1. When fault current can flow in both the directions through the relay, at its location. Therefore, it is necessary to make the relay respond for a particular defined direction, so that proper discrimination is possible. This can be achieved by introduction of directional control elements.
2. These are basically power measuring devices in which the system voltage is used as a reference for establishing the relative phase of the fault current.
Basically, an AC directional relay can recognize certain difference in phase angle between two quantities, just as a D.C. directional relay recognize difference in polarity
The polarizing quantity of a directional relay
1. It is the reference against which the phase angle of the other quantity is compared. Consequently the phase angle of the polarizing quantity must remain fixed when other quantity suffers wide change in phase angle.
2. The voltage is chosen as the “polarizing” quantity in the current-voltage induction type directional relay.
3. Four pole induction cup construction is normally used.
