Transformer Protection

INTRODUCTION ` TYPES OF FAULTS ` TYPES OF PROTECTION RELAY `  APPLICATION ` CONCLUSION `

Transformers 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 transformer  For protection ` Small rating transformers use fuses ` But large rating transformers always need some type of automatic protective relaying equipment `

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

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.

The winding winding faults f aults are called internal faults. f aults. The overheating or mechanical shocks deteriorates the winding insulation. If  the winding winding insulation-is weak, there is a possibility of  ` Phase to phase faults ` Earth faults ` Inter turn faults Persistent fault would have a possibility of oil-fire. PROTECTION ` Differential 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.

`

`

`

The open circuit in one of the three phases is causes the undesirable heating of the transformer. transformer. Open circuits are much harmless compared to other faults. Transformers Transformers can be manually disconnected from the system during such faults.

Through faults faults are the external faults which occur outside the protected zone. ` These are not detected by the differential protection. transform er gets subjected to thermal ` If faults persists the transformer and mechanical stresses which can damage the transformer. PROTECTION relays with undervoltage blocking, zero ` The overcurrent relays 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 transm ission line. `

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 ` Transformers 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. `

Other faults `

Tap changer cha nger faults

`

High voltage surges due to lightning

`

Switching surges

`

Incipient faults

 Also called as percentage differential protection ` It is based on circulating current principle protect ects s agains againstt phase phase ` It prot to phase and phase to ground faults `

`

`

`

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

`

`

`

`

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

` ` `

`

`

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 f ault relay This operates the relay

`

`

`

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

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

Even though faults in the transformers are very rare different types of possible faults were seen ` Different protection relays relays and devices were shown `  Advantages and limitations were seen `

`

`

`

`

H.C.Branes and R.H.Mcpherson ³field experiencetransformer protection´, AIEE AIEE Trans,72 part 3 M.H.Haque,2004´use M.H.Haque,2004´use of local information for protection", international power 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