Directional Relays

Directional relays, designed for unidirectional fault currents, improve power system safety and efficiency.

On power lines with a directional relay, faults to the right of the current transformer cause the fault current to lag the bus voltage by nearly 90 degrees, termed the forward direction. Conversely, left-side faults result in the fault current leading the bus voltage by almost 90 degrees, termed the reverse direction

A directional relay uses reference voltage and current inputs to define its trip and block regions based on the angle between the current and voltage.

In series with an overcurrent relay, the directional relay energizes the breaker trip coil only when the current transformer's secondary current exceeds the overcurrent relay pickup value and is in the forward-tripping direction.

Operating like a watt-hour meter, electromechanical directional relays use voltage and current coils and a rotating disc element.

Forward-direction faults create maximum positive torque on the rotating disc, closing relay contacts, while reverse faults generate maximum negative torque, restrained by mechanical stops.

For two-source systems, directional relays can overcome coordination challenges, ensuring uninterrupted service to loads during faults.