It is noted that 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 selectively trip downstream overcurrent protection. Field forcing schemes are thus 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. In the present work, an analysis of standby system fault current is presented which shows that there are many variables to consider and that, except for the most unusual circumstances, self-excited generators do a very good job of supplying short-circuit currents and clearing faults. The specification of standby generators for hospitals is considered by way of illustration.
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