Overcurrent relay is a sensing relay, which operates when the current increases beyond the operating value of the relay. Depending upon the time of operation, overcurrent relays may be categorized as instantaneous over current relay, inverse time overcurrent relay, definite time overcurrent relay, inverse definite time overcurrent relay, very inverse overcurrent relay and extremely inverse overcurrent relay. These relays are explained below in details.
Instantaneous Overcurrent Relay
In such type of relay, there is no intentional time delay is provided for operation. The relay contacts are closed immediately after the current in the relay coil exceeds the operating value. Although there will be a short time interval between the instant of pick up value and the closing of the relay contacts, no intentional time delay is provided.
This characteristic can be achieved with the help of the hinged armature relay. This 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 provided.
The most important considerations in overcurrent and overvoltage protection are 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 type of relay is used for controlling earth fault and other types of circulating current protection.
Inverse-Time Overcurrent Relay
In such type of relay, the operating current is approximately inversely proportional to the magnitude of the actuating quantity. The time-current characteristic of the inverse current relay is shown in the figure below. At values of current less than pick up value, the relay never operates. At, higher values, the operating time of the relay decrease regularly with the increase of the current. The more pronounced the effect is, the more inverse the characteristic is said to be.
In this relay, the operating time is inversely proportional to the fault current near pick-up value. The relay becomes considerably constant slightly above the pickup value of the relay as shown in the figure above. This is achieved by using a core of the electromagnet which gets saturated for current slightly greater than the pick-up current.
Very Inverse Relay
In such relay, the saturation of the current occurs at a still later stage shown in the figure above. This curve is known as very inverse characteristic curve. The time-current characteristic is inverse over a greater range and after saturation tends to the definite time. Relays with very inverse time characteristic are employed on feeders and long sub-transmission lines.
Extremely Inverse Relay
The extremely inverse relay characteristic curve is shown in the graph above. In this type of relay, the core saturation occurs at the very large stage. Such types of relays are quite suitable for the protection of transformer, cables, etc. This is because of their ability to ride through starting current and surges providing at the same time fast operation under relay fault conditions.
Relay with inverse time-current characteristic is 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.