SELF BALANCE SYSTEM

The self balance system discussed in the category ‘ protection of generators’ can also be used without any modification for transformers; but the same type of equipments should be used for both primary and secondary windings.

OVER CURRENT AND EARTH FAULT PROTECTION

The over current protection is needed to protect the transformer from sustained overloads and short circuits. Induction type over current relays are used which in addition to providing overload protection acts as back up relays for protection of transformer winding fault. Fig 10 shows the combined over current and earth fault protection. The earth fault protection is used to provide protection against any earth fault in the windings of the transformer. It works on the principle that when the transformer winding is sound the currents in all the three phases will balance and no current will spill into the earth fault relay. The arrangement is such that the relay does not respond to any out of balance current between windings caused by tap changing arrangement

Fig 10 illustrates the use of earth fault and over current relays for both star and delta connections of the transformer. Instantaneous type of earth relay is used. When the winding is delta connected the earth relay is operated by the residual current from three C.T.s connected as shown in left hand side of the fig. If the transformer winding has an earthed neutral then the residual current from the three line current transformers is balanced against the current of the current transformers provided in the neutral as on the right hand side.

When the system works normal, the sum of three currents in the C.T.s is zero and no current flows through the operating winding of the instantaneous earth fault relay and through the neutral of the transformer. However if fault is outside the protection area current flows in the neutral and lines as well, but the sum of currents in the lines is balanced by the current in neutral and hence earth relay is not operated. Now if earth fault occur within the protected zone say in the winding itself current will flow only in the neutral of the main transformer and thus there will be no balancing current in the relay circuit so, the relay is energized and the circuit breaker is opened. The trip contacts of the over current relay and earth fault relay are in parallel so, with the energisation of either over current relay or earth fault relay the circuit breaker of the concerned side will be tripped.

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