The directional power relay discussed above is unsuitable for use as a directional protective relay under short circuit conditions. When a short circuit occurs, the system voltage falls to a low value and there may be insufficient torque developed in the relay as the driving torque depends upon the system voltage, to cause its operation. This difficulty is overcome in the directional over-current relay which is designed to be independent of system voltage and power factor.
Figure 2.17 shows construction of a typical induction type directional over-current relay. It consists of two relay elements mounted on a common case viz. (i) Directional element and (ii) Non-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 & 2 of the directional element are connected in series with the secondary circuit of the overcurrent element. Therefore, the latter element can not start to operate until its secondary circuit is completed. In other words, the directional element must operate first i.e. contacts 1 & 2 should close in order to operate the over-current (non-directional) element.
It is an over-current or non-directional 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 operations of the 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 the overcurrent element and are connected to the bridge.
2.17 Directional over-current relay
Under normal operating conditions, power flows in the normal direction in the circuit protected by the relay. Therefore, directional power relay (upper magnet) does not operate, thereby keeping the overcurrent element (lower magnet) unenergized. However, when a short circuit occurs, there is a tendency for the current or power to flow in reverse direction.
Should this happen, the disc of the upper magnet element rotates as the directional element is made very sensitive so that with the lowest value of voltage is sufficient to produce torque to bridge the fixed contacts 1 and 2. This completes the circuit for the overcurrent 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.
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