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46 E L EC TR I C AL CONNEC T I ON
W I N T E R 20 1 6
INSTALLATION HARMONY
T
he notion of power quality
may be familiar to many in the
electricity sector, but it is gaining
in importance and needs to be tackled.
The reasons include an increase
in apparent power demand and the
growth in distributed generation.
The latter is not given much weight
at present because its influence
percentage-wise is low. Yet there are
already examples, even at low-voltage
(LV) distribution levels, in which the
high penetration of rooftop solar
photovoltaic (PV) generation causes
harmonics and elevated voltage effects.
In addition, the power factor is
lowered on days of high insolation.
The growth of solar and wind farms
will accentuate the importance of
power quality. These are examples of
distributed generation (DG), adding
to the familiar mix of combined cycle,
diesel and gas generation.
Although the effect of DG is
receiving attention from power
generation and distribution authorities,
the overall control of power quality
will pose challenges as the growth of
DG continues.
RENEWABLE SOURCES
The electrical contracting industry is
familiar with certain constraints placed
on electrical installations at the point of
common coupling (PCC).
The Australian Standard AS/NZS
61000-3-4 limits harmonic contribution
at the PCC for LV distribution.
A similar Standard, AS/NZS
61000.3.6, pertains to medium-voltage
(MV) distribution and high-voltage
transmission. Larger installations are
often connected to the MV network.
These are likely to be 11kV installations,
or lower voltages.
The growth in solar farms and wind
farms is mainly at distribution voltage
levels, which can be as high as 132kV.
However, connection is very much
dictated by geographical constraints, so
lower voltages will also be encountered.
These renewable energy sources
affect power quality because there is
limited control over frequency, power
factor and reactive power.
Until now, the concept of power
quality has been understood as
controlling harmonic emission from
installations. The future will be quite
different because of power quality
problems on the supply side.
LOW INERTIA SOURCES
Although many issues affecting
power quality from wind and solar
sources are beyond the scope of this
article, some aspects are highlighted
because of their importance with
higher levels of DG penetration.
Both types of DG are low-inertia
forms and therefore dependent
on the grid stability provided by
high rotational inertia synchronous
generators in base-load power stations.
The latter generators damp power
surges by virtue of this high inertia.
Wind and solar generation can result
in large phase angle differences across
distribution lines and consequent
power surges.
In the case of stiff networks, those
with a high reactance-to-resistance
ratio (X/R), power flow can take
place without much change to voltage.
For slack networks, with an X/R less
than 4, appreciable voltage effects
are noticed.
WHOSE PROBLEM IS IT?
The foregoing considerations
highlight the need for forensic power
quality analysis.
For mission critical installations, and
those in which electrical power plant is
a large part of the productive assets,
power quality monitoring will be of
increasing importance.
Take harmonics – analysis of
dominant harmonics on the incomers
may be required because of their
potential to stress the power factor
correction equipment.
The harmonic aspect is already
a vexing one. Is harmonic voltage
distortion the power supplier’s
problem, or is it brought on by
the installation?
To gain forensic information,
synchronous timing with the supplier
is an essential feature. This article
provides basic information on power
quality analysers as furnished by
participating companies.
As will be evident from a reading
POWER QUALITY ANALYSIS
SHOULD BE CONDUCTED FOR ANY
PLANNED INSTALLATION.
PHIL
KREVELD
EXPLAINS WHY.
Renewable energy effects power quality caused by
limitations on reactive power and frequency control.
LET’S GET TECHNICAL