In a case where a transformer bank protection panel shows an 87T target, what type of initial relay operation triggered the lockout?

The indication of an 87T target on a transformer bank protection panel signifies that a transformer differential relay has detected an imbalance in the currents on either side of the transformer. A differential relay is designed to monitor the difference between the current entering and leaving a transformer. Under normal operating conditions, the current difference should be negligible. However, if there is a fault, such as a short circuit or winding failure, the differential relay will sense a deviation from this expected current balance, triggering the lockout to protect the equipment from damage.

Differential relays are critical in transformer protection because they offer high sensitivity and rapid response to internal faults, ensuring that potential damage is mitigated quickly. The activation of this specific relay, represented by the 87T target, is a direct and reliable indication that the transformer is experiencing an internal fault condition, prompting the protective actions mandated by the relay settings.