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Voltage phasors at the two terminals of a transmission line of length 70 km have a magnitude of 1.0 per unit but are 180 degrees out of phase. Assuming that the maximum load current in the line is 1/5th of minimum 3-phase fault current, which one of these transmission line protection schemes will NOT pick up for this condition?

1. (A) Distance protection using mho relays with zone-1 set to 80% of the line impedance
2. (B) Directional overcurrent protection set to pick up at 1.25 times the maximum load current
3. (C) Pilot relaying system with directional comparison scheme
4. (D) Pilot relaying system with segregated phase comparison scheme

Correct answer: (B) Directional overcurrent protection set to pick up at 1.25 times the maximum load current

Solution

The directional overcurrent protection set to pick up at 1.25 times the maximum load current will not activate because the current during the fault condition is significantly higher than the maximum load current, and the protection scheme is designed to respond only to currents above that threshold.

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