8.9.5 Generator station design
The most common configuration used presently for large fossil-fired generating plants is a unitconnected
system. In this system, the generator, generator stepup transformer (GSU), the unit
auxiliary transformer(s) (UAT) and the reserve or startup transformer (RAT) are all connected as a
unit and their protection and controls are interrelated.
Figure 8.21 shows a typical design with the associated zones of protection. Since our concern
here involves only the transformers of this design we will only touch upon the protection and control
aspects of the other equipment. They are discussed in more detail in the appropriate chapters. The
GSU is usually included in the unit and overall differentials (87U and 870A). Since these see
transformer inrush they must be harmonic restrained relays. It is possible, but not common, to
use an internal transformer differential (87Tr) connected to the transformer bushing CTs. If this is
done, then high- and low-voltage lead differentials should be added. If the leads are very long, i.e.
beyond 460–610 m, then pilot-wire protection should be considered. The UAT has its own internal
differential and is not included in the unit or overall differential. However, since there are no highside
breakers to clear a UAT fault, the generator must be tripped for such faults. For sensitivity in
setting the two sets of differential relays, and to identify the fault location, the auxiliary transformer
current is subtracted from the unit and overall transformer differential circuits.
Since the GSU and UAT are connected to the generator as it is coming up to speed prior to
synchronizing to the system, they will be energized at frequencies lower than the normal 60 or
50 Hz. Transformers are designed to operate at a given flux level and flux is related to both voltage
and frequency. It is therefore necessary to provide volts/hertz relays to detect the condition either
of excessive voltage, which is possible during startup if the excitation is not monitored, or low
frequency as the unit is starting.