Handbook of Electrical Installation Practice - BeKnowledge

Standby Power Supplies 131
In some cases it may be considered advantageous to shut down some of the paralleled
sets if the load falls to a very low level. The automatic removal of generating
sets from the parallel running group and their re-introduction should load levels
increase further, complicates the control system and is normally only justified in
special cases.
The triplen harmonic content in the output of modern low voltage generators is
usually sufficiently small to allow the line and neutral connections to be paralleled.
Therefore the once common practice of employing contactors to select only one
generator neutral is unnecessary unless recommended by the manufacturer or
where generators of different age, rating or design are to be operated in parallel.
Peak lopping system
The capital investment in a standby generating set is sometimes offset by using the
equipment for peak lopping duty whereby the highest electrical tariff charges can
be avoided by running the set in parallel with the mains supply during peak periods.
There are certain minimum requirements concerning standard of equipment and
safety associated with this form of operation and Electricity Distributors vary in
their attitude towards the idea, some allowing it readily while others forbid it completely.
This is dealt with more fully in Chapter 1.
Non-linear loads
The general term non-linear applies to loads which do not have a constant impedance
to the sinusoidally varying voltage. Variations in impedance may be due to the
natural non-linear characteristics of the load, or may be due to the switching action
of circuit components such as occurs in rectifiers or thyristors. The essential characteristic
of non-linear loads is that the current waveform is not sinusoidal, even
when the supply voltage is of ideal sine-wave shape. When the load current is not
sinusoidal the internal reactance drop in the generator windings will follow the nonsinusoidal
shape of the current, so that the actual terminal voltage of the generator
will be the basic sine-wave voltage generated by the machine less the non-sinusoidal
voltage drop, and the result of this will be a non-sinusoidal terminal voltage which
may adversely affect the operation of other equipment fed from the same supply.
The amount of harmonic distortion of the output voltage will depend on how
much of the applied load is non-linear and on the internal reactance of the generator
windings. The acceptability of harmonic distortion will depend on how critical
the total load is to supply voltage waveform. If the load on the generator is totally
non-linear then, although maximum distortion of the voltage waveform will occur,
this may be acceptable as a working condition. As a broad general rule, non-linear
loads should represent no more than about 40% of the total load on conventional
generators of normal reactance.
The adverse effects of any type of non-linear load can always be reduced by
selecting an oversize generator and this is frequently specified by the generating set
manufacturer. The effect of increasing the kVA rating of the machine is to reduce
the per-unit reactance in the same proportion, and in this way voltage distortion can
be minimised to any required extent. Some reduction in the reactance of standard