Page 129 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 127

For resonance at the seventh harmonic:
1
Capacitance (4.10)
2
(2
f) L
10 6

2
(2
350) 27.5
7519 F per phase

This value of capacitor at 240 V and 50 Hz will result in a funda-
mental capacitor current of 567 A, and when operating as a three-phase
bank the kVAr rating will be 408 kVAr.
A 400-kVAr capacitor bank would be used, for example, to correct the
power factor of a 1-MVA load from 0.7 to 0.95 and is therefore likely to
be encountered in practice.
Similar calculations undertaken for the eleventh and thirteenth har-
monics lead to resonant capacitor ratings of 165 and 118 kVAr respec-
tively. Such lower ratings may well be encountered if a capacitor bank
is automatically switched in sections. A calculation for the fifth har-
monic leads to a resonant capacitor rating of 800 kVAr, which would be
unlikely to occur in practice.
Capacitors can be detuned by adding sufficient series inductance to
ensure that the series circuit is inductive at the lowest significant har-
monic (or the troublesome harmonic). The circuit will then be inductive
at all higher harmonic frequencies.

Engineering Recommendation G.5/4
With these matters in mind the Electricity Association (of the United
Kingdom) in 1976 issued Engineering Recommendation G.5/3, “Limits
for harmonics in the United Kingdom Electricity Supply System.” In
2001 a revised version was issued, Engineering Recommendation G.5/4,
“Planning levels for Harmonic Voltage Distortion and the Connection of
Non-Linear Equipment to Transmission Systems and Distribution
Networks in the United Kingdom.” G.5/4 is invoked by the United
Kingdom network operators when a consumer wishes to install nonlin-
ear equipment.
Document G.5/4 applies to all transmission and distribution sys-
tems from 400 V to 400 kV, different limits being applied to the vari-
ous voltage systems. It applies to nonlinear loads which take power
from the system, such as uninterruptible power supplies, and to gen-
erators arranged for parallel running which deliver power to the sys-
tem; in the text no distinction is made between loads and generators,

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