Depending upon the time of operation the overcurrent relays are categorized as :-
1. Instantaneous over-current relays
2. Inverse time-current relays
3. Inverse definite minimum time (IDMT) Over-current relay
4. Very inverse relay and
5. Extremely inverse relay
Now let us study the same one by one:-
1. Instantaneous Over-current relay:-
This relay as clear from it’s name is that it has instantaneous tripping without any delay. Time of operation of such relay is about 0.1 sec. This type of characteristic can be achieved by using hinged armature relays.
2. Inverse time-current relay:-
In that type of relays operating time reduces as the actuating quantity ,i.e. current In case of over-current ,increases in magnitude. More pronounced the effect more inverse will be the characteristics. They are normally more inverse near the pick up value of the actuating quantity and become less inverse as it is increased. This type of characteristic can be achieved with induction type relays by using suitable core which doesn’t saturate for large value of fault current. If saturation occurs at very early stage, the time of operation remains the same over the working range. Characteristic for the same in diagram as (a )
|Characteristics of Over current Relay|
3. Inverse definite minimum time (IDMT) Over-current relay:-
In that type of relay operating time is approximately inversely proportional to the fault current near pick up value and becomes substantially constant slightly above the pick up value of the relay as shown (b). This is achieved by using a core of the electromagnet which get saturated for current slightly greater than the pickup current.
4. Very inverse relay:-
In this relay saturation of the core occurs at later stage, characteristic is shown in (c ). The time current characteristic is inverse over greater range and after saturation rends to definite time.
5. Extremely inverse relay:-
In this relay saturation occur even at later stage. Same is shown in (d) curve. The equation describing the curve is K= I2t , where I is the operating current and t is operating time.