Question

The initial value of DC offset in a three-phase fault depends on:
1. The instantaneous value of fault current
2. Instantaneous source voltage at the instance of fault
3. Both (1) and (2)
4. Neither (1) nor (2)

Answer 1

Correct Answer - Option 2 : Instantaneous source voltage at the instance of fault

• AC circuit transient current is composed of symmetrical short circuit current (AC fault current) and DC offset.
• The value of DC offset (magnitude of the initial value of DC offset) can be between zero to the maximum value of AC fault current depending upon different values of (voltage phase angle)
• As the value of α is decided by the instantaneous source voltage at the instant of short circuit the value of DC offset is decided by the instantaneous source voltage at the instant of short circuit.