An induction type overcurrent relay giving inverse-time operation with a definite minimum time characteristic is shown in Fig. 3.25. It consists essentially of an ac energy meter mechanism with slight modification to give required characteristics. The relay has two electromagnets. The upper electromagnet has two windings, one of these is primary and is connected to the secondary of a CT in the line to be protected and is tapped at intervals.
The tappings are connected to a plug setting bridge by which the number of turns in use can be adjusted, thereby giving the desired current setting. The plug bridge is usually arranged to give seven sections of tappings to give overcurrent range from 50% to 200% in steps of 25%. If the relay is required to response for earth fault the steps are arranged to give a range from 10 % to 70 % or 20 to 80% in steps of 10%. The values assigned to each tap are expressed in terms of percentage of full-load rating of CT with which the relay is associated and represents the value above which the disc commences to rotate and finally closes the trip circuit.
Thus pick-up current equals the rated secondary current of CT multiplied by current setting. For example suppose that an overcurrent relay having a current setting of 150% is connected to a supply circuit through a CT of 500/5 A. The rated secondary current of CT is 5 A and, therefore, the pick-up value will be 1.5 x 5 i.e., 7.5 A. It means that with above current setting, the relay will actually operate for a relay current equal to or greater than 7.5 A.
ADVERTISEMENTS:
Similarly for current settings of 50,100 and 200% the relay will operate for relay currents of 2.5 A, 5 A and 10 A respectively. Adjustment of current setting is made by inserting a pin between the spring loaded jaws of the bridge socket at the tap value required. When the pin is withdrawn for the purpose of changing the setting value while the relay in service, the relay automatically adopts higher setting, thus the CT’s secondary is not open-circuited.
The second winding is energized by induction from the primary, and is connected in series with the winding on the lower magnet. By this arrangement, leakage fluxes of upper and lower electromagnets are sufficiently displaced in space and phase to set up a rotational torque on the aluminium disc suspended between the two magnets, as in the shaded pole induction disc motor. This torque is controlled by the spiral spring and also sometimes by a permanent magnet brake on the disc. The torque is given by the expression –
T = K1I2rms – K2
Where, Irms is the current through the coil and K2 is the restraining torque of the spring. The disc spindle carries a moving contact which bridges two fixed contacts (trip circuit contacts) when the disc has rotated through a preset angle. The angle can be set to any value between 0° and 360° and thereby giving desired time setting. This adjustment is known as time-setting multiplier. Time multiplier setting is generally in the form of an adjustable back-stop which decides the arc length through which the disc travels, by reducing the length of travel, the operating time is reduced.
ADVERTISEMENTS:
The time setting multiplier is calibrated from 0 to 1 in steps of 0.05. These figures do not represent the actual operating times but are multipliers to be used to convert the time known from the relay name plate curve (time-PSM curve) into the actual operating time. Thus if time setting is 0.2 and the operating time obtained from the time-PSM curve of the relay is 5 seconds, then actual operating time of the relay will be equal to 0.2 x 5 i.e., 1 second.
Since the time required to rotate the disc through a preset angle depends upon the torque which varies as the current in the primary circuit, therefore, more the torque lesser will be the time required. So the relay has inverse-time characteristic.
In more recent designs the definite minimum time characteristic is obtained by saturating iron in the upper electromagnet so that there is practically no increase in flux after the current has reached a certain value and any further increase in current will not affect the relay operation.
The ratio of reset to pick-up is inherently high in induction relays because their operation does not involve any change in the air gap. It lies between 95% and 100%.
ADVERTISEMENTS:
Current-Time Characteristics of an Overcurrent Induction Disc Relay:
A set of typical time-current characteristics of the above relay is given in Fig. 3.26. The horizontal scale is marked in terms of plug-setting multiplier and represents the number of times the relay current is in excess of the current setting. The vertical scale is marked in terms of the time required for relay-operation.
The abscissa is taken as multiple of pick-up value so that the same curves can be used for any value of pick-up i.e., if the curves are known for pick-up value of 5 A, then the characteristics remain same for 2.5 A, 6.25 A, 7.5 A, 10 A or any other pick-up value. This is possible with induction type relays where the pick-up adjustment is by coil, because the ampere-turns at pick-up are the same for each tap and hence at a given multiple of pick-up, the coil ampere-turns, and hence the torque are the same regardless of the tap used.
ADVERTISEMENTS:
These curves are normally plotted on log-log graph papers as illustrated in the figure. The advantages of plotting the curves on log-log sheets is that if the characteristic for one particular pick-up value and one time multiplier setting is known, then the characteristics for any other pickup value and time multiplier settings can be obtained.
Theoretically the time ordinates of these curves should be in proportion to the time multiplier setting so that, if the times for a given current are divided by the TMS all the curves should coincide. Owing to inertia of the disc which takes a little time for the disc to accelerate from standstill to its steady speed at low values of current they may not exactly coincide. It introduces some error in time which might affect the discrimination of the whole scheme.
The curves are used to estimate not only the operating time of the relay for a given multiple of pick-up value and time multiplier setting but also it is possible to know how far the relay moving contact would have travelled towards the fixed contacts within any time interval.
This method is also useful in finding out whether the relay will pick-up and how long it will take for the relay operation when the actuating quantity is changing as for example during the inrush current period of starting a motor etc.
ADVERTISEMENTS:
Current-time characteristics of an IDMT relay are given below:
Setting of Induction Relays:
The setting for a phase-fault relay is usually of the order of 150-200% of the full-load current. An induction relay will not operate at a current equal to or less than its setting and minimum operating current of the relay must not exceed 130% of the setting.
The setting of the earth fault relays should be in the range of 20 to 80%. An earth fault relay is subject to mal-operation if its setting is too low. This mal-operation can be due to unbalance in load currents, unbalance in output of CTs, switching surges or saturation of CTs during phase faults. As such, settings lower than 20% are not recommended. Also, the more sensitive an earth fault relay, the greater will be the chances of its mal-operation.
A time interval of 0.4 to 0.5 second may be allowed in the time settings of two adjacent induction relays for proper selectivity. Shorter interval of 0.35 second may be used with very inverse overcurrent relays.
Whenever a protective system comprises of a series of overcurrent relays with graded time settings, it is imperative that the current-time characteristics of all such relays must flatten out in the same fashion so that a flawless relay operation is ensured howsoever severe may be the nature of the fault.
Determination of Relay Operating Time:
For determination of actual operating time of a relay, the data required is:
(i) Time-PSM curve
(ii) Current setting
(iii) Time setting
(iv) Fault current
(v) CT ratio.
The actual operating time of a relay is determined by following the steps given below:
(i) Determination of relay current from fault current If and CT ratio x: y from the expression–
Relay current, IR = If × (y/x) … (3.8)
(ii) Determination of relay current setting multiplier (i.e., PSM) which is given as –
where Ip is the per cent current setting of the relay
(iii) Determination of operating time of relay corresponding to calculated PSM from Time-PSM curve.
(iv) Determination of actual operating time of relay by multiplying the time obtained in step (iii) by the time-setting multiplier in use.