Page 98 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
96 Chapter Three

cumulative load seen by the generator is 466 kWe when motor A is
accelerating, which will require 496 kWm from the engine. It follows
that the maximum or prime power required from the engine will be 496
kWm, and the continuous power will be 405 kWm, or 81.6 percent of
the maximum. The average power over a 24-h period will depend on the
duration of the initial sequence and will be a little over 81.6 percent.
The maximum step load applied to the generator is 240 kWe which
will require 255 kWm from the engine. This is 51.5 percent of the max-
imum rating, an acceptable figure since most engines will accept a
60-percent step load.
For some applications a simpler table will suffice. Table 3.1 includes
some redundant information, which may, however, be useful as it assists
in checking the arithmetic used in compiling the table. The full informa-
tion also enables the reader to see all the changes that occur during the
loading sequence.
Unless a safety margin has been allowed in the loads given, it would
be wise to increase these ratings by say 10 percent. ISO 8528 does not
allow for any overload rating, whereas ISO 3046 requires engines to be
capable of supplying 10 percent overload for 1 h.

The Generator Rating
The continuous load is 457 kVA.
The maximum step load is 597 kVA when motor B is started.
If a generator rated at 500 kVA is used the volt drop on application of
motor B would be:
Drop (597 500) 11% 13.2%

To restrict the volt drop to 12 percent would require a rating of:

500 13.2 12 550 kVA
The preceding calculations are an estimating procedure which will
give approximate results. The generating set manufacturer, in consul-
tation with the engine and generator manufacturers, will usually be in
the best position to make the final decisions.

Reliability and Redundancy
Reliability
The reliability of a standby power supply will depend on a number of
factors such as:
The design of the equipment that has been installed

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