Directional overcurrent relay consists of two elements, viz- directional element and non-directional element.
Directional elements :
It is essentially a directional power relay. Which operates when the power flows in a specific direction.
The voltage coil of this element is connected through a potential transformer.
The current coil of the element is energized through a C.T by the circuit current. This winding is placed over the upper magnet of the non-directional element.
The trip contacts (a and b) of the directional element are connected in series with the secondary circuit of the overcurrent element, therefore the latter element cannot start to operate until its secondary circuit is completed.
In other words, the directional element must operate first in order to operate the overcurrent element.
Non-directional elements
The spindle of the disc of this dement carries a moving contact which closes the fixed contacts after the operation of the directional element.
It may be noted that a plug-setting bridge is also provided in the relay for the current setting.
The tapings are provided on the upper magnet of the over-current element and are connected to the bridge.
Operation :
Under normal operating conditions, power flows in the normal direction in the circuit. Therefore directional power relay does not operate and over current element remains unenergised.
However when a short circuit occurs, and if the current or power flows in the reverse direction, the disc of the upper element rotates to bridge the fixed contacts a and b.
This completes the circuit for the overcurrent element.
The disc of this element rotates and the moving contact attached to its closes the trip circuit.
This operates the circuit breaker, which isolates the faulty section.
The two-relay elements are so arranged that the final tripping of the current controlled by them is not made till the following conditions are satisfied.
1. Current flows in a direction such as to operate the directional element.
2. Current in the reverse direction exceeds the pre-set value.
3. Excessive current persists for a period corresponding to the time setting of the overcurrent element.