Directional relays are based on the flow of
Right Answer is:
Wherever power flows in both directions, for example, in the present-day grid system having sources normally at both ends of the transmission line, directional relays are needed. The directional relay is connected in series with the Overcurrent relay so that the trip command is only given if there is a fault and also flow of power due to the fault in the forward direction, i.e. away from the bus. The directional relay is supplied with the current as well as the voltage.
The directional feature can be achieved by comparing the direction of current flow in the line with reference to the bus voltage. In other words, the directional relay measures the phase angle between voltage and current vectors. This is why the directional relay is a two-quantity relay and a phase comparator relay, i.e., voltage and current both are fed to this relay.
Therefore, the test set for the directional relay has current and voltage outputs with a facility to adjust and measure: –
- The magnitude of the voltage and current i.e Power
- The phase angle between the current and voltage
The relay should operate only for the direction in which it has been set to operate and over a certain phase angle zone or range which is measured (within 0 to 360°). If the phase angle is out of this zone then the relay should not operate. Directional relays can respond to positive sequence, negative sequence, or zero sequence inputs. This should be a consideration when applying directional relays at locations where zero-sequence voltage is minimal.