Over current relays used in a power system

Over current relay is that relay which picks up when the current in the circuit exceeds the pick up value. Over current protection includes the protection from overloads as well as from short circuits. 

Depending upon the time of operation, over current relays are classified as:

i) Instantaneous over-current relay:

The over-current relay in which no intentional time delay is provided to pick up is known as instantaneous over-current relay. In these relays, 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 relay contacts. The point to note is that this time delay is not intentionally provided.

Such relays have the advantage of reduced time of operation and are useful in case of faults very close to the source. The time of operation of such relays is approximately 0.01 sec. The instantaneous relay is more effective where the impedance between the relay and the source is small compared to the impedance of the section to be protected.

ii) Inverse-time over-current relay:

In inverse time over-current relay the operating time of the relay reduces as the actuating quantity increases in magnitude. These relays are normally more inverse near the pick up value of the actuating quantity and becomes less inverse as the actuating quantity is increased as shown in figure 1. 

Fig.1: Relationship between Actuating Quantity & Operating Time

The operating time of all over-current relays tend to become asymptotic to a definite minimum value with the increase in the value of current. This is an inherent quality of electromagnetic relays and is due to saturation of the magnetic circuit. So by varying the point of saturation different characteristics such as definite time, inverse definite minimum time, very inverse, and extremely inverse, are obtained. These time-current characteristics are shown in the figure below.

If the core is made to saturate at a very early stage, the time of operation remains same over the working range i.e. is independent of the actuating quantity or current as shown in the figure.; and is called definite time characteristics. In the inverse definite minimum time relays, the operating time is approximately inversely proportional to the operating current near the pick up value and becomes substantially constant slightly above the pick up value (as shown by the moderately inverse and inverse curve of the figure). This characteristic can be obtained by using a magnetic circuit which gets saturated for currents slightly greater than the pick up current. 

For the very inverse and the extremely inverse relays, the saturation of core occurs at a still later stage, and the characteristics is known as very inverse and extremely inverse characteristics as shown in the figure above.
In the next post you will see Induction type Non-directional Over-current relay.