Question

When do we need directional overcurrent protection?

Answer 1

If the fault current is larger than theovercurrent setting, the relay will trip.Directional overcurrent protectionschemes were replaced with line impedance relays (21) to prevent a situation like this from occurring.

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Answer 2

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The fault current flows into the polarity mark of the CT connected to Circuit Breaker 3, so the Directional Overcurrent (67) relay sees the fault in the forward direction. If the fault current is larger than the overcurrent setting, the relay will trip.