Protection Notes

7/31/2019 Protection Notes

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Transformer Protection Notes16.15 OIL AND GAS DEVICESAll faults below oil in an oil-immersed transformer resultin localised heating and breakdown of the oil; some degreeof arcing will always take place in a winding fault and the

resulting decomposition of the oil will release gases.When the fault is of a very minor type, such as a hot joint,gas is released slowly, but a major fault involving severearcing causes a very rapid release of large volumes of gasas well as oil vapour. The action is so violent that the gasand vapour do not have time to escape but instead buildup pressure and bodily displace the oil.When such faults occur in transformers having oilconservators, the fault causes a blast of oil to pass up therelief pipe to the conservator. A Buchholz relay is usedto protect against such conditions. Devices respondingto abnormally high oil pressure or rate-of-rise of oil

pressure are also available and may be used inconjunction with a Buchholz relay.16.15.1 Oil Pressure Relief Devices

The simplest form of pressure relief device is the widelyused frangible disc that is normally located at the endof an oil relief pipe protruding from the top of thetransformer tank.

The surge of oil caused by a serious fault bursts the disc,so allowing the oil to discharge rapidly. Relieving andlimiting the pressure rise avoids explosive rupture of thetank and consequent fire risk. Outdoor oil-immersedtransformers are usually mounted in a catchment pit to

collect and contain spilt oil (from whatever cause),thereby minimising the possibility of pollution.A drawback of the frangible disc is that the oil remainingin the tank is left exposed to the atmosphere afterrupture. This is avoided in a more effective device, thesudden pressure relief valve, which opens to allowdischarge of oil if the pressure exceeds a set level, butcloses automatically as soon as the internal pressure fallsbelow this level. If the abnormal pressure is relativelyhigh, the valve can operate within a few milliseconds,and provide fast tripping when suitable contacts arefitted.

The device is commonly fitted to power transformersrated at 2MVA or higher, but may be applied todistribution transformers rated as low as 200kVA,particularly those in hazardous areas.16.15.2 Rapid Pressure Rise Relay

This device detects rapid rise of pressure rather thanabsolute pressure and thereby can respond even quickerthan the pressure relief valve to sudden abnormally highpressures. Sensitivities as low as 0.07bar/s are


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attainable, but when fitted to forced-cooledtransformers the operating speed of the device may haveto be slowed deliberately to avoid spurious trippingduring circulation pump starts.16.15.3 Buchholz ProtectionBuchholz protection is normally provided on all

transformers fitted with a conservator. The Buchholzrelay is contained in a cast housing which is connectedin the pipe to the conservator, as in Figure 16.21.A typical Buchholz relay will have two sets of contacts.One is arranged to operate for slow accumulations ofgas, the other for bulk displacement of oil in the event ofa heavy internal fault. An alarm is generated for theformer, but the latter is usually direct-wired to the CBtrip relay.

The device will therefore give an alarm for the followingfault conditions, all of which are of a low order ofurgency.

a. hot spots on the core due to short circuit oflamination insulationb. core bolt insulation failurec. faulty jointsd. interturn faults or other winding faults involvingonly lower power infeedse. loss of oil due to leakageWhen a major winding fault occurs, this causes a surgeof oil, which displaces the lower float and thus causesisolation of the transformer. This action will take placefor:i. all severe winding faults, either to earth orinterphaseii. loss of oil if allowed to continue to a dangerousdegreeAn inspection window is usually provided on either sideof the gas collection space. Visible white or yellow gasindicates that insulation has been burnt, while black orgrey gas indicates the presence of, dissociated oil. Inthese cases the gas will probably be inflammable,whereas released air will not. A vent valve is provided onthe top of the housing for the gas to be released orcollected for analysis. Transformers with forced oilcirculation may experience oil flow to/from theconservator on starting/stopping of the pumps. TheBuchholz relay must not operate in this circumstance.Cleaning operations may cause aeration of the oil. Undersuch conditions, tripping of the transformer due toBuchholz operation should be inhibited for a suitable period.Because of its universal response to faults within thetransformer, some of which are difficult to detect byother means, the Buchholz relay is invaluable, whether


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regarded as a main protection or as a supplement toother protection schemes. Tests carried out by striking ahigh voltage arc in a transformer tank filled with oil,have shown that operation times of 0.05s-0.1s arepossible. Electrical protection is generally used as well,either to obtain faster operation for heavy faults, or

because Buchholz relays have to be prevented fromtripping during oil maintenance periods. Conservatorsare fitted to oil-cooled transformers above 1000kVArating, except those to North American design practicethat use a different technique.16.17 INTERTRIPPINGIn order to ensure that both the high and low voltagecircuit breakers operate for faults within the transformerand feeder, it is necessary to operate both circuitbreakers from protection normally associated with one.

The technique for doing this is known as intertripping.The necessity for intertripping on transformer-feeders

arises from the fact that certain types of fault produceinsufficient current to operate the protection associatedwith one of the circuit breakers. These faults are:a. faults in the transformer that operate the Buchholzrelay and trip the local low voltage circuit breaker,while failing to produce enough fault current tooperate the protection associated with the remotehigh voltage circuit breakerb. earth faults on the star winding of the transformer,which, because of the position of the fault in thewinding, again produce insufficient current forrelay operation at the remote circuit breaker

c. earth faults on the feeder or high voltage deltaconnectedwinding which trip the high voltagecircuit breaker only, leaving the transformerenergised form the low voltage side and with twohigh voltage phases at near line-to-line voltageabove earth. Intermittent arcing may follow andthere is a possibility of transient overvoltageoccurring and causing a further breakdown ofinsulation

Generator Protection SystemsThe following problems require consideration from the

point of view of applying protection:1 stator electrical faults2 overload3 overvoltage4 unbalanced loading5 overfluxing6 inadvertent energisation7 rotor electrical faults8 loss of excitation


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9. loss of synchronism10 failure of prime mover11 lubrication oil failure12 overspeeding13 rotor distortion14 difference in expansion between rotating and

stationary parts15. excessive vibration16core lamination faults

1.GENERATOR DIFFERENTIAL PROTECTION:

Setting : 0.5 Amp Time : Instantaneous

It is one of the important protections to protect generator winding against internal faultssuch as phase-to-phase and three phase-to-ground faults. This type of fault is very serious

because very large current can flow and produce large amounts of damage to the winding if

it is allowed to persist. One set current transformers of the generator on neutral and phaseside, is exclusively used for this protection. The differential protection can not detect turn-

to-turn fault and phase to ground within one winding for high impedance neutral grounding

generator such as ours. Upon the detection of a phase-to-phase fault in the winding, the unitis tripped with out time delay.

Relays acted : a. Flag operation at Protection panel.

b. Acting of Master relayc. Indication at Annunciation Panel.

Consequences : a. Tripping of 220KV breaker

b. Tripping of Field breaker

c. Stop command to Turbine thro Mark-IVStatus : a. Unit is at coasting down.

Once the differential protection operated, the unit can not be taken into service unless the

generator winding is thoroughly examined by the maintenance staff of any internal faults

2.GENERATOR-TRANSFORMER DIFFERENTIAL PROTECTION :Setting : 0.75 Amp Time : Instantaneous

It protects 11KV bus duct, 11/0.440KV unit auxiliary transformer, 11/20KV step-up

transformer against internal faults such as phase-to-phase and three phase-to-ground faults.

This type of fault is very serious because very large current can flow and produce largeamounts of damage to the winding if it is allowed to persist. One set current transformers

of the generator on neutral side and another set current transformer on 220KV side after

transformer, is exclusively used for the protection. Upon the detection of difference incurrent between these current transformers, the unit is tripped with out time delay.One the

generator-transformer differential protection operated, the unit can not be taken into serviceunless the 11KV bus duct, unit auxiliary transformer, power transformer are thoroughlyexamined by the maintenance staff for any internal faults.






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c. Stop command to Turbine thro Mark-IV

Status : a. Unit is at coasting down.

3.LOSS OF FIELD OR EXCITATION PROTECTION :

Setting : K1-2, K2-1, K3-2 Trip after 2 Sec.When the synchronous machine with excitation, is connected to the grid, it generates

reactive power along with active power to the grid and the rotor speed is same as that of

grid frequency. Loss of field or loss of excitation results in loss of synchronism betweenrotor flux & stator flux. The synchronous machine operates as an induction machine at

higher speed and draws reactive power from the grid. This will result in the flow of slip

frequency currents in the rotor body as well as severe torque oscillations in the rotor shaft.

As the rotor is not designed to sustain such currents or to withstand the high alternatingtorques which results in rotor overheating, coupling slippage and even rotor failure.

A loss of excitation normally indicates a problem with the excitation system. Some times it

may be due to inadvertent tripping of filed breaker, open or short circuit of field winding or

loss of source to the exciter. If the generator is not disconnected immediately when it losesexcitation wide spread instability may very quickly develop and major system shutdown

may occur.When loss of excitation alarm annunciates at annunciation panel, the machine may

probably be running with less excitation at leading MVAR power. Increase the excitation

on the machine until it reaches on lagging MVAR power. The machine trips on the sameprotection along with alarm resynchronize the machine and try to stabilize at required

MVAR power. If not possible, trip the machine immediately and inform to the maintenance

staff for thorough checking of the Automatic Voltage Regulator (AVR) and its associated

parts.Relays acted : a. Flag operation at Protection panel.

b. Acting of Master relay

c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breaker



4.NEGATIVE SEQUENCE OR CURRENT UNBALANCE PROTECTION :

Setting : Alarm 75% of 12s Time - 5 Sec.Trip 75% of 12s Time - 300 Sec.

When the machine delivering the equal currents in three phases, no unbalance or negative

phase sequence current is produced as the vector sum of these currents is zero, when thegenerator is supplying an unbalanced load to a system, a negative phase sequence current is

imposed on the generator. The system unbalance may be due to opening of lines, breaker

failures or system faults. The negative sequence current in the stator winding creates amagnetic flux wave in the air gap which rotates in opposite direction to that of rotor

synchronous speed. This flux induces currents in the rotor body, wedges, retaining rings at

twice the line frequency. Heating occurs in these areas and the resulting temperatures

depend upon the level and duration of the unbalanced currents. Under these conditions it is


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possible to reach temperatures at which the rotor material no longer contain the centrifugal

forces imposed on them resulting in serious damage to the turbine-generator set. Any

machine as per design data will permit some level of negative sequence currents forcontinuous period.

An alarm will annunciate at annunciation panel if negative sequence currents exceeds a

normal level. Reduce the MVAR power on the machine if necessary load also and keep themachine for some time till the alarm vanishes at annunciation panel. If the machine trips on

the Negative sequence protection never take the machine into service until the temperatures

on the rotor parts settle down to its lower value. Resynchronize the machine to the gridafter considerable time under grid & feeder parameters are within limits. If the unit trips

again on the same protection, stop the machine after consideration time so as to cool down

the rotor parts and inform to the maintenance staff for thorough examination of the system.

Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay

c. Indication at Annunciation Panel.


b. Tripping of Field breakerStatus : a. Unit is at FSNL.

5. OVER FLUXING OR EXCITATION OR VOLTS PER HERTZ PROTECTION:

Setting : Alarm 1.17 Time - 10 Sec.

Trip 1.17 Time - 30 Sec.Per unit voltage divided by per unit frequency commonly called Volts/Hertz is a

measurable quantity that is proportional to flux in the generator or step-up transformer

cores. Moderate over fluxing (105-110%) increases core loss resulting in increase of core

temperatures due to hysterics & eddy currents loss. Long term operation at elevatedtemperatures can shorten the life of the stator insulation. Severe over fluxing can

breakdown inter-laminar insulation followed by rapid local core melting. Over fluxing

normally can be caused by over speed of the turbine or over excitation during Off-linecondition, and load rejection or AVR mal-functioning during On-line condition.

If alarm annunciation panel, Increase/Reduce the speed of the turbine to rated generator

speed (3000RPM) and reduce the generator voltage to rated during Off-line condition.Reduce the MVAR power on the generator during On-line condition. If the machine trips

on over fluxing protection during On-line, Keep the machine at FSNL till the grid

parameters stabilize and resins. Again the machine trips on the same stop the machine for

examination of the AVR & Governor systems by maintenance staff.Relays acted : a. Flag operation at Protection panel.





6.OVER CURRENT WITH VOLTAGE RESTRAINT PROTECTION :

Setting : Alarm 85% Time - 10 Sec.


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Trip 100% Time - 0.5 Sec.

Normally generators are designed to operate continuously at rated MVA, frequency and

power factor over a range of 95 to 105% rated voltage. Operating the generator at ratedMVA with 95% voltage, 105% stator current is permissible. Operating of the generator

beyond rated KVA may result in harmful stator over current. A consequence of over

current in winding is stator core over heating and leads to failure of insulation.If alarm annunciates at annunciation panel, Reduce the stator current to the below the rated

by reducing the MVAR power on the machine. When the trips on the same protection,

Resins the machine after keeping the machine at FSNL for some time, and keep the statorcurrent below the rated.





Status : a. Unit is at FSNL.

7 STATOR EARTH FAULT PROTECTION :

Setting : 70% Time - 5 Sec.Normally the generator stator neutral operates at a potential close to ground. If a faulty

phase winding connected to ground, the normal low neutral voltage could rise as high as

line-to-neutral voltage depending on the fault location. Although a single ground fault willnot necessarily cause immediate damage, the presence of one increases the probability of a

second. A second fault even if detected by differential relay, may cause serious damage.

The usual method of detection fault is by measuring the voltage across the secondary of

neutral grounding transformer (NGT). Here are two over lapping zones to detect statorground faults in a high impedance grounded generator system, the two zones are put

together cover 100% stator winding for earth faults. A fundamental frequency neutral over

voltage relay covers about 0-95% of the stator zonal winding for all faults except thosenear the neutral. Another third harmonic neutral under voltage relay covers remaining 96-

100% of the stator zone 2 winding on neutral side.

Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay



b. Tripping of Field breakerc. Stop command to Turbine thro Mark-IV


8.ROTOR EARTH FAULT PROTECTION (64R) :Settings : Less than 80K ohm

Any rotor field winding of the generator is electrically isolated from the ground. Therefore

the existence of one ground fault in the field winding will usually not damage the rotor.However the presence of two or more ground faults in the winding will cause magnetic and

thermal imbalance plus localized heating and damage to the rotor metallic parts. The rotor

earth fault may be caused due to insulation failure of winding or inter-turn fault followed

by localized heat.


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9.RESTRICTED EARTH FAULT PROTECTION:

Settings : 0.1 Amp. Time : InstantaneousIt is similar to generator differential protection in working. It protects the high voltage

winding of 11/220KV power transformer against internal faults. One set current

transformers of the power transformer on neutral and phase side, is exclusively used for

this protection. The protection can not detect turn-to-turn fault within one winding. Uponthe detection of a phase-to-phase or phase-to-ground fault in the winding, the unit to be

tripped without time delay.



Consequences : a. Tripping of 220KV breakerb. Tripping of Field breaker


Status : a. Unit is at coasting down.Once the restricted earth fault protection operated, the unit can not be taken into service

unless the transformer winding is thoroughly examined by the maintenance staff for any

internals faults.

10.BACKUP IMPEDANCE PROTECTION:Settings ; K1-3, K2-0.71 Time 1.5 Sec.

As in name implies, it is used to protect the generator from supplying the over loaded or

faulty system. It is backup protection of the generator over current protection. In measuresratio of the voltage and current supplied by the generator and initiates trip signal when the

measured impedance is less than the preset value.

If the machine trips on the Backup protection, never take the machine into service until thetemperatures of the generator settle down to its lower value. Resynchronize the machine to

the grid after considerable time when grid & feeder parameters are within limits.






11.LOW FORWARD POWER PROTECTION:Setting : 0.5% Time : 1 Sec.

The generator will not develop output power when turbine input is less than the no load

losses and motoring action develops on the turbine. The generator is able to generate

power, usually 55 to 10% of generator capacity, within pre-determined time after closing of


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220KV breaker.




Status : a. Unit is at FSNL with potential.The unit trips on the low forward protection, Resins the machine and increase input power

to the turbine as quickly as possible within low forward power time setting. Even after two

to three attempts, the machine is tripping on the same protection; probably the governor ofturbine is faulty. Inform to maintenance staff for rectification of the same.

12.REVERSE POWER PROTECTION:

Setting : 0.5% Time - 2.0 Sec.

It is backup protection to the low forward protection. Motoring of a generator will occurwhen turbine output is reduced such that it develops less than no-load losses while the

generator is still on-line, the generator will operate as a synchronous motor and driving the

turbine. The generator will not be harmed by synchronous motoring and a steam turbine

can be harmed through over heating during synchronous motoring if continued longenough. The motoring of the turbine output can be detected by reverse power protection.

The avoid false tripping due to power swings a time delay is incorporated before trippingsignal is generated.






The unit trips on the reverse power protection. Resins the machine and increase the input

power to the turbine as quickly as possible within low forward power time setting. Evenafter two to three attempts, the machine is tripping on the same protection; probably the

governor of turbine is faulty. Inform to maintenance staff for rectification of the same.

13.POLE SLIP OR OUT-OF-STEP PROTECTION:

Setting : 6.9 ohm.

When a generator loses synchronism, the resulting high current peaks and off-frequency

operation may cause winding stresses, pulsation torques and mechanical resonances thathave the potential danger to turbine generator. Therefore, to minimize the possibility of

damage, it is generally accepted that the machine should be tripped without time delay

preferably during the first half-slip cycle of the loss of synchronism condition. Theelectrical center during loss-of-synchronous conditions can occur in the generator as a

result of increased impedance of the generator while decrease system impedance. The

protections normally applied in the generator zone such as back-up impedance, loss ofexcitation etc., will not protect a generator during loss of synchronism under normal

generator conditions.




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The unit trips on the Pole slip protection, Resynch the machine after stabilization of thegrid parameters

14.POLE DISCREPANCY PROTECITON:Setting : 0.5 Sec.

If One or two poles of generator breaker fail to close during synchronization, all poles of

the breaker trip on this protection. It may be due to mechanical failure of the breaker un

equal distribution of closing signal to the breaker from protection system.Relays acted : a. Flag operation at 220KV Breaker panel.

b. Indication at Annunciation Panel.

Consequences : a. tripping of 220KV breaker

Status : a. Unit is at FSNL with potential.The generator breaker trips on the pole discrepancy protection, Resynch the generator.

Even after two to three attempts, the machine is tripping on the same protection, probablythe generator breaker is faulty. Inform to maintenance staff for rectification of the same.

15.LOCAL BREAKER BACKUP PROTECTION:Setting : 25% Time : 0.8 Sec.

For most of the faults, the generator breaker involves tripping the generator from the

system. Failure of the breaker to open probably results in loss of protection and other

problems such as motoring action or single phasing, If one or two poles of the generatorbreaker fail to open due to mechanical failure in breaker mechanism, the result can be a

single phasing and negative phase sequence currents inducted on the rotor. The LBB

protection is energized when the breaker trip is initiated after a suitable time interval ifconfirmation of the confirmation of breaker tripping from three poles is not received. The

energized tripping signal from LBB protection will trip all 220KV generator breakers and

all 220KV feeder breakers through Bus-bar protection.Relays acted : a. Flag operation at Protection panel.

b. Acting of Master relay for all units.


Consequences : a. Tripping of 220KV breakerb. Tripping of Field breaker of all units.

Status : a. all Units are at FSNL.

Once the LBB protection operated, the entire station is in dark. First restore all essentialservices to all units such as lube oil system and turning gear etc., from battery backup and.

Checkup the faulty 220KV breaker and isolate the breaker from the system by opening the

both side of the isolators.After restoring all services from station supply, Close 220KV feeder breakers first and take

all units into service one after the other duly co-coordinating with the DE/LD.

Since it involves complex operation, it is necessary to get help from maintenance staff for

restoring the normally in the station. Never attempt to close the faulty 220KV generator in


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panic as it causes permanent damage to the generator and transformer.

16.BUS BAR PROTECTION:Setting : 0.8 Amp.

There are mainly three protection zones namely called generator zone, bus duct transformer

zone, 220KV breakers zone. The protection of generator zone and bus duct & transformerzone are covered in previous schemes. All 220KV breakers at switchyard will come under

Bus-Bar protection. Functioning of this scheme is similar to the generator differential

protection or generator-transformer differential protection. It measures all incomingcurrents from the generators at 220KV side and all outgoing currents in 220KV feeders,

and initiates trip signal if it detects any deviation more than the preset value as the

algebraic sum of all currents at 220KV bus must be less than the preset value. It isolates all

220KV generator breakers and all 220KV feeder breakers connected to 220KV bus.Relays acted : a. Flag operation at Protection panel.

b. Acting of Master relay for all units.


Consequences : a. Tripping of 220KV breakerb. Tripping of Field breaker of all units.

Status : a. all Units are at FSNL.Once the Bus-Bar protection operated, the entire station is in dark. First restore all essential

services to all units such as lube oil system and turning gear etc., from battery backup and

6.6/0.44KV Stage II reserve power supply. Checkup the entire 220KV switch yard forany wire snapping or equipment damage.

After restoring all services from station supply, Close 220KV feeder breakers first and take

all units into service one after the other duly co-ordinating with the DE/LD.

Since it involves complex operation, it is necessary to get help from maintenance staff forrestoring the normalcy in the station. Never attempt to restore the 220KV supply at switch

yard in panic unless the entire system is thoroughly examined and satisfy yourself as it

causes permanent damage to the equipment or injury/death to the person working at switchyard.

17.OVER FREQUENCY PROTECTION:Setting : 52 Hz Time - 2 Sec.

For a generator connected to a system, abnormal frequency operation is a result of a severe

system disturbance. The generator can tolerate moderate over frequency operation provided

voltage is within an acceptable limits. The machine operated at higher speeds at which therotor material no longer contain the centrifugal forces imposed on them resulting in serious

damage to the turbine-generator set. The abnormal over frequency on the machine may be

due to improper speed control adjustment or disoperation of the speed controller or severegrid disturbance or sudden load through off.






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The unit trips on the over frequency protection, Resins the machine. Even after two to threeattempts, the machine is tripping on the same protection; probably the governor of turbine

is faulty. Inform to maintenance staff for rectification of the same.

18.UNDER FREQUENCY PROTECTION:

Setting : 48 Hz Time : 2.0 Sec.

For a generator connected to a system, under frequency operation is a result of a severesystem disturbance. The generator can tolerate moderate under frequency operation

provided voltage is within an acceptable limits. The machine operated at lower higher

speeds causes severe over fluxing in the generator-transformer. The abnormal under

frequency on the machine may be due to improper speed control adjustment or disoperationof the speed controller.


b. Indication at Annunciation Panel

Consequences : a. NILStatus : a. Unit is at lower speed with potential.

Increase governor speed until machine reaches full speed. Even after two to three attempts,the machine are running at lower speed, probably the governor of turbine is faulty. Inform

to maintenance staff for rectification of the same.

19.OVER VOLTAGE PROTECTION :Setting : a. 110% Time - 2.0 Sec.

b. 120% Time - 0.3 Sec.

Generator voltage is at present value under normal operating conditions as selected by

operator in AVR. If it parts from preset value, May be due to AVR mal-functioning or asystem disturbance. Severe over voltage can cause over fluxing and winding insulation

failure. The over voltage protection can be considered as a backup to the Volts-per-Hertz

protection.Relays acted : a. Flag operation at Protection panel.




Status : a. Unit is at FSNL without potential.

Raise the generator voltage slowly with manual mode in AVR and keep generator voltagewithin the limits of normal voltage. If it is unable to control the generator voltage, trip the

field breaker and inform to the maintenance staff for rectification of the AVR.

12.2.1 Generator faults

(a) Stator faults: Stator faults involve the main current carrying conductors andmust therefore be cleared quickly from the power system by a complete shutdown

of the generator. They may be faults to earth, between phases or between turns of a

phase, singly or in combination. The great danger from all faults is the possibility of

damage to the laminations of the stator core and stator windings due to the heat

generated at the point of fault. If the damage so caused is other than superficial, the

stator would have to be dismantled, the damaged laminations and windings replaced

and the stator rebuilt, all of which is a lengthy and costly process.


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Limitation of generator stator earth-fault current by means of resistance earthing

is normal practice (see Chapter 1) and serves, among other things, to minimise core

burning.

Phase-to-phase faults and interturn faults are both less common than earth faults.

It is relatively easy to provide protection for phase-to-phase faults, but interturn

faults are, on the other hand more difficult to detect and protection is not usually

provided. Generally speaking, interturn faults quickly involve contact with earth via

the stator core and are then tripped by stator earth-fault protection.(b) Rotor faults: Rotor faults may be either to earth or between turns and may

be caused by the severe mechanical and thermal stresses acting upon the winding

insulation; these are aggravated by a variable load cycle.


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cleared by the appropriate protection. The main condition of interest is that of

an unsymmetrical fault producing negative phase sequence currents in the stator

winding. The effect of these currents is to produce a field rotating in opposite sense

to the d.c. field system producing a flux which cuts the rotor at twice the rotational

frequency thereby inducing double frequency currents in the field system and the

rotor body. These currents produce severe rotor heating and modern machines have

a limited negative phase sequence current capability.

Automatic tripping is therefore required for the higher negative phase sequencecurrent conditions.

This capability limit applies to all modem hydrogen-cooled machines and many

air-cooled machines, but some of the older air-cooled machines are designed to

withstand full negative sequence currents continuously. In large modern alternators,

particularly those employing direct cooling of the stator and rotor conductors, the

temperature rise caused by abnormally high stator currents is more rapid than in

the less highly rated machines and the capability limit is therefore lower.


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Negative phase-sequence protection

Generator transformer Buchholz surge

Generator transformer winding temperature trip

Emergency push button

Generator stator E/F relay (1 st i.d.m.t., high resistance)Loss of excitation protection

Loss of vacuum tripLow steam inlet pressure trip

Unit transformer 1.v. standby E/F

Pole-slipping protection

Loss of stator water flow

Loss of lubricating oil

Loss of speed governor tripLoss of boiler water

Power station, second supply

Generator feeder, second main protection relay 2

Unit transformer 1.v. restricted E/F

Unit transformer h.v. overcurrent

Generator stator E/F relay (2nd i.d.m.t., high resistance)

Generator stator E/F relay (i.d.m.t., low resistance)

Generator transformer, h.v. restricted E/FNegative phase-sequence protection


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Protection Notes