Directional relays, essential for managing unidirectional fault currents, enhance the safety and efficiency of power systems. On power lines equipped with directional relays, faults downstream (to the right) of the current transformer typically cause the fault current to lag the bus voltage by approximately 90 degrees, known as the forward direction. In contrast, upstream (left-side) faults may result in the fault current leading the bus voltage by nearly 90 degrees, termed the reverse direction. However, the actual phase shift depends on system impedance and specific fault conditions.

Directional relays use reference voltage and current inputs to define their trip and block regions based on the angle between the current and voltage. Typically, the reference voltage is taken from the bus or the line-to-ground voltage, and the current is taken from the current transformer (CT). When paired with an overcurrent relay, the directional relay activates the breaker trip coil only if the CT's secondary current exceeds the overcurrent relay pickup value and aligns with the forward-tripping direction.

Electromechanical directional relays function similarly to watt-hour meters. They incorporate voltage and current coils along with a rotating disc element. Forward-direction faults produce maximum positive torque on the rotating disc, closing the relay contacts. Conversely, reverse faults generate maximum negative torque, which is restrained by mechanical stops.

In two-source systems, directional relays address coordination challenges, ensuring continuous service to loads during faults. By accurately distinguishing between forward and reverse fault conditions, directional relays enable precise and reliable fault isolation, thereby maintaining system stability and minimizing disruption. Modern digital directional relays offer advanced features such as self-monitoring, communication capabilities, and precise timing adjustments, further enhancing the reliability and effectiveness of power system protection.