Determining Direction

The user must set a Characteristic Angle for each element. The Characteristic Angle is essentially a prediction of the expected fault angle when a forward fault occurs. The fault angle is defined according to the polarising method for each element and is effectively the phasor relationship between the fault current and the polarising voltage. A forward fault is one where the fault angle is within ± 90° of the Characteristic Angle.

For any given fault, the actual fault angle will be determined by network characteristics such as line resistance and reactance as well as fault characteristics such as the nature of the short circuit, solid, or arcing etc.

It is not necessary that the fault angle matches the Characteristic Angle precisely. There are only two possibilities for direction, forward and reverse. As long as the fault angle is at the Characteristic Angle ± 90° it will be recognized as a forward fault. Otherwise it is a reverse fault.

Each element takes 20 ms to determine the direction of a fault once it has been detected. This delay occurs prior to the trip timing and adds to the actual time to trip.

The figure below demonstrates a Characteristic Angle of +45° with phasors representing possible fault angles on either side. Both of these fault angles represent forward faults as they are well within the forward fault region
(135° to -45°).