What does differential protection compare to decide if a protected element should trip?

What does differential protection compare to decide if a protected element should trip?

• A. Currents entering and leaving the element
• B. Voltages at the ends of the element
• C. Temperatures of conductors
• D. Frequency at the substation

Answer: (A)

Differential protection hinges on current balance at both ends of the protected element. In normal operation, the current flowing into the element equals the current flowing out (accounting for CT ratios), so there’s no significant differential signal and the relay stays idle. If a fault occurs inside the protected zone, some current is diverted through the fault path and does not return through the other end, creating an imbalance between the two ends. The protection relay senses this difference and, if it exceeds its threshold, trips to isolate the fault. This approach is fast and selective because it directly monitors the actual current through the protected path. Voltages at the ends don’t reliably indicate internal faults, temperatures rise too slowly to indicate a fault quickly, and frequency at the substation reflects broader system conditions rather than a specific internal fault.

Differential protection hinges on current balance at both ends of the protected element. In normal operation, the current flowing into the element equals the current flowing out (accounting for CT ratios), so there’s no significant differential signal and the relay stays idle. If a fault occurs inside the protected zone, some current is diverted through the fault path and does not return through the other end, creating an imbalance between the two ends. The protection relay senses this difference and, if it exceeds its threshold, trips to isolate the fault.

This approach is fast and selective because it directly monitors the actual current through the protected path. Voltages at the ends don’t reliably indicate internal faults, temperatures rise too slowly to indicate a fault quickly, and frequency at the substation reflects broader system conditions rather than a specific internal fault.