What type of relay uses a combination of CT measurements and PT measurements?

Relay 21, known as the distance relay or impedance relay, utilizes both current transformer (CT) measurements and potential transformer (PT) measurements to determine the impedance of the line it is protecting. This relay is designed to assess the electrical distance to a fault within the power system, using the ratio of voltage (from PTs) to current (from CTs) to calculate the impedance.

By analyzing these measurements, the relay can detect various fault conditions and make decisions on whether to trip the circuit breaker or not, thus providing protection against overcurrent and short-circuit faults along the transmission lines. The operational principle of Relay 21 is particularly essential for high-voltage transmission lines where knowing the exact location and nature of a fault is crucial for maintaining system reliability and minimizing disruption.

In contrast, other relays listed perform different specialized functions, such as protective measures against specific types of faults or overloads, but do not use the combination of CT and PT measurements in the same manner as Relay 21. Relay 87, for instance, is a differential relay focusing on current comparisons, while Relay 51 is an overcurrent relay that principally uses CT measurements alone. Relay 86 is a lockout relay, which is typically used in conjunction with other