In a distance relay, what triggers its operation?

A distance relay operates based on the measurement of electrical quantities to assess the distance to a fault on the transmission line. The correct answer involves the operation being triggered when the ratio of voltage measurements taken via potential transformers (PT) and current measurements taken via current transformers (CT) falls below a set limit. This relationship is crucial because it provides an indication of the impedance along the line, which is directly related to the distance to the fault.

When a fault occurs, the impedance seen by the relay decreases, ultimately resulting in the voltage-to-current ratio dropping below the predetermined threshold. This condition is a key operational principle of distance relays, utilizing the measured values to determine if a fault is within the settings configured for protection.

In contrast, other options do not trigger the operation of a distance relay effectively. High voltage across transmission lines can occur under normal operational conditions or during transient events but does not provide direct information about fault location. Similarly, while a current exceeding a predetermined threshold could indicate a fault, it is the ratio of voltage to current (the impedance) that specifically triggers the relay’s operation for distance protection. Finally, a frequency drop in the system is more relevant to other protective schemes and does not directly serve as a triggering factor for distance relays.