1. Sensing Elements: Protective relays receive measurements from instrument transformers such as current transformers (CTs) and voltage transformers (PTs), which scale high voltages and currents to manageable levels for the relay to process. These sensing elements detect abnormal conditions like overcurrent, overvoltage, underfrequency, or differential currents and feed the signals to the relay logic for evaluation . 2. Relay Logic: The relay logic determines whether a fault condition exists based on the measured parameters. In electromechanical relays, this logic is implemented using moving parts, coils, induction disks, or solenoids that respond to electrical quantities . Static relays use electronic circuits without moving parts, while numerical or digital relays employ microprocessors to execute complex algorithms, combining multiple protection functions in a single device . 3. Trip Output Circuit: Once a fault is detected, the relay activates the trip circuit, which energizes the circuit breaker coil to isolate the faulty section. This circuit may include AC/DC power sources, relay contacts, and auxiliary devices to ensure reliable operation . The trip output is coordinated with upstream and downstream devices to prevent unnecessary outages and maintain system stability . 4. Auxiliary and Indication Circuits: Relays often include alarm, indication, and monitoring circuits to provide status information to operators. These circuits can trigger alarms, display fault locations, or log events for maintenance purposes . Modern digital relays also integrate communication interfaces for remote monitoring and control.
A typical relay protection device is housed in a compact cubicle or panel, which contains:
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