Page 104 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
102 Chapter Three

governor setting. It will make a contribution to the kVA rating of the
generator. The kilovar loading of the generator results in a rotor field
demagnetizing effect, and a voltage drop across the stator leakage reac-
tance, causing a drop in terminal voltage which is restored to its cor-
rect value by the voltage regulator. If the power factor of the load is
lower than 0.8 the rotor field demagnetizing effect may be such that
additional excitation is required which in turn may lead to a larger
frame size of generator.
Harmonic currents may be regarded as being generated by the load
and flowing through the generator of the standby set or through the
supply system. Harmonic voltages are developed across the reactance
of the generator (or of the supply system) which result in voltage dis-
tortion. Within the generator harmonic currents cause increased iron
and copper losses. The voltage distortion affects current measuring
instruments and introduces a distortion factor into calculations.
Notches in the fundamental sine wave of voltage are caused by the
delayed commutation of thyristor rectifiers and the greater the delay
the deeper the notches. Notching may affect the operation of voltage
regulators and speed governors, and some form of filtering of the sup-
plies to such devices may be necessary. The effect of notching is reduced
if the generator includes a permanent magnet pilot exciter.
The size of the engine driving a generating set is determined only by
the kilowatt loading, the size of the generator is determined by the kilo-
voltampere loading but may be increased due to the harmonic current
loading. It follows that if a generating set is to supply a low power fac-
tor load or a heavily distorting load, the generator should be “oversized”
with respect to the engine rating. An oversized generator has the
advantage of having a lower impedance than one of standard rating
and will be less affected by disturbances such as step loads, harmonic
currents, and notches.

Characteristics of Particular Loads
The electrical characteristics of the load seen by a generator may have
an effect on the quality of the output. In general where the load on the
standby supply is a mix of various loads no problems will arise, but
where one particular load predominates the likely effect should be con-
sidered at the planning stage.

Computer Loads
The dc power taken by computers is usually derived from switched
mode power supplies which take a peaky current having typically a
third harmonic component measuring 85 percent of the fundamental

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