What are the consequences of incorrect CT polarity on the performance of differential relays and how can this be diagnosed?

Incorrect CT (Current Transformer) polarity has severe consequences on the performance of differential relays. Differential relays are designed to protect equipment by comparing the current entering and leaving a protected zone. Under normal operating conditions or during external faults, the currents should be equal or nearly equal. CTs provide the secondary currents that the relay uses for this comparison. If a CT polarity is incorrect, the secondary current from that CT will be 180 degrees out of phase with the other CTs. This means that during normal operation or an external fault, the differential relay will see a large differential current, even though no internal fault exists. ....