Using transformer differential relay I/sub OP/ characteristics to measure near-trip conditions

Abstract

When a transformer differential relay operates with no obvious transformer fault, system operators have a serious decision to make: is there a transformer fault, or did the relay trip incorrectly? Testing the transformer requires significant time, with associated costs. However, re-energizing a faulted transformer can lead to catastrophic failure. This scenario of a questionable transformer trip occurs too often, especially during the commissioning of new transformers and/or differential relays. Usually a differential relay false trip is caused by either incorrect settings or incorrect current transformer (CT) secondary lead connections. These conditions can cause false trips for external faults, or simply transformer loading. Some indication is needed that the relay is not operating as desired, before an incorrect trip happens. A potential problem can be discovered by monitoring the operating condition of the differential relay. An indicator can serve as a warning if the settings or connections are not correct.