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. This will cause the relay to falsely trip, unnecessarily isolating the protected equipment. This is known as a "false trip" or a "maloperation". The consequences of a false trip can be significant, including interruption of power supply, damage to equipment, and disruption of the power system. Diagnosing incorrect CT polarity can be done using several methods. One method is the "CT polarity test". This involves injecting a small current into the primary winding of the CT and verifying the polarity of the secondary current using a DC ammeter or voltmeter. The polarity should match the markings on the CT. Another method is to use a "secondary injection test set". This device injects currents into the relay circuit and monitors the relay's response. By varying the phase angle of the injected current, the CT polarity can be verified. Modern digital relays often have built-in CT polarity check functions. These functions use the relay's internal measurements to detect incorrect polarity and issue an alarm. Comparing the phase angles of the currents from different CTs can also reveal incorrect polarity. If the phase angle difference is significantly different from what is expected (typically 0 or 180 degrees depending on the CT connections), it indicates a polarity issue. Finally, careful visual inspection of the CT wiring and connections is crucial. Ensure that the CTs are connected correctly and that the polarity markings are consistent with the relay wiring diagram. Therefore, detecting and correcting incorrect CT polarity is essential for the proper and reliable operation of differential relays.