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Differential relays compare currents entering and leaving a generator. Under normal conditions, identical current transformers keep the relay inactive since currents are equal. When an internal fault like a phase-to-ground short occurs, currents become unequal, creating differential current that flows through the relay's operating coil and triggers it to open the circuit breaker.

A balance beam relay is an electromechanical differential relay that operates by comparing electromagnetic forces. The relay trips when the force generated by the operating coil exceeds the restraining force. This force comparison method allows the relay to detect differential currents caused by internal faults while remaining stable during external faults.

No two current transformers are perfectly identical, so significant differential currents can occur during external faults even when primary currents are equal. This mismatch between transformer characteristics creates false differential signals. Relay design must account for these inherent transformer variations to prevent nuisance trips during external faults.

Bus differential protection uses one relay per phase. Each relay monitors the sum of currents entering and leaving the bus. Under normal conditions, this sum is zero. When an internal bus fault occurs, the sum becomes non-zero, triggering all three-phase circuit breakers connected to the bus to open simultaneously.

Transformers larger than 10 MVA require differential relays. Protection is complex due to different winding configurations and turn ratios. Current transformer connections must account for phase shifts in Y-Δ transformers by connecting CTs in Δ on the Y side and in Y on the Δ side, ensuring secondary currents remain in phase.

When an internal transformer fault occurs, the primary and secondary currents become unequal after adjustment by their current transformer ratios. This imbalance creates differential current flowing through the relay operating coil, causing the relay to trip and open the circuit breaker to isolate the faulted transformer.

Differential relays are one component of comprehensive system protection, working alongside other methods like radial system protection. They provide zone-based protection by comparing electrical quantities at specific points. This localized approach ensures faults are isolated quickly without affecting unrelated parts of the electrical system.