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G5/4
Explained
Engineering Recommendation G5/4, Some
of the Facts and Figures.
Scope of Engineering Recommendation
G5/4.This Guide
gives details of the background to the technical limits and the present day
procedures appropriate to the connection of non-linear loads to public supply
systems in the UK .
Limits of
Harmonic Distortion Electricity Council Recommendations G5/4
The G5/4 document has
been written to safeguard the HV
Networks from VOLTAGE DISTORTION, in
some cases the amount of non-linear load
on the LV plant may exceed the current
distortion but not the voltage
distortion. Therefore, consideration
must be given to the regulatory
requirements of ER G5/4 and the
practical application of LV Harmonic
mitigating equipment (Active Filters) to
reduce the 5th Harmonic
Current. If when sizing the Transformer
this is taken into account then the
Harmonic current may not be a problem
for the designers. Prior to the
installation of any new non-linear
equipment to an existing site, a
Harmonic Power Analyser should be
connected to monitor the
existing/background conditions, when
installation of non-linear equipment is
complete a post survey be carried out to
ensure G5/4 compliance. Any Harmonic
mitigating equipment can be retrofitted
if required.
Consideration has to be given to what
happens on the 415 volt system where
Harmonic currents are circulating and
also the resultant levels being
generated onto the 11 kV system :-
a)
Higher frequency currents
will cause extra heating in cables,
switchgear, transformers etc, hence
extra losses in energy and rotating
machines; together with additional noise
from motors and other apparatus.
b)
The supply authority may
require that Harmonic distortion be
reduced where they identify Harmonic
distortion on the network, which may be
presented to other users that may have
sensitive plant.
c)
The effects on
instrumentation control and computers
are more likely to be a problem; above a
certain level of voltage distortion,
some instrumentation and controls are
known to produce errors or malfunction.
Regulation devices and Electronic
Equipment are subject to downgraded
performance, through displacement of
zero crossing of the voltage wave.
d)
Capacitors are
susceptible to Harmonic overloading;
whilst they are designed to accept 30%
over-current. It is the increased
stress across the dielectric medium; due
to the absorbed Harmonic current and
impedance of the capacitor; which may,
exceed the permissible overload
conditions, according to its
manufacturing specification/design
criteria; which leads to early failure.
Any capacitors installed, where there is
known to be Harmonic distortion
generated, should be suitably de-rated,
if the probability of premature failure
is to be reduced. Where high current
magnification occurs, further measures
are required.
e)
Where standard capacitors
are installed ie without detuning
reactors, any Harmonic currents
generated will be magnified making the
distortion worse.
f)
Levels of
voltage distortion defined in
Electricity Council Recommendation G5/4,
and IEC 1000 for Electromagnetic
Compatibility of low frequency
disturbances, are known to be generally
tolerable at the respective defined
supply voltage levels. Experience
indicates that voltage distortion levels
in excess of the defined values within
these documents, for the given Busbar
voltage –
(irrespective of compliance with the
G5/4 document at the PCC – Point of
Common Coupling) – are known to create
operational problems with major items of
installed plant.
g)
Induced audio frequency
interference on telephone lines if these
run close to power lines and are not
suitably screened and segregated.
As pointed out earlier, Harmonic
Distortion is a wide ranging subject should you require any further assistance
don’t hesitate to give us a call……. |
