power quality

Review

What is power quality?

M.H.J. Bollen *

Department of Electric power Engineering, Chalmers University of Technology, Horsalsvagen 11, Gothenburg 412 96, Sweden

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.

Keywords: Power quality; Harmonic distortion; Voltage dips

1. 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’.

This difference in emphasis will be discussed in more

detail below. The main complaint of domestic customers

concerned the costs which were perceived too high,

especially where cross-subsidising was used to keep

prices low for industrial or agricultural customers.

This classical model of the power system, as it can be

found in many textbooks, is found in Fig. 1. The

customers are traditionally referred to as loads.

Various developments have led to a different view at

the power system. These developments are strongly

interrelated, but the three main ones are:

. The deregulation of the electricity industry makes

that there is no longer one single system but a number

of independent companies with customers.

. Electricity customers have become more aware of

their rights and demand low-cost electricity of high

reliability and quality, where the priorities are

different for different (types of) customers. Custo-

mers are certainly no longer willing to accept their

position as merely one parameter in a global optimi-

sation.

. Generation of electricity is shifting away from large

power stations connected to the transmission system

towards smaller units connected at lower voltage

levels. Examples are combined-heat-and-power and

renewable sources of energy like sun and wind.

Because of this the power system can no longer be

seen as one entity but as an electricity network with

customers. This new model is shown in Fig. 2. Note that

the physical structure of the power system/network has

not changed, it is only the way of viewing it that has

changed.

In Fig. 2 the electric power network connects some or

many customers. Customers may generate or consume

electrical energy, or even both albeit at different

moments in time. Different customers have different

* Tel.: �/46-31-772-3832; fax: �/46-31-772-1633.

E-mail address: [email protected]

(M.H.J. Bollen).

Electric Power Systems Research 66 (2003) 5�/14

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0378-7796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

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demands on voltage magnitude, frequency, waveform,

etc. Different customers have different patterns of

current variation, fluctuation and distortion, thus pol-

luting the voltage for other customers in different ways.

The power network in Fig. 2 could be a transmission

network, a distribution network, an industrial network,

or any other network owned by one single company.

For a transmission network, the customers are, e.g.,

generator stations, distribution networks, large indus-

trial customers (who could be generating or consuming

electricity at different times, based on the electricity

price at that moment), and other transmission networks.

For a distribution network, the customers are currently

mainly end-users that only consume electricity, but also

the transmission network and smaller generator stations

are customers. Note that all customers are equal, even

though some may be producing energy while others are

consuming it. The aim of the network company is only

to transport the energy, or in economic terms: to enable

transactions between customers.

The technical aim of the power network becomes one

of allowing the transport of electrical energy between

the different customers, guaranteeing an acceptable

voltage and allowing the currents taken by the custo-

mers.

With an ideal network each customer should perceive

the electricity supply as an ideal voltage source with a

zero impedance. Whatever the current is, the voltage

should be constant. As always, reality is not ideal.

Power quality concerns this deviation between reality

and ideal.

Note that this same model also becomes attractive

when considering the integration of renewable or other

environmentally�/friendly sources of energy into the

power system. The power network is no longer the

boundary condition that limits e.g. the amount of wind

power that can be produced at a certain location.

Instead the power network’s task becomes to enable

the transport of the amount of wind power that isproduced. It will be clear to the reader that the final

solution should be found in co-operation between the

customer and the network operator considering various

technical and economic constrains.

There are many aspects to the limitations set by the

network on the market. A much discussed one is the

limited ability of the network to transport energy. Note

that lack of generation capacity is not a deficiency of thenetwork but a deficiency of the market.

In this modern way of looking at power systems, the

utility no longer buys and sells energy, but instead sells

transport capacity and access to the network.

This paper will give a short overview of power quality

with emphasis on the two issues that currently receive

most attention: harmonic distortion and voltage dips.

But first another attempt will be made at defining theterm ‘power quality’.

2. Definitions

There is a lot of confusion on the meaning of the term

‘power quality’, not in the least because ‘power’ is used

as a synonym for ‘electricity’ in American English

whereas it is also the energy transport per unit of time.

Different authors use different definitions. A consistent

set of definitions is given as follows:

. Voltage quality is concerned with deviations of the

voltage from the ideal. The ideal voltage is a single-

frequency sine wave of constant amplitude andfrequency.

. Current quality is the complementary term to voltage

quality: it is concerned with the deviation of the

current from the ideal. The ideal current is again a

single-frequency sine wave of constant amplitude and

frequency, with the additional requirement that the

current sine wave is in phase with the voltage sine

wave.. Power quality is the combination of voltage quality

and current quality.

. Quality of supply is a combination of voltage quality

and the non-technical aspects of the interaction from

the power network to its customers.

. Quality of consumption is the complementary term to

quality of supply.

Note that not all these terms are equally commonly

used, especially current quality and quality of consump-tion are used more frequently. Also note that other

sources give other, often conflicting, definitions. All

definitions given above apply to the interface between

Fig. 1. Classical model of the power system.

Fig. 2. Modern model of the power system.

M.H.J. Bollen / Electric Power Systems Research 66 (2003) 5�/146

the network (company) and the customer. This may be

for example a domestic customer and the public low-

voltage distribution network, an individual plant and

the industrial medium-voltage distribution network, a

power station and a transmission network, or a trans-

mission network and a distribution network. The term

power quality is certainly not restricted to the interac-

tion between the power grid and end-user equipment.The term electromagnetic compatibility (EMC) has in

this context a more restricted meaning: it applies only to

the interaction between equipment and its electromag-

netic environment (e.g. the power system). Strictly

speaking it would thus only apply to low-voltage

networks but the terminology is also being applied to

higher voltage levels. Note that in the international

(IEC) standards power quality is treated as a subset of

EMC.

Power quality disturbances (i.e. deviations of voltage

and/or current from the ideal) come in two types, based

on the way a characteristic of voltage or current is

measured:

. Variations are small deviations of voltage or current

characteristics from its nominal or ideal value, e.g.

the variation of voltage r.m.s. value and frequency

from their nominal values, or the harmonic distortion

of voltage and current. Variations are disturbances

that are measured at any moment in time. Harmonic

distortion will be discussed as an example of a powerquality variation.

. Events are larger deviations that only occur occa-

sionally, e.g. voltage interruptions or load switching

currents. Events are disturbances that start and end

with a threshold crossing. Voltage dips will be treated

below as an example.

The difference between variations and events is not

always obvious, and related to the way in which the

disturbance is measured. The best way of distinguishing

between the two is as follows: variations can be

measured at any moment in time; events require waiting

for a voltage or current characteristic to exceed a pre-

defined threshold. As the setting of a threshold is always

somewhat arbitrary there is no clear border between

variations and events. Still the distinction between them

remains useful and it is in fact done (implicitly of

explicitly) in almost any power quality study. However,

note that here also there is no consistency in terminol-

ogy. For example, measurements of the r.m.s. voltage

can be the basis for a variation (when 10-min averages

are continuously recorded) but also for an event

(starting and ending when the r.m.s. voltage dips below

90% of the nominal voltage).

The definitions of power quality events and variations

as given here are much wider than the general inter-

pretation of power quality. This has to do with the fact

that power quality remains in most cases as a part of the

phrase ‘bad power quality’. A power quality disturbance

is only seen as an issue when it causes problems, either

for the customer or for the network operator. Voltagedips and harmonics are seen as a power quality issue by

many; but voltage and frequency variations are not seen

as a power quality issue because the latter were

incorporated in the design of power systems many years

ago.

3. Harmonic distortion

The term harmonics refers to the decomposition of a

non-sinusoidal but periodic signal into a sum of

sinusoidal components:

f (t)�X�

h�1

Ah cos(2phf0�8 h)

with Ah and 8h amplitude and phase angle for harmonic

order h , f0�/1/T and T the period. For a power system

operating at 50 Hz, any non-sinusoidal voltage or

current can be decomposed into a fundamental (50

Hz) component plus a number of harmonic components

with frequencies that are a multiple integer of 50 Hz.

The latter are called harmonic components. The 150-Hzcomponent (h�/3) is referred to as the third harmonic,

etc.

A more appropriate term would be ‘waveform

distortion’ where one could distinguish between:

. Harmonic distortion is distortion where the waveform

is non-sinusoidal but periodic with a period equal to

the period of the power system frequency (50 or 60

Hz). Most of the literature on waveform distortion

only considers this harmonic distortion, which is anacceptable approximation in many cases. However

most power quality studies consider more or less

exceptional situations, so that we cannot limit

ourselves to harmonic distortion only.

. The presence of a dc component can be seen as a

special case of harmonic distortion, but is often

treated separately due to difference in measurement

techniques and consequences.. Interharmonic distortion is mathematically the same

as harmonic distortion. The difference with harmonic

distortion is that the period is a multiple of the period

of the power system frequency. For example, a 50 Hz

signal with a 180 Hz interharmonic component has a

period of 100 ms (5 cycles of 50 Hz, 18 cycles of 180

Hz). Mathematically, a frequency component at an

irrational multiple of the power system frequencywould lead to a non-periodic signal, but that case

does not need to be considered in practice. Inter-

harmonic distortion is discussed in more detail in [1].

M.H.J. Bollen / Electric Power Systems Research 66 (2003) 5�/14 7

. Subharmonic components are components with a

frequency less than the power system frequency.

They can be considered as interharmonic distortion,

but are often treated separately because their con-sequences are different from those of higher fre-

quency components.

. Voltage flicker , or more accurately, voltage fluctua-

tions leading to light flicker , are mathematically

another special case of interharmonic distortion.

The special interest in this type of disturbance is

again due to the consequences. Even very small

fluctuations in the r.m.s. voltage with frequenciesbetween 1 and 15 Hz lead to light-intensity variation

for which our eyes are very sensitive.

. Noise , are all non-periodic frequency components.

The power system is not a static entity but it changes

all the time, so that strictly applying the above defini-

tions would imply that everything is noise. To distin-

guish between the different types of distortion is indeed

not always possible. A way of distinguishing would be

by taking the spectrum of the signal over a reasonable

number of cycles, e.g. 50 cycles (1 s). Harmonics and

interharmonics show up as sharp lines in the spectrum

whereas noise is seen as a continuous spectrum. Light

flicker cannot be observed directly from the spectrum,

although the presence of frequency components within

10 Hz of the fundamental component is a good

indication. For the analysis of light flicker, the flicker-

meter algorithm has been developed.

Fig. 3 shows the spectrum of a current signal contain-

ing several types of waveform distortion. The spectrum

was obtained by applying a discrete Fourier transform

to a 20-s window of the measured current to an arc

furnace. The harmonic distortion shows up as the

spectral lines at integer multiples of 50 Hz. The spectral

components close to the spectral lines are due to time

variations in the amplitude of these harmonics. Inter-

harmonic distortion shows up as spectral lines in

between the harmonic lines. In this case there are no

clear interharmonic components present, instead there is

a significant amount of noise, especially below 100 Hz.

Part of this noise is the cause of light flicker. Note thataccording to the IEC standard for measurement of

harmonic distortion (IEC 61000-4-7) all spectral content

in between integer harmonics is counted as interharmo-

nic distortion even if the term noise would be more

appropriate.

3.1. Origin of waveform distortion

Harmonic distortion is due to the presence of non-

linear elements in the power system (i.e. either in the

network or in the loads). The main distortion is due to

power-electronic loads like computers, televisions, en-

ergy-saving lamps. Such loads can be found in increas-

ing numbers with domestic and commercial customers

leading to an increasing level of distortion in thenetwork. An example of the non-sinusoidal current

due to a normal computer is shown in Fig. 4. This

waveform is typical for many loads at home and in the

office.

Also adjustable-speed drives and arc furnaces are

famous for the distortion they cause. But these loads are

mainly found with large industrial customers where

mitigation methods are applied to limit the resultingvoltage distortion. Therefore the resulting voltage dis-

tortion is mainly determined by small non-linear loads

and not by the large ones, although large non-linear

loads sometimes cause local problems. The daily varia-

tion of the harmonic distortion shows clearly the pattern

of domestic load, mainly televisions. This pattern is

visible round the globe as shown in Figs. 5 and 6, where

each figure shows the 5th harmonic and the THDobtained as averages over 10-min intervals.

Interharmonic distortion is much more related to

industrial loads, so is the noise component of waveform

distortion.

Capacitor banks are often incorrectly mentioned as a

source of harmonic distortion. They are not a cause of

Fig. 3. Spectrum of a signal with different types of waveform

distortion. (Current in A, Frequency in Hz.)

Fig. 4. Example of voltage at the terminals of a computer (sine wave)

and the resulting non-sinusoidal current.


harmonic distortion but their resonance with (mainly

transformer) impedances leads to an amplification of the

harmonic currents and voltages generated by non-linear

loads.

The harmonic distortion due to rotating machines is

discussed in detail in [2].

3.2. 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 differentmethods of harmonic analysis.

. Time domain study : the system (i.e. network and load)

are modelled in detail after which a time-domain

study is done resulting in the actual waveforms. The

harmonic components are obtained by applying a

Fourier transform to the waveforms.

. Frequency domain study : a separate system model is

made for each frequency component included in thestudy. Each single-frequency model is relatively

simple as it only needs to be valid for that specific

frequency. The resulting models are the same as used

for fundamental-frequency analysis resulting in com-

plex voltages and currents. The main difference, and

also the main difficulty, is in the choice of the

impedance values. Especially for higher frequency

components different models are needed becausevarious capacitive currents become significant, but

the calculation methods remain the same. More

details of frequency-domain studies are found in [3]

The term ‘harmonic analysis’ is normally used for the

second method, but the first method will equally result

in a harmonic spectrum. The reason that the second

method is most commonly used is its simplicity: the

same analysis methods can be applied to harmonic

components as to the fundamental frequency. The basic

assumptions behind this method are that the non-linearity is restricted to a limited number of components

(in most cases loads) and that the current waveform of a

non-linear component is not significantly affected by the

voltage waveform. Harmonic analysis studies for large

transmission systems are discussed in detail in [3].

3.3. Consequences

Harmonic voltage distortion leads to harmonic cur-

rents through linear loads. These harmonic currents may

cause extra losses in the loads which in turn requires de-

rating of the load. The effect is especially severe for

lower-order voltage harmonics at the terminals ofrotating machines. Negative-sequence voltages have

the same effect. Rotating machines are designed for a

given maximum amount of voltage unbalance. The

presence of voltage distortion limits the immunity of

the machine for voltage unbalance. The effect of

harmonic distortion in rotation machines is discussed

in detail in [2].

Whereas machines are mainly affected by lower-orderharmonics, capacitor banks are mainly affected by

higher-order harmonics.

Some sensitive electronic loads are negatively affected

by high harmonic voltage distortion. The effect on such

loads is however not so much related to the harmonic

spectrum but to the actual waveform, e.g. notching and

multiple zero-crossings. An indirect effect of harmonic

voltage distortion is that the efficiency of rectifiersbecomes less when the crest factor (the maximum of

the voltage waveform) decreases. Loads also become

more sensitive to voltage dips.

Fig. 5. Harmonic voltage distortion (5th harmonic and THD)

measured over a 6-day period in Gothenburg, Sweden.

Fig. 6. Harmonic voltage distortion (5th harmonic and THD)

measured over a 6-day period in Shanghai, China.


A high crest factor (harmonic overvoltage) on the

other hand may cause faster ageing of the insulation.

The main effect of harmonic current distortion is

overheating of series components like transformersand cables. The heating is proportional to the r.m.s.

current; whereas, the transported energy is related to the

fundamental component. For a given active power, the

heating increases with increasing current distortion. The

effect, however, is more severe than would follow from

this reasoning as the resistance of transformers increases

with frequency. The higher order harmonics thus

produce more heating per Ampere than the fundamentalcomponent. Heavily distorted current waveforms re-

quire a de-rating of transformers. The effect is also

present for cables and lines, but to a lesser extent.

The rating of power-electronic series components like

UPS and static transfer switch is determined mainly by

the peak value of the current, not so much by its r.m.s.

value. A current with a high crest factor (as is very

common with electronic load) will require a significantderating.

Third harmonic currents lead to a large sum current

through the neutral conductor. This current may cause

overheating if the neutral conductor is designed not to

carry any significant current and is not equipped with

overload protection. Many single-phase loads cause a

large third harmonic current which could lead to neutral

overload. The problem is especially present in low-voltage installations with large amounts of computers or

energy-saving lighting.

3.4. Mitigation

Mitigation is in the harmonic context often seen as

synonym to reduction of harmonic voltage or current

distortion. However the problem can also be mitigated

by improving the immunity of equipment. De-rating oftransformers and motors is a way of mitigating the

harmonic problem, albeit not necessarily the most

economic solution.

A more common way of tackling the harmonic

problem is by installing filters, typically LC-series

connections that shunt the unwanted harmonic current

components back to the load. The harmonic currents

remain high but they do not spread through the systemand do not cause much harmonic voltage distortion.

The disadvantages of these so-called passive filters (high

risk of overload, introduction of new resonances) has

led to the development of so-called active filters where

the current is fully controlled and adjusted to the

existing voltage or current distortion.

Other mitigation methods include improvements in

the network (de-rating of transformers, splitting sensi-tive and polluting loads) and improvements in the load.

The latter includes a more sinusoidal current waveform

(reduced emission) but also an increased immunity to

voltage distortion. Reduced emission is seen by many as

the preferred long-term solution of the harmonic

distortion problem. One may however wonder if this is

indeed the cheapest solution. As the number of concreteproblems due to harmonic distortion remains relatively

small, keeping the distortion at its current level or even

allowing a further increase may be a cheaper overall

solution.

An important component in addressing harmonic

problems is in defining limits to harmonic voltage and

current distortion. The limits on harmonic voltage

distortion as mentioned in various national and inter-national standards are mainly a formalisation of the

already existing distortion. For harmonic current limits,

IEC and IEEE use two principally different approaches.

The IEC standards set limits to the amount of emission

of individual equipment, whereas the IEEE harmonic

standard limits the emission per customer. Under the

IEEE standard the responsibility lies with the customer

who may decide to install filters instead of buying betterequipment. Under the IEC standards the responsibility

lies with the manufacturers of polluting equipment. The

difference can be traced back to the aim of the

documents: the IEEE standard aimed at regulating the

connection of large industrial customers, whereas the

IEC document mainly aims at small customers that do

not have the means to choose between mitigation

options.

3.5. Future research directions

Most of the research on harmonic waveform distor-

tion has been done at universities, with emphasis on

harmonic analysis studies in large transmission systems.

Further calibration with measurements is required to

test the various network and load models. The avail-

ability of a growing amount of monitoring equipmentmake such studies feasible.

An important question that remains to be answered is

where the ‘optimal distortion level’ is. The consequences

of harmonic distortion should be studied, both for

existing distortion levels and for higher levels. The

discussion is ongoing about how much the distortion

level may increase before serious problems occur.

Another direction of research is in improved equip-ment. Large PWM converters are not only able to

produce a sinusoidal waveform, they are even able to

mitigate the distortion produced by other loads. The

installation of additional control algorithms on equip-

ment with PWM converters (wind turbines, large drives)

may lead to a reduction of harmonic distortion without

much extra costs. The development of these algorithms

may be encouraged by the network operators by settingup a harmonic-distortion market. Such a market

requires some additional fundamental research in find-

ing adequate market mechanisms. Another develop-


ment-related research direction is on active harmonic

filters.

4. 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 ofovercurrents leading to voltage dips are motor starting,

transformer energising and faults. Also capacitor en-

ergising and switching of electronic load lead to short-

duration overcurrents, but the duration of the over-

current 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 andearth 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.

An example of a measured voltage dip is shown in

Fig. 7, where the three voltage waveforms are given. A

more common way of presenting a voltage dip is

through the r.m.s. voltages as a function of time. The

r.m.s. voltage is calculated over a window of typicallyone cycle duration and updated one or more times per

cycle. Fig. 8 shows the r.m.s. voltage for the dip in Fig.

7; the calculation is updated every sample in this

example. The voltage dip shown is due to a phase-to-

phase fault in an underground cable that develops into a

three-phase fault within two cycles.

Voltage dips are generally seen as undesired events,

but a more positive viewpoint could equally well seethem as a consequence of the high reliability of the

power supply. Without the wide-spread use of protec-

tion equipment any fault would lead to the loss of

supply for a large fraction of the customers. The

protection significantly limits the numbers of customers

that experience a long interruption, in many cases to

zero. However many customers who would experience

an interruption without protection now experience a

voltage dip. This way of protection has been good

enough for many years, but recently more and more

problems with end-user equipment are reported due tothese voltage dips. Not only has, especially electronic,

equipment become more susceptible to voltage dips,

companies have also become less tolerant of production

stoppages.

4.1. Consequences

As mentioned before, a voltage dip is a reduction involtage. This reduction in voltage leads to a reduction in

energy-transfer capability of the system. This has, for

long been, a well-known basis of transient-stability

studies: the undervoltage due to a fault leads to a

reduction of the power transfer from the generators to

the motors; motors slow down, generators speed up.

This phenomenon limits e.g. the fault-clearing time in

transmission systems and also rules the connection ofwind farms to the grid. Motor-starting dips equally

become a concern when they lead to excess loss of speed

for neighbouring motors (or speed gain for generators).

Typical limits for stability concerns in distribution

systems are 70% voltage during 1 s. These events do

not occur very often in the public supply, and in

industrial power systems the stability issues are a

standard part of the design.Many modern (power)-electronic devices like compu-

ters, process-controllers, and adjustable-speed drives

already experience operational problems when the

voltage drop below 85% for 40 ms. These events occur

ten times per year or more, causing a serious concern.

What most sensitive equipment has in common is that

it is connected to the power system through a rectifier

that converts a.c. to d.c. The d.c. voltage is thenconverted to the actual application voltage. A voltage

dip on the a.c. side of the rectifier leads very fast to a

drop in d.c. voltage. This in turn causes problems withFig. 7. Example of a voltage dip.

Fig. 8. R.m.s. voltages for the voltage dip shown in Fig. 7.


the application voltages. An additional problem is the

large inrush current that occurs when the voltage

recovers after the dip (i.e. upon fault clearing). This

recovery inrush may lead to damage in the rectifiercomponents. For three-phase rectifiers, also the unba-

lance of the currents through the rectifier and the ripple

in the d.c. voltage are of concern. An example of the d.c.

bus voltage behind a three-phase diode rectifier is shown

in Fig. 9. The simulated event is a drop in the voltage

between two phases to 50% of its pre-event value. The

effect of voltage dip on adjustable-speed drives is

discussed in more detail in [4].

4.2. Characterisation and indices

To be able to characterise voltage dip events somekind of processing of the sampled voltage waveforms is

needed. This is defined rather well in the standard

document IEC 61000-4-30, where remaining voltage and

duration are defined as the two main characteristics to

quantify a voltage dip. Both are obtained from the r.m.s.

voltage as a function of time (see Fig. 8). The

characterisation of three-phase measurements remains

a point of discussion. The current practice (using theworst channel) is not very satisfactory. The character-

isation of three-phase measurements is discussed in

more detail in [5].

Having defined the characteristics of a single event, it

becomes possible to describe the performance of a site

and even of a whole network. For processing of event

indices into site and system-indices, there exist two

different schools of thought which for the time beinghave shown incompatible.

. The principle ‘don’t throw away too much informa-

tion’ , typically results in a table with the number of

events per year for different remaining voltage and

duration. There is an ongoing discussion about how

to group remaining voltage and duration into bins.

Examples are the Unipede disdip table, IEC 61000-2-8 and IEEE Std. 493 and 1346.

. The opposite principle ‘keep it simple’ , results in a

small number of indices, ideally just one index. The

commonly-used SARFI indices belong to this school

as well as several proposals to quantify supply

performance by just one number.

Important in the discussion on voltage-dip indices is

to consider that they can be obtained by measurements

as well as by simulations. Measurement (power quality

monitoring) is a good way of assessing the performanceof a site or system, in the end measurement is the only

exact method. But measurements have limited predictive

value due to the large year-to-year and site-to-site

differences. To predict voltage-dip performance a large

number of monitors are needed for a long period of

time. Stochastic prediction methods are much more

suitable for performance prediction, e.g. for comparing

different mitigation methods.

4.3. Mitigation methods

What has to be mitigated here is the tripping ofequipment due to voltage dips. This can be done in a

number of ways:

. Reducing the number of faults . There are several well-

known methods for this like tree-trimming, animal

guards, and shielding wires, but also replacing over-

head lines by underground cables. As most of the

severe dips are due to faults, this will directly affect

the dip frequency.

. Faster fault clearing . This requires improved protec-tion techniques. Much gain can be obtained in

distribution networks, but at transmission level the

fault-clearing time is already very short. Further

improvement at transmission level would require

the development of a new generation of circuit

breakers and relays.

. Improved network design and operation . The network

can be changed such that a fault will not lead to asevere dip at a certain location. This has been a

common practice in the design of industrial power

systems, but not in the public supply. Possible

options are to remove long overhead feeders from

busses supplying sensitive customers, and connecting

on-site generators at strategic locations. Also the use

of very fast transfer switches can be seen as a

network-based solution.. Mitigation equipment at the interface . The most

commonly-used method of mitigating voltage dips

is connecting a UPS or a constant-voltage transfor-

Fig. 9. Voltage at the d.c. bus of a three-phase adjustable-speed drive

before and during a voltage dip. Solid curve: large capacitor; dashed

curve: small capacitor.


mer between the system and the sensitive load. For

large loads the static series compensator of DVR

(dynamic voltage restorer) is a possible solution.

Power-electronic solutions are discussed in moredetail in [6].

. Improved end-user equipment . Making the equipment

immune against all voltage dips would also solve the

problem, but it is for most equipment not (yet)

feasible. Methods of improving equipment behaviour

will be discussed in more detail in [4].

The ongoing discussion on voltage-dip mitigation

concerns the responsibility sharing between the custo-

mer and network: should the solution be sought in the

network or with the customer. In some cases the costs of

mitigation equipment are shared or power quality

contracts define the responsibility. In the long run an

agreement has to be reached between what are ‘normal

dips’ and what are ‘abnormal dips’. For normal dips

end-user equipment is expected to be immune, whereas

abnormal dips should have a small frequency of

occurrence, see Fig. 10.

4.4. Future research

Research on voltage dips includes development-re-

lated research on mitigation equipment and improved

end-user equipment. It also includes education-related

research on the relation between voltage-dip frequency

and system design and operation.

Fundamental research is needed on voltage dip

characteristics and indices, especially on methods for

extracting system indices with a limited number of

monitors and on suitable single-index methods. Related

work is needed on the extraction of additional informa-

tion from voltage-dip recordings. This is one of the

possible applications for signal-processing techniques as

are discussed in [7].

Fundamental research is also needed on stochastic

prediction methods, including a large number of com-

parisons with monitoring results to find the limitations

of stochastic prediction.Most of the work on consequences of voltage dips has

been directed towards adjustable-speed drives. With the

increase in embedded (renewable) generation more work

should be done on the effect of voltage dips on

generation, especially on inverter-based interfaces.

5. 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-frequencywaveform 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 power-

quality 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 theIEC standards [19�/27] and the IEEE standards [28�/34]

on power quality contain useful basic knowledge and

overviews.

Harmonics, voltage dips and interruptions will be-

come a normal part of power system design and

operation and of the design of end-user equipment.

Transients and high-frequency distortion will require

much more attention before they reach this stage.The fact that power quality is becoming more mature

does not mean that it will disappear as a subject that

deserves attention from academics. There remain inter-

esting research topics that await being taken up. Some

examples for harmonics and voltage dips are presented

in this paper. Another important task for academics is

to incorporate power quality issues in education.

Spreading knowledge on potential power quality pro-blems (and not only to power engineers) will make it

more likely that future problems will be addressed

before they actually occur.

Acknowledgements

The measurements presented in this paper were

obtained with the help of Christian Roxenius (Goteborg

Energi Nat), Mats Hager (STRI) and Gu Zengti.Fig. 10. Distinction between events that are the responsibility of the

customer and those that are the responsibility of the network operator.


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