Unit Auxiliary Transformer (UAT) Differential Protection 87UAT

Unit Auxiliary Transformer (UAT) Differential Protection:

Unit Auxiliary Transformer (UAT) differential protection is used to protect the generator and transformer’s winding internal short circuit, inter turn short, winding earth fault etc. UAT is an auxiliary transformer which is directly connected with the Generator, before the circuit breaker. The transformer gets energize as the generator terminal voltage increases. So that there is no breaker in between the generator and the transformer, the UAT differential protection (including the generator), should trip the unit either fault on the generator or transformer. It provides extra protection for the generator windings.

UAT differential protection
UAT differential protection

Principle of Unit Auxiliary Transformer (UAT) Differential Protection:

Vector sum of the equal magnitude and opposite phase current in a circuit is equal to zero. Simply to say it operates under the principle of differential protection.

In this, scheme to protect Unit Auxiliary Transformer windings and generator windings, a biased differential relay (87UAT) is connected between the generator & secondary winding of UAT through respective CTs. Refer the diagram, The CT1 is connected at the generator neutral terminal side and another one CT2 is connected in UAT secondary side.

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In this, to match the current transformer CT ratio between the generator side and transformer secondary side, interposing current transformer is used.

During normal condition any current flowing in generator winding CT will flow out from secondary winding CT & there will not be any difference between CT currents. Also their polarity will be same, so differential current through relay operating coil of 87UAT will be zero & relay will not operate.

A fault inside the protected zone i.e. the current flow in the generator winding and the transformer winding varies. Hence the differential current flow in the relay operating coil. if the current exceeds the pickup current of the relay, the relay trips the entire unit along with Generator Circuit Breaker & Excitation system & Turbine.

UAT Differential Protection ANSI code: 87 UAT

Relay Acted:

  • 87 UAT opted
  • 86H Gen master trip opted
  • 86H Turbine trip opted


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