These relays can operate for power flow in a specified direction. Directional power relay operates on the same principle as that of induction type watt-hour relay. The difference is that in the case of an induction type watt-hour relay the operating torque is produced by the interaction of magnetic fields given by the current in the circuit through CT.
But in a directional power relay, the operating torque is produced by the interaction of fields given by both voltage and current sources of the circuit it protects. The relay operates when the current exceeds a predetermined set value in a specified direction. Let us see the construction and working of induction type directional relay.
Construction of Induction Type Directional Relay :
The schematic diagram of the directional power relay is shown in the below figure. It consists of an aluminum (metallic) disc which rotates freely in between two electromagnets. The central limb of the upper electromagnet consists of winding called potential coil. The potential coil is connected to the system voltage through a potential transformer.
The lower electromagnetic has a separate winding called current coil which is magnetized proportional to operating current. It is connected in series with the line through a current transformer and is tapped at intervals. The tappings are connected to a plug setting bridge to get the required current setting of the relay which can be done by changing the number of turns of the current coil.
The torque is controlled by a spiral spring. A moving contact is present on the disc and lt links two fixed contacts when the disc rotates through a preset angle. This angle can be adjusted to any value (0-360°) and the moving contact path can be set. Hence the relay can be assigned to any time setting.
Working of Induction Type Directional Relay :
The two fluxes are produced by the two quantities for the production of torque. Let the two fluxes be Φ1 and Φ2. The current in the potential coil lags behind the applied voltage V nearly by 90°. Hence the flux Φ1 produced by the potential coil also lags behind the applied voltage.
While the flux Φ2 produced by the current coil will be in phase with the line current. The torque produced on the disc is due to the interaction of eddy currents with the flux imposed by the potential and current coils and it is called driving torque. The driving torque is given by,
From the above expression, it is clear that the relay connection for the power in the circuit decides the direction of driving torque produced on the disc of the relay. Under normal conditions i.e., when there is no reverse power flow in the circuit.
The driving torque and restraining torque (produced due to spring) on the disc will be in such a way that, the moving contact on the disc turns away from the fixed contact and it doesn't make the trip circuit close. Hence the relay does not operate.
However, when the operating current in the circuit reverses, it reverses the driving torque produced on the disc. The disc starts rotating in the reverse direction once the reversed driving torque becomes large enough. This causes moving contact to close the fixed contact i.e., the trip circuit closes and the trip signal to disconnect the circuit is initiated by the relay to the circuit breaker.