The truth about standby generator excitation support systems

Many who specify standby generators for critical applications assume that some type of excitation support system is required for self-excited generators. The rationale behind this thinking is that self-excited generators are not capable of producing enough short-circuit current to trip downstream overcurrent protection selectively. Field forcing schemes are used to produce 300% of rated current for at least 10 s to ensure that faults are cleared quickly enough and selectively enough to preserve the integrity of the power supply. This analysis of standby system fault currents shows the many variables to be considered and demonstrates that, except for the most unusual circumstances, self-excited generators do a good job of supplying short-circuit currents and clearing faults. The analysis is carried out in the context of a hospital