TESTING OF BUS DIFFERENTIAL RELAYING

The testing of bus differential relays and associated systems requires special care and accuracy on the part of the tester. The consequences of an inadvertent trip, due to tester error or of a false trip due to incorrect or incomplete test practices, are usually severe in that numerous circuits can be affected.

Practices have been developed which considerably ease the problems facing the relay tester and improve quality and thoroughness. Specific practices are as follows.

Test Switches
Many utilities provide a test switch in each output circuit and in the coil of the lockout relay or other auxiliary devices used to control multiple breakers. These test switches are used to ensure that no inadvertent trip occurs when removing the relay from service or returning it to service.

Test switches are also of significant value when performing trip tests in that the breakers can be tripped and closed one at a time to verify the integrity of the relay output circuitry. Covers are usually provided which cannot be put in place until the test switches are returned to their normal position.

Permanent Test Facilities
Permanently installed test equipment has been designed to check some types of differential relay circuits while the power circuit is in operation. These facilities use indicating meters to monitor pre selected points in the relay circuit by means of a test selector switch. These types of test systems are described in the manufacturers’ literature.

Verification of Relay Input Sources
Many problems with bus differential relay systems can be attributed to improper connections of current transformers and associated circuitry. Consequently, it is important to verify the ratio and polarity of the current transformers and the connections to the relays.

Procedures for performing the above tests are well known throughout the industry. However, if not properly performed, failure to trip or false trips will result.