Induction Type Directional Over current Relay:

In order to make directional over current relay to operate to a fault flowing in a specified direction, directional element is added to non directional relay.Directional Overcurrent Relay which is designed to be almost independent of system voltage and power factor

Constructional details: Fig. 21.19 shows the constructional details of a typical Induction Type Directional Overcurrent Relay. It consists of two relay elements mounted on a common case viz.

1. Directional element and
2. Non-directional element.

1. Directional element: It is essentially a directional power relay which operates when power flows in a specific direction. The potential coil of this element is connected through a potential transformer (P.T.) to the system voltage. The current coil of the element is energised through a C.T. by the circuit current. This winding is carried over the upper magnet of the non-directional element. The trip contacts (1 and 2) 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 (i.e. con­tacts I and 2 should close) in order to operate the overcurrent element.

2. Non-directional element: It is an over current element similar in all respects to a non-direc­tional over current relay . The spindle of the disc of this element carries a moving contact which closes the fixed contacts (trip circuit contacts) after the operation of direc­tional element.

It may be noted that plug-setting bridge is also provided in the relay for current setting but has been omitted in the figure for clarity and simplicity. The tappings are provided on the upper magnet of over current element and are connected to the bridge.

Operation: Under normal operating conditions, power flows in the normal direction in the circuit protected by the relay. Therefore, Induction Type Directional Over current Relay (upper element) does not operate, thereby keeping the over current element (lower element) unenergised. However, when a short-circuit occurs, there is a tendency for the current or power to flow in the reverse direction. Should this happen, the disc of the upper element rotates to bridge the fixed contacts 1 and 2. This completes the circuit for over current element.

The disc of this element rotates and the moving contact attached to it closes the trip circuit. This operates the circuit breaker which isolates the faulty section.