INDUCTION TYPE DIRECTIONAL POWER RELAY
The step of relay operates when the, power in the circuit flows, in a specific direction. A directional power relay is so designed that it obtains its operating torque by the interaction of magnetic field derived from both voltage and current source of the circuit it protects. The direction of torque depends upon the current relative to voltage.
Constructional Details:- Figure shows the essential pails of a typical induction type directional power relay. It consists of an aluminum disc, which is free to rotate in between the poles of two electromagnet. The upper electromagnet carries a winding called potential coil on the central limb, which is connected through a potential transformer (PT.) to the circuit voltage source. The lower electromagnet has a separate winding called current coil connected to the secondary of CT. in the line to be protected.. The current coil is provided with a number of tappings connected to the plug setting bridge. This permits to have any desired current setting. The restraining torque is provided by a spiral spring. The spindle of the disc carries a moving contact which bridges two fixed contacts when the disc has rotated through a preset angle. By adjusting this angle, desired time setting can be obtained.
Operation:- The flux Ф1 due to current in the potential coil will be nearly 90° lagging behind the applied voltage V. The flux Ф2 due to current coil will be nearly in phase with the operating current I, as in the vector diagram. The interaction of fluxes Ф1 and Ф2 with the eddy currents induced in the disc produces a driving torque given by:
T α Ф1 Ф2 sin α.
Ф1 α V, Ф2 α I and α.= 90 – θ
α V I sin (90 – θ)
α V I cos θ
α Power in the circuit
It is clear, that the direction of driving torque on the disc depends on the direction of power flow in the circuit to which the relay is associated. When the power in the circuit flows in the normal direction the driving torque and the restraining torque help each other to turn away the moving contact from the fixed contacts. Thus the relay remains in operative. But with reversal of current in the circuit the direction of driving torque on the disc reverses. When the reversed driving torque is large enough, the disc rotates in reverse direction, and then the moving contact closes the trip circuit