Power plant auxiliary system is usually pretty complicated and consists of dozen of motors, transformers, capacitor banks, variable frequency drives, PLCs, and other electrical devices. Therefore, protection relaying cannot be simple, and it requires special attention for each component of a power and control system as well as coordination between them.
Auxiliary system of a unit-connected generator
A portion of a typical auxiliary system of a unit-connected generator is shown in Figure 1 below.
The 4 kV auxiliary bus is fed directly from the 20 kV generator leads or from the startup transformer and is the source for the major motors. As unit sizes increase, the auxiliary load increases proportionately, requiring higher rated transformers and higher rated, higher voltage motors. This has resulted in higher bus voltages, such as 6.9 kV and 13 kV.
Phase fault currents also increased, requiring switchgear with higher interrupting capacity. In sizing the switchgear there are two contradictory factors that must be considered. The impedance of standard transformers increases as their ratings increase.
Since the normal and short-circuit currents are also increasing, there is a greater voltage drop between the auxiliary bus and the motor.
