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Friday, 20 March 2015

Induction type over-current relays used in power system protection

Electromagnetic induction relays are the most widely used relays for the protection of primary distribution in the country. These relays operate on the principle of electromagnetic induction and therefore can be used on AC circuit only. Torque or the actuating force is produced in these relays when one alternating magnetic flux reacts with the current (eddy currents) induced in the rotor by another alternating flux displaced in time and space but having the same frequency.

Construction of Induction Relay:

Depending on the type of rotor whether a disc or a cup, the relay is known as an induction disc or an induction cup relay. 

In induction disc type of relays, disc is the moving element on which the moving contact of relay is fixed whereas in the case of induction cup the contact is fixed on the cup. 

There are two structures of the induction disc type of relay:

1.      Shaded pole structure, and
2.      Watt-hour meter structure.

Most of the induction relays are of watt-hour meter structure. The construction of this relay is similar to the watt-hour or the AC energy meter. It consists of two electromagnets. The upper electromagnet carries two windings; primary winding and the secondary winding. 

The advantage of this type of construction is that it can provide a larger phase angle between the two fluxes and hence a higher torque. An important feature of this type of relay is that its operation can be controlled by opening or closing the secondary winding. If the circuit is opened, no torque will be produced and thus the relay is made inoperative.

The primary winding has tapings at fixed intervals and these tapings are connected to a plug setting bridge. With the help of plug setting bridge, the number of turns can be adjusted and hence the desired current setting can be achieved.

The plug setting bridge usually provides 7 sections of tapings to give over-current range from 50% to 200% in steps of 25%. If the relay is required to respond to earth faults, the tapings are arranged to give a range from 10% to 70% in steps of 10%. It is so because the magnitude of earth fault current is usually low compared to the phase fault currents and the earth fault relay is set independent of load current. 

The values of each tap are expressed in terms of percentage of full load rating of C.T. It gives the value above which the disc commences to rotate and finally closes the trip circuit. Adjustment of current setting is made by inserting a pin between the spring loaded jaws of the bridge socket at the required tap. When the pin is withdrawn for changing the setting while the relay is in service, the relay automatically adopts higher settings, thus avoiding the open circuit condition in C.T. secondary. Figure 1 shows an Induction type Over-current relay.

Fig.1: Induction type Over-current relay.

The secondary winding of the upper electromagnet is energized by induction from the primary winding. It is connected in series with the winding on the lower magnet. An aluminum disc is placed between the poles of the two electromagnets. The spindle of the disc carries a moving contact. The disc rotates through an angle which is adjustable between 0 degree to 360 degrees. By adjusting this angle, the travel of the moving contact can be adjusted so that the relay can be given any desired time setting.

The time required to rotate the disc through a preset angle depends upon the torque. More the torque lesser will be the time required. So the relay has inverse time characteristics. The IDMT characteristics can be obtained by saturating the magnetic circuit of the upper electromagnet. Thus, there is practically no increase in the flux after the current reached a certain value and any further increase in current will not affect the relay operation.