVR) is generally used to control the DC field voltage through Thyristor gate pulse control. The basic block diagram of the system as shown in Fig-3. µP based DVR in static exciter system already supplied at Kolaghat TPP . The basic block diagram of the DVR is shown in Fig-3. The main control functions are same as Static Excitation System of electronic control. Only difference is Control , interlock & monitoring parts of the Regulator panel has been converted to DVR. Thyristor panels , Field breaker ,Field Flashing and Excitation transformer will remain same. Control function : a. b. c. d. e. Regulation of generation Rotor current limits Rotor Angle limit Stator current limit Power system stability

Adv of DVR : a. b. c. d. e. f. Fast transformer 10 msce Internal condition monitoring Reproduce able set valve Less not Electronic module High noise animosity Possibility of comprehensive measuring and settings

(See Fig-3) Development of Excitation System in the following way:1. 2. 3. 4. D.C. Excitation System - using DC exciter and voltage regulator AC Exciter system -using Ac exciter, static rectifier and voltage regulation Brush less excitation System - Ac exciter, rotating diode and voltage regulation. S.E.E. –Static Excitation System without any rotating machine, Thyristors have been used for controlled DC of Main Generator.

In respect of field current controlling system of generator -in Direct Excitation system or of Main Exciter (AC or DC) in Indirect Excitation System, gradual development occurred from magnetic amplifier (Amplidyne) to Electronic Controller to Microprocessor based digital controller (DVR) .

BRUSH-LESS EXCITATION SYSTEM
Automatic Voltage Regulation : Electronic Controller

It is a closed loop control of generator terminal voltage as shown in the Fig-4. 389

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Brief description of different functional blocks as follows:Voltage Regulator: Voltage error signal is formed by comparing the set point and actual terminal voltage of Generator and also take care of sum up of influencing factor if applicable i.e.; a. Reactive current compensation hence the influence is 10% maximum. b. Under excitation limiter both proportional and integral c. Over excitation limiter d. V/Hz limiter e. PSS Current regulator : The error signal passes through PI controlled voltage regulator . It forms the set point reference of excitation current regulator (high gain proportional Control), which gives the output for gate control signal comparing with the actual field current of Generator. Under steady state condition current set point and actual field current feed back have approximately the same. Any difference of this setting value multiplied by gain of the current regulator goes to gate control card for Thyristor gate control pulses. Out put of current regulation i.e. UST signal will be over ruled by the following conditions:• • UST will go directly to the full +ve value (+15V) for inverter operation-in case of de-excitation. UST will be in the tune of no load voltage (-1.13V) if GCB open.

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Actual value of field current i.e. output current of Thyristor set is sensed by two transducers connected to separate shunt resistors. Converter has been used for converting mv output of shunt to voltage for control card. One converter is for manual channel. Other one is for auto channel. IF2N =23.0 VF2N = 66.75V IF2MAX=46.50 VF2NMAX = 135.0V for 10 sec. The actual field current signal is taken for permissive interlock for conduction monitoring unit of auto channel when current > 4.0A. This actual field current signal is also taken for ‘field forcing limiter faulty’ annunciation whenever the field current exceed the ceiling current limitation i.e. 1.1 x 1F2MAX. This condition may arise any way the field current ceiling limiter IF2MAX does not act properly. GATE control set : This will generate 3 phase controller-firing pulses in double pulse mode. 3 phase input AC voltage of thyristers is taken for synchronizing the generated pulse with the input power to the Thyristor. The control voltage will change the firing angles ( ∞ ) of Thyrister. For –ve control voltage ∞< 90° in rectifier mode i.e. supplying power to the field winding of main exciter and +ve control voltage ∞ > 90° in inverter mode these reversing the Thyrister set output voltage and drawing power from field winding to exciter resulting is the field current falling towards zero. There are two gate control card one for Auto Ch, and other for manual Ch. Pulse output of any one of the cards will be blocked according to the channel in service. Excitation Build up and field forcing During voltage build up - (speed >90% AND FB ON) and run back, voltage regulator output i.e. set point input of current regulator will be limited in the following manner. In auto mode absence of feed back signal voltage regulator error calling for maximum excitation of the Generator. The Thyristers set accordingly supply higher voltages than actually required under steady state. At the beginning of Generator start up cycle, output Voltage of the ramp function generator is ‘0’ so that Thyrister output current limited to ‘0’ when the speed value and FB close reaches ramp friction Generator gets its input voltage to run up its maximum within approximately 20 sec. This gradually increases the Generator Terminal Voltage.

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On shutdown of the M/C ramp function generator run back to output zero after removing the run up command i.e. speed >90% disappearance i.e. field breaker not tripped till then . Run back time is 5 Sec. (max.) This ramp function Gen. output taken for field Bkr. closing interlock for monitoring the ‘0’ volt (< 0.8V for FB close permissive ). In GCB off condition field forcing may be limited by rectifier limiter of selected to 1.1 x IFN rated excitation). This is required in case of failure of diodes in the rotating rectifier between main exciter & Generator.

Field suppression :
A de-excitation command from the generator protection system or a field breaker off command from control desk will initiate (a) Thyristers set operation from Converter to inverter Mode thereby reversing the main exciter field winding voltage and reduce the Thyristers sets output current to ‘0’. (b) Energize to open the FB contact and close the field discharge register to ensure proper de-excitation even in the event of failure of the electronic de-excitation circuit via inverter operation. The field breaker is automatically tripped during generator shut down by speed criteria ( i.e. speed <90% ) as described above. If, not tripped earlier by power relay(Low forward or Reverse power) an emergency field breaker may be tripped manually by emergency push button on control desk. FB off PB require an interlock of generator circuit breaker to prevent de-excitation of the loaded generator. Local mechanical tripping of FB is not permissible. For emergency de-excitation, a switch is locally provided inside the AVR cubical. Emergency de-excitation may also be possible by tripping the MCB for pulse power supply unit. Auto Manual Change over : Voltage regulator include a control system for automatic voltage control ( auto channel ) and excitation control in manual channel. The manual system is provided for taking the generator characteristics( short circuit and open circuit test) and stability test of protection system during the commissioning and also for take care of generator voltage in case of the failure of auto channel. The desired change over could be done from control desk as well as voltage regulator cubical. Three push actuators are provided in the Control desk-(a) Auto (b) Match & (c) Manual. For channel change over match button must be actuated prior to balancing of auto & man Channel voltage set point by raise / lower push button. The same set of push actuator will be used for both auto & man channel voltage control. When the matching stage reached auto /man balancing voltmeter will show zero position. The change over to man or auto is only be possible after match condition has been achieved. Change over is also being blocked for regulator alarm system, when excitation current regulator (man) or auto channel became faulty. This change over will initiate pulse blocking of the Gate control card. During any fault in the auto channel AVR will change to Man channel. For this a continuous follow up of excitation current set of man channel has been provided. Following conditions will initiate auto to man change over-

1. Power Supply (±15 volt) & (±24volt) faulty.
2. Generator actual voltage faulty-tripping VT MCB of actual voltage monitor by fault. 3. Faulty over excitation-initiated by field forcing limit monitor and additionally by generator reactive current monitor response above 1.1 X ceiling excitation limit. 4. Any failure of the Thyristor through conduction monitoring unit (current < 4.0Amp)

Manual controlIt is the constant excitation mode open loop control in respect of voltage feedback. However to maintain this constant excitation actual excitation current feedback signal is taken.

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The set point is tracked automatically by the follow up control so that in case of failure of auto channel a smooth change over would possible without any delay. A separate gate control card (pulse generation card) with Thyristor set and separate control power supply has been provided for this. A fuse monitoring system for the power Thyristor fuse has been provided through the MCB system (0.1Amp). By tripping of the MCB on failure of Thyristor will provide the alarm “man channel faulty” only.

Limitation of Excitation in Auto channelUnder excitation limiterIn the under excitation range, the under excitation limiter ensure that the minimum excitation required for stable parallel operation of generator. The response characteristics are formed, as shown in the Capability Curve, on the basis of reactive current, active current and generator terminal voltage. There is no time-delayed action for under excitation limiter to increase the excitation on action.

Over excitation limiterFor reactive load demand, excitation will be increased as per requirement to meet the terminal voltage constant. This is also effective for any fault condition. This may cause thermal loading of the rotor. Over excitation limiter limit the field current in the tune of 105% of the rated field current, whenever it cross the set point, output response of the error amplifier will be such that maximum 140% of rated (23amp) If will be allowed to pass through a integrator of time constant 10sec. i.e a delayed response will be occurred and then it will control the excitation on response of limiter by reducing the field current resulting the terminal voltage drop up to 80% of the rated voltage. Below 80% VGN over excitation limiter will no longer be effective. This over excitation limiter is supplemented by Stator Current limiter, which in principle works in the same way. Whenever the stator current crosses the 105% of rated stator current (IGN=9050Amp) it will act to reduce the excitation. An integrator with time constant 10sec have be incorporated in this system for a delayed action in case of system fault feeding condition.

V/Hz LimiterVoltage regulator keeps the voltage constant irrespective of Generator frequency. Excessive under frequency will increase the thermal stress and over fluxing of Generator Transformer & Unit Aux. Transformers. To avoid this, V/Hz limiter is used to reduce the generator voltage by the way of reducing the excitation current with a delayed response as stated below:

V/Hz
1.2 1.3

Response Time
18 secs 10 secs

V/Hz limiter action will be permitted to act in regulator circuit if the following condition prevail1) AVR in Auto 2) Generator voltage > 50%

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54
GENERATOR SEAL OIL SYSTEM
High capacity Generators are normally cooled by hydrogen gas. To keep this hydrogen gas sealed inside the generator casing proper sealing arrangement is incorporated. The following two types of Sealing arrangements are provided to prevent any leakage of gas :

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Static Sealing : It consists of Gasket, ‘O’ rings and other sealing arrangement on the stationary part of the Generator. Dynamic Sealing : This is provided in the annular gap in between stator & rotor of the Generator. The sealing is done by supplying seal oil. The seal oil is pressed in the special channel to form an oil film at a higher pressure than gas pressure. Thus gas leakage is prevented.

GENERATOR SEALING SYSTEM SHOULD ENSURE THE FOLLOWING: 1. Minimum hydrogen gas loss through seals. 2. Frictional losses should be minimum to affect generator efficiency. 3. Frictional loss should be negligible . 4. Sealing system should be reliable with minimum maintenance involvement. 5. Sealing system should function smoothly with variation of generator casing gas pressure. TYPES OF GENERATOR SEALING SYSTEM : • Thrust Type • Ring Type THRUST TYPE SEALING SYSTEM: In Thrust type sealing system Generator Rotor should have collars at its both ends. A circular sealing ring is kept pressed against this collar by either a spring or oil pressure. The surface of the seal ring coming in contact with the collar is lined with babbit metal. Seal oil is supplied at higher pressure than the gas pressure and this oil pressure maintains an oil film in between the Collar & the seal ring babbit metal. Quantity of oil flow is so designed that increase of seal oil temperature, due to frictional loss, remains well within permissible limit. The Seal Ring is housed in Seal Body and Seal Body is fixed with End Shield.

Thrust Type Seal Assembly consists of : a. Seal Ring. b. Synthetic Rubber Gaskets. c. Oil Wipers. & d. Seal Body. a. Seal Ring :

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Seal Ring is housed inside the Seal Body. One face of the Seal Ring, which is located towards collar of the rotor, has a babbit metal liner .This Seal ring liner consists of a grooved end face. From Seal Oil Chamber High pressure seal oil is supplied and through this grooves oil is spread by the centrifugal force between the collar and the seal liner. The gap between liner and the collar can be adjusted by varying thrust oil pressure or by spring compression. Seal Ring is free to move/ float axially inside the Seal Body. b. Synthetic Rubber Gaskets: Seal Oil Chamber and Thrust Oil chamber are isolated by synthetic rubber liner. These Rubber cords are placed inside the grooves located at periphery of the seal liner. c. Oil Wipers: Oil Wipers are located at gas side of the sealing system to prevent oil ingress through generator shaft. d. Seal Body: Seal ring is housed inside the Seal Body. It also contains seal oil chamber and thrust oil chamber. Seal ring is played inside the seal body. OPERATION : Seal oil from the annular seal oil chamber flows through radial holes to the space in between seal babbit and collar of the shaft. From this space oil flows either side i.e. air side and gas side along the shaft. Maximum oil is drained at air side and flows along bearing drain oil. Gas side oil is arrested by oil catcher / wipers and directed it to the hydraulic seal oil tank. Gap between shaft collar and Babbitt liner is very minimum; hence there is a possibility to flow shaft current through this system. To prevent it shaft is normally earthed near front end pedestal and Seal Body, Oil pipes etc. at the exciter end are kept well insulted from the stator casing. RING TYPE SEALING SYSTEM: In Seal Ring system, the Seal Ring floats radically inside the housing of Seal Body. Seal Body is fixed with the End Shield. Oil Sealing is maintained in between contact surface of Rotor Shaft and the Seal Ring. The contact surface of the Seal Ring is lined with Babbitt metal like the thrust sealing system to minimize the frictional losses.

Ring Sealing system are of two types – 1. Single Oil Flow System & 2. Double Oil Flow system.

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In Single Oil Flow System only one oil is supplied to the seal ring which gets divided to flow through Air Side as well as Gas side. For Double Flow System one oil is supplied at gas side and other oil is supplied at air side Advantages / Disadvantages of Thrust Type Sealing : Sl.No. 1. ADVANTAGES DISADVANTAGES

2.

Very less quantity of oil flows towards A separate collar of the Rotor shaft is to be hydrogen side, as a result hydrogen purity provided. In case of damage of the contact surface maintains at higher level. of the collar it is to be machined and machined amount of packing is to be provided behind the end shield. Since very small amount of oil flows towards It creates problem in movement of the Rotor shaft hydrogen side , there is less possibility to when expands due to increase of shaft ingress oil inside the Generator Casing. temperature.

Advantages / Disadvantages of Thrust Type Sealing : Sl.No. 1. ADVANTAGES DISADVANTAGES

2.

3.

Since sealing is maintained at the journal Oil flow towards gas side is more, hence portion of the rotor, there is no restriction of possibility of huge amount of oil ingress inside the expansion due to increase of temperature. generator casing, especially for single oil flow system. The seal ring floats above the journal portion Excessive oil ingress inside generator casing may of the rotor. Hence there is less possibility of deteriorate gas purity. seal Babbitt damage or shaft journal due to sudden impact coming from rotor. This seal oil system operates under a close loop and is separate from lub oil system. Thus purity of oil is maintained.

RING TYPE SEALING SYSTEM: This system comprises of the following accessories: 1. Seal Ring. 2. Seal Body. 3. Oil Catcher. 4. Oil Wiper Ring. 5. Seal Oil Chamber. 6. Pressure oil groove. 7. Seal Oil groove. Seal Ring : Seal Ring is a steel alloy circular ring splitted in two halves for easy assembly on the rotor shaft. It is screwed with the seal body to avoid its radial movement. Oil flow through shaft seal is designed according to increase oil of temperature may be allowed. This oil flow control is done by allowing clearance between shaft journal and seal ring inner diameter. Seal Body: Seal body holds the seal ring and it is fixed with the End Shield. Seal body is also made of two halves for easy assembly. Seal body is insulated from End Shield to restrict flow of shaft current. Seal Body has the provision for Seal oil supply to the pressure oil chamber.

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Seal Oil Chamber: Oil is supplied to the sealing surface from oil chamber through radial holes of the seal ring. Oil Catcher & Oil Wiper Ring: A portion of oil may pass towards gas side along the surface of the shaft. This oil flow caught in oil catcher. Similarly other part of the oil flows towards air side. It is arrested by Oil Wiper Ring. Seal Oil System comprises the following equipments: 1. Seal Oil Pump. 2. Seal Oil Coolers. 3. Seal Oil Filters. 4. Differential Pressure Regulator (DPR). 5. Pressure Regulating Valve. 6. Vacuum Pump. 7. Damper Tank etc. Seal Oil Pump: Seal Oil Pump supply sufficient seal oil at desired pressure. Normally one A.C. power driven seal oil pump and another one D.C. driven seal oil pump maintain seal oil supply, where a third source of supplying seal oil is from Relay Oil / Control Oil provided. Otherwise there are two 415V A.C. driven and one 220V D.C. driven Seal Oil Pumps are provided to serve this purpose. One Seal oil Pump runs as main pump and other pumps are kept available to take start automatically in case of failure of main pump to meet the demand. Seal Oil Pumps are normally screw pump with positive displacement pumps with inbuilt self recirculation arrangement. Seal Oil Coolers: After passing through the annular gap between shaft collar / shaft journal & seal ring the temperature of the oil increases. To cool this seal oil Seal Oil Coolers (2 nos.) are provided. One cooler remains in service and the other as stand by. Seal Oil temperature is maintained at around 40 0C. Cooling water for these coolers may be D.M water or clarified water. Coolers are located at the discharge end of the pumps. Seal Oil Filters: Seal oil quality should be maintained as it passes through a very narrow gap between the shaft collar / journal & seal ring. The oil should be free from dust, metal chips etc. otherwise these may damage seal ring babbit or shaft collar/journal. To prevent these seal oil is passed through very fine Filters. Seal Oil Filters consists of fine mesh wire cloth and also provided some magnetic bars to separate out any magnetic particles from seal oil. Magnetic Bars create magnetic field. Seal Oil first passes through this magnetic field and magnetic material are separated out. Remaining dirts are retained by fine mesh wire cloth.

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Differential Pressure Regulator (DPR : Differential Pressure Regulator (DPR) regulates seal oil pressure in such a way that Oil/gas differential pressure is maintained at a preset point irrespective of its gas pressure or machine load. Vacuum Pump: Seal oil comes in contact with hydrogen gas. Hence seal oil should be free from air / oxygen to maintain purity of hydrogen gas. As such seal oil is pre-treated and entrapped air in seal oil is removed by vacuum pump. DAMPER TANK: Damper tanks are provided where thrust type seals are provided. Normally seal oil is fed in this tank after passing through DPR and from this tank seal oil is fed to the seal oil chambers of seal oil system This tank stores a good quantity of seal oil and it provides seal oil supply for 10-12 minutes to the seal system in case of total seal oil supply failure. It is located about 6 M above the centre of the Generator Shaft.

GENERATOR SEAL OIL SYSTEM LINE DIAGRAM

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Seal Oil Flow Regulators: It provides adequate seal oil flow through seal ring under all operating conditions. Two regulators are deployed for both side seals. By adjusting the orifice, flow rates are regulated... The seal oil flow rates varies on the cross sectional area of the orifice which is controlled by the movement of the diaphragm. Movement of this diaphragm again controlled by the differential pressure acting on the diaphragm via a bore in the cone. The differential pressure is created by the high pressure of seal oil before orifice and low oil pressure after the orifice. THRUST TYPE SEAL OIL SYSTEM: Seal Oil after Filter is divided in two streams- one stream goes to Differential Pressure Regulator (DPR) and other stream goes to Pressure Oil Regulator (POR). At all operating conditions down stream pressure of seal oil is maintained by the DPR at 0.6 – 0.8 kg/cm2 above the gas pressure. Two signals i.e. H2 gas pressure and seal oil pressure operate the DPR to maintain this oil/gas differential pressure. From the outlet of DPR seal oil is fed in Damper Tank and from damper tank seal oil is supplied to individual seal system. H2 gas impulse line is connected at the top of the Damper Tank. A bypass line with a regulating valve is normally provided to bypass DPR in case of malfunctioning of DPR. Seal oil after passing through Pressure Oil Regulator (POR) is fed in the thrust oil chamber located in seal body. POR regulates its outlet pressure at a predetermined set value of thrust oil pressure Under all operating conditions thrust oil presses the seal liner against the rotor shaft collar. In case of seal oil supply failure, oil is supplied to POR from Damper Tank through a line with a NRV. RING TYPE SEAL OIL SYSTEM:

55
A BRIEF NOTE ON GENERATOR PROTECTION
Introduction: Strictly speaking a fault is any abnormal state of a system. The faults in general consist of short circuits as well as open circuits. Open circuit faults are much more unusual than short circuits. In terms of seriousness of the effects of

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a fault short circuits are of far greater concern than open circuits, although some open circuits may result in major hazards. Protective relays and relaying systems detect the faults in electrical circuits and operate automatic switchgears to isolate faulty equipment from the rest of the system as fast as possible. This limits the extent of damage at the fault location and prevents the effects of fault spreading into the system.

It is to be noted that a protective relay cannot prevent the appearance of faults; it comes into action only after fault has occurred. Current and Voltage Transformers: When the primary values of currents and voltages in a power circuit are too high to permit direct connection at measuring and protective instruments, coupling is made through current and voltage transformers. They also isolate the equipment electrically from the higher voltage levels of the power systems. Current Transformers (C.T.): - The primary winding of a C.T. is connected in series with the circuit the current of which is to be measured and across the secondary the current circuit of the measuring instrument is connected. Current Transformers reduce the system current to a lower value. Normally C.T. secondary are 1 ampere or 5 ampere rated. The C.T.s used in metering circuits are known as metering C.T.s. These require comparatively higher accuracy over the range of around 10% to 120% of rated current and saturation point of these are relatively lower. The ‘knee point’ marks the boundary between the saturated and unsaturated regions of a C.T. It is defined as the point at which a 10% increase in secondary voltage would result in 50% increase in exciting current. Protective C.T.s require linear characteristics up to maximum fault current level and so their knee point is much higher up at the excitation curve. Voltage Transformers (V.T.) [Also known as Potential Transformers (P.T.)]: - Primary of these transformers is connected across the points at which the voltage is to be measured. Voltage transformers step down the system voltage to a lower value. Rated secondary voltage of a V.T. is 110/√3 volts (between phase and neutral) and has been regarded as standard. There are two types of voltage transformers – conventional electromagnetic voltage transformer and capacitive voltage transformer (C.V.T.). C.V.T. consists of a capacitance voltage divider, to which is connected an intermediate transformer of much lower voltage rating than the system voltage at which it is used, e.g. a C.V.T. for 220 kV system has intermediate transformer of around 14 kV primary rating. Other than being used at measurement and protection purposes C.V.T. finds its application in Power Line Carrier Communication (P.L.C.C.) also. Apart from normal C.T.s and V.T.s, sometimes interposing current transformer (I.C.T.) and interposing voltage transformer (I.V.T.) are used for some specific purposes.

Protective Relays: Basic Characteristics

: Any protective relay has to satisfy four basic functional characteristics : Ability of a relay to respond to the minutest abnormality is known as sensitivity. A relay is more sensitive than other if it can respond to more minute fluctuations of actuating quantity than other : It should be able to select which part of the system is faulty and only isolate that part from the rest of the system that is healthy otherwise. Selectivity is achieved in two ways – unit system of protection and non-unit system of protection. In unit system of protection the relays and protection schemes respond to faults those occur within their zones of protection only. In non-unit system of protection selectivity is obtained by current grading and time grading of the relays situated at different locations, all of which may experience a change in actuating quantity for a particular fault. : Occurrence of faults is generally not frequent in power systems, so that for most of the time relays remain idle, but whenever any fault occurs they should be reliable enough to ensure their action. Careful design and simple construction of

Sensitivity

Selectivity

Reliability

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Bakreswar Thermal Power Project : WBPDCL the protective relays must satisfy this condition. : A protective relay must operate at the required speed it is intended for. Proper isolation of faults in time will reduce the amount of damage incurred

Speed

Classification of Protective Systems: Protective systems may mainly be classified on the basis of their actuating parameters, types of protection and equipments to be protected. If we classify the systems based on actuating parameters, then they are current relays, voltage relays etc. They measure the actuating quantities continuously and monitor the deviations from the set point. If the amount of deviation exceeds the allowable limit, the relay operates after the set or evaluated time delay. Types of protection can be classified in two categories, primary protection (also called main protection) and backup protection. Except L.T. switchgears, everywhere backup protection is incorporated additionally for redundancy. Backup protection is expected to operate in the case of primary protection failure so, every effort is made not to leave anything common between these two types. If classifications are made depending on the equipments to be protected, the systems are called equipment protections. Examples of these are generator protections, transformer protections etc. For protections of individual equipment against electrical fault, the selections of protective relays depend on the nature and behaviour of the equipment during normal and abnormal conditions. The protective relays used for one type of equipment may be distinctly different from those used for others.

Generator and associated equipments: The core of an electrical power system is the generator. The 210 MW generating unit driven by steam turbine is a complex system comprising the generator stator winding and associated transformers, and the rotor with its field winding and excitation systems. The generator is connected to one ‘generator transformer’ (G.T.) through which it is coupled to 400kV / 220kV switchyards. No switchgear is provided between generator and G.T. Two numbers of unit auxiliary transformers (U.A.T.) are tapped off the interconnection for the supply of auxiliary power to the unit. The generator, G.T. and U.A.T.s are interconnected through isolated phase bus ducts.

Neutral Grounding of Generator: The grounding method applied to generator neutral is ‘transformer grounding’. The transformer used is a singlephase distribution type transformer with adequate rating (75 kVA, 15.75kV/ 240V, AN). Primary of this transformer is connected between the generator neutral and the ground, and the secondary is loaded with a resistor of lower value (0.456 , 340 amps. for 5 minutes). This resistance presents a high equivalent value when referred through the transformation ratio in the generator circuit. With this method of grounding the generator ground fault current can be limited within 5 amps. for a solid phase to ground fault at the generator terminals.

Classes of protection: One generating unit consists of a single steam generator (boiler) that supplies steam to a single steam turbine coupled to an A.C. generator. Protection classes are therefore divided into three categories- Class A, Class B and Class C; each of which is again subdivided as ‘group one’ and ‘group two’. All the above-mentioned classes and groups comprise of a number of high speed tripping relays.

Operation of either group of a class has the same effect of operation of that class. The following table details the effects of operation of various classes of protection. Equipment Boiler Turbine G.T. Circuit Breaker Class A Trip Trip Trip Class B Trip Trip Class C

Trip

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Bakreswar Thermal Power Project : Monitoring of Synchronism: For monitoring the correct synchronism state of the generator with the grid to which it is to be connected ‘synchronism check relay’ is used. This relay measures the differences in phase, frequency and voltage between running (grid) and incoming (generator) voltage vectors and when all the values are within the setting limit, it closes its output contact to permit circuit breaker closure. Another relay used is ‘guard relay’. For synchronisation, closing command to G.T. H.V. circuit breaker is to be issued during the persistence of permissive from synchronism check relay. If closing command is issued before initiation of permissive from synchronism check relay and maintained, then guard relay prevents the circuit breaker closure when permissive is achieved subsequently.

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Types of faults: Faults of many kinds can occur within a generator system for which diverse protective means are needed. The amount of protection applied takes into account the value of the machine and its importance to the power system as a whole. Depending upon the types of faults those occur on a generator, its protection system has been divided into stator protection, rotor protection and protection from abnormal operating conditions. A list of different relays and relaying systems applied for generator protection at different categories is given below.

Stator protection 1. Generator differential relay 2. 100% Stator earth fault relay 3. Generator stator back-up earth fault relay 4. Inter-turn fault protection 5. Back-up impedance protection Rotor protection Rotor earth fault protection Protections from abnormal operating conditions 1. Low forward power relay 2. Reverse power relay 3. Over voltage protection 4. Overspend protection 5. Under frequency protection 6. Loss of synchronism (Pole slipping relay) 7. Field failure protection 8. Negative sequence current relay 9. Overload protection Overall differential relay: This relay is used to protect the generator and the G.T. as one unit apart from a number of protections applied to those separately. The zone of protection covered by this relay is from the neutral side of the generator to the high voltage side of the G.T. including the interconnecting bus ducts and excluding both the UATs. In case of any fault within this zone this relay will operate to trip the unit instantaneously.

Generator differential relay: This relay covers all the three phases of generator stator windings and provides both phase and earth fault protections for those. A stabilizing resistance is added in series with the relay in each phase to achieve stability during through fault conditions.

100% Stator earth fault relay: For a ground fault near the neutral end of the stator winding, there will be proportionately less voltage available to drive the current through the ground, resulting in a lower fault current and a lower neutral displacement voltage; and a fault at the neutral will result in no fault current at all. Therefore, possibility is there for an earth fault to remain undetected if the fault current is below the differential relay setting range. 100% Stator earth fault relay is used to

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detect earth faults occurring in the regions of generator windings closer to the neutral ends. This relay comprises of two protective units – Neutral displacement detection unit and third harmonic voltage comparator unit.

Neutral displacement detection unit: - This unit receives input voltage from the secondary side of generator neutral grounding transformer. If the voltage developed at this point exceeds the set value as the effect of a fault it initiates an internal IDMT timer which when operates after the evaluated time delay energizes its output. Third harmonic voltage comparator unit: - Generators during operation produce a certain magnitude of third
harmonic voltage in their windings. Under healthy conditions of running the third harmonic voltage generated is shared between the phase to ground capacitive reactance at generator terminal and the neutral to ground impedance (consisting of grounding resistance and neutral to ground capacitance) at the generator neutral. The third harmonic voltages at the generator line end (VL3) and neutral end (VN3) maintains a reasonable ratio, but this ratio follows a non-linear pattern and hence is not a constant. It varies over the entire range of generator active and reactive power loading – starting from no load up to full load. In case of occurrence of a fault, the ratio of VL3 and VN3 undergoes a change from that during healthy running conditions. The amount of deviation from normal will depend on the location and resistance of fault. Due to inherent non-linearity in the ratio of VL3 and VN3 a blind zone is present and therefore any fault resulting in a position in the blind zone will remain undetected by this module. Every fault outside the blind zone will be detected and after the set time delay the relay will operate. However, the neutral displacement module will detect the faults within the blind zone. Therefore, the combined operation of both the relay modules provides complete protection for generator stator windings.

Generator stator back-up earth fault relay: It is over current relay of IDMT characteristic. The input current of this relay is driven by a C.T., which is connected in the resistance loaded secondary circuit of generator neutral grounding transformer. Being time delayed in nature, this relay will operate in the event of primary protection failure.

Inter-turn fault protection: The faults those involve the windings of the same phase are termed as inter-turn faults. This type of fault is uncommon, but not unknown. One method for detection of inter-turn fault is transverse differential protection or cross differential protection. This method is applicable to generators having two identical windings per phase. Another method is ‘inter-turn fault protection by zero sequence voltage measurement’. In this scheme a voltageoperated relay is used. The relay receives phase side zero sequence voltage components from the broken delta secondary windings of one I.V.T. (interposing voltage transformer), the primary of which has star connection with grounded neutral. The primary of I.V.T. receives three-phase voltage from generator P.T. secondary. Neutral side voltage component is directly drawn to the relay from the secondary side of generator neutral grounding transformer. Under normal operating conditions there are no zero sequence voltage components on side, phase or neutral. For an inter-turn fault zero sequence voltage components is developed towards phase side and no earth component is developed and hence the relay will actuate. For an earth fault, the relay will remain stable as the same of the neutral side will balance phase side zero sequence voltage components and hence discrimination is achieved from earth faults.

Back-up impedance protection: Generator is to be provided with back-up protection against external faults, so that failure of main protections of the external equipments does not cause any damage to generator. Three types of schemes are available for this protection – a) voltage controlled over current relay, b) single step definite time impedance relay and c) single step definite time offset mho relay. Over current relays those operate after a time delay are not sufficient for reliable operation on generators because the fault current decreases rapidly as the generator reactance changes from sub-transient to transient to synchronous. Voltage controlled over current relays is also not suitable, as A.V.R. equipped with the generator tends to maintain normal voltage at the terminals. If impedance type relay is used for this protection, its characteristic could create grading problems as the relay reach covers a vast area on the impedance diagram.

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Offset mho type relay is most suitable for this type of protection. The protection is divided into two categories, presynchronising and post-synchronising. Back-up impedance relay (pre-synchronising) covers up to generator transformer within its reach and is interlocked with G.T. high voltage circuit breaker off condition for its operation. Back-up impedance relay (post-synchronising) covers G.T. and one I.B.T. within its reach for operation. Being back-up protection, both of the relays include timers in their operation.

Rotor Earth fault protection: The relay is used to detect high and low ohmic earth faults in the excitation circuits of generator. Any further earth fault in the excitation circuit results in a double earth fault, which mechanically endangers the rotor due to magnetic unbalance and thermally endangers the rotor due to high fault current. For this reason, the single earth fault should either be alarmed or initiate trip. This protection has two stages; gradual deterioration of insulation initiates an alarm and a solid earth fault initiates a trip. To measure high ohmic earth faults, this relay applies a D.C. voltage between the neutral of the main exciter and the rotor shaft and from the resulting current the ohmic insulation resistance to earth is evaluated. The connection is done with the rotating parts of the machine by means of slip rings and brushes. D.C. voltage is used for measurement to eliminate the influence of rotor earth capacitance. After an adequate long settling time from the instant of voltage application a steady state condition of measurement is reached. The polarity of the applied voltage is consecutively reversed whenever a stabilized end value of the measurement is achieved and repeated measurements are carried out. So, the switching frequency of the applied D.C. voltage is not constant; it depends on the values of ‘Earth resistance’ and ‘Earth capacitance’ of the excitation circuit. Generally the switching frequency is between 0.8 and 1.8 Hz. If measured insulation resistance value is ≤ 80 k the relay will initiate alarm and will initiate trip for a value ≤ 5 k . The relay has a built in measuring instrument that continuously points the measured resistance values over a scale of 0 to 180 k .

Low forward power relay and Reverse power relay: When a turbine fails or trips with its generator in operation and connected to grid, the generator remains in synchronism with the system and continues to run as a synchronous motor, drawing sufficient power to drive the turbine. This reverse power operation of a turbine is dangerous and under many conditions of failure may cause further damage to it. During this type of failure the electrical conditions remain balanced, unlike those arising out of electrical faults. Low forward power relay and Reverse power relay are used to disconnect the generator from the grid in the case of its prime mover failure. Low forward power relay: - When the power output of a generator falls downs and reaches a minimum preset value (0.5% of rated power), following a turbine trip, this relay operates to disconnect the same from the grid, with a time delay. Tripping of the generator with a minimum forward power is carried out to prevent its motoring and is interlocked with T.L.R. operation as a condition of prime mover failure, before tripping. Low forward power relay is also used for over speed protection of the turbine-generator set (the same has been discussed in the related section). Reverse power relay: - If in any case the low forward power relay fails to prevent motoring action of

generator, the reverse power relay comes into action with a time delay if the power drawn by the generator from the system exceeds a maximum preset value (0.5% of rated power). This relay disconnects the generator from the system to protect the prime mover. Over voltage protection: Over voltage is mainly classified into two categories, transient and power frequency over voltage. Transient over voltage: - These originate largely in the transmission systems due to switching and atmospheric disturbances. The surge voltages generally cannot reach generator terminals as adequate protection is provided with the E.H.V. installations. However, the generator is also equipped with surge protective devices at its terminals, which comprise of one surge capacitor and one surge arrester in each phase. Power frequency over voltage: - This is sustained in nature and can occur on a generator due to the following reasons – Defective operation of automatic voltage regulator A sudden reduction of load, in particular the VAR component when the regulator is operating under manual control.

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Over voltage protection of the generator has been divided into two stages. Over voltage relay stage-1 has been set at 110% of the rated voltage with a time delay of two seconds and setting of stage-2 is 120% with a time delay of one second. Operation of stage-1 initiates automatic changeover to manual mode of A.V.R. and that of stage-2 trips the unit. Both the stages separately annunciate the abnormality. Overspend protections: The speed of a turbo-generator set rises when the steam input is in excess of that required for the load. The speed governor normally controls the speed and also a generator connected to the grid cannot accelerate independently. However, if load is suddenly lost when the H.V. circuit breaker is tripped, the set will begin to accelerate. The turbine is equipped with speed governor, load shedding relay and other protective devices to prevent a dangerous speed rise. The risk of overspend during a machine trip is avoided mainly in two ways. During severe electrical faults, the associated protection system is arranged to trip the turbine simultaneously with the generator and thus ensures a complete stop of steam input immediately with the opening of generator circuit breaker. During a turbine trip, the generator circuit breaker is not allowed for immediate opening because this could result in a speed rise. In this case, disconnection of generator from the system is delayed until the electrical output is reduced to 0.5% of its rating. Upon initiation of turbine trip, the generator rotor falls back from the load angle towards the no load position as the electrical output reduces. Some over swing takes place and this causes power oscillations sufficient to give a momentary false indication of motoring, although the steam conditions may not be safe enough for disconnection of the generator from the grid. This problem is overcome by introducing a definite time delay at the power relays outputs. However, if acceleration of the turbo-generator set takes place with the excitation system in service, due to improper operation of any of the preventive measures, the resulting over frequency is detected by the over frequency relay. This relay is intended to trip the turbine if the running frequency exceeds its setting (53.5 Hz with a time delay of 2 seconds).

Under frequency protection: The frequency of a generator connected to grid is always equal to the system frequency. The system frequency is maintained within the reasonable operating limits by a balance between load and generation. A sudden shortage of generation from the balanced state of power system caused due to an outage of a major generating unit is either compensated by load shedding or by an increase in generation at the other units in operation. If this compensation is not fast enough to meet the requirements of balance in time, there will be a drop in system frequency. Under frequency operation is very much harmful for turbine blades and therefore should not be allowed to sustain at the lower limit, which is decided by the turbine. Under frequency relay is provided to monitor the frequency of the generator during operation. It comprises of both alarm and trip stages. If the generator frequency drops below 48 Hz, the relay operates after a time delay of 2 seconds to initiate alarm. If the frequency drops further and becomes less than 47.4 Hz, the relay operates after a time delay of 3 seconds to disconnect the generator from the grid. Loss of synchronism (Pole slipping relay): Generator may lose synchronism with the power system, without failure of excitation, because of a severe system fault disturbance or operation at a high load with leading power factor and hence a relatively weak field. During out of step operation of synchronous generator violent oscillations of torque take place with wide variations in current, power and power factor. The resulting high peak currents and off-frequency operation can cause winding stresses and pulsating torque can be potentially damaging to the shafts. To protect the generator from the effects of this abnormal operating condition pole-slipping relay is used. The quantity that undergoes maximum change when a generator loses synchronism is the impedance measured at the stator terminals. The pole-slipping relay is a combination of an over-current unit, a timer unit, a directional unit and a blinder unit. The directional and blinder units have straight-line characteristics on the impedance diagram. The over-current unit serves as a starter for the other units. When a pole slip condition exists, the extremity of the impedance vector moves along one of the power swing loci. The relay operates if the time taken by the pole-slipping locus to pass between the two characteristics exceeds the timer setting. Operation of this relay causes disconnection of the generator from the grid.

Field failure protection: -

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Loss of field supply to the generator can be caused by some fault in the excitation circuits or by incorrect opening of the field circuit breaker. On loss of field, the generator operates as asynchronous generator excited by reactive power drawn from the system to which it is connected and speeds up slightly higher than synchronous speed. Operation as asynchronous generator causes the flow of slip frequency current in the rotor and so abnormal heating of rotor takes place. Furthermore, the wattless current that the generator draws as magnetizing current from the system overloads the stator. Due to loss of field of a generator, the terminal voltage will begin to decrease and the current to increase, resulting in a decrease of impedance and also a change in power factor. The relay that is being used is a circular mho relay with its characteristic in the negative reactance area and offset from the origin on the impedance diagram. If the field supply fails, the locus of the machine terminal impedance moves inside the relay characteristic and it operates.

To avoid incorrect operation of field failure relay due to system transients, it is necessary to include a set of timers and auxiliary relays in the protection circuit. Generator negative sequence current relay: The currents in a generator are normally balanced in all the three phases, but in the cases of faults in the system this balance can be disturbed. Single phase or phase-to-phase faults, phase failure or unbalanced loading in the system may cause current unbalance in generators, which in turn causes the generation of negative sequence currents. These currents generate a flux in the stator, which rotates at synchronous speed but in the opposite direction to that of the rotor. Relative to the rotor, this flux therefore rotates at double the synchronous speed and generates eddy currents in the rotor. The high frequency of these eddy currents causes the outer parts of the rotor and the winding to become heated. If the negative sequence current is of high magnitude or persists for a long time the rotor parts can be damaged due to overheating. The negative sequence withstand capacity of a generator beyond certain minimum value is given by the equation –

( Insc / IM )2 . t = K , where, Insc = negative sequence current IM = rated current of the machine t = time in seconds K = a constant in seconds. This constant represents the length of time the machine can withstand a negative sequence current equal to its rated current. The relay has both alarm and trip stages; alarm has been set at 6% of IM and trip has been set with a value of K= 8. Overload protection: A generator connected to the grid is normally not in much danger of overloading as it operates within the safe limits of its capability curve. However, a thermal overload relay has been provided to protect the generator from accidental overloading. Overloading causes overheating of the stator and allowable duration of operation under this condition depends on the amount of overload and the heating time constant of the generator. The relay has been set to match its thermal characteristic closely with that of the generator. On detection of overload the allowable operating time is evaluated by the relay on the basis of the thermal content of the generator and the relay operates when this thermal content reaches the tripping level. Operation of overload relay disconnects the generator from the system.

Different techniques, which are applied to protect the generator against various faults and abnormal conditions, have been discussed in the foregoing sections. As the generator is directly connected to G.T. and U.A.T.s without any circuit breaker in between, a number of protections related to those also because tripping of the same. However, those are transformer protections of course.

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56
GENERATOR PROTECTION RELAYS
Introduction: The faults in general consist of short circuits as well as open circuits. Open circuit faults are much more unusual than short circuits. In terms of seriousness of the effects of a fault short circuits are of far greater concern than open circuits, although some open circuits may result in major hazards. Protective relays and relaying systems detect the faults in electrical circuits and operate automatic switchgears to isolate faulty equipment from the rest of the system as fast as possible. This limits the extent of damage at the fault location and prevents the effects of fault spreading into the system.

It is to be noted that a protective relay cannot prevent the appearance of faults; it comes into action only after fault has occurred. Protective Relays: Basic Characteristics: - Any protective relay has to satisfy four basic functional characteristics.

a)

Sensitivity: - Ability of a relay to respond to the minutest abnormality is known as sensitivity. A relay is more sensitive than other if it can respond to more minute fluctuations of actuating quantity than other. b) Selectivity: - It should be able to select which part of the system is faulty and only isolate that part from the rest of the system that is healthy otherwise. Selectivity is achieved in two ways – unit system of protection and non-unit system of protection. In unit system of protection the relays and protection schemes respond to faults those occur within their zones of protection only. In nonunit system of protection selectivity is obtained by current grading and time grading of the relays situated at different locations, all of which may experience a change in actuating quantity for a particular fault. c) Reliability: - Occurrence of faults is generally not frequent in power systems, so that for most of the time relays remain idle, but whenever any fault occurs they should be reliable enough to ensure their action. Careful design and simple construction of the protective relays must satisfy this condition. d) Speed: - A protective relay must operate at the required speed it is intended for. Proper isolation of faults in time will reduce the amount of damage incurred. Classification of Protective Systems: Protective systems may mainly be classified on the basis of their actuating parameters, types of protection and equipments to be protected. If we classify the systems based on actuating parameters, then they are current relays, voltage relays etc. They measure the actuating quantities continuously and monitor the deviations from the set point. If the amount of deviation exceeds the allowable limit, the relay operates after the set or evaluated time delay. Types of protection can be classified in two categories, primary protection (also called main protection) and backup protection. Except L.T. switchgears, everywhere backup protection is incorporated additionally for redundancy. Backup protection is expected to operate in the case of primary protection failure so, every effort is made not to leave anything common between these two types. If classifications are made depending on the equipments to be protected, the systems are called equipment protections. Examples of these are generator protections, transformer protections etc. For protections of individual equipment against electrical fault, the selections of protective relays depend on the nature and behaviour of the equipment during normal and abnormal conditions. The protective relays used for one type of equipment may be distinctly different from those used for others.

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Generator and associated equipments: The core of an electrical power system is the generator. The 210 MW generating unit driven by steam turbine is a complex system comprising the generator stator winding and associated transformers, and the rotor with its field winding and excitation systems. The generator is connected to one ‘generator transformer’ (G.T.) through which it is coupled to 400kV / 220kV switchyards. No switchgear is provided between generator and G.T.. Two numbers of unit auxiliary transformers (U.A.T.) are tapped off the interconnection for the supply of auxiliary power to the unit. The generator, G.T. and U.A.T.s are interconnected through isolated phase bus ducts.

Neutral Grounding of Generator: The grounding method applied to generator neutral is ‘transformer grounding’. The transformer used is a singlephase distribution type transformer with adequate rating (75 kVA, 15.75kV/ 240V, AN). Primary of this transformer is connected between the generator neutral and the ground, and the secondary is loaded with a resistor of lower value (0.456 , 340 amps. for 5 minutes). This resistance presents a high equivalent value when referred through the transformation ratio in the generator circuit. With this method of grounding the generator ground fault current can be limited within 5 amps. for a solid phase to ground fault at the generator terminals.

Classes of protection: One generating unit consists of a single steam generator (boiler) that supplies steam to a single steam turbine coupled to an a.c. generator. Protection classes are therefore divided into three categories- Class A, Class B and Class C; each of which is again subdivided as ‘group one’ and ‘group two’. All the above-mentioned classes and groups comprise of a number of high speed tripping relays.

Operation of either group of a class has the same effect of operation of that class. The following table details the effects of operation of various classes of protection. Equipment Class A Boiler Trip Turbine Trip G.T. Circuit Breaker Trip Monitoring of Synchronism: Class B Trip Trip Class C

Trip

For monitoring the correct synchronism state of the generator with the grid to which it is to be connected ‘synchronism check relay’ is used. This relay measures the differences in phase, frequency and voltage between running (grid) and incoming (generator) voltage vectors and when all the values are within the setting limit, it closes its output contact to permit circuit breaker closure. Another relay used is ‘guard relay’. For synchronisation, closing command to G.T. H.V. circuit breaker is to be issued during the persistence of permissive from synchronism check relay. If closing command is issued before initiation of permissive from synchronism check relay and maintained, then guard relay prevents the circuit breaker closure when permissive is achieved subsequently.

Types of faults: Faults of many kinds can occur within a generator system for which diverse protective means are needed. The amount of protection applied takes into account the value of the machine and its importance to the power system as a whole. Depending upon the types of faults those occur on a generator, its protection system has been divided into stator protection, rotor protection and protection from abnormal operating conditions.

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Provided for back up protection against the failure of the Main protection of Line. General O/C relay not used as the Fault current reduces sharply from sub-transient to transient to synchronous the will causes a delay in operation The voltage control O/C feature is also ineffective as AVR will try to maintain the voltage to normal. That’s why distance measuring type (Impedance MHO) preferred. Generally the longest Line is consider for setting of the relay. Here UAT & GT have been considered . Set value 1.5 ohm Time delay: 0.5 sec.

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This relay comprises two operating element-

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(1) Neutral displacement detector: It is a voltage operated relay covering 95% to 100% of the winding setting value: 6volt and IDMT characteristics for time delay TMS:7. (2) Third harmonic comparator: Principle of operation is Third Harmonic component of the generator. AC generator produce some third harmonic voltage in their winding. There will be no third harmonic voltage appeared across the phases of either star or delta connected generator, though a certain magnitude of third harmonic voltage between each phase and ground of machine will be existed. Under healthy condition these third harmonic voltage generated by the machine are shared between the phase to ground capacitive impedance and neutral to ground impedance of the machine neutral. These third harmonic voltages VL3 & VN3 should bear a reasonable constant ratio. But the non-linearity appear due to coupling of GT & UAT voltage system as well as active /reactive power output of the machine. Variation of the linearity will be plotted under different healthy condition. To balance this inequality a signal multiplier “a” has been used with VN3 i.e. “a*VN3”. At the balance condition aVN3~VL3 should be zero . To avoid any mal-operation a Dead Band setting has been adopted which is the proportionate value (ratio) of differential value of third harmonic and total harmonic (VL3+VN3) called K. Setting 45%(15-75%).

A high ac voltage 50 volt PP are superimpose upon the Excitation DC voltage. This ac voltages are filtered out by low pass filter and eliminating the noise the measurement of Insulation resistance are done

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Due to loss of Field (AVR Failure / FB open) machine operates as an Induction Generator excited by the reactive power drawn from system. This causes the instability of the system and overheating of Rotor especially if the cylindrical rotor type, without damper winding ( Thermal ). Generally Hydro Machines are Salient type with damper winding. To avoid mal operation during synchronizing surge and transient condition time delay is adopted for tripping or Load shedding action (if adverse field condition persisted for longer period)

This relay is in use for opening the Gen. Ckt. Breaker at Switch Yard.Interlocks of Turbine Tripping and Generator Field CB have been used.Setting:0.5%, Time:2.0 sec Ch.angle:90°

Setting value >1MW Special feature of this relay is that it is required to operate at low level of power and at low power factor. This is very precession in terms of relay design.For this special class accuracy of CTs (0.2%) have be used. Torque=IR x Iv x Sinθ Iv is proportional and lags Vby by 90º

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Setting: Alarm 48.00Hz Time 2.0sec Trip 47.41Hz Time 3.0sec

Setting: Trip 53.5Hz Time 2.0sec

To protect the Synchronous Generator against the possibility of the machine running in the unstable region of the power angle curve, resulting in Power Oscillation and pole slip. Relay consists of basically of one Directional

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Relay and one blinder relay operating in conjunction with 40-80msec static timer.An over current relay used at starter for the relay operation. Setting: O/C starter 100% Blinder Ch. Angle 75°, Dir. Unit Ch. Angle 75°. R=1.0*0.82*2.0=1.6 Time T= 40mses

Protection against the excessive rotor heating –may occur due to unbalance phase currents. These current generate a machine stator flux that has the same rotational speed as the rotor flux but rotate in opposite direction. Relative to rotor these stator flux rotate double the power system frequency and generate eddy current in the rotor These high frequency eddy current causes the overheating in outer part of the rotor and the winding. If it is persisted for long period rotor may damage. Data for sustain the negative seq current- t=K(Im/Insc)2 Im: Rated current of machine Insc: Negative Seq. Current t: Time in Sec K:Constant in Sec supplied by the Manufacturer=8 Assuming that all energy generated by Neg Seq. current is transmitted in the form heat to the rotor without any losses to the surroundings. The magnitude of the current which can be tolerated for an unlimited period of time without risking thermal damages to the rotor, varies depending on type of generator and rotor cooling. Furthermore in the salient pole generator in Hydro Generator the eddy current occur to a great extent in the damper winding which is heavily dimensioned. Therefore able to withstand much higher negative Seq. Current than non salient pole generator. Setting: Alarm 6% of Im (Im=0.9 of In) Trip 8% of Im Trip time 1-63 sec as per t=(Im/Inseq)²XK K=8 Alarm time 0.1 sec

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Setting:

100% of 10000Amp, 20sec

MotPRO relay used

An IDMT(10/3) relay additionally employed with a setting 10% (15A) of maximum fault current to protect 90% of the winding. Setting: Plug setting 10%, TMS 0.5

In EHV substation, reliability of fault detection I enhanced by – • Duplication of protection Circuit • Duplication of Trip coil • Both the protection connected to each trip circuit However any mechanical, pneumatic / hydraulic equipment are not duplicated and hence a failure in this part of the tripping chain has to be catered. This is achieved by provision of Breaker Failure Protection This will trip all other associated Breaker with repot end tripping (for line)

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This is the earliest protection system for distribution feeder specially for Transmission system using the Telecom line was used .

cable and later on for overhead

Limitation of length for pilot wire protection max-35/40km FSK system is adopted for transmission of carrier 10ms block of carrier and 10ms block of no carrier.

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Generally the time gap between the carrier block represent the phase difference of current at both end. If it exceed generally 30deg then line will trip at both end.

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