GENERATOR PROTECTION
The generator protection scheme includes the main connections and the windings of the generator transformer. There is an overlap between the protection arrangement of the generator and its transformer. Figure 25.3 lists the generator protective systems.
Stator Ground (Earth) Faults—Low-Impedance Grounding
The generator star point is grounded through a low-resistance liquid grounding (earthing) resistor, as shown in Fig. 25.4. The objective is to limit the fault current to 300 A for all ratings of generators. A single 300-A current transformer (CT) in the generator neutral line supplies two relays connected in series. These relays have an instantaneous and inverse time characteristic, respectively. The inverse time characteristic relay is set at 5 percent and the instantaneous relay at 10 percent. This method was replaced with the one described in the following section due to core burning events that occurred at a ground fault current up to 300 A.
Stator Ground Faults—High-Resistance Grounding
The modern practice for generators rated around 660 MW is to design the stator ground fault protection system for neutral currents of 10 to 15 A. The previous practice for design- ing for 2 to 3 A has led to problems in locating the ground faults which activated the protection system.
Figure 25.5 illustrates the ground fault protection system using resistance grounding through a distribution transformer. It includes two protection methods. Relay R1 is oper- ated by a current transformer and relay R2 by a voltage transformer (VT). Both methods use the same relay which consists of an induction disk with an adjustable inverse time/volt- age characteristic (Fig. 25.6). The interposing voltage transformer limits the maximum voltage across the relay during fault conditions to a value lower than its continuous voltage.
Stator Phase-to-Phase Faults
Phase-to-phase faults occur normally in the end windings. However, they can also occur in the slots if the slots contain windings of different phases. In this case, the phase-to-phase fault will cause a ground fault within a short time. The ground fault protection will clear the fault after a time delay due to the inverse time characteristic of the relay. The circulating current principle is used to provide fast protection against phase-to-phase faults. It relies on a differential protection using either biased relays or high-impedance relays.