Relay Control

Protective relay systems detect and isolate faults.

Time-delayed phase and ground relays are coordinated with fuses and reclosers further out on the circuit. They are instantaneous units with inverse TCCs to coordinate with fuses and reclosers further downstream. Relays are usually set to trip feeder breakers and protect the fuse in the event of temporary faults beyond the fuse. CTs measure large currents as well as insulate relays from high-voltage circuits. The primary winding of the CT is connected in series with the circuit to be measured. The nominal current ratio of the CT is the inverse of the turns ratio.

A variety of relay protocols exist. Directional comparison relay systems are set in twos; relay A responds to faults to its right, and relay B responds to faults to its left. Both will respond simultaneously only if the fault is on the line they are both protecting. One communication channel informs A about the state of B, and another informs B about the state of A.

Phase comparison relaying uses a communication channel to compare information about the phase currents based on the fact that external faults are essentially equal in magnitude and opposite in direction while internal faults are in phase.

Associations: DynamicProtectiveDevice (only valid for Circuit Breaker and Recloser subtypes)

Subtypes: None

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