EXPERIMENT 26

26.1Object: Study of Overcurrent Relay (I.D.M.T. Type) and Determination of the Time-Current Characteristic

26.2An overcurrent relay consists of an aluminum disc free to rotate between the poles on an electromagnet 

Current flowing through the operating coil produces flux in the air gap which is split into two out-of-phase fluxes by copper “shading rings” placed on part of the pole faces. If ϕ_s and ϕ_u are the fluxes due to shaded and unshaded portions of the pole, the torque on the disc is proportional to ϕ_s ϕ_u sin α

where α—angle by which ϕ_s lags ϕ_u.

Assuming that ϕ_s, and ϕ_u each are proportional to the current through operating coil torque I² sin α.

Figure 26.1

The direction of the torque depends upon which flux is leading the other.

By plug setting multipliers the effective number of turns on the operating coil and hence the pickup value of the relay can be changed.

The disc spindle carries a moving contact which bridges two fixed contacts. By means of time setting multipliers, the distance between moving and fixed contact and hence the operating time can be varied; a braking magnet is used to produce the time lag.

EXPERIMENTAL RESULTS

Figure 26.2

The ammeter range is 10 A. An avometer is used as a voltmeter, with a range of 10 V. An auto transformer is used for course adjustment, and a rheostat is used for fine setting.

(1)We make connections as shown in Figure 26.2. Find the pickup values and reset values of the relay for minimum and maximum plug setting. Also measure the voltage across the operating coil.

(2)To plot the time current characteristic, adjust the plug setting to its minimum. Bring the time dial setting to 0.6.

Allow the currents equal to 3.5, 4, 4.5 . . . 10 times of the pickup values to flow through the operating coil, and for each current note the operating time.

Repeat the same procedure for the time dial setting of B lag. Plot the time-current characteristic of the relay for a 0.6 and 0.8 time dial setting.

26.3Questions and Answers on the Experiment

Q1. What arrangement is provided in the relay to save the G.T. from being open circuited when the plug setting is changed?

Ans. The upper electromagnet has a primary and secondary winding. The primary is connected to the secondary of C.T. in the line to be protected and is provided with tapping. These tappings are connected to a plug setting bridge which is usually arranged to give seven selections of tapping. The overcurrent range is 507 to 2007 in steps of 25% of the element, which tends to operate the relay.

Figure 26.3

When the pin is withdrawn for the purpose of changing the setting while the relay is in service, the relay automatically adopts a high setting, thus ensuring that the C.T. secondary is not open circuited and that the relays remain operative for fault during the process of changing the setting.

Q2. Why it is preferred to take on the X axis of the time-current characteristic of the relay multiples of the pick-up value instead of the actual value of current in amps?

Ans. The curve time current characteristics are the inverse definite minimum time type, since the time of operation is approximately inversely proportional to the smaller values of current and tends to a definite minimum time as the current increases without limit. The multiple of the pickup value of the current is taken along the X-axis.

Since the actual value of the current in amperes will provide only the quantity of current and the current setting multiplier gives the number of times the relay current is in excess of the current setting. Therefore, the multiples of the pickup value current are preferable to the actual value of the current.