Power Quality - schneider- · PDF fileCahier Technique Schneider Electric no. 199 / p.3 Power Quality One of the properties of electricity is that some of its characteristics depend

...................................................................................

Cahier technique no. 199

Power Quality

Ph. Ferracci

Collection Technique

"Cahiers Techniques" is a collection of documents intended for engineersand technicians, people in the industry who are looking for more in-depthinformation 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 developmentsin the electrotechnical field and electronics. They also provide betterunderstanding of various phenomena observed in electrical installations,systems and equipment.Each "Cahier Technique" provides an in-depth study of a precise subject inthe fields of electrical networks, protection devices, monitoring and controland industrial automation systems.

The latest publications can be downloaded from the Schneider Electricinternet web site.Code: http://www.schneider-electric.comSection: 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 ofinformation or diagrams reproduced in this document, and cannot be heldresponsible for any errors or oversights, or for the consequences of usinginformation and diagrams contained in this document.

Reproduction of all or part of a "Cahier Technique" is authorised with theprior consent of the Scientific and Technical Division. The statement"Extracted from Schneider Electric "Cahier Technique" no. ….." (pleasespecify) is compulsory.

no. 199Power Quality

ECT 199(e) october 2001

Philippe FERRACCI

Graduated from the "École Supérieure d’Électricité" in 1991, he wrotehis thesis on the resonant earthed neutral system in cooperation withEDF-Direction des Etudes et Recherches.He joined Schneider Electric in 1996, where he now conductsadvanced research into the area of electrotechnical and electricalpower systems.

Cahier Technique Schneider Electric no. 199 / p.2


Power Quality

One of the properties of electricity is that some of its characteristics dependnot only on the electricity producer/distributor but also on the equipmentmanufacturers and the customer. The large number of players combinedwith the use of terminology and definitions which may sometimes beimprecise partly explain why this subject area is so complex.

This "Cahier Technique" aims to facilitate exchanges on this topic betweenspecialists and non-specialists, as well as customers, manufacturers,installers, designers and distributors. The clear terminology used shouldhelp avoid confusion. It describes the main phenomena causingdegradation in Power Quality (PQ), their origins, the consequences forequipment and the main solutions. It offers a methodology for measuringthe PQ in accordance with differing aims. Illustrated with practicalexamples for the implementation of solutions, it shows that only byobserving best practice and by applying strict methodology (diagnostics,research, solutions, implementation and preventive maintenance) canusers obtain the right quality of power supply for their requirements.

Contents

1 Introduction 1.1 Context p.41.2 Objectives of Power Quality measurement p.5

2 Degradation of PQ: origins - 2.1 General p.6characteristics - 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.12and processes 3.2 Harmonics p.13


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


5.4 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


1 Introduction

1.1 Context

The widespread use of equipment which issensitive to voltage disturbance and/orgenerates disturbance itselfAs a consequence of their numerousadvantages (flexible operation, excellentefficiency, high performance levels, etc.), wehave seen the development and widespread useof automated systems and adjustable speeddrives in industry, information systems, and fluo-compact lighting in the service and domesticsectors. These types of equipment are bothsensitive to voltage disturbance and generatedisturbance themselves.

Their multiple use within individual processesrequires an electrical power supply which canprovide ever increasing performance in terms ofcontinuity and quality. The temporary shutdown ofjust one element in the chain may interrupt thewhole production facilities (manufacture of semi-conductors, cement works, water treatment,materials handling, printing, steelworks,petrochemicals, etc.) or services (data processingcentres, banks, telecommunications, etc.).

Consequently, the work of the IEC onelectromagnetic compatibility (EMC) has led tostricter and stricter standards andrecommendations (limitations on disturbancesemission levels, etc.).

The opening up of the electricity market

The rules governing the electricity sector areundergoing radical change: electricity productionhas opened up to competition, production isdecentralised, and (large) electricity consumersnow have the opportunity to choose their supplier.

In 1985, the Commission of the EuropeanCommunities states (directive 85/374) thatelectricity is to be considered a product and as aconsequence made it necessary to define itsessential characteristics clearly.

In addition, in the context of liberalising energymarkets, the search for competitiveness byelectricity companies now means that quality hasbecome a differentiating factor. A guarantee ofquality is a potential criterion of choice for industrialusers when looking for an energy supplier.

The quality of electricity has become a strategic issuefor electricity companies, the operating, maintenanceand management personnel of service sectorand industrial sites, as well as for equipmentmanufacturers, for the following main reasons:c the economic necessity for businesses toincrease their competitiveness,c the widespread use of equipment which issensitive to voltage disturbance and/orgenerates disturbance itself,c the opening up of the electricity market.

The economic necessity for businessesto increase their competitivenessc Reduction of costs linked to loss of supplycontinuity and problems of non-qualityThe cost of disturbance (interruptions, voltage dips,harmonics, lightning overvoltages, etc.) is substantial.These costs must take into account losses inproduction and raw materials, restarting ofproduction facilities, non-quality of productionand delivery delays. The malfunction orshutdown of vital equipment such as computers,lighting and safety systems may put lives at risk(e.g. in hospitals, airport lighting systems, publicand high-rise buildings, etc.).Costs also include high quality, targetedpreventive maintenance measures foranticipating possible problems. There is anincreasing transfer of responsibility from theindustrial user to the equipment manufacturer forthe provision of site maintenance; manufacturersare now becoming electricity suppliers.c Reduction of costs linked to oversizedinstallations and energy billsOther less obvious consequences of PQdegradation are:v A reduction of installation energy efficiency,leading to higher energy billsv Overloading of the installation, causingpremature ageing and increasing the risk ofbreakdown, leading in turn to oversizing ofdistribution equipmentThis is why professional users of electricity arekeen to optimise the operation of their electricalinstallations.


1.2 Objectives of Power Quality measurement

The measurement parameters and accuracymay differ depending on the application.

Contractual application

Within the context of a deregulated market,contractual relations may exist not onlybetween the electricity supplier and the enduser, but also between the power productioncompany and transmission company or betweenthe transmission company and distributioncompany. A contractual arrangement requiresthat terms are defined jointly and mutuallyagreed upon by all parties. The parameters formeasuring quality must therefore be definedand the values compared with predefined, i.e.contractual limits.This arrangement frequently requires theprocessing of significant quantities of data.

Corrective maintenance

Even where best practice is observed (single-line diagram, choice of protective devices andneutral point connection, application ofappropriate solutions) right from the designphase, malfunctions may occur duringoperation:c Disturbances may have been ignored orunder-estimated.c The installation may have changed (newloads and/or modification).

Troubleshooting is generally required as aconsequence of problems of this nature.The aim is frequently to get results as quicklyas possible, which may lead to premature orunfounded conclusions.Portable measurement systems (for limitedperiods) or fixed apparatus (for continuousmonitoring) make it easier to carry outinstallation diagnostics (detection andarchiving of disturbances and triggering ofalarms).

Optimising the operation of electricalinstal lat ions

To achieve productivity gains (operationaleconomies and/or reduction of operatingcosts) correct operation of processes andsound energy management are required, bothof which are factors dependent on PQ.Operating, maintenance and managementpersonnel of service sector and industrial sitesall aim for a PQ which matches theirrequirements.

Complementary software tools to ensurecontrol-command and continuous monitoring ofthe installation are thus required.

Statistical surveys

Such research requires a statistical approach onthe basis of wide-ranging results from surveysgenerally carried out by the operators oftransmission and distribution power systems.

c Benchmark the general performances of apower systemThese can be used, for example, to:v Plan and target preventive actions by mappingdisturbance levels on a network. This helpsreduce operating costs and improve control ofdisturbance. An abnormal situation with respectto an average level can be detected andcorrelated with the addition of new loads.Research can also be carried out into seasonaltrends or excessive demand.v Compare the PQ of various distributioncompanies in different geographical areas.Potential customers may request details of thereliability of the electricity supply before installinga new plant.

c Benchmark performances at individual pointson the power systemThese can be used to:v Determine the electromagnetic environment inwhich a future installation or a new piece ofequipment may have to operate. Preventivemeasures may then be taken to improve thedistribution power system and/or desensitise thecustomer power system.v Specify and verify the performance levelsundertaken by the electricity supplier as part ofthe contract. This information on the electricityquality are of particular strategic importance forelectricity companies who are seeking toimprove competitiveness, satisfaction of needsand customer loyalty in the context of liberalisingenergy markets.


2.1 General

2 Degradation of PQ: origins - characteristics - definitions

Electromagnetic disturbances which are likely todisturb the correct operation of industrialequipment and processes is generally ranked invarious classes relating to conducted andradiated disturbance:c low frequency (< 9 kHz),c high frequency (u 9 kHz),c electrostatic discharge.

Measurement of PQ usually involvescharacterising low frequency conductedelectromagnetic disturbances (the range iswidened to include transient overvoltages andtransmission 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 eachtype of disturbance.The types can be placed in four categories,affecting the magnitude, waveform, frequencyand symmetry of the voltage. Several of thesecharacteristics may be modified simultaneouslyby any one type of disturbance. Disturbancescan also be classified according to theirpermanent, semi-permanent or random nature(lightning, short-circuit, switching operations,etc.).

2.2 Voltage dips and interruptions

DefinitionsA voltage dip is a sudden reduction of thevoltage at a point in an electrical power systemfollowed by voltage recovery after a short periodof time from a few cycles to a few seconds(IEC 61050-161 ). A voltage dip is normallydetected and characterised by the calculation ofthe root mean square value "rms (1/2)" over onecycle every half-cycle -each period overlaps theprior period by one half-cycle- (see fig. 1).There is a dip to x % if the rms (1/2) value fallsbelow the dip threshold x % of the referencevalue Uref. The threshold x is typically set below90 (CENELEC EN 50160, IEEE 1159). Thereference voltage Uref is generally the nominalvoltage for LV power systems and the declaredvoltage for MV and HV power systems. A slidingreference voltage, equal to the voltage beforethe beginning of the disturbance is useful tostudy transference factor between differentvoltage 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, theduration is dependent on the selected dipthreshold value (set by the user according to theobjective). The duration is typically defined asthe time interval during which the rms (1/2) islower than 90 %. The shape of the envelope (forexample in case of complex multi-step and notsimple one step dip) may be assessed usingseveral dip thresholds set and/or wave formcapture. Time aggregation techniques maydefine an equivalent dip characterised by thesmallest rms (1/2) value measured during the dipand the total duration of the dip. For three-phasesystems phase aggregation techniques (mainlyused for contractual applications) may define asingle phase equivalent dip (characterised forexample by the greatest depth on the threephases and the total duration).Interruptions are a special type of voltage dip toa few percentage of Uref (typically within therange 1-10 %). They are characterised by oneparameter only: the duration. Short interruptionslast less than one minute (extended to threeminutes depending on network operatingconditions) and often result from tripping andautomatic reclosure of a circuit breaker designed


to avoid long interruptions which have longerduration. Short and long interruptions differ inboth their origins and the solutions required toprevent 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 dependingon 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 measuredvoltages may be between live conductors(between phases or between phase andneutral), between live conductors and earth (Ph/earth or neutral/earth), or between liveconductors 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 andcharacterised separately on each phase.A voltage dip is regarded as occurring on a3-phase system if at least one phase is affectedby the disturbance.

Originsc Voltage dips and short interruptions aremainly caused by phenomena leading to highcurrents, which in turn cause a voltage dropacross the network impedances with amagnitude which decreases in proportion to theelectrical distance of the observation point fromthe source of the disturbance.Voltage dips and short interruptions have variouscauses:v Faults on the transmission (HV) or distribution(LV and MV) networks or on the installation itselfThe occurrence of faults causes voltage dips forall users. The duration of a dip is usuallyconditioned by the operating time of theprotective devices. The isolation of faults byprotective devices (circuit breakers, fuses) willproduce interruptions (long or short) for usersfeeded by the faulty section of the powersystem. Although the power source is no longerpresent, network voltage may be maintained bythe residual voltage provided by asynchronousor synchronous motors as they slow down (0.3to 1 s) or voltage due to the discharge ofcapacitor banks connected to the power system.

Short interruptions are often the result of theoperation of automated systems on the networksuch as fast and/or slow automatic reclosers, orchangeover 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/orshort interruptions caused by intermittent arcfaults, sequence of automatic reclosing (onoverhead or mixed radial networks) intended toextinguish transient and semi-permanent faultsor voltage feedback intended to locate the fault.v Switching of large loads (asynchronousmotors, arc furnaces, welding machines, boilers,etc.) compared to the short-circuit power.

c Long interruptions are the result of thedefinitive isolation of a permanent fault(requiring to repair or to replace any componentbefore re-energising) by means of protectivedevices or by the intentional or unintentionalopening of a device.Voltage dips and interruptions are propagatedto lower voltage levels via transformers. Thenumber of phases affected and the depth ofthe voltage dips depend on the type of faultand the transformer coupling.

-1

1

0,5

0

10

70

90100110

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)


Overhead networks, which are exposed tobad weather, are subject to more voltage dipsand interruptions than undergroundnetworks. However, an underground feederconnected to the same busbar system asoverhead or mixed networks will suffer voltagedips which are due to the faults affectingoverhead lines.

c Transients (∆T < T/2) are caused, forexample, by the energisation of capacitor banks,the isolation of a fault by a fuse or a fast LVcircuit breaker, or by commutation notchesfrom polyphase converters.

2.3 Harmonics and interharmonics

Summary:All periodic functions (of frequency f) can bebroken down into a sum of sinusoidal waves offrequency h x f (h is an integer). h is theharmonic order (h > 1). The first ordercomponent is the fundamental component.

y(t) Y Y 2 sin(2 h f )0 hh 1

h = + +=

∞∑ π ϕ

The rms is:

Y Y Y Y Y ...eff 02

12

22

h2= + + + +

The THD (Total Harmonic Distortion) factormeasures the signal distortion:

THDYY

h

1h 2=

=

∞∑

2

Harmonics are mainly produced by non-linearloads which draw current of a different waveform from the supply voltage (see fig. 2). Thespectrum of the harmonics depends on thenature of the load. Harmonic voltages occuracross network impedances resulting distortedvoltages which can disturb the operation ofother users connected to the same supply. Thevalue of the supply impedance at different

harmonic frequencies thus has a vital role inlimiting the voltage distortion. Note that if thesource impedance is low (Scc is high), voltagedistortion is low.

Main sources of harmonicsThese are loads which can be distinguishedaccording to their domain, i.e. industrial ordomestic.c Industrial loadsv Power electronic equipment: drives, rectifiers(diode or thyristor), inverters or switching powersupplies;v Loads using electric arcs: arc furnaces,welding machines, lighting (discharge lamps,fluorescent tubes). Starting motors usingelectronic starters and power transformersenergisation also generates (temporary)harmonics.Note that because of its multiple advantages(operating flexibility, excellent energy efficiency,high performance levels, etc.), the use of powerelectronic equipment is becoming morewidespread.c Domestic loads with power inverters or switchingpower supplies such as television, microwaveovens, induction hotplates, computers, printers,photocopiers, dimmer switches, electrodomesticequipments, fluorescent lamps.

Fig. 2: Degradation of network voltage caused by a non-linear load.

EZ

U = E - ZI I

Harmonicsgenerator

Other loadsVoltage source


Although their individual power ratings are muchless than for industrial loads, the combination oflarge numbers and simultaneous use over longperiods creates significant sources of harmonicdistortion. Note that the use of this type ofequipment is increasing, as in some cases is thepower rating.

Harmonic levels

These generally vary according to the operatingmode of the device, the hour and the season(heating and air conditioning).The sources usually generate odd harmoniccomponents (see fig. 3). Power transformerenergisation, polarised loads (half-wave rectifiers)and arc furnaces generate even harmonics inaddition to odd harmonics components.

Interharmonics are sinusoid components withfrequencies which are not integer multiples ofthe fundamental component (they are locatedbetween harmonics). They are due to periodicor random variations in the power drawn byvarious devices such as arc furnaces, weldingmachines and frequency inverters (drives,cycloconverters). The remote control frequenciesused by the power distributor are alsointerharmonics.The spectrum may be discrete or continuous andvary randomly (arc furnaces) or intermittently(welding machines).To study the short, medium and long termeffects, the various parameters must bemeasured at time intervals which are compatiblewith the thermal time constant of the devices.

Fig. 3: Characteristics of certain harmonics generators.

Non-linear loads Current waveform Spectrum THD

Adjustable speed drive

44 %

Rectifier/charger

28 %

Data processing load

115 %

Fluorescent lighting

53 %

A

t

h

h

A

t

A

t

A

t

1

%

50

0

100

%

100

%

100

5 7 11 13 17 19

19

23 25

1

50

05 7 11 13 17

h1

50

03 5 7 9 11 13

%

100

h1

50

03 5 7 9 11 13


2.4 Overvoltages

Where voltage is applied to a device and thepeak value exceeds the limits defined in astandard or specification, this is an overvoltage(see "Cahiers Techniques" nos. 141, 151 and179).

Overvoltages are of three types:c temporary,c switching,c lightning.

They can appear:c in differential mode (between live conductors:ph/ph – ph/neutral),c in common mode (between live conductorsand the exposed-conductive-part or earth).

Temporary overvoltages

By definition, these occur at power frequency(50/60 Hz). They have various origins:

c An insulation faultWhen an insulation fault occurs between phaseand earth in an isolated neutral system orimpedance earthed neutral system, the voltage ofthe healthy phases to earth may reach the phaseto phase voltage. Overvoltages on LVinstallations may come from HV installations viathe earth of the HV/LV station.

c FerroresonanceThis is a rare non-linear oscillatory phenomenonwhich can often be dangerous for equipment andwhich is produced in a circuit containing acapacitor and a saturable inductance.Ferroresonance is often the apparent cause ofmalfunctions or the destruction of devices (see"Cahier Technique" no. 190).

c Break of the neutral conductorDevices powered by the phase with the leastload witness an increase in voltage (sometimesup to the phase to phase voltage).

c Faults on alternator regulators or tap changertransformer

c Overcompensation of reactive powerShunt capacitors produce an increase in voltagefrom the source to their location.This voltage is especially high during periods oflow load.

Switching overvoltagesThese are produced by rapid modifications in thenetwork structure (opening of protective devices,etc.). The following distinctions are made:

c switching overvoltages at normal load,

c overvoltages produced by the switching on andoff of low inductive currents,

c overvoltages produced by the switching ofcapacitive circuits (no-load lines or cables,capacitor banks). For example, the energisationof a capacitor bank produces a transientovervoltage in which the first peak may reach 2rtimes the rms value of the nominal voltage and atransient overcurrent with a peak value of up to100 times the rated current of the capacitor (see"Cahier Technique" no. 142).

Lightning overvoltagesLightning is a natural phenomenon occurringduring storms. A distinction is made betweendirect lightning strike (on a line or structure) andthe indirect effects of lightning (inducedovervoltages and increase in earth potential)(see "Cahiers Techniques" nos. 151 and 179).

2.5 Voltage variations and fluctuations

Voltage variations are variations in the rms valueor the peak value with an amplitude of less than10% of the nominal voltage.

Voltage fluctuations are a series of voltagechanges or cyclical or random variations in thevoltage envelope which are characterised by thefrequency of variation and the magnitude.

2.6 Unbalance

A 3-phase system is unbalanced if the rms valueof the phase voltages or the phase angles betweenconsecutive phases are not equal. The degree ofunbalance is defined using the Fortescuecomponents, comparing the negative sequencecomponent (U1i) (or zero sequence component(U1o)) of the fundamental to the positivesequence component (U1d) of the fundamental.

c Slow voltage variations are caused by theslow variation of loads connected to the network.

c Voltage fluctuations are mainly due to rapidlyvarying industrial loads such as weldingmachines, arc furnaces or rolling mills.

∆ ∆Ui U1

U1 and Uo

U1

U1

i

d

o

d

= =

The following approximate formula can also be

used: ∆Ui maxV Vavg

Vavgii= −


where Vi = phase voltage i and

Vavg V1 V2 V3

3= + +

The negative sequence (or zero sequence)voltage is produced by voltage drops along thenetwork impedances due to negative sequence

(or zero sequence) currents produced byunbalanced loads leading to non-identicalcurrents on the three phases (LV loadsconnected between phase and neutral, or single-phase or 2-phase MV loads such as weldingmachines and induction furnaces).Single-phase or 2-phase faults produceunbalance until tripping of the protective devices.

2.7 Summary

Disturbances Voltage Overvoltages Harmonics Unbalance Voltagedips fluctuations

Characteristicwaveforms

Origin of disturbance

c Power system

v Insulation fault, break of the neutral conductor... v Switching, ferroresonance v Lightning

c Equipment

v Asynchronous motor v Synchronous motor v Welding machine v Arc furnace v Converter v Data processing loads v Lightingv Inverterv Capacitor bank

: Occasional phenomenon : Frequent phenomenon


3 Effects of disturbance on loads and processes

c Deferred effects: energy losses, acceleratedageing of equipment due to overheating andadditional electro-dynamic stress caused by thedisturbance.The financial impact (e.g. on productivity) ismore difficult to quantify.


Voltage dips and interruptions disturb manytypes of devices connected to the network.They are the most frequent cause of PowerQuality problems. A voltage dip or interruptionof a few hundred milliseconds may havedamaging consequences for several hours.

The most sensitive applications are:c complete continuous production lines wherethe process cannot tolerate any temporaryshutdown of any element in the chain (printing,steelworks, paper mills, petrochemicals, etc.),c lighting and safety systems (hospitals, airportlighting systems, public and high-risebuildings, etc.),c computer equipment (data processingcentres, banks, telecommunications, etc.),c essential auxiliary plant for power stations.

The paragraphs below cover the mainconsequences of voltage dips andinterruptions on equipment used in theindustrial, service and domestic sectors.

Asynchronous motors

When a voltage dip occurs, the torque of anasynchronous motor (proportional to thesquare of the voltage) drops suddenly whichslowdowns the motor. This slowdown dependson the magnitude and duration of the dip, theinertia of the rotating masses and the torque-speed characteristics of the driven load. If thetorque developed by the motor drops belowthe resistant torque, the motor stops (stalls).Following an interruption, at the time of voltagerecovery, the motor tends to re-accelerate andabsorb current whose value is nearly its startingcurrent, the duration of which depends on theduration of the interruption. Where there areseveral motors in an installation, thesimultaneous restarting may produce a voltagedrop in the upstream impedances on the networkwhich will increase the duration of the dip andmay make restarting difficult (long restarts withoverheating) or even impossible (motor torquelower than the resistive torque).

Generally speaking, the effects of all disturbancescan be classified in two ways:c Instantaneous effects: unwanted operation ofcontactors or protective devices, incorrect operationor shutdown of a machine. The financial impactof the disturbance can be estimated directly.

Rapidly reconnecting (~ 150 ms) the power toan asynchronous motor which is slowing downwithout precautionary measures may lead toreclosing in opposition to the phase betweenthe source and the residual voltage inasynchronous motors. In this case the firstcurrent peak may reach three times the start-up current (15 to 20 In) (see "CahierTechnique" no. 161).The overcurrents and consequent voltagedrops have consequences for the motor(excessive overheating and electro-dynamicforce in the coils, which may cause insulationfailures and torque shocks with abnormalmechanical stress on the couplings andreducers, leading to premature wear or evenbreakage) as well as other equipment suchas contactors (wear or even fusion of thecontacts). Overcurrents may cause tripping ofthe main general protective devices of theinstallation causing the process to shutdown.

Synchronous motorsThe effects are almost identical to those forasynchronous motors. Synchronous motorscan however withstand deeper voltage dips(around 50 %) without stalling, owing to theirgenerally greater inertia, the possibilities ofoverexcitation and the fact that their torque isproportional to the voltage. In the event ofstalling, the motor stops and the entirecomplex start-up process must be repeated.

ActuatorsThe control devices (contactors, circuit breakerswith voltage loss coils) powered directly from thenetwork are sensitive to voltage dips whosedepth exceeds 25 % of Un. Indeed, for astandard contactor, there is a minimum voltagevalue which must be observed (known as thedrop-out voltage), otherwise the poles willseparate and transform a voltage dip (lasting afew tens of milliseconds) or a short interruptioninto a long interruption until the contactor isreenergized.


Computer equipmentComputer equipment (computers, measurementapparatus) today occupy a dominant position inthe monitoring and control-command ofinstallations, management and production. All ofthis equipment is sensitive to voltage dips withdepth greater than 10 % Un.The ITIC (Information Technology IndustryCouncil) curve – formerly CBEMA curve – showson a duration-amplitude scale, the typicalwithstand of computer equipment to voltage dips,interruptions and overvoltages (see fig. 4).Operation outside these limits leads to loss ofdata, incorrect commands, and shutdown ormalfunction of equipment. The consequences of

the loss of equipment functions depend inparticular on the restart conditions when voltage isrestored. Certain equipment, for example, has itsown voltage dip detection devices which enabledata to be backed up and ensure safety byinterrupting calculation processes and anyincorrect commands.

Adjustable speed machines

The problems of voltage dips applied to variablespeed drives are:

c It is not possible to supply sufficient voltage tothe motor (loss of torque, slowdown).

c The control circuits supplied directly by thenetwork cannot function.

c There is overcurrent when voltage recovers(the drive filter capacitor is recharged).

c There is overcurrent and unbalanced current inthe event of voltage dips on a single phase.

c There is loss of control of DC drives functioningas inverters (regenerative braking).

Adjustable speed drives usually trip out when avoltage dip deeper than 15 % occurs.

Lighting

Voltage dips cause premature ageing ofincandescent lamps and fluorescent tubes.Voltage dips deeper than or equal to 50 % with aduration of around 50 ms will extinguishdischarge lamps. The lamp must then be left offfor several minutes to cool the bulb before it isturned on again.

3.2 Harmonics

The consequences of harmonics are linked to theincrease in peak values (dielectric breakdown),rms values (excessive overheating) and to thefrequency spectrum (vibration and mechanicalstress) of voltages and currents.The effects always have an economic impactresulting from the additional costs linked to:c degradation in the energy efficiency of theinstallation (energy loss),c oversizing of equipment,c loss of productivity (accelerated ageing ofequipment, unwanted tripping).

Malfunctions are probable with a harmonicdistortion factor of greater than 8 % of thevoltage. Between 5 and 8 %, malfunctions arepossible.

c Instantaneous or short term effectsv Unwanted operation of protective devices:harmonics have a harmful influence mainly onthermal control devices. Indeed, when protectivedevices of this type calculate the rms value ofthe current from the peak value, there is a risk of

error and unwanted operation even duringnormal operation with no overload.v Disturbances induced by low current systems(remote control, telecommunications, hi-fisystems, computer screens, television sets).v Abnormal vibrations and acoustic noise(LV switchboards, motors, transformers).v Destruction of capacitors by thermal overloadIf the actual frequency of the upstreamcapacitor-network system is similar to aharmonic order, this causes resonance andamplification of the corresponding harmonic.v Loss of accuracy of measurement instrumentsA class 2 induction energy meter will produce incurrent and voltage, a 0.3 % additional error inthe presence of 5 % of harmonic 5.

c Long term effectsCurrent overload produces excessive overheatingand leads to premature ageing of equipment:v Overheating of sources: transformers,alternators (through increased joule and ironlosses).

Fig. 4: Typical withstand as defined by the ITIC curve.

500U (%)

200

140120

∆T (s)

100 11090

7080

00 10-3 3.10-3 100,020,01T 0,5


3.3 Overvoltages

The consequences are extremely variedaccording to the period of application, repetitivity,magnitude, mode (common or differential),gradient and frequency:c Dielectric breakdown, causing significantpermanent damage to equipment (electroniccomponents, etc.).c Degradation of equipment through ageing(repetitive rather than destructive overvoltages).c Long interruptions caused by the destruction ofequipment (loss of sales for distribution

v Mechanical stress (pulse torque inasynchronous machines).v Overheating of equipment: phase and neutralconductors through increased joule anddielectric losses. Capacitors are especiallysensitive to harmonics as their impedancedecreases in proportion to the harmonic order.v Destruction of equipment (capacitors, circuitbreakers, etc.).

Overload and excessive overheating of theneutral conductor may result from the presenceof third harmonic (and multiples of 3) currents inthe phase conductors which add in the neutral.The TNC neutral earthing system uses thesame conductor for neutral and protectionpurposes. This conductor interconnects the

installation earth, including the metal structuresof the building. Third harmonic (and multiplesof 3) currents will flow through these circuitsand produce variations in potential with thefollowing results:v corrosion of metal parts,v overcurrent in the telecommunication linksbetween the exposed-conductive-part of twodevices (for example, printer and computer),v electromagnetic radiation causing screendisturbance (computers, laboratory apparatus).

The table in figure 5 summarises the main effectsof harmonics and the normal permitted levels.

Interharmonics affect remotely-controlleddevices and produce a phenomenon known asflicker.

Fig. 5: Effects of harmonics and practical limits.

HVF U hhh

==∑ 2

2

13 (Harmonic Variation Factor according to IEC892)

Equipment Effects Limits

Power Overheating, premature ageing (breakdown), I < 1.3 In, (THD < 83 %)capacitors resonance. or U < 1.1 Un

for 12 hrs/days at MVor 8 hrs/days at LV

Motors Losses and excessive overheating. HVF i 2 %Reduction of capacity for use at full load. for usual asynchronousPulse torque (vibrations, mechanical stress) motorsNoise pollution.

Transformers Losses (ohmic-iron) and excessive overheating.Mechanical vibrations. Noise pollution.

Circuit breakers Unwanted tripping (exceeding voltage peak Uh / U1 i 6 to 12 %values, etc.).

Cables Additional dielectric and ohmic losses THD i 10 %(especially in the neutral conductor if third harmonic Uh / U1 i 7 %currents present).

Computers Operating problems. Uh / U1 i 5 %

Power Problems related to waveformelectronics (commutation, synchronisation).

company, loss of production for industrialcompanies).c Disturbance in control system and low currentcommunication circuits (see "Cahier Technique"no. 187).c Electrodynamic and thermal stress (fire)caused by:v Lightning (usually)Overhead networks are most vulnerable tolightning, but installations supplied byunderground networks may also be affected by


stress due to high voltage if lightning strikesclose to the site.v Switching overvoltages: these are repetitiveand their probability of occurrence is

3.4 Voltage variations and fluctuations

As fluctuations have a magnitude no greaterthan ± 10 %, most equipment is not affected.The main effect of voltage fluctuations is afluctuation in the luminance of lamps (flicker).The physiological strain (visual and nervousfatigue) depends on the magnitude of thefluctuations, the repetition rate of the variations,

the composition of the spectrum and theduration of the disturbance.There is however a perceptibility threshold (theamplitude as a function of the variationfrequency) defined by the IEC below whichflicker is no longer visible.

considerably higher than that of lightning, witha longer duration.They can lead to degradation as serious as thatcaused by lightning.

3.5 Unbalance

The main effect is the overheating of 3-phaseasynchronous machines.In fact, the zero sequence reactance of anasynchronous machine is equivalent to itsreactance during the start-up phase. The currentunbalance factor will thus be several times thatof the supply voltage. Phase currents can thus

differ considerably. This increases theoverheating of the phase(s) which the highestcurrent flows through and reduces the operatinglife of the machine.In practice, a voltage unbalance factor of 1 %over a long period, and 1.5 % over a few minutesis acceptable.

3.6 Summary

Equipment Sensitivity to disturbanceVoltage dips Overvoltages Harmonics Unbalance Voltage< 0.5 s > 0.5 s fluctuation

c Asynchronous motor c Synchronous motor c Actuatorc Speed drive c Data processing load,numerical control c Induction furnace c Lightingc Capacitor bankc Transformerc Inverterc Circuit breakerc Cable


4 Level of Power Quality

Contractual application

The contract must state:c Its duration.c The parameters to be measured.c The contractual values.c The measurement point(s).c The voltages measured: these voltages(between phases and/or between phase andneutral) must be the equipment supply voltages.c For each parameter measured the choice ofmeasurement method, the time interval, themeasurement period (e.g. 10 minutes and 1 yearfor the voltage amplitude) and the referencevalues; for voltage dips and interruptions, forexample, the reference voltage, detectionthresholds and the distinction between long andshort interruptions must be defined.c The measurement accuracy.c The method of determining penalties in theevent of one party failing to honour the terms ofthe contract.c Clauses in the event of disagreementconcerning the interpretation of themeasurements (intervention of third parties, etc.).c Data access and confidentiality.

Corrective maintenance

This is generally the consequence of incidents ormalfunctions during operation requiringtroubleshooting in order to apply correctivemeasures.The usual steps are:

c Data collection

This involves the collection of information suchas the type of load, the age of the networkcomponents and the single-line diagram.

c Search for symptoms

This involves identifying and locating the equipmentsubject to disturbance, determining the time anddate (fixed or random) when the problem occurred,any correlation with particular meteorologicalconditions (strong winds, rain, storm) or recentmodification of the installation (installation of newmachines, modification of the power system).

c Examination of the installation

This phase is sometimes sufficient for quicklydetermining the origin of the malfunction.Environmental conditions such as humidity, dustand temperature must not be overlooked.The installation, especially the wiring, circuitbreakers and fuses, have to be checked.

c Monitor the installation

This step consists in equipping the site withmeasurement apparatus to detect and record theevent where the problem originated. It may benecessary to place instruments at several pointsin the installation, especially (where possible)close to the equipment subject to disturbance.

The apparatus detects events when thethresholds of the parameters used to measurethe Power Quality are exceeded, and recordsthe data characterising the event (for exampledate, time, depth of voltage dip, THD). Thewaveforms just before, during and after thedisturbance can also be recorded. The thresholdsettings must match the sensitivity of theequipment.

When using portable apparatus, the duration ofthe measurements must be representative of theoperating cycle of the factory in question (e.g.one week). It must always be assumed that thedisturbance will recur.

Fixed apparatus can be used for continuousmonitoring of the installation. If the apparatussettings are correct, it will carry out preventionand detection by recording each occurrence ofdisturbance. The data can be displayed locally orremotely via an Intranet or Internet connection.This can be used to diagnose events as well asto anticipate problems (preventive maintenance).This is the case with apparatus in the PowerLogic System range (Circuit Monitor - PowerMeter), Digipact and the latest generation ofMasterpact circuit breakers fitted withMicrologic P trip release (see fig. 6).

Records of disturbance from the distributor’spower system which have caused damage(destruction of equipment, production losses,etc.) may also prove useful when negotiatingcompensation claims.

4.1 Evaluation methodology


c Identification of origin

The signature (waveform, profile of rms value) ofthe disturbance can in general be used byexperts to locate and identify the source of theproblem (fault, motor starting, capacitor bankenergisation, etc.).The simultaneous recognition of the signature forthe voltage and the current can be used todetermine if the disturbance is sourced upstreamor downstream of the measurement point. Thedisturbance may come from either theinstallation or the distribution power system.

c Definition and choice of mitigation solutions

A list of solutions and costings is prepared. Thechoice of solution is often made by comparingthe cost with the potential lost earnings in theevent of disturbance.After implementing a solution, it is important toverify, via measurement, that it is effective.

Optimising the operation of electricalinstallationsThe operation of electrical installations can beoptimised through three complementary actions:

c Saving energy and reducing energy bills:

v making users aware of costs,v assigning costs internally (by site, departmentor product line),

v locating potential economies,v managing peaks in consumption (loadshedding, standalone sources),v optimising the power contract (reduction insubscribed power demand),v improving the power factor (reduction inreactive power).

c Ensuring the Power Quality:

v displaying and monitoring the measurementparameters for Power Quality,v detecting problems in advance (monitoring ofharmonics and neutral current, etc.) forpreventive maintenance purposes.

c Ensuring continuity of service:

v optimising maintenance and operation,v becoming acquainted with the network in real time,v monitoring the protection plan,v diagnosing faults,v reconfiguring a network following a fault,v ensuring an automatic source transfer.

Software tools are used for the control-commandand monitoring of the installation. They can beused for example to detect and archive events,monitor circuit breakers and protection relays inreal time, control circuit breakers remotely, andgenerally make use of the possibilities ofcommunicating devices (see fig. 6).

Sepam

Digipactpower meter

Circuit Monitormeasurement and control device

Masterpactcircuit breaker

Compact NScircuit breaker

Digipact DC150dataconcentrator

Fig. 6: Some communicating products (Merlin Gerin brand).


4.2 EMC and planning levels

Electromagnetic compatibility (EMC)Electromagnetic compatibility is the ability of anequipment or system to fonction satisfactorily in itselectromagnetic environment without introducingintolerable electromagnetic disturbances toanything in that environment (IEC 60050-161).

The aim of electromagnetic compatibility is toensure that:c The emission of disturbances from eachseparate source is such that the combinedemission from all sources does not exceed theexpected levels of disturbance in the environment.

c The immunity level of the equipment gives theappropriate level of performance for theexpected disturbance in three classes ofenvironment (see fig. 7).

Note that the environment is also determined bythe characteristics specific to the user installation(single-line diagram, types of load, etc.) and bythe characteristics of the supply voltage.

One way of ensuring compatibility levels is tospecify the emission limits of user installations witha sufficient margin below the compatibility level. Inpractice this is possible for large installations (IEC61000-3-6, IEC 61000-3-7). For other installations(e.g. LV) the "product" standards specify emissionlimits for families of equipment (e.g. standard IEC61000-3-2 imposes emission limits on currentharmonics for loads under 16 A).

In certain cases, technical solutions must beapplied to keep the emission levels below theprescribed levels.

Voltage characteristicsThe method used to evaluate the actual voltagecharacteristics at a given point on the networkand to compare them to the predefined limits is

based on a statistical calculation over a givenmeasurement period. For example, for theharmonic voltage the measurement period is oneweek: 95 % of the rms values calculated oversuccessive periods of 10 minutes must notexceed the specified limits.

Planning levelsThese are the internal quality objectivesspecified by the network operator which areused to evaluate the impact of all disturbance-producing loads on the network. They areusually equal to or below the compatibilitylevels.

Summary

Figure 8 summarises the relations between thevarious levels of disturbance.

Fig. 7: Compatibility levels according to IEC 61000-2-4.

Fig. 8: Relations between the various levels ofdisturbance.

Susceptibility of equipment

Immunity level(specified test value)

Compatibility level(conventional value)

Planning level

Emission level

Voltagecharacteristic

Disturbance level

Probability density

Disturbances Class 1 Class 2 Class 3

Voltage variations ∆U/UN ± 8 % ± 10 % +10 % -15 %

Voltage dips(1)

∆U / UN 10 % to 100 % 10 % to 100 % 10 % to 100 %∆T (number of half-cycle) 1 1 to 300 1 to 300

Short interruption (s) none – i 60

Voltage unbalance Ui / Ud 2 % 2 % 3 %

Frequency variations ∆f / fN ± 1 % ± 1 % ± 2 %

(1) These values are not compatibility levels: they are given for indicative purposes only.


A degradation of quality may lead to a change inbehaviour, performance or even the destructionof equipment and dependent processes withpossible consequences for the safety ofpersonnel and additional economic costs.This assumes three elements:

c one or more generators of disturbance,

c one or more loads sensitive to the disturbance,

c a channel for the disturbance to be propagatedbetween them.

The solutions consist in taking action with regardto all or part of the three elements, either globally(the installation) or locally (one or more loads).The solutions can be implemented to:

c correct a malfunction in an installation,

c take preventive action when polluting loads areto be connected,

c ensure the installation conforms to a standardor to the power distributor’s recommendations,

c reduce energy bills (reduction of subscribedpower in kVA, reduction in consumption).

Loads are not sensitive to the same disturbanceand have different levels of sensitivity, thesolution adopted, as well as being the best froma technical and economic point of view, mustensure an appropriate level of PQ which meetsactual requirements.It is vital that specialists carry out a priordiagnosis to determine the nature of the

disturbance to be prevented (e.g. remedies maydiffer depending on the duration of aninterruption). This determines the effectivenessof the chosen solution. The definition, choice,implementation and maintenance (to ensurelong-term effectiveness) of solutions must alsobe carried out by specialists.The value of the choice and implementation of asolution depends on:

c The required level of performance

Malfunction is not permitted if it would put lives atrisk (e.g. in hospitals, airport lighting systems,lighting and safety systems in public buildings,auxiliary plant for power stations, etc.).

c The financial consequences of malfunction

Any unprogrammed stop, even when very short,of certain processes (manufacture of semi-conductors, steelworks, petrochemicals, etc.)results in loss or non-quality of production oreven restarting of production facilities.

c The time required for a return on the investment

This is the ratio of financial losses (rawmaterials, production losses, etc.) caused by thenon-quality of electrical power and the cost(research, implementation, operation,maintenance) of the solution.

Other criteria such as practices, regulation andthe limits on disturbance imposed by thedistributor must also be taken into account.


The network architecture, automated powerrestart systems, the reliability of equipment, thepresence of a control-command system andmaintenance policy all play an important role inthe reduction and elimination of interruptions.

Correct diagnosis is vital before choosing aneffective solution. For example, at the point ofcommon coupling (the customer’s electricityinput), it is important to determine whether thevoltage dip is coming from the customer’sinstallation (with a corresponding increase incurrent) or from the distribution power system (noincrease in current).Different types of solution exist.

Reducing the number of voltage dips andinterruptions

Distributors can take certain measures such asmaking their infrastructure more reliable(targeted preventive maintenance,modernisation, underground installation) orrestructuring power systems (shorteningfeeders). For impedance earthed neutral powersystems, they can also replace auto-reclosingcircuit breakers with shunt circuit breakers whichpresent the major advantage of not causinginterruptions on a damaged feeder in the eventof a transient earth fault (reducing the number ofshort interruptions).

5 Solutions for improving PQ


These circuit breakers allow the extinction oftransient earth faults by cancelling the voltage tothe fault terminals for at least 300 ms by earthingthe single faulty phase at the substation busbars.This does not alter the voltage between phasessupplying the customer.

Reducing the duration and depth of voltagedips

c At power system level

v Increasing the possibilities of ring connections(new substations, ring closing switch)v Improving the performance of electricalprotective devices (selectivity, automatic powerrestart, remote control devices on the network,remote management, replacement of spark gapswith surge arresters, etc.)v Increasing the network short-circuit power

c At equipment level

Decrease the power consumed by the switchedlarge loads with real time reactive compensatorsand soft starters which limit current peaks (andmechanical stress).

Increasing immunity of industrial and serviceinstallations

The general principle for ensuring that equipmentis immune to voltage dips and interruptions is tocompensate for a lack of power with an energystorage device between the distribution powersystem and the installation. The availability of thestorage device has to be greater than the durationof the disturbances to which the system has to beimmune to.The information required when choosingmitigation solutions is:

c the quality of the source (maximum level ofexisting disturbances),

c the load requirements (voltage sag ride-through capability in the duration-depth scale).Only by careful analysis of the process and of thetechnical and financial consequences ofdisturbances can these two elements bereconciled. There are various possible solutionsto provide immunity depending on the powerrequired by the installation and the duration ofthe voltage dip or interruption. It may well behelpful to study solutions by making a distinctionbetween power supplies for control systems and

regulation systems and those for motors andlarge power consumers. Indeed, a voltage dip orinterruption (even of short duration) may besufficient to open all of the contactors whosecoils are supplied by the power circuit. Loadscontrolled by the contactors are thus no longersupplied when the voltage is restored.

Increasing immunity of the control system

The increase of immunity of a process is ingeneral based on providing immunity to thecontrol system.In general, the control system is not of highpower and is thus extremely sensitive todisturbances. It is therefore often moreeconomical to immunise only the control systemrather than the equipment power supply.Maintaining control of machines assumes:

c There will be no risk to the safety of personnelor equipment when the voltage is restored.

c The loads and processes tolerate a shortinterruption in the power circuit (high inertia orslowdown is tolerated) and can be restarted onthe fly when the voltage is restored.

c The source can ensure that all of theequipment can be supplied simultaneously (inthe case of a replacement source) and providethe inrush current caused by the simultaneousrestart of several motors.

The solutions consist in powering all of thecontactor coils from a reliable auxiliary source(battery or rotating set with flywheel), or in usingan off-delay relay, or in using a rectifier and acapacitor connected in parallel with the coil.

Increasing immunity of the equipment powersupply

Certain loads cannot withstand the expecteddisturbance levels, i.e. neither voltage dips norinterruptions. This is the case for "priority" loadssuch as computers, lighting and safety systems(hospitals, airport lighting systems, publicbuildings) and continuous production lines(manufacture of semi-conductors, dataprocessing centres, cement works, watertreatment, materials handling, paper industry,steelworks, petrochemicals, etc.).The following different technical solutions arepossible depending on the power required by the


installation and the duration of the voltage dip orinterruption.

c Solid state uninterruptible power supply (UPS)

A UPS consists of three main elements:v a rectifier-charger, powered from the mainsupply, to convert AC voltage to DC,v a flywheel and/or battery (kept charged) whichon interruption provide the necessary power forthe load via the inverter,v a DC-AC inverter.

Two technologies are currently in use: on-lineand off-line.v On-line technologyDuring normal operation, power is suppliedcontinuously via the inverter without drawing onthe battery. This, for example, is the case forMGE-UPS brand Comet and Galaxy UPS units.They ensure continuity (no changeover delays)and quality (voltage and frequency regulation)of the power supply for sensitive loads rangingfrom a few hundred to several thousand kVA.Several UPS can be connected in parallel toobtain more power or to provide redundancy.In the event of overload, power is provided bythe static contactor (see fig. 9) from network 2(which may be combined with network 1).Power is maintained without interruption via amaintenance by-pass.

v Off-line (or stand-by) technologyThis is used for applications of no more than afew kVA. During normal operation, power issupplied from the network. In the event of loss ofnetwork power or if the voltage exceeds theprescribed tolerances, use is transferred to theUPS. The changeover causes an interruption of2 to 10 ms.

c Sources transfer

A device is used to control transfer between themain source and a replacement source (and viceversa) for supply to priority loads and ifnecessary orders the shedding of non-priorityloads.There are three types of transfer depending onthe duration of transfer (∆t):v synchronous (∆t = 0),v delayed (∆t = 0.2 to 30 s),v pseudo-synchronous (0.1 s < ∆t < 0.3 s).

The devices require special precautions (see"Cahier Technique" no. 161). For example, ifthere are several motors in the installation,simultaneous restart may produce a voltagedrop which could prevent restart or lead toexcessively long restarts (with the risk ofoverheating). It is therefore prudent to install aPLC which will restart the priority motors atintervals, especially with a replacement(backup) source with a low short-circuit power.

Fig. 9: Schematic diagram of an on-line uninterruptible power supply (UPS).

Battery

Batterycircuit breaker (NC)

SwitchorCircuit breaker(NC)

Switch(NC)

Sensitiveequipment

Network 2

Network 1

Supply networkfeeders

Switch(NC)

Static contactor

Manual maintenance by-pass (NO)

Rectifier / charger Inverter

NO : normally open

NC : normally closed


This solution is selected where the installationcannot withstand a long interruption of more thana few minutes, and/or requires a large amount ofpower. It can also be used in conjunction with aUPS.

c Zero-time set

In certain installations, the autonomy required inthe event of interruption makes it necessary toinstall a generating set (large batteries would betoo expensive, or cause technical or installationproblems). Here, in the event of any loss ofpower supply, the battery or flywheel is used toprovide sufficient time for starting and running upthe stand-by engine generator, load shedding (ifnecessary) and interruption-free coupling bymeans of an automatic source changeover.

c Electronic conditionersThese are modern electronic devices tocompensate voltage dips and interruptions to acertain extent with a short response time: for

example the real time reactive compensatorcompensates the reactive power in real time andis especially well suited to loads with rapid, largevariations (welding machines, lifts, presses,crushers, motor starting, etc.).

Clean stopIf a stoppage is acceptable, it is especiallyadvisable to prevent uncontrolled restarting if anunwanted restart would present a risk for themachine operator (circular saws, rotating electricalmachines) or for the equipment (compressionchambers while still under pressure, staggeredrestarts of air-conditioning compressors, heatingpumps or refrigeration units) or for the application(necessity of controlling production restart). Theprocess may be automatically restarted by aPLC using a predetermined restart sequencewhen conditions return to normal.

Summary (see table below)

Installation Duration (indicative values) Immunisation solutionpower and technical requirements

0 to 100 ms 400 ms 1 s 1 min > 3 min100 ms to 400 ms to 1 s to 1 min to 3 min

A few VA Time-delayed contactorscontacteurs.

DC power with capacitor storage

< 500 kVA Rotating set with flywheel.

< 1 MVA Transfer source with diesel set

< 300 kVA Between 15 minutes and several hours depending on DC power with battery storagebattery capacity

< 500 kVA Transfer time to a backup source may cause a short Rotating set with flywheel andinterruption thermal motor or backup source

< 500 kVA Between 15 minutes and several hours depending on DC motor connected to batterybattery capacity and alternator

< 1 MVA (up to Between 10 minutes (standard) and several hours UPS4800 kVA with depending on battery capacityseveral UPS inparallel)

Effective mitigation solution

Ineffective mitigation solution


Z (Ω)

f (Hz)fr far

Zone where harmonics are present

Network onlywith capacitorwith anti-harmonic choke

5.2 Harmonics

v Derate equipmentv Segregate polluting loadsAs a first step, the sensitive equipment must beconnected as close as possible to the powersupply source.Next, the polluting loads must be identified andseparated from the sensitive loads, for exampleby powering them from separate sources or fromdedicated transformers. These solutions involvework on the structure of the installation and are,of course, usually difficult and costly.

v Protective devices and oversizing of capacitorsThe choice of solution depends on theinstallation characteristics. A simple rule is usedto choose the type of equipment where Gh is theapparent power of all generators of harmonicssupplied from the same busbar system as thecapacitors, and Sn is the apparent power of theupstream transformer(s):- If Gh/Sn i 15 %, standard equipment is suitable- If Gh/Sn > 15 %, there are two possiblesolutions.1 - For polluted networks(15 % < Gh/Sn i 25 %): the current rating of theswitchgear and in-series links must be oversized,as must the voltage rating of the capacitors.2 - For very polluted networks(25 % < Gh/Sn i 60 %): anti-harmonic chokesmust be connected to the capacitors and set to afrequency lower than the frequency of the lowestharmonic (for example, 215 Hz for a 50 Hznetwork) (see fig. 10). This eliminates any risk ofresonance and helps to reduce harmonics.

There are three possible ways of suppressing orat least reducing the influence of harmonics. Onesection will examine the question of protectivedevices.

c Reducing generated harmonic currents

v Line chokeA 3-phase choke is connected in series with thepower supply (or integrated into the DC bus forfrequency inverters). It reduces the line currentharmonics (especially high number harmonics) andtherefore the rms value of the current consumptionand the distortion at the inverter connection point.It is possible to install the choke without affectingthe harmonics generator and to use chokes forseveral drives.

v Using 12-phase rectifiersHere, by combining currents, low-orderharmonics such as 5 and 7 are eliminatedupstream (these often cause the mostdisturbance owing to their large amplitude). Thissolution requires a transformer with twosecondary windings (star and delta), and onlygenerates harmonics numbered 12 k ± 1.v Sinewave input current devices(see "Cahier Technique" no. 183)This method consists in using static converterswhere the rectifier uses PWM switching toabsorb a sinusoidal current.

c Modifying the installation

v Immunise sensitive loads with filtersv Increase the short-circuit power of theinstallation

Fig 10: Effects of an anti-harmonic choke on network impedance


c Filtering

Where Gh/Sn > 60%, specialists must calculateand install the harmonics filter (see fig. 11).

v Passive filtering (see "Cahier Technique"no. 152)This involves connecting a low impedance by-pass to the frequencies to be attenuated usingpassive components (inductor, capacitor,resistor). Several passive filters connected inparallel may be necessary to eliminate severalcomponents. Careful attention must be paid tothe sizing of harmonic filters: a poorly designedpassive filter may lead to resonance and amplifyfrequencies which did not cause disturbancebefore installation of the filter.

v Active filtering (see "Cahier Technique" no. 183)This consists in neutralising the harmonicsinduced by the load. First an analysis of thecurrent identifies them in amplitude and phase.Then the same but opposite harmonics areproduced by the active filter. It is possible toconnect several active filters in parallel. Anactive filter may for example be connected to aUPS to reduce harmonics which have beeninjected upstream.

v Hybrid filteringThis consists of an active filter and a passivefilter set to the order of the dominant harmonic(e.g. 5) which supplies the necessary reactivepower.

Fig. 11: Principles and characteristics of passive, active, and hybrid filtering.

Filter Principle Characteristics

Passive By-pass series LC circuit tuned to each c No limits in harmonic current.harmonic frequency to be eliminated. c Compensation of reactive power.

c Elimination of one or more harmonic orders(generally 5, 7, 11). One filter for one or twoorders to be compensated.

c Risk of amplification of harmonics in the eventof network modification.

c Risk of overload caused by external pollution.

c "Network" filter (global).

c Case by case engineering study.

Active Generation of current cancelling out all c Solution particularly suited to "machine" filteringharmonics created by the load. (local).

c Filtering on a wide frequency band (eliminationof harmonic orders 2 to 25).

c Self-adapting:v network modification has no effect,v adapts to all variations in load and harmonicspectrum,v open-ended, flexible solution for each type ofload.c Simple engineering study.

Hybrid Offers the advantages of passive and activefiltering solutions and covers a wide range ofpower and performance:

c filtering on a wide frequency band (eliminationof harmonics numbered 2 to 25),

c compensation of reactive power,

c large capacity for current filtering,

c good technical-economic solution for "network"filtering.

NetworkLoad(s)

Active filter

Network Pollutingload(s)

Passivefilter(s)

Network

Load(s)

Hybridfilter

Active filterPassive filter


c Special case: circuit breakers(see "Cahier Technique" no. 182)

Harmonics may cause unwanted tripping ofprotective devices: care must be taken whenchoosing protective devices to avoid this.Circuit breakers can be fitted with two types oftrip device, thermal-magnetic or electronic.The heat sensors of thermal-magnetic circuitbreakers are particularly sensitive to harmonicsand can identify the actual load on the conductorscaused by the presence of harmonics. They arethus well suited to use on low current circuits,essentially in domestic or industrial applications.The method used by electronic circuit breakers to

calculate the current being carried may present arisk of unwanted tripping and care must thereforebe taken when choosing these devices that thetrue rms value of the current is measured. Thesedevices have the advantage of being better ableto track changes in the temperature of cables,particularly in the case of cyclical loads, as theirthermal memory is superior to that of indirectlyheated bimetallic strips.

c Derating

This solution is applicable to some equipmentand is a simple and frequently adequateresponse to disturbance caused by harmonics.

5.3 Overvoltages

Correct insulation co-ordination involvesensuring the protection of personnel andequipment against overvoltages, with the bestbalance between technical and economicconsiderations.This requires (see "Cahier Technique" no. 151):

c knowledge of the level and energy of theovervoltages which may occur on the network,

c selection of the level of overvoltagewithstand of the power system components tomeet constraints,

c use of protective devices where necessary,in fact, the appropriate solutions depend onthe type of overvoltage encountered.

Temporary overvoltages

c Switch off all or some of the capacitorsduring periods of low load.

c Avoid configurations susceptible toferroresonance or introduce losses (reducingresistors) to damp the phenomenon (see"Cahier Technique" no. 190).

Switching overvoltages

c Limit the capacitors energisation transientsby installing a fixed reactor and pre-insertionresistors. Static automatic reactivecompensators which control closing instant areespecially suitable for LV applications whichcannot withstand transient overvoltages(PLCs, computer systems).

c Connect line chokes upstream of thefrequency inverters to limit the effects oftransient overvoltages.

c Use main residual current circuit breakers ofdiscriminatory type (type "S") for LV and circuitbreakers of type "si" (I∆n = 30 mA and 300 mA).Their use avoids unwanted tripping due totransient leakage currents : lightning andswitching overvoltages, energisation of circuits

with a high capacitance to earth (capacitivefilters connected to earth, extended cablenetworks, etc.) which flow through the networkdownstream of the RCD (residual currentdevice) via the network capacitance to earth.

Lightning overvoltages

c Primary protectionThis protects the building and its structurefrom direct lightning strikes (lightningconductors, Faraday cages, overhead earthwire/earthing wire).

c Secondary protectionThis protects equipment against theovervoltages which follow lightning.Surge arresters (spark gaps are now used lessand less) are installed on the particularlyexposed points of HV and MV networks and atthe input to MV/LV installations (see "CahierTechnique" no. 151).On LV installations, they are installed as farupstream as possible (to offer maximumprotection) and as close as possible to theload. It is sometimes necessary to cascadesurge arresters: one at the head of theinstallation, and one close to the load (see"Cahier Technique" no. 179). An LV surgearrester is always connected to adisconnection device. In addition, the use ofmain residual current circuit breakers ofdiscriminatory type on LV installations avoidsany current flow to earth via the surge arrestertripping the circuit breaker at the head of theinstallation, which would be incompatible withsome equipment (freezers, controllers, etc.).Note that overvoltages can be propagated tothe equipment by other routes than theelectrical power supply: telephone lines(telephone, fax), coaxial cables (computerlinks, TV aerials). Suitable protective devicesare commercially available.


5.4 Voltage fluctuations

Fluctuations produced by industrial loads mayaffect a large number of consumers supplied fromthe same source. The fluctuation magnitudedepends on the ratio between the impedance ofthe device generating the disturbance and theimpedance of the power supply. The solutions are:

c Changing the type of lightingFluorescent lamps are less sensitive thanincandescent lamps.

c Installing an uninterruptible power supplyThis may be a cost-effective solution if users subjectto disturbance are identified and grouped together.

c Modify the device generating the disturbanceChanging the starting mode of motors whichhave to start frequently, for example, can reduceovercurrents.

c Modify the networkv Increase the short-circuit power by connectinglighting circuits to the nearest power supplypoint.v Increase the "electrical distance" between thedisturbance-generating load and lighting circuitsby powering the disturbance-generating loadfrom an independent transformer.

c Use a reactive compensatorThis device provides real time reactivecompensation for each phase. Flicker can bereduced from 25 % to 50 %.

c Connect a reactance in seriesBy reducing the inrush current, a reactancedownstream from the connection point of an arcfurnace can reduce flicker by 30 %.

The solutions are:c balancing single phase loads on all three phases,c reducing the power system impedanceupstream of the devices causing the unbalance byincreasing the transformer rated power and thecable cross-section,

c fitting the appropriate protective device for themachines,c using carefully connected LC loads (Steinmetzconnection).

5.5 Unbalance

Type ofdisturbanceVoltagevariationsandfluctuationsVoltagedips

Interruptions

Harmonics

Inter-harmonics

Transientovervoltages

Voltageunbalance

Origins

Large load variations (weldingmachines, arc furnaces, etc.).

Short-circuit, switching oflarge loads (motor starting,etc.).

Short-circuit, overloads,maintenance, unwantedtripping.

Non-linear loads (adjustablespeed drives, arc furnaces,welding machines, dischargelamps, fluorescent tubes, etc.).

Fluctuating loads (arcfurnaces, welding machines,etc.), frequency inverters.

Operation of switchgear andcapacitors, lightning.

Unbalanced loads (largesingle-phase loads, etc.).

Consequences

Fluctuation in the luminance of lamps(flicker).

Disturbance or shutdown of process:loss of data, incorrect data, opening ofcontactors, locking of drives, slowdownor stalling of motors, extinguishing ofdischarge lamps.

Overloads (of neutral conductor,sources, etc.), unwanted tripping,accelerated ageing, degradation ofenergy efficiency, loss of productivity.

Interruption of metering signals, flicker.

Locking of drives, unwanted tripping,destruction of switchgear, fire, operatinglosses.

Inverse motor torque (vibration) andoverheating of asynchronous machines.

Examples of mitigation solutions(special equipment and modifications)

Electromechanical reactive power compensator, realtime reactive compensator, series electronicconditioner, tap changer.

UPS, real time reactive compensator, dynamicelectronic voltage regulator, soft starter, serieselectronic conditioner.Increase the short-circuit power (Scc).Modify the discrimination of protective devices.

UPS, mechanical source transfer, static transferswtich, zero-time set, shunt circuit breaker, remotemanagement.

Anti-harmonic choke, passive or active filter, hybridfilter, line choke.Increase the Scc.Contain polluting loads.Derate the equipment.

Series reactance.

Surge arrester, surge diverter, controlled switching,pre-insertion resistor, line chokes, static automaticcompensator.

Balance the loads.Shunt electronic compensator, dynamic electronicvoltage regulator. Increase the Scc.

5.6 Summary


6.1 Hybrid filtering

Description of the installationSki-lifts are powered by an MV/LV transformer(800 kVA).The connected loads are the chair lifts togetherwith other loads such as payment booths, ski-pass validation systems, the official timingsystem for competitions and a telephonenetwork.

Problems encounteredWhen the chair lifts are running, the low voltagenetwork powered by the MV/LV transformer issubject to disturbance.The measures taken at the site pinpointed a highpre-existing harmonic distortion factor in thevoltage (THD ≈ 9 %) from the MV power systemas well as harmonic pollution from the chair-liftfeeder. The resulting distortion of the supplyvoltage (THD ≈ 12 %) disturbed sensitiveequipment (payment booths, timing system, etc.).

SolutionsThe aim of the device is to ensure the simultaneousreactive compensation when harmonics are presentand neutralisation of harmonics likely to disturb theinstallation.The solution chosen (see fig. 12) was to install ahybrid filter (see fig. 13) consisting of a passive filtertuned to the order of the dominant harmonic (H5)which provides the required reactive power(188 kvar), and an active filter rated at 20 A isdedicated to the elimination of all other harmonics.

After commissioning, measurements show thatthe device reduces the magnitude of theharmonics over a wide frequency spectrum inboth current and voltage (see fig. 14) andreduced the voltage distortion factor from 12.6 %to 4.47 %. It also increased the power factor ofthe installation from 0.67 to 0.87. This solutionsolved all of the problems as no malfunction wassubsequently detected.

0THD H5 H7 H11 H17 H23

%

2

4

68

10

12

14No filterH5 filterHybrid filter

05

101520253035

40

a

b

THD H5 H7 H11 H17 H23

No filterH5 FilterHybrid filter

%

Fig. 12: Implementation of the solution.

Fig. 13: Rectiphase hybrid filter device (Merlin Gerinbrand).

Fig 14: Spectrums showing the effectiveness of ahybrid filter: [a] in voltage [b] in current.

6 Case studies

Linear loads

Harmonicsgenerator

5th orderpassivefilter

MV/LVtransformer

SCC

Activefilter

Hybrid filter


6.2 Real time reactive compensation

Description of the installationA car equipment manufacturer’s plant in Concord(Ontario - Canada) is supplied by a transformerrated at 2000 kVA - 27.6 kV / 600 V - Yy -Ucc = 5.23 %.It manufactures exhaust assemblies from steelplate using spot welders and seam welders.

Problems encounteredc Visual and nervous fatigue in personnel, due tothe fluctuation in brightness of lamps (flicker)when welding equipment was in operation.c Noise pollution and premature mechanicalageing of equipment caused by vibrations mainlyin the transformer and the main switchgear whenwelding equipment was in operation.c Inability to add equipment for fear that theinstallation would be overloaded (peak currentswhen welders were fired were greater that thenominal current of the main circuit breaker).Expansion of the installation would thus requiresubstantial investment, either to upgrade theexisting installation or to build a new powersupply facility.c Annual penalties of 5 kE for exceeding thereactive power consumption limit (0.75 powerfactor).c Defective parts caused by welding faultsappeared at the end of the manufacturingprocess when the tubes are bent into shape.All these factors reduced company productivity.

SolutionsThe measures taken during the operation of thewelding equipment showed a nominal voltage of584 V, voltage dips of 5.8 %, current peaks of2000 A, and reactive power peaks of 1200 kvar(see fig. 15).

The problems clearly stemmed from voltagefluctuations caused by the operation of welderswith loads which vary rapidly and frequently andwhich consume significant reactive power.A voltage dip of 6 % produces a reduction of12 % (1-0.942) in the power available forwelding. This was the reason for the largenumber of defective welds.Standard devices for reactive powercompensation use electromechanicalcontactors which cannot achieve the requiredresponse times; the operation of capacitor stepsis deliberately time delayed to reduce the numberof operations and avoid reducing the service lifeof the contactors through premature wear, aswell as to enable the capacitors to discharge.The solution chosen was to connect a real timereactive compensator (see fig. 16). Thisinnovative device offers:c ultra-rapid compensation of the variations inreactive power within one cycle (16.6 ms at60 Hz), which is especially suitable for loadswith rapid, large variations (welding machines,lifts, presses, crushers, motor starting, etc.);c transient-free switch through controlledswitching, which is especially useful with loadswhich cannot withstand transient overvoltages(PLCs, computer systems, etc.);c increased service life of capacitors andcontactors owing to the absence of movingmechanical parts and overvoltagesWith compensation of 1200 kvar it would bepossible to minimise voltage dips, but 800 kvarwas deemed sufficient to maintain the voltage at

Fig. 15: Improvements due to the real time reactivecompensator.

Fig. 16: Real time reactive compensator [a] principle,[b] practical implementation.

L1

a b

L2 L3

Before After

Voltage (V) 584 599

Voltage dipc Depth (%): 5.8 3.2c Duration (cycle) 20 to 25 10 to 15

Currentc Average 1000 550c Peak 2000 1250

Reactive power (kvar) 600 0to 1200 to 300

Power factor 0.75 > 0.92


an acceptable level for all processes in the plantunder all load conditions.The results of implementing the solution are(see fig. 17):

c a reduction in current peaks to 1250 A and theaddition of loads without modification of theinstallation, with improved installation efficiencythrough reduction of joule losses;c a reduction in reactive power peaks to300 kvar and an increase in the power factor

to over 0.92, thus avoiding power factorpenalties;c an increase in the nominal voltage to 599 Vand a reduction in voltage dips to 3.2 % (seefig. 16). This is a consequence of the increase inthe power factor and reduction in the currentamplitude (see fig. 18). Visual and nervousfatigue in personnel due to the flicker was alsoeliminated. Welding quality improved, as didproductivity.

Fig. 17: Measurement of current, voltage and reactive power: [a] without compensation [b] with compensation.

Fig. 18: Reduction in voltage drop obtained using a real time reactive compensator.

Voltage(V)

a340

335

330

1500

1000

1000

500

350

340

1250

750

5000

Current(A)

kvar

Voltage(V)

b

Phase 1

Current(A)

kvar

d = 3 s

d = 1.5 s

Phase 2 Phase 3 Average

Reactivecompensator

With Without

VS

∆V

∆V

VS VS

VL VLVL

∆V

Welder


6.3 Protection against lightning

Description of the installationThe site consists of offices (computer hardware,lighting and heating unit), a security post (firealarm, burglar alarm, access control, videosurveillance) and three buildings for themanufacturing process on 10 hectares in theAvignon region of France (probability of lightningis 2 strikes per km2 per year).There are trees and metal structures (pylons) inthe vicinity of the site. All of the buildings arefitted with lightning conductors. The MV and LVpower supplies are underground.

Problems encounteredA storm struck the site, destroying the LVinstallation in the security post and causing36.5 kE of operating losses. The presence oflightning conductors prevented the structure fromcatching fire, but the electrical equipment whichwas destroyed was not protected by surgearresters, contrary to the recommendation instandards UTE C-15443 and IEC 61024.

SolutionsAfter analysing equipotentiality and earthing ofthe power system, followed by verification ofthe installation of lightning conductors andchecking of the values of the earth electrodes,the decision was taken to install surgearresters.

Surge arresters were installed at the head of theinstallation (main LV distribution board) and incascade in each manufacturing building (seefig. 19). As the neutral point connection wasTNC, protection would only be provided incommon mode (between phases and PEN).

In conformity with guide UTE C-15443 regardingoperation in the presence of lightningconductors, the characteristics of theMerlin Gerin PF65 and PF8 surge arresters (seefig. 20) are as follows:

c At the head of the installation

In = 20 kA – Imax = 65 kA – Up = 2 kV

c In cascade (at least 10 m apart)

In = 2 kA – Imax = 8 kA – Up = 1.5 kV

In cascade, good protection is provided for thesecondary distribution boards (offices andsecurity post).

As the neutral point connection was converted toTNS, protection had to be provided in commonmode (between phase and PE) and differentialmode (between phases and neutral). Thedisconnection devices in this case are circuitbreakers with a breaking capacity of 22 kA.

PF8PF8

PF65 PF65

3LN

3LN

LN

LN

Main LV distribution boardBuilding 2

Main LV distribution boardBuilding 1

Secondary distribution boardBuilding 1

Secondary distribution boardBuilding 1

Fig. 19: Installation diagram for several surge arrestersin cascade.

Fig. 20: Low voltage surge arresters (Merlin GerinPF65 and PF8).

PF65

PF8


Electrical disturbance may originate in thedistribution power system, in the installation ofthe user who is subject to disturbance or in theinstallation of a nearby user.

The consequences of the disturbance varyaccording to the economic context and the areaof application: from inconvenience to shutdownof production facilities - it can even put lives atrisk.

The search to improve company competitivenessand the deregulation of the electricity market

mean that the quality of electricity has become astrategic issue for electricity companies, theoperating, maintenance and managementpersonnel of service sector and industrial sites,as well as for equipment manufacturers.

However, problems of disturbance should not beregarded as insurmountable, as solutions doexist. Employing specialists to define, implementand maintain these solutions while observingbest practice will provide users with the rightquality of power supply for their requirements.

7 Conclusion


Bibliography

c Harmonic disturbances in networks, and their treatment.C. COLLOMBET, J.-M. LUPIN, J. SCHONEK,Cahier Technique no. 152.c Inverters and harmonics (case studies of non-linearloads).J.-N. FIORINA, Cahier Technique no. 159.

c Harmonics upstream of rectifiers in UPS.J.-N. FIORINA, Cahier Technique no. 160.

c Automatic transferring of power supplies in HV andLV networks.G. THOMASSET, Cahier Technique no. 161.

c HV industrial network design.G. THOMASSET, Cahier Technique no. 169.

c Earthing systems in LVB. LACROIX, R. CALVAS, Cahier Technique no. 172.

c Earthing systems worldwide and evolutionsB. LACROIX, R. CALVAS, Cahier Technique no. 173.

c The IT earthing system (unearthed neutral).F. JULLIEN, I. HERITIER, Cahier Technique no. 178.

c LV surges and surge arresters.LV insulation co-ordination.Ch. SERAUDIE, Cahier Technique no. 179.

c LV circuit-breakers confronted with harmonic,transient and cyclic currents.M. COLLOMBET, B. LACROIX,Cahier Technique no. 182.

c Active harmonic conditioners and unity power factorrectifiers.E. BETTEGA, J.-N. FIORINA, Cahier Technique no. 183.

c Cohabitation of high and low currents.R. CALVAS, J. DELABALLE,Cahier Technique no. 187.

c Ferroresonance.Ph. FERRACCI, Cahier Technique no. 190.

Other publications

c Guide de l’ingénierie électrique des réseaux internesd’usines - Collection ELECTRA.

c Method of symmetrical co-ordinates applied to thesolution of polyphase networks - Trans. Amer. Inst.Electr. Engrs, June, 1918 - C.L. FORTESCUE.

c Guide to quality of electrical supply for industrialinstallations Part 2: voltage dips and short interruptionsWorking Group UIE Power Quality 1996.

c Supply Quality Issues at the Interphase betweenPower System and Industrial Consumers - PQA 1998,A. ROBERT.

c Real time reactive compensation systems for weldingapplications - PQ 1998, R. WODRICH.

c Low voltage hybrid harmonic filters, technical &economic analysis - PQ 1999, J. SCHONEK.

Standards

c IEC 61000-X-X – Electromagnetic compatibility (EMC):

v 2-1: Description of the environment.v 2-2: Compatibility levels (public low-voltage powersupply systems).v 2-4: Compatibility levels in industrial plants for low-frequency conducted disturbances.v 2-5: Classification of electromagnetic environments.v 3-2: Limits for harmonic current emissions(equipment input current i 16A per phase).v 3-3: Limitation of voltage fluctuations and flicker inlow-voltage supply systems for equipment with ratedcurrent i 16 A.v 3-5: Limitation of voltage fluctuations and flicker inlow-voltage supply systems for equipment with ratedcurrent > 16 A.v 3-6: Assessment of emission limits for distortingloads in MV and HV power systems.v 3-7: Assessment of emission limits for fluctuatingloads in MV and HV power systems.v 4-7: Harmonics and interharmonics measurementsv 4-11: Voltage dips, short interruptions and voltagevariations immunity tests.v 4-12: Oscillatory waves immunity test.v 4-15: Flickermeter.

c Other standards and laws

v European Union "Council Directive 85/374 on theapproximation of the laws of the Member Statesrelating to the liability for defective products", OfficialJournal (07.08.1985).v EN 50160 Characteristics of electricity supplied bypublic distribution systems (07-1994).v Application Guide to the European StandardEN 50160 - July 1995 - UNIPEDE.v IEEE Std 1159-1995: Recommended Practice forMonitoring Electric Power Quality.v IEEE Std 1000-1992: IEEE Recommended Practicefor Powering and Grounding Sensitive ElectronicEquipment.v IEC 60071-1: Insulation coordination.v IEC 60050-161: International ElectrotechnicalVocabulary.

Cahiers Techniques Schneider Electric

c Residual current devices.R. CALVAS, Cahier Technique no. 114.

c Electrical disturbances in LV.R. CALVAS, Cahier Technique no. 141.

c Switching MV capacitor banks.D. KOCH, Cahier Technique no. 142.

c EMC: Electromagnetic compatibility.F. VAILLANT, Cahier Technique no. 149.

c Overvoltages and insulation coordination in MV and HV.D. FULCHIRON, Cahier Technique no. 151.

Schneider Electric Direction Scientifique et Technique,Service Communication TechniqueF-38050 Grenoble cedex 9Fax : 33 (0)4 76 57 98 60

Real: LAI Ltd - Tarporley - Cheshire -GB.Edition: Schneider ElectricPrinting: Imprimerie du Pont de Claix - Claix - 1000.- 100 FF- ©

200

0 S

chne

ider

Ele

ctric

00-0094836

Documents

Power Quality - schneider- · PDF fileCahier Technique Schneider Electric no. 199 / p.3 Power Quality One of the properties of electricity is that some of its characteristics depend