Protective relay

A protective relay monitors electrical quantities in a power system, detects abnormal conditions, and signals circuit breakers or other devices to isolate faults.

Current transformer: An instrument transformer that produces a reduced secondary current proportional to a power circuit's current. It supplies relays with scaled current signals while separating them from high-voltage conductors.
Power system fault: An unintended electrical connection or interruption that disrupts normal power-system operation. Faults are the abnormal conditions protection is designed to detect.
Distance protection: Transmission-line protection that estimates fault distance from measured voltage and current relationships. Distance relays provide fast, selective protection for transmission lines.
Electromechanical relay: A relay that uses electromagnetic forces and moving mechanical parts to perform switching or protection functions. It is the traditional relay design, using physical motion to make operating decisions.
Protection coordination: The arrangement of protection settings so devices operate selectively and in a planned sequence. Coordination lets the relay nearest a fault trip before upstream protection.
Voltage transformer: An instrument transformer that produces a reduced secondary voltage proportional to a power circuit's voltage. Voltage-dependent relay functions use its scaled voltage signal.
Short circuit: An unintended low-impedance connection between electrical conductors at different potentials. It commonly produces the large currents that trigger relay protection.
Transformer differential protection: A protection scheme that compares currents on a transformer's sides to detect internal faults. It adapts differential protection to transformer ratios and phase shifts.
Solid-state relay: A relay that uses electronic components rather than moving contacts to perform switching functions. It replaces mechanical operating elements with electronic circuitry.
Selectivity (electrical protection): The ability of a protection system to isolate a faulted part while keeping unaffected parts energized. Relay settings determine whether a trip is limited to the faulted section.

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