What is the primary function of a voltage differential relay in an electrical protection system?

The correct answer is: 4) It detects the difference in voltage between two or more points and trips the system if the voltage difference exceeds a set threshold.

Explanation:
The primary function of a voltage differential relay is to:

• Compare voltages at two or more points in the system (e.g., across a transformer winding, busbar, or transmission line).
• Detect imbalances (differences) in these voltages.
• Trip the circuit breaker if the voltage difference exceeds a predefined threshold, indicating a fault (e.g., short circuit, open circuit, or insulation failure).

Option Analysis
Phase angle differences in currents → This describes a phase comparison relay, not a voltage differential relay.

Impedance measurement → This is the function of an impedance relay (used in distance protection).

Current entering/leaving a zone → This describes a current differential relay, which operates based on Kirchhoff’s current law (not voltage).

Thus, the correct choice is 4).