...................................................................................
Cahier technique no. 199
Power Quality
Ph. Ferracci
Collection Technique
"Cahiers Techniques" is a collection of documents intended for engineers
and technicians, people in the industry who are looking for more in-depth
information in order to complement that given in product catalogues.
Furthermore, these "Cahiers Techniques" are often considered as helpful
"tools" for training courses.
They provide knowledge on new technical and technological developments
in the electrotechnical field and electronics. They also provide better
understanding of various phenomena observed in electrical installations,
systems and equipment.
Each "Cahier Technique" provides an in-depth study of a precise subject in
the fields of electrical networks, protection devices, monitoring and control
and industrial automation systems.
The latest publications can be downloaded from the Schneider Electric
internet web site.
Code: http://www.schneider-electric.com
Section: The expert's place
Please contact your Schneider Electric representative if you want either a
"Cahier Technique" or the list of available titles.
The "Cahiers Techniques" collection is part of Schneider Electric’s
"Collection technique".
Foreword
The author disclaims all responsibility subsequent to incorrect use of
information or diagrams reproduced in this document, and cannot be held
responsible for any errors or oversights, or for the consequences of using
information and diagrams contained in this document.
Reproduction of all or part of a "Cahier Technique" is authorised with the
prior consent of the Scientific and Technical Division. The statement
"Extracted from Schneider Electric "Cahier Technique" no. ….." (please
specify) is compulsory.
no. 199
Power Quality
ECT 199(e) october 2001
Philippe FERRACCI
Graduated from the "École Supérieure d’Électricité" in 1991, he wrote
his thesis on the resonant earthed neutral system in cooperation with
EDF-Direction des Etudes et Recherches.
He joined Schneider Electric in 1996, where he now conducts
advanced research into the area of electrotechnical and electrical
power systems.
Cahier Technique Schneider Electric no. 199 / p.2
Cahier Technique Schneider Electric no. 199 / p.3
Power Quality
One of the properties of electricity is that some of its characteristics depend
not only on the electricity producer/distributor but also on the equipment
manufacturers and the customer. The large number of players combined
with the use of terminology and definitions which may sometimes be
imprecise partly explain why this subject area is so complex.
This "Cahier Technique" aims to facilitate exchanges on this topic between
specialists and non-specialists, as well as customers, manufacturers,
installers, designers and distributors. The clear terminology used should
help avoid confusion. It describes the main phenomena causing
degradation in Power Quality (PQ), their origins, the consequences for
equipment and the main solutions. It offers a methodology for measuring
the PQ in accordance with differing aims. Illustrated with practical
examples for the implementation of solutions, it shows that only by
observing best practice and by applying strict methodology (diagnostics,
research, solutions, implementation and preventive maintenance) can
users obtain the right quality of power supply for their requirements.
Contents
1 Introduction 1.1 Context p.4
1.2 Objectives of Power Quality measurement p.5
2 Degradation of PQ: origins - 2.1 General p.6
characteristics - definitions 2.2 Voltage dips and interruptions p.6
2.3 Harmonics and interharmonics p.8
2.4 Overvoltages p.10
2.5 Voltage variations and fluctuations p.10
2.6 Unbalance p.11
2.7 Summary p.11
3 Effects of disturbance on loads 3.1 Voltage dips and interruptions p.12
and processes 3.2 Harmonics p.13
3.3 Overvoltages p.15
3.4 Voltage variations and fluctuations p.15
3.5 Unbalance p.15
3.6 Summary p.15
4 Level of power quality 4.1 Evaluation methodology p.16
4.2 EMC and planning levels p.18
5 Solutions for improving PQ 5.1 Voltage dips and interruptions p.19
5.2 Harmonics p.23
5.3 Overvoltages p.25
5.4 Voltage fluctuations p.26
5.5 Unbalance p.26
5.6 Summary p.26
6 Case studies 6.1 Hybrid filtering p.27
6.2 Real time reactive compensation p.28
6.3 Protection against lightning p.30
7 Conclusion p.31
Bibliography p.32
Cahier Technique Schneider Electric no. 199 / p.4
1 Introduction
1.1 Context
The widespread use of equipment which is
sensitive to voltage disturbance and/or
generates disturbance itself
As a consequence of their numerous
advantages (flexible operation, excellent
efficiency, high performance levels, etc.), we
have seen the development and widespread use
of automated systems and adjustable speed
drives in industry, information systems, and fluo-
compact lighting in the service and domestic
sectors. These types of equipment are both
sensitive to voltage disturbance and generate
disturbance themselves.
Their multiple use within individual processes
requires an electrical power supply which can
provide ever increasing performance in terms of
continuity and quality. The temporary shutdown of
just one element in the chain may interrupt the
whole production facilities (manufacture of semi-
conductors, cement works, water treatment,
materials handling, printing, steelworks,
petrochemicals, etc.) or services (data processing
centres, banks, telecommunications, etc.).
Consequently, the work of the IEC on
electromagnetic compatibility (EMC) has led to
stricter and stricter standards and
recommendations (limitations on disturbances
emission levels, etc.).
The opening up of the electricity market
The rules governing the electricity sector are
undergoing radical change: electricity production
has opened up to competition, production is
decentralised, and (large) electricity consumers
now have the opportunity to choose their supplier.
In 1985, the Commission of the European
Communities states (directive 85/374) that
electricity is to be considered a product and as a
consequence made it necessary to define its
essential characteristics clearly.
In addition, in the context of liberalising energy
markets, the search for competitiveness by
electricity companies now means that quality has
become a differentiating factor. A guarantee of
quality is a potential criterion of choice for industrial
users when looking for an energy supplier.
The quality of electricity has become a strategic issue
for electricity companies, the operating, maintenance
and management personnel of service sector
and industrial sites, as well as for equipment
manufacturers, for the following main reasons:
c the economic necessity for businesses to
increase their competitiveness,
c the widespread use of equipment which is
sensitive to voltage disturbance and/or
generates disturbance itself,
c the opening up of the electricity market.
The economic necessity for businesses
to increase their competitiveness
c Reduction of costs linked to loss of supply
continuity and problems of non-quality
The cost of disturbance (interruptions, voltage dips,
harmonics, lightning overvoltages, etc.) is substantial.
These costs must take into account losses in
production and raw materials, restarting of
production facilities, non-quality of production
and delivery delays. The malfunction or
shutdown of vital equipment such as computers,
lighting and safety systems may put lives at risk
(e.g. in hospitals, airport lighting systems, public
and high-rise buildings, etc.).
Costs also include high quality, targeted
preventive maintenance measures for
anticipating possible problems. There is an
increasing transfer of responsibility from the
industrial user to the equipment manufacturer for
the provision of site maintenance; manufacturers
are now becoming electricity suppliers.
c Reduction of costs linked to oversized
installations and energy bills
Other less obvious consequences of PQ
degradation are:
v A reduction of installation energy efficiency,
leading to higher energy bills
v Overloading of the installation, causing
premature ageing and increasing the risk of
breakdown, leading in turn to oversizing of
distribution equipment
This is why professional users of electricity are
keen to optimise the operation of their electrical
installations.
Cahier Technique Schneider Electric no. 199 / p.5
1.2 Objectives of Power Quality measurement
The measurement parameters and accuracy
may differ depending on the application.
Contractual application
Within the context of a deregulated market,
contractual relations may exist not only
between the electricity supplier and the end
user, but also between the power production
company and transmission company or between
the transmission company and distribution
company. A contractual arrangement requires
that terms are defined jointly and mutually
agreed upon by all parties. The parameters for
measuring quality must therefore be defined
and the values compared with predefined, i.e.
contractual limits.
This arrangement frequently requires the
processing of significant quantities of data.
Corrective maintenance
Even where best practice is observed (single-
line diagram, choice of protective devices and
neutral point connection, application of
appropriate solutions) right from the design
phase, malfunctions may occur during
operation:
c Disturbances may have been ignored or
under-estimated.
c The installation may have changed (new
loads and/or modification).
Troubleshooting is generally required as a
consequence of problems of this nature.
The aim is frequently to get results as quickly
as possible, which may lead to premature or
unfounded conclusions.
Portable measurement systems (for limited
periods) or fixed apparatus (for continuous
monitoring) make it easier to carry out
installation diagnostics (detection and
archiving of disturbances and triggering of
alarms).
Optimising the operation of electrical
instal lat ions
To achieve productivity gains (operational
economies and/or reduction of operating
costs) correct operation of processes and
sound energy management are required, both
of which are factors dependent on PQ.
Operating, maintenance and management
personnel of service sector and industrial sites
all aim for a PQ which matches their
requirements.
Complementary software tools to ensure
control-command and continuous monitoring of
the installation are thus required.
Statistical surveys
Such research requires a statistical approach on
the basis of wide-ranging results from surveys
generally carried out by the operators of
transmission and distribution power systems.
c Benchmark the general performances of a
power system
These can be used, for example, to:
v Plan and target preventive actions by mapping
disturbance levels on a network. This helps
reduce operating costs and improve control of
disturbance. An abnormal situation with respect
to an average level can be detected and
correlated with the addition of new loads.
Research can also be carried out into seasonal
trends or excessive demand.
v Compare the PQ of various distribution
companies in different geographical areas.
Potential customers may request details of the
reliability of the electricity supply before installing
a new plant.
c Benchmark performances at individual points
on the power system
These can be used to:
v Determine the electromagnetic environment in
which a future installation or a new piece of
equipment may have to operate. Preventive
measures may then be taken to improve the
distribution power system and/or desensitise the
customer power system.
v Specify and verify the performance levels
undertaken by the electricity supplier as part of
the contract. This information on the electricity
quality are of particular strategic importance for
electricity companies who are seeking to
improve competitiveness, satisfaction of needs
and customer loyalty in the context of liberalising
energy markets.
Cahier Technique Schneider Electric no. 199 / p.6
2.1 General
2 Degradation of PQ: origins - characteristics - definitions
Electromagnetic disturbances which are likely to
disturb the correct operation of industrial
equipment and processes is generally ranked in
various classes relating to conducted and
radiated disturbance:
c low frequency (< 9 kHz),
c high frequency (u 9 kHz),
c electrostatic discharge.
Measurement of PQ usually involves
characterising low frequency conducted
electromagnetic disturbances (the range is
widened to include transient overvoltages and
transmission of signals on a power system):
c voltage dips and interruptions,
c harmonics and interharmonics,
c temporary overvoltages,
c swell,
c transient overvoltages,
c voltage fluctuations,
c voltage unbalance,
c power-frequency variations,
c DC in AC networks,
c signalling voltages.
It is not generally necessary to measure each
type of disturbance.
The types can be placed in four categories,
affecting the magnitude, waveform, frequency
and symmetry of the voltage. Several of these
characteristics may be modified simultaneously
by any one type of disturbance. Disturbances
can also be classified according to their
permanent, semi-permanent or random nature
(lightning, short-circuit, switching operations,
etc.).
2.2 Voltage dips and interruptions
Definitions
A voltage dip is a sudden reduction of the
voltage at a point in an electrical power system
followed by voltage recovery after a short period
of time from a few cycles to a few seconds
(IEC 61050-161 ). A voltage dip is normally
detected and characterised by the calculation of
the root mean square value "rms (1/2)" over one
cycle every half-cycle -each period overlaps the
prior period by one half-cycle- (see fig. 1).
There is a dip to x % if the rms (1/2) value falls
below the dip threshold x % of the reference
value Uref. The threshold x is typically set below
90 (CENELEC EN 50160, IEEE 1159). The
reference voltage Uref is generally the nominal
voltage for LV power systems and the declared
voltage for MV and HV power systems. A sliding
reference voltage, equal to the voltage before
the beginning of the disturbance is useful to
study transference factor between different
voltage systems.
A voltage dip is characterised by two parameters
(see fig. 1b for x equal to 90):
c depth: ∆U (or its magnitude U),
c duration ∆T.
In case of a non-rectangular envelope, the
duration is dependent on the selected dip
threshold value (set by the user according to the
objective). The duration is typically defined as
the time interval during which the rms (1/2) is
lower than 90 %. The shape of the envelope (for
example in case of complex multi-step and not
simple one step dip) may be assessed using
several dip thresholds set and/or wave form
capture. Time aggregation techniques may
define an equivalent dip characterised by the
smallest rms (1/2) value measured during the dip
and the total duration of the dip. For three-phase
systems phase aggregation techniques (mainly
used for contractual applications) may define a
single phase equivalent dip (characterised for
example by the greatest depth on the three
phases and the total duration).
Interruptions are a special type of voltage dip to
a few percentage of Uref (typically within the
range 1-10 %). They are characterised by one
parameter only: the duration. Short interruptions
last less than one minute (extended to three
minutes depending on network operating
conditions) and often result from tripping and
automatic reclosure of a circuit breaker designed
Cahier Technique Schneider Electric no. 199 / p.7
to avoid long interruptions which have longer
duration. Short and long interruptions differ in
both their origins and the solutions required to
prevent or reduce their occurrence.
Voltage disturbances lasting less than a half-
cycle T (∆T < T/2) are regarded as transient.
Different terms are used in the USA depending
on the length of the dips (sags) and interruptions:
c instantaneous (T/2 < ∆T < 30 T),
c momentary (30 T < ∆T < 3 s),
c temporary (3 s < ∆T < 1 min),
c sustained interruption and undervoltage
(∆T > 1 min).
Depending on the context, the measured
voltages may be between live conductors
(between phases or between phase and
neutral), between live conductors and earth (Ph/
earth or neutral/earth), or between live
conductors and the protective conductor.
In a 3-phase system, the characteristics ∆U and
∆T in general differ for each of the three phases.
This is why a voltage dip must be detected and
characterised separately on each phase.
A voltage dip is regarded as occurring on a
3-phase system if at least one phase is affected
by the disturbance.
Origins
c Voltage dips and short interruptions are
mainly caused by phenomena leading to high
currents, which in turn cause a voltage drop
across the network impedances with a
magnitude which decreases in proportion to the
electrical distance of the observation point from
the source of the disturbance.
Voltage dips and short interruptions have various
causes:
v Faults on the transmission (HV) or distribution
(LV and MV) networks or on the installation itself
The occurrence of faults causes voltage dips for
all users. The duration of a dip is usually
conditioned by the operating time of the
protective devices. The isolation of faults by
protective devices (circuit breakers, fuses) will
produce interruptions (long or short) for users
feeded by the faulty section of the power
system. Although the power source is no longer
present, network voltage may be maintained by
the residual voltage provided by asynchronous
or synchronous motors as they slow down (0.3
to 1 s) or voltage due to the discharge of
capacitor banks connected to the power system.
Short interruptions are often the result of the
operation of automated systems on the network
such as fast and/or slow automatic reclosers, or
changeover of transformers or lines. Users are
Fig. 1: Characteristic parameters of a voltage dip [a]
waveform [b] rms (1/2).
subjected to a succession of voltage dips and/or
short interruptions caused by intermittent arc
faults, sequence of automatic reclosing (on
overhead or mixed radial networks) intended to
extinguish transient and semi-permanent faults
or voltage feedback intended to locate the fault.
v Switching of large loads (asynchronous
motors, arc furnaces, welding machines, boilers,
etc.) compared to the short-circuit power.
c Long interruptions are the result of the
definitive isolation of a permanent fault
(requiring to repair or to replace any component
before re-energising) by means of protective
devices or by the intentional or unintentional
opening of a device.
Voltage dips and interruptions are propagated
to lower voltage levels via transformers. The
number of phases affected and the depth of
the voltage dips depend on the type of fault
and the transformer coupling.
-1
1
0,5
0
10
70
90
100
110
rms (1/2)
(%)
V(p.u.)
U
(magnitude)
t (ms)
∆T = 140 ms
∆U = 30 %
(depth)
-0,5
0
0 50 100 150 200 250 300
t
a
b
(duration)
Cahier Technique Schneider Electric no. 199 / p.8
Overhead networks, which are exposed to
bad weather, are subject to more voltage dips
and interruptions than underground
networks. However, an underground feeder
connected to the same busbar system as
overhead or mixed networks will suffer voltage
dips which are due to the faults affecting
overhead lines.
c Transients (∆T < T/2) are caused, for
example, by the energisation of capacitor banks,
the isolation of a fault by a fuse or a fast LV
circuit breaker, or by commutation notches
from polyphase converters.
2.3 Harmonics and interharmonics
Summary:
All periodic functions (of frequency f) can be
broken down into a sum of sinusoidal waves of
frequency h x f (h is an integer). h is the
harmonic order (h > 1). The first order
component is the fundamental component.
y(t) Y Y 2 sin(2 h f )0 h
h 1
h = +