What is the primary function of 'Percentage Differential Protection' in an alternator?

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RRB JE EE 22 Apr 2025 Shift 2 CBT 2 Official Paper

Answer 1

Percentage Differential Protection of an alternator

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It is a protection scheme primarily used to detect and protect against internal faults within the alternator windings.
It uses a "percentage" or "bias" principle to avoid tripping during external faults or minor CT errors, making it stable and selective.
It works by comparing the current entering and leaving the alternator windings, and operates only when there is a significant difference between them, indicating an internal fault.
A differential relay monitors the difference between these currents. If the alternator is healthy, I₁ ≈ and I₂, so no differential current flows → the relay does not trip.
If there's an internal fault, part of the current leaks to ground or between phases → I1 ≠ I2, producing a significant I_diff and relay trips.