What characteristic defines the operation of a distance relay?

The operation of a distance relay is fundamentally defined by its ability to utilize the relationship of current to voltage to assess distance to a fault on a transmission line. This type of protection device calculates the impedance, which is a combination of voltage and current measurements taken by the relay. When a fault occurs, the current that flows through the relay and the voltage measured across the line provide vital information that indicates how far along the transmission line the fault is located.

Distance relays operate based on the principle that the impedance from the relay location to the fault changes, allowing the relay to determine its position relative to the fault. By calculating the impedance, the relay can effectively trip circuit breakers if the value falls below a predetermined threshold, thus protecting the electrical system from damage due to faults.

This characteristic makes distance relays particularly effective for long transmission lines, where assessing the distance to a fault is critical for maintaining system reliability and efficiency. The other characteristics listed, such as monitoring voltage levels, measuring frequency variations, and identifying phase imbalances, are essential functions in power system monitoring and protective relaying but do not specifically define the operational principle of distance relays.