Directional Overcurrent Protection Principle:

For the main bus-bars in the power stations, due to their importance in the operating conditions, it is required that the disconnection be without any delay in the case of faults. Hence it is imperative to use a differential current protection without time delay.

The Directional Overcurrent Protection is based on simple circulating current principle that under normal operating conditions or under external fault conditions the sum of currents entering into a bus-bar will be equal to the sum of currents leaving the bus-bar. In case the sum of these currents (for a given conductor) is not zero, it must be due to a short circuit either a ground fault or phase-to-phase fault. Hence this directional overcurrent protection scheme is applicable to both types of faults i.e., phase-to-phase faults as well as ground faults.

Figure 13.12(a) shows the application of differential circulating current principle to a bus with four circuits. The CTs are inserted in each phase of the incoming and outgoing feeders of the bus-bar and the secondaries are connected in parallel with due considerations to polarity and phase and the relay operating coil is connected across the pilot wires in such a way that the summation current of secondaries flows through it. All the CTs must of the same ratio, regardless of the capacities of various circuits. Flow of current in the relay is an indication of a fault within the protected zone and will initiate opening of the breakers of each generator and feeder.

Drawbacks:

The main drawback of differential overcurrent protection is the difference in the magnetic conditions of the iron-cored CTs which may cause false operation of the relay at the time of an external fault. Even with identical CTs having large iron cores to avoid the saturation with maximum fault currents the dc transient component creates problem due to its slow decay. Biasing of differential relays improves the stability considerably but does not solve the problem completely.

Better discrimination between internal and external faults can be had if high impedance bus differential relay is used in place of usual low impedance relay. High impedance relay is an overcurrent relay with a series resistance. Such a relay remains stable against spill currents due to external faults or CTs inaccuracies.

Another method of protecting bus-bar sections is by means of voltage differential protection, which overcomes the difficulties of iron-cored CTs. In this scheme CTs without iron cores, known as linear couplers are employed so that they have a much larger number of secondary turns than an iron-core CT. In this scheme secondary windings of CTs are connected together in series and the differential relay coil connected across them, as illustrated in Fig. 13.12 (b). Under normal operating conditions or under external fault conditions, the sum of voltages induced (proportional to the primary currents) in the secondary windings is zero but in the event of an internal fault on the bus-bar, the voltages of the CTs in all source circuits add to cause the flow of current through the secondary windings and the differential relay operating coil.

This scheme provides high speed protection for a relatively small net voltage in the differential circuit.