The generator CT's are often shared by relaying and switchboard metering so these CT's
are usually rated at about 25% more than rated full load current so that the meter deflections
will be less than full scale. This will not usually be possible if the transformer connects to a
double breaker arrangement, such as that found in a ring bus, breaker-and-a-half, or double
bus-double breaker arrangement. In these cases, the CT's may have to be rated according to
the normal current through the two breakers, which may be greater than the generator rated
current.
The generator CT's are subject to CT errors due to manufacturing tolerances and differences
in secondary burden. It is recommended that identical units be used in the generator
relaying, if possible, to minimize CT ratio errors.
The arrangement shown in Figure 18.1 is common for large generators, where the generator
and generator step-up (GSU) transformer operate as a unit and are protected as a unit.
The CT's used for the transformer protection should be chosen using the methods discussed
in Chapter 17. The question arises regarding the effect of magnetizing inrush current on this
transformer protection and the need for harmonic or other restraint. In normal operation, the
transformer is brought up to full voltage gradually, as the generator is brought from turning
gear to full speed. Therefore, at startup, the generator step-up transformer is never switched
on suddenly.
There is the possibility of magnetizing inrush in the GSU transformer following a system
fault near the generator that is cleared by opening and then reclosing line breakers. This is
analyzed by considering the three-terminal equivalent of the transformer, as shown in Figure
18.2.