One of the problems with increasing limits, is that the
prevailing harmonic levels also seem to follow the trend.
limits for "external" harmonic voltages and existing consumer PFC
Consumers supplied via a utility HV grid
It can be seen although the overall voltage THD limit
has remained at 3%, the EPCC and NRS limit for individual 3rd and 5th
harmonic voltage have effectively doubled, compared to the pre-1992 limit
This increase in harmonic limits can allow harmonic
levels on utility supply networks to increase beyond limits upon which
consumer equipment was originally designed, thereby shifting responsibility
for any related adverse effects in the consumers system onto the consumer.
Case Study -
The consumer had PFC capacitor banks designed and
installed at his dedicated 22 kV supply from the utility in 1988, when the
prevailing 5th harmonic voltage at the utility 132 kV supply was about
0,5%, or half the recommended 'limit of the day'. These banks operated
successfully for over 6 years, after which spurious tripping occurred due
to harmonic overload.
Upon investigation, it was found that the banks were
tripping due to series resonance amplification of an "external"
5th harmonic voltage source of about 1,5% present on the 132 kV supply,
i.e. 50% higher that the accepted recommended limit at the design stage,
and three times the original level recorded.
However, as the 5th harmonic voltage limit at 132 kV had
in the interim effectively doubled from 1% to 2,1% as listed by the EPCC,
the new 5th harmonic voltage was now within limits.
In order to continue to use his PFC, the customer was
forced to convert his capacitor banks to harmonic filters at significant
Consumers supplied via a utility MV grid
The voltage THD limit increased from 3% to 5% in 1992
(EPCC limit), thereafter increasing to 8% in 1997 (NRS limit). This
represents an effective 270% increase in THD limit.
The situation for individual harmonic voltages is even
worse, with an effective 500% and 600% increase in recommended limit for
3rd and 5th harmonic voltage respectively.
Although increased limits alone will obviously not
result in consumer PFC problems, it appears inevitable that increasing
harmonic limits eventually results in higher harmonic levels on the utility
For consumers with existing PFC installed and a
dedicated supply from the utility via a utility MV network (typically from
11 kV to 44 kV), the potential for spurious PFC tripping is therefore even
greater. Of course if PFC protection is not set up correctly, indications
of a problem could first appear through PFC equipment failure such as
capacitor can failure.
For example, consider a 'typical' consumer who is
provided with a 6,6 kV supply by the utility via a 44/6,6 kV transformer.
The consumer had a 4 MVAr capacitor bank installed in 1990 to improve power
factor at his 6,6 kV supply, the PFC design being based at the time on
existing and future harmonic generation in the consumer's plant, together
with a potential "external" 5th harmonic voltage source at the
utilities' 44 kV supply of 1% - being the accepted limit. However, by the
time 1997 arrived, two sets of revised harmonic limits had allowed the
utility to move the harmonic goalposts.
With an increase in MV 5th harmonic voltage to 4% (1992
EPCC limit), the consumer would experience a 5th harmonic voltage at his
6,6 kV supply of about 7% due to series resonance effects. Although not an
ideal situation, it would be a liveable for the consumer.
However, the utility is now in a position to 'allow' the
5th harmonic voltage source at their 44 kV supply to approach 6% and still
remain within the NRS limit. This consumer would now experience a 5th
harmonic voltage at his supply of over 10%, leading to tripping of his PFC
and related financial penalties.
The future ?
As small and
medium consumers cannot go on strike and presently do not have the option
of an alternative utility to stimulate competition (although one wonders whether
this will ever occur in SA), they effectively have very little say in
respect of imposed limits. Certainly in respect of the increased
"external" harmonic levels allowed on the utility supply by
revised limits, it is the consumer who will bear the brunt of implementing
remedial measures to alleviate any harmonic overloading of PFC. This PFC
being correctly designed according to previous harmonic limits.
hopefully the latest harmonic limits as specified in NRS-048, although
termed both 'constructive' and 'scandalous', are here to stay for at least
the foreseeable future. This will provide a certain clarity in what seems
over the last few years to have been a grey area in respect of allowed
harmonic limits, as well as providing the PFC design engineer with fixed
although it is difficult to see how the harmonic limits can be increased
beyond NRS-048, I suppose anything is possible. As Winston Churchill once
remarked "we must beware of needless innovation, especially when
guided by logic".