28-05-2012, 10:48 AM
Review - What is power quality?
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Abstract
This paper introduces the terminology and various issues related to ‘power quality’. The interest in power quality is explained in
the context of a number of much wider developments in power engineering: deregulation of the electricity industry, increased
customer-demands, and the integration of renewable energy sources. After an introduction of the different terminology two power
quality disturbances are discussed in detail: voltage dips and harmonic distortion. For each of these two disturbances, a number of
other issues are briefly discussed, which are characterisation, origin, mitigation, and the need for future research.
# 2003 Elsevier Science B.V. All rights reserved.
Introduction
Classically, the aim of the electric power system is to
generate electrical energy and to deliver this energy to
the end-user equipment at an acceptable voltage. The
constraint that was traditionally mentioned is that the
technical aim should be achieved for reasonable costs.
The optimal level of investment was to be obtained by
means of a trade-off between reliability and costs. A
recurring argument with industrial customers concerned
the definition of reliability: should it include only long
interruptions or also short interruptions or even voltage
dips. The term power quality came in use referring to the
other characteristics of the supply voltage (i.e. other
than long interruptions). But, immediately, the first
confusion started as utilities included the disturbances
generated by the customers in the term ‘power quality’.
Harmonic analysis
Harmonic analysis aims at predicting the harmonic
distortion at one or more locations in the power
network. Such a study can be done to estimate the
effect of a new non-linear load or of the installation of a
harmonic filter. There are two distinctly different
methods of harmonic analysis.
Voltage dips
Voltage dips are short-duration reductions in r.m.s.
voltage caused by short-duration increases of the
current, typically at another location than where the
voltage dip is measured. The most common causes of
overcurrents leading to voltage dips are motor starting,
transformer energising and faults. Also capacitor energising
and switching of electronic load lead to shortduration
overcurrents, but the duration of the overcurrent
is too short to cause a significant reduction in
the r.m.s. voltage. These events are normally not
referred to as voltage dips but as voltage notches or
voltage transients. Voltage dips due to short circuit and
earth faults are the cause of the vast majority of
equipment problems. Most of the recent emphasis on
voltage dips is directed towards these fault-related dips.
Conclusions
Power quality is a very wide and dispersed area that
somewhat accidentally became viewed as one subject.
The two examples presented in this paper (harmonics
and voltage dips) show the variety of aspects related to
even these two disturbances. Other disturbances that
would deserve an equal amount of attention are (long
and short) interruptions, transients, and high-frequency
waveform distortion. Note also that flicker is presented
here as a subset of waveform distortion, even though it
is commonly (and more correctly) treated as a separate
disturbance.
For more information on these and other powerquality
disturbances, the reader is referred to the
extended literature on power quality. Good overviews
can be found in some of the books [8/18], but also the
IEC standards [19/27] and the IEEE standards [28/34]
on power quality contain useful basic knowledge and
overviews.