Numerical relay

A protective relay that samples electrical signals and processes them digitally to detect power-system faults and initiate protective actions.

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Current transformer: An instrument transformer that produces a reduced current proportional to current in a power circuit. It supplies an isolated, scaled current signal for relay measurement.
Overcurrent protection: Protection that operates when current exceeds a specified threshold or time-current characteristic. A numerical relay can apply pickup settings and timing curves to measured current.
Protective relay: A device that detects abnormal electrical conditions and initiates actions to protect power-system equipment. Numerical relays are digital implementations of this broader device class.
Electromechanical relay: A protective relay that uses electromagnetic forces and mechanical parts to detect conditions and operate contacts. It performs protection through physical mechanisms rather than digital sampling.
IEC 61850: An international standard for communication networks and systems in electrical substations. It enables numerical relays to exchange standardized data and protection messages.
Voltage transformer: An instrument transformer that produces a reduced voltage proportional to voltage in a power circuit. Its voltage signal lets the relay assess system conditions alongside current.
Distance protection: Transmission-line protection that estimates fault distance from measured voltage and current impedances. The relay calculates apparent impedance to identify faults within configured line zones.
Phasor: A complex-number representation of a sinusoidal quantity's magnitude and phase. Many relay functions operate on estimated voltage and current phasors.
Static relay: A protective relay that uses analog electronic circuits rather than moving measuring mechanisms or digital computation. It represents the solid-state analog generation preceding numerical relays.
Digital fault recorder: A device that records electrical waveforms and event data during power-system disturbances. Numerical relays can capture disturbance records that support fault analysis.