Explain construction and operation of induction type directional over current relay.

1 Answer

Answer :

Directional overcurrent relay:

Construction: It consists of two units: i) Non-directional Over-current unit ii) Directional power unit


The directional unit consists of upper electromagnet excited from potential transformer (PT), so it produces flux proportional to voltage. The lower electromagnet is excited from current transformer (CT), so it produces flux proportional to current. These two electromagnets are placed at the edge of aluminium disc. Eddy currents are produced in the disc and their interaction with the fluxes causes torque to rotate the disc. The coils on electromagnets are so placed that the torque is produced only for a particular direction of power flow. If the power flow is reversed, there will not be torque and so no rotation of the disc. The disc carries an arm to close the contacts when it rotates through some set angle.

The non-directional over-current unit also has upper and lower electromagnets. The upper electromagnet has two windings: primary is excited from CT as shown in the figure, and the secondary winding is connected to coil on lower electromagnet through contacts of directional unit, as shown in the figure. Thus when directional unit contacts remain open, the over-current unit remains unenergized.

Operation: Under normal operating conditions, power flows in the normal direction in the circuit. For this direction of power flow, the directional power relay does not operate and overcurrent element remains unenergized. However, when a short circuit occurs, and the current or the power flows in the reverse direction, the disc of the directional unit rotates to bridge the fixed contacts 1 & 2. This completes the circuit for over current element. The disc of this element rotates and moving contact attached to it closes the trip circuit. This operates circuit breaker which isolates the faulty section. 

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