Page 131 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 129

sumers. The point of common coupling will be regarded as the point
of supply (the consumer’s terminals).
■ The consumer is supplied at low voltage by a dedicated cable from a
nearby substation. The point of common coupling will be regarded as
the substation low-voltage busbars.
■ The consumer is supplied at high voltage and owns high-voltage
switchgear, one or more transformers, and high- and low-voltage dis-
tribution systems as appropriate. The point of common coupling will
be regarded as the high-voltage intake terminals.

The third pattern of distribution in which power is taken at high volt-
age is worthy of comment. The high-voltage network is likely to have a
low impedance and the network operator will allow a large harmonic
load to be connected to it, much larger than would be allowed if the
point of common coupling was on the low-voltage side. If a large dis-
torting load is connected to the low-voltage system the entire con-
sumer’s low-voltage system will be heavily distorted and damage and
malfunction may result. As the point of common coupling is on the
high-voltage side the network operator is neither responsible for nor
concerned with distortion on the low-voltage side; any corrective action
required will be the responsibility of the consumer.
If nonlinear equipment is to be installed within such a system the
problem can be avoided by stating, within the contract documents, that
Engineering Recommendation G.5/4 applies and the point of common
coupling is to be regarded as the equipment terminals or some other
clearly defined point within the low-voltage system.

The Effect of Bridge Rectifier Loads on Local
Generators
Impedance of the Supply
If the electricity supply is derived from a local generator, all the effects
mentioned in the previous section may be experienced but, as the
source impedance will almost certainly be higher than that of a supply
system, the effects are more likely to be troublesome. There are other
additional effects which occur owing to the nature of synchronous
machines, and these are described in subsequent paragraphs.
The magnitude of the effects described in this section depend on the
rating of the local generator. If a rectifier is supplied from a very large
generator the effects will be negligible and the supply may be regarded
as a low-impedance system as considered in the previous section.
However, many standby generating sets supply uninterruptible power
supplies which have rectifiers as their input modules and the effects
may well become relevant.

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