Depending upon the time of operation, overcurrent relays may be categorized as: 1. Instantaneous Overcurrent Relay 2. Inverse-Time Overcurrent Relay 3. Definite Time Overcurrent Relay 4. Inverse Definite Minimum Time (IDMT) Relays 5. Very Inverse Relay 6. Extremely Inverse Relay.
1. Instantaneous Overcurrent Relay:
An instantaneous overcurrent relay is one in which no intentional time delay is provided for operation. In such a relay, the relay contacts close immediately after the current in the relay coil exceeds that for which it is set. Although there will be a short time interval between the instant of pick-up and the closing of the relay contacts, no intentional time delay is provided. This characteristic can be achieved with the help of hinged armature relays. Such relay has a unique advantage of reducing the time of operation to a minimum for faults very close to the source where the fault current is the greatest. The instantaneous relay is effective only where the impedance between the relay and source is small compared with the impedance of the section to be protected.
One of the most important considerations in overcurrent and overvoltage protection is the speed of operation. With hinged armature relays, the time of operation of 0.01 second at three times the setting can be obtained. Such relays are employed for restricted earth-fault and other types of circulating current protection. With so fast an operation it is likely that the relay may operate on transients beyond the normal range of setting.
2. Inverse-Time Overcurrent Relay:
An inverse time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity.
Below given figure illustrates that the time-current characteristics of an inverse-current relay. At values of current less than pick-up value, the relay never operates:
At higher values, the operating time of the relay decreases steadily with the increase of current. The more pronounced the effect is the more inverse the characteristic is said to be. In fact, all time-current curves are inverse to a greater or lesser degree. They are normally more inverse near the pick-up value of the actuating quantity and become less inverse as it is increased.
The operating time of all overcurrent relays tends to become asymptotic to a definite minimum value with increase in the value of actuating quantity. This is inherent in electromagnetic relays due to saturation of the magnetic circuit. So by varying the point of saturation different characteristics are obtained.
(i) Definite time
(ii) Inverse definite minimum time
(iii) Very inverse and
(iv) Extremely inverse
These characteristics can be obtained by induction disc and induction cup relays.
3. Definite Time Overcurrent Relay:
If the core is made to saturate at a very early stage, the time of operation remains same over the working range. This characteristic is shown by curve I in Fig. 3.24 and is known as definite time characteristic. Such a relay operates after a specified time irrespective of the magnitude of the fault current.
The definite-time relays are used in:
(i) Radial or loop circuits having a few sections
(ii) As backup protection for other types of protection and
(iii) On systems with wide variations of fault current due to source impedance.
Selectivity amongst such relays is obtained if there is difference of 0.5 s in the time settings of the two successive relays.
4. Inverse Definite Minimum Time (IDMT) Relays:
Such a relay is one in which operating time is approximately inversely proportional to fault current near pick-up value and becomes substantially constant slightly above the pick-up value of the relay, as illustrated by curve II in Fig. 3.24. This is achieved by using a core of the electromagnet which gets saturated for currents slightly greater than the pick-up current.
5. Very Inverse Relay:
In such a relay the saturation of the core occurs at a still later stage, as illustrated by curve III in Fig. 3.24. This curve is known as very inverse characteristic curve. The time-current characteristic is inverse over a greater range and after saturation tends to definite time. Relays with very inverse time-current characteristics are employed on feeders and long sub-transmission lines.
6. Extremely Inverse Relay:
The curve IV in Fig. 3.24 illustrates extremely inverse characteristic i.e., core saturation occurs at a very late stage. The equation describing the curve IV in the figure is approximately of the form 12t = K where I is the operating current and I is the operating time. Such relays are quite suitable for the protection of transformers, cables etc., as it is possible to achieve accurate discrimination with fuses and auto-reclosures in their case, which can seldom be made selective with standard IDMT relays. This is because of their ability to ride through starting currents and surges, providing at the same time fast operation under fault conditions. They are, thus more suitable for installations with large inrush currents after an outage.
Relays with inverse time-current characteristics are widely employed in distribution networks and industrial plant systems. Their relatively flat time-current characteristic permits them to achieve reasonably fast operation over a wide range of short-circuit currents.