1. Current Transformers (CTs) and Voltage Transformers (VTs) These instrument transformers provide reduced, isolated signals from high-voltage or high-current equipment, allowing the relay to safely monitor electrical quantities such as current, voltage, phase angle, and frequency. The primary winding of the CT is connected in series with the line to be protected, while the secondary winding supplies the relay with a proportional current or voltage signal for measurement and decision-making . 2. Protective Relay The relay is the decision-making device that continuously monitors electrical quantities. It applies protection logic such as overcurrent, differential, distance, or ground fault detection. When abnormal conditions are detected, the relay closes its contacts to initiate the tripping process . 3. Tripping Circuit The tripping circuit connects the relay output to the circuit breaker. It includes a power source (AC or DC), the trip coil of the circuit breaker, and the stationary contacts of the relay. When the relay operates, it energizes the trip coil, causing the breaker to open and isolate the faulty section . 4. Circuit Breaker (CB) The circuit breaker is the interrupting device that physically opens the power circuit to clear the fault. It works in coordination with the relay but does not make protection decisions itself . 5. Auxiliary Components These may include auxiliary contacts, lockout relays, and supervision circuits to ensure reliable operation and provide alarms, event recording, or communication with other devices .
When a fault occurs, the CTs and VTs detect abnormal electrical quantities and send signals to the relay. The relay evaluates these signals against preset thresholds and, if necessary, closes its contacts to energize the tripping circuit. The trip coil of the circuit breaker is activated, opening the breaker and isolating the faulty section, thereby protecting equipment and maintaining system stability . This coordinated system ensures fast, selective, and reliable protection of electrical networks.
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