Handbook of Electrical Installation Practice - BeKnowledge

Standby Power Supplies 133
Capacitive loads
Most industrial loads operate at a power factor which is less than unity and is most
frequently lagging. It is generally agreed that a power factor less than 0.8 lagging is
disadvantageous and steps are then taken to correct the power factor to a value in
excess of this figure. The usual procedure is to connect capacitors across the load
circuits which then draw a leading current which partially cancels out the lagging
current of the main load, thus raising the overall power factor to the required value.
In some installations arrangements are made to adjust the value of the capacitors
automatically with changing loads, and this is the ideal situation as the overall power
factor is then retained at its desired value.
However, in some commercial installations the capacitor is fixed at an average
level of compensation based on the load expectations of the equipment, and if the
actual reactive pattern of the load does not match this assumed value, overcompensation
will occur and the total load on the supply will operate at a leading power
factor. Two examples of such situations could occur with welding transformers and
fluorescent lighting installations where the power factor correction capacitor are
arranged for total correction and are not split into individual units to match the
separate load elements.
The field excitation current requirement for a loaded generator depends very much
on the power factor of the load current and in the usual case of lagging power factor
the field current increases as the power factor reduces. On the other hand, with
leading power factor loads the effect is the opposite and as the power factor reduces,
the field current requirement for a given output will go down. If the leading power
factor of the load circuit is reduced sufficiently, a condition may be reached in which
the field current requirement becomes zero and this is known as self-excitation. At
this critical point the AVR sensing the generator output voltage would lose control,
and any further reduction of the leading power factor would cause overexcitation
and the voltage would rise without any corrective action from the AVR. Such a condition
is obviously most undesirable and could cause damage to the connected load.
Because self-excitation depends on armature reaction effects from the stator
winding, modern a.c. generators which have high synchronous reactance will reach
this point of voltage instability at a lower level of reactive load than older type
machines which tended to have lower synchronous reactance values. Circuits which
have any risk of self-excitation should be carefully examined before arranging to
feed them from a generating set supply and in doubtful cases it may be necessary
to adjust the values of the power factor correction capacitors.
To prevent voltage instability and voltage transients it is recommended that
power factor correction capacitor banks should be disabled when only the standby
generation is running.
Unbalanced loads
Ideally the phase currents of an a.c. generator should be equal, as in this way the
line-to-line and line-to-neutral voltages are held within their normal tolerances.
Large differences in the individual phase currents may cause voltage values outside
normal limits and because of this it is recommended that efforts should be made to
balance the loads to within about 20%.