transformer protections

7/28/2019 Transformer Protections

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TRANSFORMER

PROTECTIONS

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The Types of faults that the transformers are

subjected to are classified as:

INTERNAL FAULTS

EXTERNAL FAULTS

( THROUGH FAULTS)



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INTERNAL FAULTS

FAULTS ARISING IN THE TRANSFORMER ITSELF

ELECTRICAL FAULTS: Faults which cause

immediate serious damage such as

1. Phase to earth fault

2. Phase to phase faults on HV or LV windings

3. Short circuit between turns on HV and LV

winding (inter turn faults)

4. Problem in tap changer gear



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INCIPIENT FAULTS: Which are initially minor

faults, causing slowly developing damage,

such as1. Poor electrical connection of conductors

2. Breakdown of insulation of laminations, core

bolt faults, clampings.

3. Coolant failure

4. Continuous uneven load sharing betweentransformers in parallel causing overheating

due to circulating current.



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EXTERNAL FAULTS (through faults)

The faults in the system to which the

transformer is connected Due to external faults (like phase-phase, phase

to earth) the effect is excessive over currentand electromagnetic stresses proportional tosquare of short circuit current.

1. Short circuits: Phase to phase, phase to earth

2. High Voltage disturbance3. High Frequency disturbance

4. Earth faults



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PROTECTION PRINCIPLE

1. Circuit breaker

2. Relay

3. Trip coil of C.B

5.Battery

6.Relay contacts

7.Bus bars

8.Current T/F9.Pilot-wires

a.Aux.switch of C.B

x.Protected Power T/F



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THE FOLLOWING RELAYS ARE EMPLOYED TO PROTECTTHE TRANSFORMER AGAINST DIFFERENT FAULTS

1. Over load

2. Over current3. Over voltage

4. Restricted Earth Fault

5. Differential protection

6. Buchholz relay

7. Pressure relief value (PRV)

8. Winding temperature

9. Oil temperature10. OLTC buchholz (oil surge relay)

11. Over fluxing

12. Over/Under frequency



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OVER LOAD

This protection is required for continuous over load ofthe transformer

Protection is based on measurement of windingtemperature which is measured by thermal imagetechnique.

Thermal sensing element is placed in small pocketlocated near the top transformer tank in the hot oil. Asmall terminal of one phase is also located in thispocket and produced a local temp, rise, similar to thatof main winding and proportional to copper losses,

above general temp of oil. Winging temperature high alarm/trip is provided

through mercury switches in the winding temp,indicators.



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OVER LAOD PROTECTION The permissible overload and their duration

depends upon the type of cooling andinsulation class of transformer. Higheroverloads are permissible for a shorterduration.

Hence for most of the transformers, overloadprotection is generally arranged to initiatealarm.

Overload % 125 150 175 200 300

Duration

(minutes)

125 45 15 10 1



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THERMAL OVER HEATING PROTECTION

Thermo couples or RTD are kept near each

winding to detect the temperature. When thetransformer temperature increases cooling

system comes in to picture.

Fans get ON : 60 0C OFF : 50 0C

Alarm : 90 0C

Trip : 104 0C



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OVER CURRENT

This protection is required for short circuit

faults and earth faults IDMT relays should be chosen

Instantaneous trip facility for high speedclearance of terminals short circuit should beprovided

Setting depends on transformer reactance or

percentage impedance, faults MVA. Setting of over current relays can be slightly

higher than rated full load current (say 120percent of FL) with proper discrimination.



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EARTH FAULT PROTECTIONThis protection is required for earth faults

Combined over current and earth fault protection

(un-restricted earth fault)



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The increase in current of phases causes correspondingincrease in respective secondary currents. The secondarycurrent flows through respective relay units. Very oftenonly two phase relays are provided instead of three,

because in case of phase faults current in at lease any twophases must increase. Hence two rely units are enough.The earth fault relay is residually connected

Over current and Earth fault protections are used as a mainprotection for medium transformers where differentialprotection is not provided. Differential protection isgenerally uneconomical for power transformers below 1MVA. In such cases, over current protection is employed asmain protection against phase faults. For transformersabove 1 MVA, if differential protection is used as a mainprotection, over current protection is used in addition, as abackup for sustained through faults. Earth fault protectionis provided in addition to phase fault protection.



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RESTRICTED EARTH FAULT PROTECTION

Transformer earth fault protection should notoperate for faults beyond the transformerprotection zone. This is ensured by the restrictedearth fault protection. This protection operateson the principle of Kirchoofs law and requiresCTs of identical ratio and ratings as the phasesand neutral earth connection. For external earthfault, the associated CTs have dissimilar polaritiesforming a series connection. Thus, the resulting

current through the relay is negligible. Forinternal fault, however, the CTs have similarpolarities, forming a parallels connection, thisadding up the current in the relay branch. Thisensures positive operation of the relay.



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RELAY DOES NOT OPERATE FOR EXTERNAL EARTH FAULT



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RELAY OPERATES FOR INTERNAL FAULT



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EARTH FAUALT PROTECTION

of transformers feeding through cables by zero

sequence relay (core balance currenttransformer)

In absence of an earth fault in the transformeror the supply cable, the current in thesecondary of zero sequence CT is negligible.Under balanced load and without earth fault

Ir = (Ia + Ib + Ic ) = 0 During tan earth fault (Ia + Ib + Ic ) 0 flux

in the core 0



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CORE BALANCE CT



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DIFFERENTIAL PROTECTION

This protection is also for internal faults of the transformerand responds to the vector difference between two

similar quantities. Two basic requirements that the differential relay

connections are to be satisfied are:

It must not operate for external faults It must operate for internal faults

As on load tap change facilities are invariably provided inthe grid transformer may departure from the nominal tapposition will result in spill currents in the relay circuits.

Further the CTs are often of different types and havedissimilar magnetization characteristics, again resulting inspill current during heavy through fault conditions.

To avoid unwanted relays operation under the above twoconditions a percentage bias differential relays is used.



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DIFFERENTIAL PROTECTION



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DIFFERENTIAL PROTECTIONSTAR/STAR TRANSFORMER



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DIFFERENTIAL PROTECTIONDELTA/STAR TRANSFORMER



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The current flowing through the operating coil of the relayshould be nearly zero during normal operating conditionsand when external short circuit occurs.

While setting the differential relay on a transformer, the( mismatch) current through differential element at normaltap and positive and negative extreme taps are to becomputed . Differential element pickup setting and/ orbias setting is adopted based on maximum percentagemismatch adding some safety margin.

In this connection the CT secondarys are connected instar or delta and pilot wires are connected between the CTsof each end. The CT connections and CT ratios are such

that currents fed into the pilot wires from both the endsare equal during normal conditions and for through faults.During the internal faults such as phase to phase or phaseto ground, the balance is disturbed



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The out of balance current (I1- I2) flows throughthe relay operating coils (O.C). To avoidunwanted operation on through faults,

restraining bias coils are provided in series withpilot wires. The ampere-turns provided bias coilor restraining coil or proportional to (I1- I2)/2, I1and I2 are vectors. Three sets of CTs primaries areconnected on primary side and 3 sets onsecondary side. CT secondarys on STAR side ofmain transformer are connected in DELTA , CTsecondarys on DELTA side of main transformerare connected in STAR. This ensures correct flowof circulating currents in pilot wires.



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COMBINED DIFFERENTIAL PROTECTION

for Generator Main Transformer

(Overall differential)The zone of combined differential

protection may include generator statorwinding, main step up transformer and Unitauxiliary transformer. In addition to normaldifferential protection of generator, overall

biased differential protection relay isconnected to protect the unit. This is asupplementary protection for individualdifferential protection of the generator.



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OVER VOLTAGE PROTECTION

Two stage protection is provided. Thedelayed trip is set at 110 percent of the rated

voltage with two second time delay and

Instantaneous setting is kept at 115-120

percent of the rated voltage. During voltage

fluctuations the AVR (Automatic Voltage

Regulator ) will care to avoid over voltagecondition if fluctuations are within its

operating limits (for generator transformers).



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SURGE ARRESTER PROTECTIONSurge arresters (lighting arresters) are connected

between phase and earth near the terminal of each

phase of the transformer. The surge arrester providesprotection against Lightning surges and switchingsurges. Lightning surges occur due to lightningstrokes on transmission line conductor. The surgetravels up to transformer and can damage thetransformer winding insulation. The surge arresterintercepts the surge and diverts it to the earth and

protects the transformer. Switching surges originatein the system while opening of inductive currentsand closing of transmission lines. The surge arresterintercepts the switching surge and diverts it to the

earth and protects the transformer.



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POWER FREQUENCY TEMPORARY OVERVOLTAGEPROTECTION

When load or line is thrown off (switched off )the power frequency (50 Hz) overvoltage occursat the bus bars of receiving substation. The onload tap changers act and bring down the voltageto permissible value. If overvoltage does notdrop in short time the transformers may fail. Thep.f voltage above 1.1 per unit is harmful totransformer. Inverse Definite Minimum Time

over voltage relay is connected to secondary ofprotective bus PT. This relay is arranged to tripthe circuit breakers feeding the transformersinstantaneously for overvoltage of 1.3 p.u. Forover voltages of 1.2 p.u time delay provided.



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OVERFLUXING PROTECTIONThe magnetic flux in the transformer core is

proportional to V/f, where V is the voltage and f is the

frequency. With rated voltage = Vr and ratedfrequency fr the V/f ratio is Vr / fr = 1 p.u . Transformercore can tolerate the V/f ratio of 1.1 p.u for longduration without harm. If V/f ratio is higher than 1.1the condition of over fluxing arises. The core gets hotand core insulation is damaged. The core loss4esincreases. The transformer becomes hot. The overfluxing condition in generator transformers and unitauxiliary transformers if the generator excitation is

increased before reaching synchronous speed. Overfluxing relay is connected in generator excitationsystem. This relay does not allow the excitationcurrent of generator till synchronous speedcorresponding to the rated frequency is reached.



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Over fluxing can occur in transformers of generator unit. Thiscondition arises during abnormal operating conditions i.e heavy

voltage fluctuations at lower frequency conditions. This condition is

experienced by the transformer during heavy power swings,cascade tripping of the generator sets and HT line in the grid,

interstate system separation conditions and due to AVR

malfunctioning during start up or shutting down in case of

generator transformers. The power frequency over voltage cause

both stress on insulation and proportionate increase in the

magnetizing flux inside the transformer due to which the iron losses

are increased and the core bolts get maximum component of flux,

thereby rapidly heating and damaging its own insulation and coil

insulation. Reduction in frequency during high voltage fluctuation

has the same effect.

V/f over fluxing factor 1.1 1.2 1.25 1.3 1.4

Duration of withstand

limit (minutes)

Contin

uous

2 1 0.5 0



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BUCHHOLZ RELAYThe incipient faults in transformer tank below oil

level actuate Buchholz relay so as to give an alarm. The

arc due to fault causes decomposition of transformeroil. The product of decomposition contain more than70% of hydrogen gas. Which being light, rises upwardsand tries to go into the conservator, The Bnuchholzrelay is fitted in the pipe leading to the conservator.The gas gets collected in the upper portion of theBuchholz relay, thereby the oil level in the Buchholzrelay drops down. The float , floating in the oil in thebuchholz relay tilts down with the lowering oil level.

While doing so the mercury switch attached to thefloat is closed and the mercury switch closes the alarmcircuit. Buchholz relay gives an alarm so that thetransformer can be disconnected before the incipientfault grows in to a serious one.



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BUCHHOLZ RELAY



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ARRANGEMENT OF BUCHHOLZ RELAY



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PRESSURE RELIEF VALUE (PRV)This is different from rate of rise pressure relay.

Pressure relief valve is mounted on transformer tank.It releases gas pressure to the atmosphere during

1. High overload peaks

2. Prolonged overloads

3. Arcing faults within oil

The PRV is spring loaded and has a seal seat. Whenpressure inside the tank increases above a certain

value, the force on movable sub assembly exceeds thespring force and valve operates. The contacts areclosed. After release of PRV the valve may be resetmanually.



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PRESSURE RELIEF VALUE (PRV)

DURING NORMAL POSITION



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PRESSURE RELIEF VALUE (PRV)

DURING INTERNAL PRESSURE



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OIL TEMPERATURE

An oil thermometer, which is common with all oilfilled transformers, can be considered as a partiallyeffective protective device when equipped with alarmcontacts connected to give remote warning ofabnormally high oil temperature. Its location is suchthat it naturally monitors the hottest fluid that exists inthe transformer. The same thermo meter is often usedto start fan motors ON and OFF.

WINDING TEMPERATURE

The thermometer bulb is located in a pocket near

the winding. The bulb is surrounded by hot circulatingoil. The bulb is also heated by a small heater connectedacross CT secondary . Thereby the heat given to thebulb is a function of load current as well as thetemperature of oil near winding.



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PROTECTION CHART OF POWER TRANSFORMER

Sl.

No.

ABNORMAL CONDITION PROTECTION REMARKS

1. Incipient fault below oil level Buchholz relay (Gasactuated relay

Buchholz relay fortransformers of rating 500

kVA and above

2. Large internal faults phase to phase, phase to

ground

1.Buchholz relay trips the

circuit breaker

2. Percentage differential

Protection

3.High speed high set over

current relay

Buchholz relay slow and

less sensitive

Percentage differential

protection for

transformers above 5

MVA

3. Saturation of Magnetic circuit 1.Overfluxing protection

with

voltage regulator

2. Over voltage protection

of

Bus bars

For generator transformer

and unit auxiliary

transformer



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Sl.

No.

ABNORMAL CONDITION PROTECTION REMARKS

4. Earth faults 1. Differential Protection

and

2. Earth fault relay in

residually connected

circuit

For transformers of and

above 5 MVA

(a) Instantaneous

restricted E/F

Relay

(b) Time lag E/F relay

5. Through faults (External faults feeding through

the transformer)

1. Graded time lag over

current relay

2. HRC fuses

Protection of transformers

Small distribution

transformers up to 500

kVA

6. Over loads 1. Thermal over load

relays

2. Temperature relays

3. Over current relayswith

inverse characteristic

Fans started at certain

temperature. Alarm and

tripping actions later

7. High voltage surges due to lightning switching 1. Horn gaps

2.Surge arresters

3.R.C surge suppressors

Not favored for important

transformers



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transformer protections

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