loss of block loads and any overshoot resulting from
the recovery of a dip.
Cost Impact. If stringent transient-response
requirements are specified, the manufacturer may
select engine and generator models which have
nominal rating much larger than the service load;
may use an unnecessarily expensive governor; and may
use a higher inertia flywheel. The designer should
investigate what may actually be provided so that
the cost estimate will be reasonably accurate and to
confirm the selected transient requirements are not
unnecessarily stringent. A maximum size for the
engine-generator set may be needed to avoid the
problems associated with a small load on a large
capacity set.
The designer must determine the cost benefits of
providing an uninterruptible power system for
transient ride-through versus purchasing a generator
with stringent transient response requirements. In
determining the allowable voltage and frequency
variation and recovery times, analyze the effects on
equipment performance and recovery. Consult the
NEMA utilization equipment standards to determine
the maximum allowable voltage dips/overshoots
(excursions).
Maximum Voltage Deviation. Select the 5% Maximum
Voltage Deviation option only if communication
equipment or other sensitive electronic equipment
are a critical part of the load, and there is no UPS
provided. Fluorescent lights can tolerate a maximum
of 10% voltage variation. NEMA induction motors and
control relays can tolerate a maximum of 10%
variation, for 30 cycles and one cycle respectively.
Solenoids (brakes, valves, clutches) and ac & dc
starter coils can tolerate a maximum of minus 30%
variation, for 1/2 cycle, 2 cycles (dropout), and 5
- 10 cycles (dropout) respectively. (The times
listed in cycles are not given to define the
recovery time back to bandwidth, but to assist the
designer in defining the maximum allowable voltage
deviation.) The designer should realistically asses
the need for limiting the transient voltage dip to
less than 30%.
Maximum Frequency Deviation. Computers can usually
tolerate only + 0.5 Hz variation, so an UPS is
normally required where computer services should not
be interrupted, or where system recovery times are
critical. Inverters can tolerate + 2 Hz variation.
NEMA induction motors and control relays can
tolerate a maximum of 5% frequency variation. (The
times listed in cycles are not given to define the
recovery time back to bandwidth, but to assist the
designer in defining the maximum allowable frequency
deviation.) The designer must be realistic in
assessing the needs of the facility to be served so
SECTION 16263A
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