Transformer Protection

TRANSFORMER PROTECTION

INTRODUCTION TYPES OF FAULTS TYPES OF PROTECTION RELAY APPLICATION CONCLUSION

CONTENTS

Transformers are static devices without any rotating part and are totally enclosed.

Chances of faults occurring on transformers are much rare as compared to generators

If fault occurs, they have to be cleared quickly If not they develop into major faults which may

be very serious for the transformerFor protection Small rating transformers use fuses But large rating transformers always need some

type of automatic protective relaying equipment

INTRODUCTON

The various possible transformer faults are, 1. Overheating 2. Winding faults 3. Open circuits 4. External faults 5. Over fluxing

TYPES OF FAULTS

They are due to Sustained overloads and short circuits Failure of cooling system

Protection The thermal overload relays and temperature relays,

sounding the alarm are used The thermocouples or resistance temperature

indicators are also provided near the winding. These are connected in a bridge circuits. When the bridge balance gets disturbed for more

than the permissible duration circuit breaker trips.

OVERHEATING

The winding faults are called internal faults. The overheating or mechanical shocks deteriorates the winding insulation. If the winding insulation-is weak, there is a possibility of

Phase to phase faults Earth faults Inter turn faultsPersistent fault would have a possibility of oil-fire.PROTECTION Differential protection Overcurrent protection is also used as a backup protection For earth fault protection, the restricted earth fault

protection system, neutral current relays or leakage to frame protection system is used.

WINDING FAULTS

The open circuit in one of the three phases is causes the undesirable heating of the transformer.

Open circuits are much harmless compared to other faults.

Transformers can be manually disconnected from the system during such faults.

OPEN CIRCUIT

Through faults are the external faults which occur outside the protected zone.

These are not detected by the differential protection. If faults persists the transformer gets subjected to

thermal and mechanical stresses which can damage the transformer.

PROTECTION The overcurrent relays with undervoltage blocking, zero

sequence protection and negative sequence protection are used to give protection

The setting of the overcurrent protection not only protects transformer but also covers the station busbar and portion of a transmission line.

THROUGH FAULTS

The flux density in the transformer core is proportional to the ratio of the voltage to frequency i.e. V/f

Transformers are designed to work with certain value of flux density in the core. In the generator transformer unit, if full excitation is applied before generator reaches its synchronous speed then due to high V/f the overfluxing of core may occur.

PROTECTION V/f relay called volts/hertz relay is provided to

give the protection against overfluxing operation.

OVERFLUXING

Other faults Tap changer faults High voltage surges due to lightning Switching surges Incipient faults

Also called as percentage differential protection

It is based on circulating current principle

It protects against phase to phase and phase to ground faults

Merz –Price protection

The primary of the power transformer is star connected while the secondary is delta connected.

To compensate for the phase difference, the C.T. secondaries on primary side must be connected in delta while the C.T. secondaries on delta side must be connected in star

The restraining coils are connected across the C.T. secondary windings while the operating coils are connected between the tapping points on the restraining coils and the star point of C.T. secondaries

It compares the currents at entering and leaving end

The vector difference(I1 –I2)/2 passes through the operating coil

The average current (I1+I2)/2passes through the restraining coil

The two currents at the two ends are equal so balance is maintained

No current flows through the operating coil and the relay is inoperative

Principle

During normal operating conditions the C.T. secondaries carry equal currents which are inphase.

With an internal fault in power transformer windings, the balance in the C.T.s get disturbed.

The operating coils of differential relay carry currents proportional to the difference of current between the two sides of a power transformer.

This causes the relay operation which trips the main circuit breakers on both the sides of the power transformer

Working

This method is to provide earth fault protection

The frame of the switch gear is grounded When there is a fault the fault leaks from

the frame and passes through the earth connection provided

The primary of the C.T senses due to which current passes through the sensitivity earth fault relay

This operates the relay

Frame leakage protection

It is a gas operated relay used for the protection of oil immersed transformers

When faults occur decomposition of oil occurs

Gas accumulates at the top hence decreasing the oil level

Buchholz relay

Oil reduction makes the hollow floats to tilt And close the contacts of the mercury

switch attached to it Hence completing the alarm circuit to sound

an alarm The transformer could hence be

disconnected to test the gas sample

Advantages

Buchholz relay gives the fault indication at a very early stage

It is the simplest transformer protection

Limitations

Only used for oil immersed transformers Only faults below oil level are detected The relay is slow having minimum operating

time of 0.1sec and average time of 0.2 sec.

Local overheating Bad and loose electrical contacts Winding earth faults Short circuited laminations Winding short circuited Loss of oil and reduction of oil level

Applications

Even though faults in the transformers are very rare different types of possible faults were seen

Different protection relays and devices were shown

Advantages and limitations were seen

Conclusion

H.C.Branes and R.H.Mcpherson “field experience-transformer protection”, AIEE Trans,72 part 3

M.H.Haque,2004”use of local information for protection", international power engineering conference

G.Verbic and F.Gubina ,”a new concept on protection based on distance relaying schemes” ,IEEE transaction on power delivery19.

www.howstuffworks.com

References

Thank you