DIFFERENTIAL PROTECTION OF SINGLE PHASE TRANSFORMER IN A THREE PHASE BANK WITH SPARE TRANSFORMER

With the increase in use of single-phase transformers in three-phase banks with spare transformers, the question frequently arises of how to best include the spare in a transformer differential scheme.

Differential relay connections are dependent to some extent on transformer connections, location of CTs, and whether the spare power transformer will be energized all the time.

If the differential zone extends to circuit breakers on both sides of a transformer, then changing the CT secondary circuits is not required to place the spare transformer in service.

However, if the spare transformer is to remain energized all the time, consideration should be given on how to protect the spare when not in use.


When the transformer bank differential is used to protect the spare transformer, the result is not always as sensitive to protection of the spare as of the transformers in service.

If bushing CTs are used on both sides of the transformer bank, a separate relay for the spare could be used to ensure that it is put in service rapidly.

This is true whether or not it will remain energized. It will provide an energized spare with adequate protection. To connect a differential relay for three-phase and single-phase transformers.

The most difficult situation to handle is that in which CTs on one side of a transformer bank are located in a circuit breaker, and those on the other side are in the transformer. Unfortunately, this is a common occurrence.

In such cases, CT secondary circuits have to be switched or rewired to place the spare power transformer in service.

With any of the above combinations of transformer connections, it is possible to switch or rewire the CT secondaries.

However, switching CT secondary circuits is not recommended as a good practice without a thorough analysis of the switching device and the risks of an open CT connection during the switching or as a result of a defective switching contact.