Over-Current Protection Schemes
This basic method is widely used in distribution networks and as a back-up in transmission systems. It is applied to generators, transformers and feeders. The arrangement of the components is shown in Figure 11.20. In the past the relay normally
Figure 11.20 Circuit diagram of simple overcurrent protection scheme, (a) CTs in star connection, (b) Phasor diagram of relay currents, star connection, (c) CTs in delta connection
Figure 11.21 Application of overcurrent relays to feeder protection
employed was the induction-disc type (Figure 11.14), but numerical over-current relays are now used.
On an underground cable feeder, fault currents only reduce slightly as the position of the fault moves further from the in-feed point of supply. The impedance of the feeder is small compared with that of the source. Therefore it is necessary to use different operating times to provide discrimination. Grading of relays across transformers, which introduce a large impedance in the circuit relies more on the variations in fault current.
A simple underground cable feeder is shown in Figure 11.21. Assume that the distribution network has slow-acting circuit breakers operating in 0.3 s and the relays have true inverse-law characteristics. The relay operating times are graded to ensure that only that portion of the feeder remote from the in-feed side of a fault is disconnected. The operating times of the protection with a fault current equal to 200% of full load are shown.
Selectivity is obtained with a through-fault of 200% full load, with the fault between D and E as illustrated because the time difference between relay operations is greater than 0.3 s. Relay D operates in 0.5 s and its circuit breaker trips in 0.8 s. The fault current ceases to flow (normal-load current is ignored for simplicity) and the remaining relays do not close their contacts. Consider, however, the situation when the fault current is 800% of full load. The relay operating times are now: A, 0.5 s [i.e. 2 x (200/800)]; B, 0.375 s [i.e. 1.5 x (200/800)]; C, 0.25 s; D, 0.125 s; and the time for the breaker at D to open is 0.125 + 0.3 = 0.425 s. By this time, relays B and C will have operated and selectivity is not obtained. This problem can only be addressed by extending the settings on relays A to C. This illustrates the fundamental drawback of this system, that is for correct discrimination to be obtained the times of operation close to the supply point become large.
Directional Over-Current Protection Schemes
To obtain discrimination in a loop or networked system, relays with an added directional property are required. For the system shown in Figure 11.22, directional and non-directional over-current relays have time lags for a given fault current as shown. Current feeds into fault at the location indicated from both directions, and the first relay to operate is at B (0.6 s). The fault is now fed along route ACB only,
Figure 11.22 Application of directional overcurrent relays to a loop network. $ Relay responsive to current flow in both directions; ! relay responsive to current flow in direction of arrow
and next the relay at C (1 s) operates and completely isolates the fault from the system. Assuming a circuit-breaker clearance time of 0.3 s, complete selectivity is obtained at any fault position. Note, however, that directional relays require a voltage input, non-directional over-current relays do not.