Self Balance Protection System of Generator:

It necessarily consists of two cables connected to the two ends of each phase and the cables are passed through the circular aperture of the ring type CTs, as shown in Fig. 11.6. Under normal operating conditions the current flowing through two cables in the central aperture of the CT will be in opposite directions and so there will be no magnetization of the CT. In case there is an earth fault on any phase, the fault current passes only once through the circular ring of the CT of that particular phase. This Self Balance Protection System of Generator will set up magnetic flux and so induce an emf in the relay circuit which will trip the circuit breaker. This system is also effective for phase-to-phase faults but not useful for protection against fault between the turns of the same phase for ordinary generator winding. In case the winding of each phase is designed in two parallel paths such a protection will be useful.

Self Balance Protection System of Generator

This Self Balance Protection System of Generator is very sensitive type of earth fault protection scheme but it has got some drawbacks, as enumerated below:

1. Under heavy short circuits large electromagnetic forces are developed in the CTs ring.

2. A different layout of the cable leads is required in this scheme.

3. For protection of the lead cable from the circuit breaker to stator terminal the ring CTs are to be placed near the circuit breaker which is again quite cumbersome. However, this difficulty can be overcome by providing a metallic sheath around the cable (but insulated from it) and earthing it in a manner illustrated in Fig. 11.7. In case the lead punctures anywhere, there will be flow of current through the sheet, as illustrated in the figure and relay will be energized resulting in tripping of the circuit breaker. In Fig. 11.7 connections for only one phase are shown.

Self Balance Protection System of Generator

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