Page 93 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
Additional Information Relating to the Standby Supply Installation 91

The generator must be capable of supplying the total required elec-
trical load to the distribution system. This will be equal to the power
rating of the engine as derived above less the losses within the genera-
tor (5 or 6 percent).
While the basic engine rating is expressed in kWm the basic genera-
tor rating is in kVA, which is the phasor sum of kWe and kVAr. The
relationship is defined by the well-known equations:

kW kVA cos (3.1)

and kVAr kVA sin (3.2)

The Effect of UPS Loads on the Generator
kVA Rating
Most standby power generators provide power to one or more uninter-
ruptible power supply systems which may use six-pulse rectifiers as
their input modules. As described in Chap. 4, a six-pulse rectifier gen-
erates harmonic currents of the orders 5, 7, 11, 13, etc., the theoretical
total harmonic distortion being 30 percent. In practice it is usually
higher. In passing through the generator these harmonic currents will
cause voltage distortion which is undesirable for many reasons. If the
total harmonic distortion of the generator voltage is to be limited to say,
10 percent, the total six-pulse rectifier load on a generator should not
exceed 30 or 40 percent of its kVA rating, depending on whether it has
high or low subtransient reactance. A precise calculation requires the
use of data which will be available to the generator manufacturer.

The Step Loading Ability of the Generator
Generators experience no difficulty in accepting step loads up to 100
percent or more of their rating. The resulting transient voltage drop,
however, has to be considered as it must be within the limits defined by
the voltage performance class (G1, G2, G3, or G4).
When load is applied to a generator, there is an immediate drop in out-
put voltage due to the stator subtransient reactance (X″) and resistance
(R). The voltage regulator will respond quickly, but the exciter and rotor
fields are highly inductive and, owing to their time constants, there is
delay in increasing the excitation of the machine. The generator rotor
will also suffer a loss of speed which will further reduce the voltage. For
self-excited machines (in which the exciter field is energized from the
generator output) the voltage drop at the generator terminals is trans-
ferred to the exciter field, still further increasing the drop. It is likely
that the output voltage drop will be increased by armature reaction

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