Current Graded Overcurrent Protection:

An alternative to time grading or in addition to time grading, current graded overcurrent protection can be applied when the impedance between two substations is sufficient. It is based on the fact that the short-circuit current along the length of the protected circuit decreases with the increase in distance between the supply end and the fault point. If the relays are set to operate at a progressively higher current towards the supply end then the drawback of long time delays occurring in graded time lag system can be partially overcome. This is known as current grading. Current graded overcurrent protection systems normally employ high-speed high-set overcurrent relays.

A simple current graded overcurrent protection scheme applied to a radial feeder is shown in Fig. 14.9. It consists of high-set overcurrent relays at A, B and C with settings such that relay at A would operate for faults between A and B, the relay at B for faults between B and C and the relay at C for faults beyond C. The current setting diminishes progressively from the supply end to the remote end of the line.

In practice, however, this protection scheme poses some difficulties which are given below:

1. The relay cannot differentiate between faults which are very close to, but are on each side of B i.e., if a fault is very near to station B in section BC, the relay at A may feel that it is in section AB because there may be very little difference in the fault currents and the relays do not discriminate between the fault in the next section and the end of first section. This is because:

• the difference in the fault currents would be extremely small,
• the magnitude of fault currents cannot be accurately determined, and
• the accuracy of the relays under transient conditions is likely to be different.

Hence for discrimination the relays are set to protect only part of the line, usually 80 %. For this reason current grading alone cannot be employed and this protection system should be supplemented by time graded inverse definite minimum time (IDMT) relay system.

2. The fault currents are different for different types of faults and so a certain difficulty is experienced in relay setting.

3. For ring mains, parallel feeders, interconnected systems, where power can flow to the fault from either direction, a system without directional control is not suited.