Unit Protection Schemes
With the ever-increasing complexity of modern power systems the methods of protection so far described may not be adequate to afford proper discrimination, especially when the fault current flows in parallel paths. In unit schemes, protection is limited to one distinct part or element of the system that is disconnected if any internal fault occurs. The protected part should be tripped for an internal fault, but remain connected with the passage of current flowing into an external fault.
At the extremities of the zone to be protected, the currents are continuously compared and balanced by suitable relays. Provided that the currents flowing into and out of the zone are equal in magnitude and phase, no relay operation will occur. If, however, an internal fault (inside the protected zone) occurs, this balance will be disturbed (see Figure 11.24) and the relay will operate. The current transformers at
Figure 11.24 Circulating current, differential protection (one phase only shown), (a) Current distribution with through-fault-no current in relay, (b) Fault on line, unequal currents from current transformers and current flows in relay coil. Relay contacts close and trip circuit breakers at each end of the line
Figure 11.25 (a) Differential protection-circuit connections (one phase only)-relay with bias, (b) Characteristic of bias relay in differential protection. Operating current plotted against circulating or restraint current
the ends of each phase should have identical characteristics to ensure perfect balance on through-faults. Unfortunately, this is difficult to achieve and a restraint, or bias, is applied (see Figure 11.25) that carries a current proportional to the full system current and restrains the relay operation on large through-fault currents. The corresponding characteristic is shown in Figure 11.25b. This principle (circulating current) may be applied to generators, feeders, transformers, and busbars, and provides excellent selectivity. By suitable connections and current summation, teed or multi-ended circuits can be protected using the same principles.