218091515 protection of tf gen

8/10/2019 218091515 Protection of TF Gen

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Protection of Generators

& TransformersPresented By:Er. H.S. DhaliwalAssistant Professor,Deptt. Of Electrical EnggBHSBIET, Lehragaga.

1Punjab Edusat Society


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Introduction

In a generating station the generator and transformerare the most expensive equipments and hence it isdesirable to employ a protective system to isolate thefaulty equipment as quickly as possible to keep thehealthy section in normal operation and to ensureuninterruptable power supply.

The basic electrical quantities those are likely tochange during abnormal fault conditions are current,voltage, phase angle and frequency . Protective relaysutilizes one or more of these quantities to detectabnormal conditions in a power system.



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Protection ofTransformers



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Transformer

A Transformer is a device that transferselectrical energy from one circuit to anotherthrough inductively coupled conductors thetransformer's coils. A varying current in the

first or primary winding creates a varyingmagnetic flux in the transformer's core, andthus a varying magnetic field through thesecondary winding. This varying magneticfield induces a varying electromotive force(EMF) or "voltage" in the secondary winding.This effect is called mutual induction.



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Why Protection is required ? A fault in electrical system is defined as a defect

in the electrical circuit due to which the flow ofcurrent is diverted from its intended path. Faultcurrents being excessive , they can damage notonly the faulty equipment, but also theinstallation through which the fault current isfed. Fault in important equipment like

transformer can effect the stability of powersystem. To improve the system stabilityprotection is required.



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Transformer Fault Categories

1. Winding and terminal faults

2. Sustained or uncleared external faults

3. Abnormal operating conditions such asoverload, overvoltage and over fluxing.

4. Core faults

Punjab Edusat Society 9


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Faults on Transformer 10-12% of total faults occurring in the power

system are due to the faults on the Transformers.The following are the major causes of faults in

transformers: 1. Insulation Failure 2. Faults in tap-changer 3. Faults in bushing 4. Faults in protection circuit 5. Overloading, Overvoltage



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Transformer Connections



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Transformer Connections



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Phasor Diagrams of Transformerconnections



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Pressure Relief and Pressure Relay

Pressure relay and pressure relief device is mountedon transformer tank. It releases gas pressure to theatmosphere during:

High overload peaks Prolonged overloads Arcing faults within oil

Pressure relief valve is spring loaded and has a

seal-seat. When the pressure inside the tankincreases above a certain value, the force onmovable sub-assembly exceeds spring force andthe valve operates. The alarm contacts are closed.



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Pressure Relief and Pressure Relay



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Buccholz Relay

Buchholz relay is a gas- actuated relayinstalled in oil-immersed transformers forprotection against all kind of faults. It is usedto gives an alarm in case of slow developing

faults or incipient faults in the transformerand to disconnect the transformer from thesupply in the event of severe internal faults. Itis installed in the pipe between theconservator and main tank as shown in fig11below. This relay is used in oil-immersedtransformers of rating above 750 kVA.



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Buccholz Relay



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Petcock

Counter balanceweight

Oil level

From transformer

Aperture adjuster

Deflector plateDrain plug

Trip bucket

To oilconservator

Mercury switch

Alarm bucket

Buchholz Relay



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Mounting of Buccholz Relay



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Differential Protection In a power transformer, the currents in primary and

secondary are to be compared. As these twocurrents are usually different, therefore the use ofidentical transformers will give differential currentand operate the relay even under no loadconditions. The difference in magnitude of currentsin primary and secondary of power transformers iscompensated by different turns ratios of C.T.s. If T isthe turns ratio of power transformer, then the turnsration of C.T.s on LV side is made T times the turns

ration of the C.T.s on HV side. When this condition isfulfilled the secondaries of the two C.T.s will carrysame current under normal conditions. And thus nocurrent will flow through the relay and it remainsinoperative.



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Differential Protection


Correct application of differential protection requires CT ratio and windingconnections to match those of transformer.

CT secondary circuit should be a replica of primary system. Consider :

(1) Difference in current magnitude(2) Phase shift(3) Zero sequence currents


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Differential Connections



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Biased Differential Scheme



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OVER CURRENT PROTECTION

The over current protection is needed toprotect the transformer from sustainedoverloads and short circuits. Induction typeover current relays are used which in additionto providing overload protection acts as backup relays for protection of transformer

winding fault.



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Requirements

Fast operation for primary short circuits

Discrimination with downstream protections Operation within transformer withstand

Non-operation for short or long term overloads Non-operation for magnetising inrush



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EARTH FAULT PROTECTION The earth fault protection is used to provide

protection against any earth fault in thewindings of the transformer. It works on the

principle that when the transformer winding issound the currents in all the three phases willbalance and no current will spill into the earthfault relay. The arrangement is such that the

relay does not respond to any out of balancecurrent between windings caused by tapchanging arrangement



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COMBINED OVER CURRENT ANDEARTH FAULT PROTECTION



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Combined Differential andEarth Fault Protection



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Frame leakage Protection

The transformer is lightly insulated from earthby mounting it on a concrete plinth. The tankis connected to earth through a CT acrosswhich an instantaneous earth fault relay isconnected. Earth fault current due toinsulation breakdown in any winding of the

transformer will flow through the tank andsingle earth connection, thus energizing thecurrent transformer and operating the relay.



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Overfluxing Protection Generator transformers Grid transformers

Usually only a problem during run-up or shut down, but canbe caused by loss of load / load shedding etc.

Flux Vf

Effects of overfluxing : Increase in magnetising current Increase in winding temperature Increase in noise and vibration Overheating of laminations and metal parts

Protective relay responds to V/f ratio Stage 1 - lower A.V.R. Stage 2 - Trip field



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m2

Ie

m

V = kf

Overfluxing Basic Theory

CAUSES Low frequency High voltage Geomagnetic disturbances

EFFECTS Tripping of differential element (Transient

overfluxing) Damage to transformers (Prolonged overfluxing)



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Inter-Turn Fault Protection



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Protection of

Generators



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Alternator design



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Principle of operation Alternators generate electricity using the same

principle as DC generators, namely, when the magneticfield around a conductor changes, a current is inducedin the conductor. Typically, a rotating magnet, calledthe rotor turns within a stationary set of conductorswound in coils on an iron core, called the stator. Thefield cuts across the conductors, generating an inducedEMF(Electro-Magnetic Field), as the mechanical inputcauses the rotor to turn.

The rotating magnetic field induces an AC voltage in

the stator windings. Often there are three sets of statorwindings, physically offset so that the rotatingmagnetic field produces a three phase current,displaced by one-third of a period with respect to eachother.



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Alternating current (AC) flows



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Faults on Alternator

6-8% of total faults occurring in the power systemis due to the alternator faults. The following arethe major causes of fault on an alternator:

1. Stator Faults 2. Rotor Faults 3. Abnormal conditions 4. Faults in associated equipment 5. Faults in protective system



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Alternator Protection The following types of protection can be provided to the

alternators:- Differential Protection- Stator Earth fault protection- Negative phase sequence protection- Protection against unbalanced loading- Inter-turn fault protection- Reverse power protection- Field failure protection- Rotor earth fault protection

- Temperature sensors in slots- Over current relays in stator & rotor circuits-Surge arrestors for surge overvoltage



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Differential Protection CTs are provided at each end of the generator

winding which is to be protected.(as shown infig.)

When there is no fault the differential current (I1-I2) through the relay is zero. So the relay will notoperate.

When the fault occurs the balance is disturbed

and differential current (I1-I2) flows through theoperating coil of the relay causing relay operationand the trip circuit of the circuit breaker is closed.



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ff l b l


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Differential Protection using balancingresistor



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Modified Differential Protection

Generally protection is made for 80 to 85% of thewinding. If any fault occurs near the neutral point then the

fault current is very small and relay does not

operate. Modified differential protection scheme is used

to over come this. Two phase elements (PC and PA) and balancing

resistor(BR) is connected in star and the earthrelay(ER) is connected between the star point andneutral pilot wire.



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Percentage Differential protection



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The disadvantage of the current differentialprotection is that current transformers mustbe identical, otherwise there will be current

flowing through the current relays for faultsoutside of the protected zone or even undernormal conditions. Sensitivity to the

differential current due to the currenttransformer errors is reduced by percentagedifferential relays.



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In percentage differential relays, the current fromeach current transformer flows through arestraint coil. The purpose of the restraint coil isto prevent undesired relay operation due tocurrent transformer errors. The operating coilcurrent | i 1 - i2 | required for tripping is apercentage of the average current through therestraint coils. It is given by

where k is the proportion of the operating coilcurrent to the restraint coil current. For exampleif k = 0.1, the operating coil current must be morethan 10% of the average restraint coil current inorder for the relay to operate.



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Restricted or balanced Earth fault Protection

In case of small size generators star point isnot available because it is made inside thegenerator and grounded through some low

resistance then percentage differential relayfor ground fault is provided and is known asrestricted earth fault protection.

This scheme can be used only for groundfaults but not for phase faults.



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M P i i


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Merz Price protection This type of protection provides a continuous check on

faults within the points, where the C.T.s are used. Thereare two sets of C.T.s; each set is mounted on either end ofthe stator phase. The secondaries of these currenttransformer sets are connected in star and their ends areconnected together through pilot wires. The over currentinverse time relay is connected across the equipotentialpoints in the pilot wires, which will naturally be in centerof these wires. These relays are kept adjacent to the maincircuit breaker; by inserting balancing resistance in serieswith the pilot wires to achieve equipotential point nearthe main circuit breaker. If there is no fault in thegenerator, same current will pass through the C.T.s atboth the ends and there will be no current spilling intothe relay.



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Stator Winding Faults

The stator fault includes the following:1. Phase-to-phase fault2. Phase-to-earth fault3. Stator inter-turn fault

The most common fault of the above is the phase-to-earth fault. This fault is sensed and protected by theearth fault protection.

From these phase faults and inter turn faults are lesscommon ,these usually develop into an earth faults.

This causes- Arcing to core Damage of conductor and insulation



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Cross Differential Protection

Used in case of hydro-electric generator havingdouble winding armature.

As shown in figure relayRc provides protectionagainst phase to groundand phase to phase fault.

The relay R1 providesprotection against inter-turn faults.



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Rotor Faults Faults in the rotor circuit may be either earth

faults or between the turns of the field winding . Field circuits are normally operated un-earthed.

So a single earth fault will not affect its operation. But when a second fault arises then field winding

is short circuited and produce unsymmetricalfield system which leads to unbalanced forces onrotor and results in excess pressure and bearingand shaft distortion.



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Rotor Earth fault Protection The rotor earth fault protection is done by dc

injection method or ac injection method . The dc or ac voltage is impressed between the

field circuit and ground through a sensitiveovervoltage relay and current limiting resistor orcapacitor(in case of ac).

But dc source is generally used as over-current

relay in case of dc is more sensitive than ac. A single earth fault in rotor circuit will complete

the path and the fault is sensed by the relay.



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Rotor Earth fault Protection



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Rotor Temperature alarm

It is provided in largegenerators.

It indicates the level oftemperature but not theactual hot spottemperature.

The relay measures thetemperature bymeasuring the resistance(as shown in fig)



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Loss of Excitation Protection

When the excitation of generator is lost it operate as aInduction generator. It derives excitation from thesystem and supply power at leading power factor.Which may cause-

A fall in voltage & so loss of synchronism & systeminstability.Over heating of rotor due to induction current on it.

A protection having MHO characteristicis used to detect loss of field.



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Overvoltages Atmospheric surge voltages are caused by direct

lightening strokes to the aerial lines in the HVsystem. These high voltage surges can reach tothe generator via transformer. To protect the

generator from severe voltage surges, surgearrestors and surge capacitors may be used.

Switching surges may cause high transient

overvoltage if the restriking occurs across thecontacts of the circuit breakers. These surges canalso be eliminated by surge arrestors.



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Over Voltage Protection

Overvoltage protection is required in case of hydro-electric or gas turbine generators but not in case ofturbo generators.

Over voltage may be caused due to-

Transient over voltage in the transmission line due tolightening.Defective operation of the voltage regulator.Sudden loss of load due to line tripping.The protection is provided with an over voltage relay.It is usually of induction pattern with an IDMTCharacteristic



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Reasons for Overvoltage

Overvoltage may occur in the vehicle electricalsystem as the result of:

- Regulator failure- Influences originating from the ignition- Switching off of devices with a predominantly

inductive load- Loose contacts- Cable breaks



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Overload Protection Overloading of the machine causes overheating

in the stator winding. This can be prevented by using over-current relay

with time delay adjustment. But overheating not only depends on over-

current but also the failure of the cooling systemin the generator.

So temperature detector coils such as thermistorsor thermocouples are used at various points instator winding for indication of the temperature.



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Thermal Overloading

Continuous overloading may increase thewinding temperature to such extent that theinsulation will be damaged.

Temperature rise can also be caused by thefailure of cooling system. In large machinedthe cooling system is utmost requirement.

Failure of cooling system will cause severe risein temperature of winding.



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Unbalanced loading Continuous unbalance loads equal to the rated

current can cause dangerous heating of thecylindrical rotor in turbo-generators.Unbalanced loading on generator may be due to:- unsymmetrical faults near the generatingstation- mal-operation of a circuit-breaker near thegenerating stationNegative sequence protection senses theunbalanced loading of alternator.



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Reverse Power Protection

When prime-mover fails machine startsmotoring and draws electrical power from thesystem and this is known as inverted

operation . The generator can be protected from inverted

operation by using single-element directional

power relay(reverse power relay) whichsenses the direction of power flow.


Negative Phase Sequence


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Negative Phase SequenceProtection

Unbalance may cause due to single phasefault or unbalanced loading and it gives rise tonegative sequence current .

This current in rotor causes rotor overheatingand damage to the rotor.

This can be protected by negative sequence

current filter with over current relay.


Negative Phase Sequence


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Negative Phase SequenceProtection



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Over current & Earth fault protection



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Field Winding Faults

Rotor faults include rotor inter turn faults andconductor-to-earth faults. These are caused bymechanical and temperature stresses. Theunbalanced loading on generator gives rise to

negative sequence currents, which gives rise tonegative sequence field & this negative sequencefield rotates in opposite direction of the mainfield and gives rise to rotor heating. Rotor

temperature indicators are used for detection ofrotor overheating due to unbalanced loading ofgenerator.



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Cooling and noise Due above all to the heat developed by the alternator when

converting mechanical power into electrical power, and alsodue to the effects of heat from the engine compartment(engine and exhaust system), considerable increases in thealternator component temperature take place.

In the interests of functional reliability, service life, and

efficiency, it is imperative that this heat is dissipatedcompletely. Depending upon alternator version, maximum permissible

ambient temperature is limited to 80...120

C, and futuretemperatures are expected to reach to 135

C.

Cooling must guarantee that even under the hostile under-hood conditions encountered in everyday operation,component temperatures remain within the specified limits("worst-case" consideration).



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Other Protections

Rotor temperature alarm Stator heating protection Loss of field protection Over speed protection Bearing insulation

Vibration protection Bearing overheating protection



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THANKSPresented By:

Er. H.S. [email protected] 098150-74800
mailto:[email protected]:[email protected]

218091515 protection of tf gen

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