Page 107 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
Additional Information Relating to the Standby Supply Installation 105

maximum power that will be regenerated (assuming no overloading),
and the generator set maker can advise the power required to drive the
set at rated speed. Any surplus power must be absorbed by other build-
ing loads or by a dummy load provided for the purpose. Under regenera-
tive conditions the governor will already be on minimum fuel and can
have no effect. The power absorbed by generating sets depends on speed
and size, but varies between 7.5 and 25 percent of the kW rating.

Step Loads
Any step load applied to a generating set will produce dips of both volt-
age and frequency. The voltage dip will be approximately proportional
to the reactive kVA component; full load kVA at 0.8 power factor will
result in a drop in voltage, depending on the type of excitation, of up to
16 percent. At lower loads it will be proportionally less, thus at 50 per-
cent load a drop of up to 8 percent might be expected. The frequency dip
will be proportional to the kW component and the maximum step load
permitted by the engine will probably result in a speed dip of say 8 per-
cent. Both effects are transient, the voltage regulator and the speed
governor will restore voltage and frequency to within tolerance within
a few seconds.

Uninterruptible Power Supplies
Any uninterruptible power supplies within the installation will form
part of the essential load. The input modules of many types of power
supplies are rectifiers which take a current rich in harmonics; theoret-
ically the current taken by an idealized three-phase bridge rectifier has
an rms harmonic content equal to 30 percent of the fundamental, which
can be reduced to 14 percent by using two phase shifted bridges. The
most troublesome harmonic currents are the fifth and seventh or, for
the phase shifted bridges, the eleventh and thirteenth, there are no
triplen or even harmonics. The harmonic currents cause additional
heating in the windings and in the rotor but, provided that the power
supply loading on the generator does not exceed say, 50 percent of the
generator rating, difficulties are unlikely to arise.
There are two types of rectifier in use, diode and thyristor and the pre-
ceding comments apply to both; diode rectifiers use natural commutation
at the voltage changeover point, and thyristor rectifiers use delayed com-
mutation and can cause severe notching of the generator output voltage.
Notching may affect the operation of voltage regulators and speed gov-
ernors, and some form of filtering of the supplies to such devices may be
necessary. The effect of notching is reduced if the generator includes a
permanent magnet pilot exciter. Notching is discussed in Chap. 4.

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