Coordinating time-delay overcurrent relays in complex radial systems and directional overcurrent relays in multi-source transmission loops can be challenging. Impedance relays address these issues by responding to the voltage-to-current ratio, specifically measuring the apparent impedance of a line. These relays become more sensitive during faults as current increases and voltage decreases, thereby reducing the apparent impedance.

Under normal conditions, low load currents keep the measured impedance outside the relay's tripping region, preventing unnecessary trips. During a fault, the impedance drops into the relay's tripping region, causing it to trip. Although typically non-directional, impedance relays can be equipped with directional capabilities through a directional restraint or a modified impedance relay.

The reach of an impedance relay indicates its fault detection range. Typically, a single impedance relay can be set to operate in multiple zones, each with progressively increasing reaches and longer delays. Ground fault impedance relays operate using line-to-neutral voltages and line currents, effectively responding to single line-to-ground and double line-to-ground faults, though they remain insensitive to line-to-line faults.

For line-to-line faults, phase relays using line-to-line voltages and the differences in line currents are used. While these relays are effective for line-to-line faults, they are less sensitive to single line-to-ground faults. This differentiation in relay sensitivity ensures that the appropriate relay responds to specific fault types, enhancing the overall protection and reliability of the power system.

Modern digital impedance relays offer advanced features such as self-monitoring, communication capabilities, and precise timing adjustments, further improving the reliability and effectiveness of power system protection. By carefully coordinating the settings of these relays, power systems can achieve high levels of fault detection and isolation, maintaining stability and minimizing disruptions.