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 circuits and provide power to the remaining load. When the second generator is in synchronism,
it will be paralleled automatically with the first. After the generators are paralleled, power is
restored to all or part of the dumped loads. This system is the ultimate in automatic systems
requiring more complexity and cost than would be appropriate in most activity requirements.
(a) If one generator fails, it is immediately disconnected. A proportionate share of the
load is dumped to reduce the remaining load to within the capacity of the remaining generator.
When the failed generator is returned to operation, the dumped load is reconnected.
(b) When the normal source is restored, the load is transferred back to it and the
generators are automatically disconnected and shut down.
1.6.4.3 Peak Load Control System. With the peak load control system shown in Figure
1-13, idle standby generator sets can perform a secondary function by helping to supply power
for peak loads. Depending on the load requirements, this system starts one or more units to feed
peak loads while the utility service feeds the base loads.
1.6.4.4 Combined Utility-Generator Operation. The system shown in Figure 1-14
provides switching and control of utility and on-site power. Two on-site buses are provided, (1)
supply bus (primary) supplies continuous power for computer or other essential loads, and (2) an
emergency bus (secondary) supplies on-site generator power to emergency loads through an
automatic transfer device if the utility service fails.
In normal operation, one of the generators is selected to supply continuous power to the primary
bus (EG1 in Figure 1-14). Simplified semiautomatic synchronizing and paralleling controls
permit any of the idle generators to be started and paralleled with the running generator to
alternate generators without load interruption. Anticipatory failure circuits permit load transfer
to a new generator without load interruption. If the generator enters a critical failure mode,
however, transfer to a new generator is made automatically with load interruption.
1.6.5 Engine Generator Operation.
1.6.5.1 Governors and Regulation. Governors can operate in two modes, droop and
isochronous. With droop operation, the engine's speed is slightly higher at light loads than at
heavy loads, while an isochronous governor maintains the same steady speed at any load up to
full load:
speed regulation = (no-load rpm) - (full-load rpm) X 100%
(full-load rpm)
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