Page 103 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
Additional Information Relating to the Standby Supply Installation 101

open the test load switch. If the set is not attended it will be necessary to
include a contactor or circuit breaker electrically interlocked with the
supply changeover device or standby supply circuit breaker.
For larger installations the advantages of a facility enabling the nor-
mal and standby supplies to be paralleled follow from the above. The
set can be test started, paralleled, and loaded onto the real load before
the normal supply is disconnected. It should be noted that this proce-
dure does not test any loading sequence, a separate test should be
devised and occasionally undertaken to prove this.

Kilowatts, Kilovars, and the Harmonic
Components of the Load Current

Within an installation the standby supply will have a much higher
impedance than the normal supply, and loads and load changes will
have a much greater effect upon it.
The currents taken by the different loads may conveniently be con-
sidered as comprising four components in various proportions:
■ The in-phase component of the fundamental frequency current, or
power component, which on its own has a power factor of unity and
represents kilowatts.
■ The phase-quadrature component of fundamental frequency current,
or reactive component, which on its own has a power factor of zero
and represents kilovars.
■ The harmonic or distorting currents which have no particular phase
relationship with the fundamental current; they are parasitic and
represent neither kilowatts nor kilovars.
■ The notches in the fundamental sine wave of voltage caused by the
delayed commutation of thyristor rectifiers.

For normal loads the kilowatt loading of a generating set will be the
major component of the load, it is the component which determines the
engine rating and will be larger than the kilovar or the harmonic load-
ing. The application of kilowatt loading to an engine will cause a drop
in speed which is restored to its correct value by the speed governor.
The kilowatt loading is seen to a small extent by the generator, which
experiences a slight drop in voltage due to the stator leakage reac-
tance, but the resulting quadrature voltage drop has only a small
effect on the terminal voltage which is restored to its correct value by
the voltage regulator.
The kilovar loading of a generating set, conversely, has no effect
whatever on the engine rating and will have no effect on the speed or

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