Phase Overcurrent Directional Blocking

The direction of a phase overcurrent fault can be determined by the relationship between the phase current and phase voltage in the faulted phases.

The tripping direction for phase faults may be:

• Trip Forward

• Trip Reverse

• Trip Forward and Reverse (that is non directional)

In order to minimize the effect of reduced voltage in faulted phases, phase protection uses quadrature voltage for polarization.

However the quadrature voltage angle is compensated by 90° to offset the inherent 90° phase shift. This gives a fault angle that relates to the phase angle between the phase current and the phase voltage.

Example

Consider a single A phase fault where the A phase current lags the A phase – earth voltage by 45°,

The fault current would be IA and the polarising voltage would be the quadrature voltage V_B-C + 90°.

The fault angle would be (V_B-C + 90°) → I_A.

Assuming balanced voltages and ABC rotation with A phase at 0°, V_B-C would be at 270°.

Compensated V_B-C would be 360° or 0°.

Therefore the fault angle would be (360 → -45) = 315° = -45°.