Page 130 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
128 Chapter Four

they are all regarded as nonlinear loads. The assessment procedure is
in three stages:

Stage 1 allows the connection of equipment to 400-V systems without
individual assessment provided that certain criteria are met. It applies
to small equipment not exceeding 16 A per phase complying with BS
EN 61000-3-2, and six-pulse and 12-pulse rectifiers not exceeding 12
and 50 kVA, respectively. Harmonic current emissions (up to the order
50) from aggregate loads are required to be within specified limits.
Stage 2 is applicable to installations supplied at a high voltage below
33 kV. It also applies to 400-V systems where a Stage 1 assessment
is not appropriate due to the rating of the equipment, the emission
levels, or the network characteristics. Harmonic current emissions
from aggregate loads are required to be within specified limits. The
voltage distortion caused by the new load is assessed and the effect
of adding this to the existing distortion is predicted. The predicted
distortion is required to be within specified limits.
Stage 3 is applicable to installations supplied at 33 kV and above. It
also applies to installations at lower voltages which are found to be
not acceptable under Stage 2. The procedure is somewhat similar to
Stage 2, but the calculations are more rigorous and there are no har-
monic current emission limits.

For Stages 2 and 3 the consumer has to provide the network opera-
tor with sufficient data to enable the appropriate calculations to be
undertaken. If the consumer wishes to undertake calculations associ-
ated with Stages 2 or 3 the network operator will have to provide infor-
mation relating to the supply system.
There is an associated guidance document, Engineering Technical
Report 122 (ETR 122), which provides background information and
includes additional information and worked examples.

The Point of Common Coupling
The point of common coupling is the point on the public electricity dis-
tribution system, electrically nearest to the consumer’s installation,
where other consumers are or may be connected. It is the point at
which the network operator applies the limits defined in Engineering
Recommendation G.5/4 and it is important that consumers understand
the concept and the results of applying it.
The electrical distribution to premises usually falls into one of three
patterns:

■ The consumer is supplied at low voltage by a short length of feeder
cable connected to a distributor cable which supplies other con-

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