Large generators are invariably connected to their own step-up transformer and the protective scheme usually covers both items. A typical scheme is shown in Figure 11.26, in which separate differential circulating-current protection schemes are used to cover the generator alone and the generator plus transformer. When differential protection is applied to a transformer the current transformer on each side of a winding must have ratios which give identical secondary currents.
In many countries the generator neutral is often grounded through a distribution transformer. This energizes a relay which operates the generator main and field breakers when a ground fault occurs in the generator or transformer. The ground fault is usually limited to about 10 A by the distribution transformer or a resistor, although an inductor has some advantages. The field circuit of the generator must be opened when the differential protection operates in order to avoid the machine feeding the fault.
Figure 11.26 Protection scheme for a generator and unit transformer
The relays of the differential protection on the stator windings (see Figure 11.27) are set to operate at about 10-15% of the circulating current produced by full-load current in order to avoid current-transformer errors. If the phase e.m.f. generated by the winding is E, the minimum current for a ground fault at the star-point end, and hence with the whole winding in circuit, is E/R, where R is the neutral effective resistance. For a fault at a fraction x along the winding from the neutral
Figure 11.27 Generator winding faults and differential protection, (a) Phase-to phase fault, (b) Intertum fault, (c) Phase-to-earth fault
(Figure 11.27(c)), the fault current is xE/R and 10-15% of the winding is unprotected. With the neutral grounded via the transformer, R is high and earth faults are detected by a sensitive relay across the transformer secondary. With an interturn fault (turn-to-turn short circuit) on a phase of the stator winding (Figure 11.27(b)), current balance at the ends is retained and no operation of the differential relay takes place, the relays operate only with phase-to-phase and ground faults.
On unbalanced loads or faults the negative-sequence currents in the generator produce excessive heating on the rotor surface and generally (l2,t) must be limited to a certain value for a given machine (between 3 and 4 (p.u.)2 s for 500 MW machines), where t is the duration of the fault in seconds. To ensure this happens, a relay is installed which detects negative-sequence current and trips the generator main breakers when a set threshold is exceeded.
When loss of excitation occurs, reactive power (Q) flows into the machine, and if the system is able to supply this, the machine will operate as an induction generator, still supplying power to the network. The generator output will oscillate slightly as it attempts to lock into synchronism. Relays are connected to isolate the machine when a loss of field occurs, which can be readily detected by a reactance relay.