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Cahier technique no. 199

Power Quality

Ph. Ferracci "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.

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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

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.

ECT 199(e) october 2001 Cahier Technique Schneider Electric no. 199 / p.2 Power Quality

One of the properties of 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 dips and interruptions p.6 2.3 Harmonics and interharmonics p.8 2.4 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 p.30 7 Conclusion p.31 Bibliography p.32

Cahier Technique Schneider Electric no. 199 / p.3 1 Introduction

1.1 Context

The quality of electricity has become a strategic issue The widespread use of equipment which is for electricity companies, the operating, maintenance sensitive to voltage disturbance and/or and management personnel of service sector generates disturbance itself and industrial sites, as well as for equipment As a consequence of their numerous manufacturers, for the following main reasons: advantages (flexible operation, excellent c the economic necessity for businesses to efficiency, high performance levels, etc.), we increase their competitiveness, have seen the development and widespread use c the widespread use of equipment which is of automated systems and adjustable speed sensitive to voltage disturbance and/or drives in industry, information systems, and fluo- generates disturbance itself, compact lighting in the service and domestic c the opening up of the . sectors. These types of equipment are both sensitive to voltage disturbance and generate The economic necessity for businesses disturbance themselves. to increase their competitiveness Their multiple use within individual processes c Reduction of costs linked to loss of supply requires an electrical power supply which can continuity and problems of non-quality provide ever increasing performance in terms of The cost of disturbance (interruptions, voltage dips, continuity and quality. The temporary shutdown of harmonics, lightning overvoltages, etc.) is substantial. just one element in the chain may interrupt the These costs must take into account losses in whole production facilities (manufacture of semi- production and raw materials, restarting of conductors, cement works, water treatment, production facilities, non-quality of production materials handling, printing, steelworks, and delivery delays. The malfunction or petrochemicals, etc.) or services (data processing shutdown of vital equipment such as computers, centres, banks, telecommunications, etc.). lighting and safety systems may put lives at risk Consequently, the work of the IEC on (e.g. in hospitals, airport lighting systems, public electromagnetic compatibility (EMC) has led to and high-rise buildings, etc.). stricter and stricter standards and Costs also include high quality, targeted recommendations (limitations on disturbances preventive maintenance measures for emission levels, etc.). anticipating possible problems. There is an increasing transfer of responsibility from the The opening up of the electricity market industrial user to the equipment manufacturer for The rules governing the electricity sector are the provision of site maintenance; manufacturers undergoing radical change: electricity production are now becoming electricity suppliers. has opened up to competition, production is c Reduction of costs linked to oversized decentralised, and (large) electricity consumers installations and energy bills now have the opportunity to choose their supplier. Other less obvious consequences of PQ In 1985, the Commission of the European degradation are: Communities states (directive 85/374) that v A reduction of installation energy efficiency, electricity is to be considered a product and as a leading to higher energy bills consequence made it necessary to define its v Overloading of the installation, causing essential characteristics clearly. premature ageing and increasing the risk of In addition, in the context of liberalising energy breakdown, leading in turn to oversizing of markets, the search for competitiveness by distribution equipment electricity companies now means that quality has This is why professional users of electricity are become a differentiating factor. A guarantee of keen to optimise the operation of their electrical quality is a potential criterion of choice for industrial installations. users when looking for an energy supplier.

Cahier Technique Schneider Electric no. 199 / p.4 1.2 Objectives of Power Quality measurement

The measurement parameters and accuracy Complementary software tools to ensure may differ depending on the application. control-command and continuous monitoring of the installation are thus required. Contractual application Within the context of a deregulated market, Statistical surveys contractual relations may exist not only Such research requires a statistical approach on between the electricity supplier and the end the basis of wide-ranging results from surveys user, but also between the power production generally carried out by the operators of company and transmission company or between transmission and distribution power systems. the transmission company and distribution c Benchmark the general performances of a company. A contractual arrangement requires power system that terms are defined jointly and mutually These can be used, for example, to: agreed upon by all parties. The parameters for v measuring quality must therefore be defined Plan and target preventive actions by mapping and the values compared with predefined, i.e. disturbance levels on a network. This helps contractual limits. reduce operating costs and improve control of This arrangement frequently requires the disturbance. An abnormal situation with respect processing of significant quantities of data. to an average level can be detected and correlated with the addition of new loads. Corrective maintenance Research can also be carried out into seasonal Even where best practice is observed (single- trends or excessive demand. line diagram, choice of protective devices and v Compare the PQ of various distribution neutral point connection, application of companies in different geographical areas. appropriate solutions) right from the design Potential customers may request details of the phase, malfunctions may occur during reliability of the electricity supply before installing operation: a new plant. c Disturbances may have been ignored or c Benchmark performances at individual points under-estimated. on the power system c The installation may have changed (new These can be used to: loads and/or modification). v Determine the electromagnetic environment in Troubleshooting is generally required as a which a future installation or a new piece of consequence of problems of this nature. equipment may have to operate. Preventive The aim is frequently to get results as quickly measures may then be taken to improve the as possible, which may lead to premature or distribution power system and/or desensitise the unfounded conclusions. customer power system. Portable measurement systems (for limited v Specify and verify the performance levels periods) or fixed apparatus (for continuous undertaken by the electricity supplier as part of monitoring) make it easier to carry out the contract. This information on the electricity installation diagnostics (detection and quality are of particular strategic importance for archiving of disturbances and triggering of electricity companies who are seeking to alarms). improve competitiveness, satisfaction of needs and customer loyalty in the context of liberalising Optimising the operation of electrical energy markets. installations 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.

Cahier Technique Schneider Electric no. 199 / p.5 2 Degradation of PQ: origins - characteristics - definitions

2.1 General

Electromagnetic disturbances which are likely to c transient overvoltages, disturb the correct operation of industrial c voltage fluctuations, equipment and processes is generally ranked in c voltage unbalance, various classes relating to conducted and c radiated disturbance: power- variations, c c low frequency (< 9 kHz), DC in AC networks, c c high frequency (u 9 kHz), signalling . c electrostatic discharge. It is not generally necessary to measure each type of disturbance. Measurement of PQ usually involves characterising low frequency conducted The types can be placed in four categories, electromagnetic disturbances (the range is affecting the magnitude, waveform, frequency widened to include transient overvoltages and and symmetry of the voltage. Several of these transmission of signals on a power system): characteristics may be modified simultaneously by any one type of disturbance. Disturbances c voltage dips and interruptions, can also be classified according to their c harmonics and interharmonics, permanent, semi-permanent or random nature c temporary overvoltages, (lightning, short-circuit, switching operations, c swell, etc.).

2.2 Voltage dips and interruptions

Definitions In case of a non-rectangular envelope, the A voltage dip is a sudden reduction of the duration is dependent on the selected dip voltage at a point in an electrical power system threshold value (set by the user according to the followed by voltage recovery after a short period objective). The duration is typically defined as of time from a few cycles to a few seconds the time interval during which the rms (1/2) is (IEC 61050-161 ). A voltage dip is normally lower than 90 %. The shape of the envelope (for detected and characterised by the calculation of example in case of complex multi-step and not the value "rms (1/2)" over one simple one step dip) may be assessed using cycle every half-cycle -each period overlaps the several dip thresholds set and/or wave form prior period by one half-cycle- (see fig. 1). capture. Time aggregation techniques may define an equivalent dip characterised by the There is a dip to x % if the rms (1/2) value falls smallest rms (1/2) value measured during the dip below the dip threshold x % of the reference and the total duration of the dip. For three-phase value Uref. The threshold x is typically set below systems phase aggregation techniques (mainly 90 (CENELEC EN 50160, IEEE 1159). The used for contractual applications) may define a reference voltage Uref is generally the nominal single phase equivalent dip (characterised for voltage for LV power systems and the declared example by the greatest depth on the three voltage for MV and HV power systems. A sliding phases and the total duration). reference voltage, equal to the voltage before Interruptions are a special type of voltage dip to the beginning of the disturbance is useful to a few percentage of Uref (typically within the study transference factor between different range 1-10 %). They are characterised by one voltage systems. parameter only: the duration. Short interruptions A voltage dip is characterised by two parameters last less than one minute (extended to three (see fig. 1b for x equal to 90): minutes depending on network operating c depth: ∆U (or its magnitude U), conditions) and often result from tripping and c duration ∆T. automatic reclosure of a designed

Cahier Technique Schneider Electric no. 199 / p.6 to avoid long interruptions which have longer duration. Short and long interruptions differ in a both their origins and the solutions required to prevent or reduce their occurrence. V(p.u.) Voltage disturbances lasting less than a half- 1 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: 0,5 c instantaneous (T/2 < ∆T < 30 T), t c momentary (30 T < ∆T<3s), 0 c temporary (3 s < ∆T < 1 min), c sustained interruption and undervoltage (∆T>1min). -0,5 Depending on the context, the measured voltages may be between live conductors -1 (between phases or between phase and neutral), between live conductors and earth (Ph/ earth or neutral/earth), or between live b conductors and the protective conductor. rms (1/2) In a 3-phase system, the characteristics ∆U and (%) ∆ T in general differ for each of the three phases. 110 This is why a voltage dip must be detected and 100 characterised separately on each phase. 90 ∆U = 30 % A voltage dip is regarded as occurring on a (depth) 3-phase system if at least one phase is affected 70 by the disturbance. ∆T = 140 ms U Origins (duration) (magnitude) c Voltage dips and short interruptions are 10 mainly caused by phenomena leading to high t (ms) currents, which in turn cause a voltage drop 0 across the network impedances with a 0 50 100 150 200 250 300 magnitude which decreases in proportion to the Fig. 1: Characteristic parameters of a voltage dip [a] electrical distance of the observation point from waveform [b] rms (1/2). the source of the disturbance. Voltage dips and short interruptions have various causes: subjected to a succession of voltage dips and/or v Faults on the transmission (HV) or distribution short interruptions caused by intermittent arc (LV and MV) networks or on the installation itself faults, sequence of automatic reclosing (on The occurrence of faults causes voltage dips for overhead or mixed radial networks) intended to all users. The duration of a dip is usually extinguish transient and semi-permanent faults conditioned by the operating time of the or voltage feedback intended to locate the fault. protective devices. The isolation of faults by v Switching of large loads (asynchronous protective devices (circuit breakers, fuses) will motors, arc furnaces, welding machines, boilers, produce interruptions (long or short) for users etc.) compared to the short-circuit power. feeded by the faulty section of the power c Long interruptions are the result of the system. Although the power source is no longer definitive isolation of a permanent fault present, network voltage may be maintained by (requiring to repair or to replace any component the residual voltage provided by asynchronous before re-energising) by means of protective or synchronous motors as they slow down (0.3 devices or by the intentional or unintentional to 1 s) or voltage due to the discharge of opening of a device. banks connected to the power system. Voltage dips and interruptions are propagated Short interruptions are often the result of the to lower voltage levels via . The operation of automated systems on the network number of phases affected and the depth of such as fast and/or slow automatic reclosers, or the voltage dips depend on the type of fault changeover of transformers or lines. Users are and the coupling.

Cahier Technique Schneider Electric no. 199 / p.7 Overhead networks, which are exposed to c Transients (∆T < T/2) are caused, for bad weather, are subject to more voltage dips example, by the energisation of capacitor banks, and interruptions than underground the isolation of a fault by a fuse or a fast LV networks. However, an underground feeder circuit breaker, or by commutation notches connected to the same system as from polyphase converters. overhead or mixed networks will suffer voltage dips which are due to the faults affecting overhead lines.

2.3 Harmonics and interharmonics

Summary: harmonic thus has a vital role in All periodic functions (of frequency f) can be limiting the voltage distortion. Note that if the broken down into a sum of sinusoidal waves of source impedance is low (Scc is high), voltage frequency h x f (h is an integer). h is the distortion is low. harmonic order (h > 1). The first order Main sources of harmonics component is the fundamental component. These are loads which can be distinguished ∞ =+∑ πϕ + according to their domain, i.e. industrial or y(t) Y0 Yh 2 sin(2 h fh ) domestic. h1= c Industrial loads The rms is: v Power electronic equipment: drives, rectifiers (diode or thyristor), inverters or switching power =2 ++++2 2 2 Yeff YYYY...0 1 2 h supplies; v Loads using electric arcs: arc furnaces, The THD (Total Harmonic Distortion) factor welding machines, lighting (discharge lamps, measures the signal distortion: fluorescent tubes). Starting motors using 2 electronic starters and power transformers ∞  Y  energisation also generates (temporary) THD = ∑ h   harmonics. h2=  Y1  Note that because of its multiple advantages Harmonics are mainly produced by non-linear (operating flexibility, excellent energy efficiency, loads which draw current of a different wave high performance levels, etc.), the use of power form from the supply voltage (see fig. 2). The electronic equipment is becoming more spectrum of the harmonics depends on the widespread. nature of the load. Harmonic voltages occur c Domestic loads with power inverters or switching across network impedances resulting distorted power supplies such as television, microwave voltages which can disturb the operation of ovens, induction hotplates, computers, printers, other users connected to the same supply. The photocopiers, dimmer , electrodomestic value of the supply impedance at different equipments, fluorescent lamps.

Other loads Voltage source E Z

U = E - ZI I

Harmonics generator

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

Cahier Technique Schneider Electric no. 199 / p.8 Although their individual power ratings are much Interharmonics are sinusoid components with less than for industrial loads, the combination of frequencies which are not integer multiples of large numbers and simultaneous use over long the fundamental component (they are located periods creates significant sources of harmonic between harmonics). They are due to periodic distortion. Note that the use of this type of or random variations in the power drawn by equipment is increasing, as in some cases is the various devices such as arc furnaces, welding power rating. machines and frequency inverters (drives, cycloconverters). The remote control frequencies Harmonic levels used by the power distributor are also These generally vary according to the operating interharmonics. mode of the device, the hour and the season The spectrum may be discrete or continuous and (heating and air conditioning). vary randomly (arc furnaces) or intermittently The sources usually generate odd harmonic (welding machines). components (see fig. 3). Power transformer To study the short, medium and long term energisation, polarised loads (half-wave rectifiers) effects, the various parameters must be and arc furnaces generate even harmonics in measured at time intervals which are compatible addition to odd harmonics components. with the thermal time constant of the devices.

Non-linear loads Current waveform Spectrum THD

Adjustable speed drive A % 100

t 50 44 %

0 h 1 5 7 11 13 17 19 23 25

Rectifier/charger A % 100

t 50 28 %

0 h 1 5 7 11 13 17 19

% Data processing load A 100

t 50 115 %

0 h 1 3 5 7 9 11 13

% Fluorescent lighting A 100

t 50 53 %

0 h 1 3 5 7 9 11 13

Fig. 3: Characteristics of certain harmonics generators.

Cahier Technique Schneider Electric no. 199 / p.9 2.4 Overvoltages

Where voltage is applied to a device and the c Break of the neutral conductor peak value exceeds the limits defined in a Devices powered by the phase with the least standard or specification, this is an load witness an increase in voltage (sometimes (see "Cahiers Techniques" nos. 141, 151 and up to the phase to phase voltage). 179). c Faults on alternator regulators or tap changer Overvoltages are of three types: transformer c temporary, c Overcompensation of reactive power c switching, Shunt produce an increase in voltage from the source to their location. c lightning. This voltage is especially high during periods of They can appear: low load. c in differential mode (between live conductors: ph/ph Ð ph/neutral), Switching overvoltages c in common mode (between live conductors These are produced by rapid modifications in the and the exposed-conductive-part or earth). network structure (opening of protective devices, etc.). The following distinctions are made: Temporary overvoltages c switching overvoltages at normal load, By definition, these occur at power frequency c overvoltages produced by the switching on and (50/60 Hz). They have various origins: off of low inductive currents, c An insulation fault c overvoltages produced by the switching of When an insulation fault occurs between phase capacitive circuits (no-load lines or cables, and earth in an isolated neutral system or capacitor banks). For example, the energisation impedance earthed neutral system, the voltage of of a capacitor bank produces a transient the healthy phases to earth may reach the phase overvoltage in which the first peak may reach 2r to phase voltage. Overvoltages on LV times the rms value of the nominal voltage and a installations may come from HV installations via transient overcurrent with a peak value of up to the earth of the HV/LV station. 100 times the rated current of the capacitor (see "Cahier Technique" no. 142). c Ferroresonance This is a rare non-linear oscillatory phenomenon Lightning overvoltages which can often be dangerous for equipment and Lightning is a natural phenomenon occurring which is produced in a circuit containing a during storms. A distinction is made between capacitor and a saturable inductance. direct lightning strike (on a line or structure) and Ferroresonance is often the apparent cause of the indirect effects of lightning (induced malfunctions or the destruction of devices (see overvoltages and increase in earth potential) "Cahier Technique" no. 190). (see "Cahiers Techniques" nos. 151 and 179).

2.5 Voltage variations and fluctuations

Voltage variations are variations in the rms value c Slow voltage variations are caused by the or the peak value with an of less than slow variation of loads connected to the network. 10% of the nominal voltage. c Voltage fluctuations are mainly due to rapidly Voltage fluctuations are a series of voltage varying industrial loads such as welding changes or cyclical or random variations in the machines, arc furnaces or rolling mills. voltage envelope which are characterised by the frequency of variation and the magnitude.

2.6 Unbalance

A 3-phase system is unbalanced if the rms value U1 U1 of the phase voltages or the phase angles between ∆∆==i o consecutive phases are not equal. The degree of Ui and Uo unbalance is defined using the Fortescue U1d U1d components, comparing the negative sequence The following approximate formula can also be component (U1i) (or zero sequence component V − Vavg (U1o)) of the fundamental to the positive ∆ = i used: Ui maxi sequence component (U1d) of the fundamental. Vavg

Cahier Technique Schneider Electric no. 199 / p.10 where Vi = phase voltage i and (or zero sequence) currents produced by unbalanced loads leading to non-identical V1 ++ V2 V3 currents on the three phases (LV loads Vavg = 3 connected between phase and neutral, or single- phase or 2-phase MV loads such as welding The negative sequence (or zero sequence) machines and induction furnaces). voltage is produced by voltage drops along the Single-phase or 2-phase faults produce network impedances due to negative sequence unbalance until tripping of the protective devices.

2.7 Summary

Disturbances Voltage Overvoltages Harmonics Unbalance Voltage dips fluctuations

Characteristic waveforms

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 Lighting v Inverter v Capacitor bank

: Occasional phenomenon : Frequent phenomenon

Cahier Technique Schneider Electric no. 199 / p.11 3 Effects of disturbance on loads and processes

Generally speaking, the effects of all disturbances c Deferred effects: energy losses, accelerated can be classified in two ways: ageing of equipment due to overheating and c Instantaneous effects: unwanted operation of additional electro-dynamic stress caused by the contactors or protective devices, incorrect operation disturbance. or shutdown of a machine. The financial impact The financial impact (e.g. on productivity) is of the disturbance can be estimated directly. more difficult to quantify.

3.1 Voltage dips and interruptions

Voltage dips and interruptions disturb many Rapidly reconnecting (~ 150 ms) the power to types of devices connected to the network. an asynchronous motor which is slowing down They are the most frequent cause of Power without precautionary measures may lead to Quality problems. A voltage dip or interruption reclosing in opposition to the phase between of a few hundred milliseconds may have the source and the residual voltage in damaging consequences for several hours. asynchronous motors. In this case the first The most sensitive applications are: current peak may reach three times the start- c complete continuous production lines where up current (15 to 20 In) (see "Cahier the process cannot tolerate any temporary Technique" no. 161). shutdown of any element in the chain (printing, The overcurrents and consequent voltage steelworks, paper mills, petrochemicals, etc.), drops have consequences for the motor (excessive overheating and electro-dynamic c lighting and safety systems (hospitals, airport force in the coils, which may cause insulation lighting systems, public and high-rise failures and torque shocks with abnormal buildings, etc.), mechanical stress on the couplings and c computer equipment (data processing reducers, leading to premature wear or even centres, banks, telecommunications, etc.), breakage) as well as other equipment such c essential auxiliary plant for power stations. as contactors (wear or even fusion of the The paragraphs below cover the main contacts). Overcurrents may cause tripping of consequences of voltage dips and the main general protective devices of the interruptions on equipment used in the installation causing the process to shutdown. industrial, service and domestic sectors. Synchronous motors Asynchronous motors The effects are almost identical to those for When a voltage dip occurs, the torque of an asynchronous motors. Synchronous motors asynchronous motor (proportional to the can however withstand deeper voltage dips square of the voltage) drops suddenly which (around 50 %) without stalling, owing to their slowdowns the motor. This slowdown depends generally greater inertia, the possibilities of on the magnitude and duration of the dip, the overexcitation and the fact that their torque is inertia of the rotating masses and the torque- proportional to the voltage. In the event of speed characteristics of the driven load. If the stalling, the motor stops and the entire torque developed by the motor drops below complex start-up process must be repeated. the resistant torque, the motor stops (stalls). Following an interruption, at the time of voltage Actuators recovery, the motor tends to re-accelerate and The control devices (contactors, circuit breakers absorb current whose value is nearly its starting with voltage loss coils) powered directly from the current, the duration of which depends on the network are sensitive to voltage dips whose duration of the interruption. Where there are depth exceeds 25 % of Un. Indeed, for a several motors in an installation, the standard contactor, there is a minimum voltage simultaneous restarting may produce a voltage value which must be observed (known as the drop in the upstream impedances on the network drop-out voltage), otherwise the poles will which will increase the duration of the dip and separate and transform a voltage dip (lasting a may make restarting difficult (long restarts with few tens of milliseconds) or a short interruption overheating) or even impossible (motor torque into a long interruption until the contactor is lower than the resistive torque). reenergized.

Cahier Technique Schneider Electric no. 199 / p.12 Computer equipment the loss of equipment functions depend in Computer equipment (computers, measurement particular on the restart conditions when voltage is apparatus) today occupy a dominant position in restored. Certain equipment, for example, has its the monitoring and control-command of own voltage dip detection devices which enable installations, management and production. All of data to be backed up and ensure safety by this equipment is sensitive to voltage dips with interrupting calculation processes and any depth greater than 10 % Un. incorrect commands. The ITIC (Information Technology Industry Council) curve Ð formerly CBEMA curve Ð shows Adjustable speed machines on a duration-amplitude scale, the typical The problems of voltage dips applied to variable withstand of computer equipment to voltage dips, speed drives are: interruptions and overvoltages (see fig. 4). c It is not possible to supply sufficient voltage to Operation outside these limits leads to loss of the motor (loss of torque, slowdown). data, incorrect commands, and shutdown or c The control circuits supplied directly by the malfunction of equipment. The consequences of network cannot function. c There is overcurrent when voltage recovers (the drive filter capacitor is recharged). c U (%) There is overcurrent and unbalanced current in 500 the event of voltage dips on a single phase. c There is loss of control of DC drives functioning as inverters (regenerative braking). Adjustable speed drives usually trip out when a 200 voltage dip deeper than 15 % occurs. 140 120 Lighting 110 100 80 90 Voltage dips cause premature ageing of 70 incandescent lamps and fluorescent tubes. 0 Voltage dips deeper than or equal to 50 % with a 0100,01T -3 3.10-3 0,02 0,5 10 ∆T (s) duration of around 50 ms will extinguish discharge lamps. The lamp must then be left off Fig. 4: Typical withstand as defined by the ITIC curve. for several minutes to cool the bulb before it is turned on again.

3.2 Harmonics

The consequences of harmonics are linked to the error and unwanted operation even during increase in peak values (dielectric breakdown), normal operation with no overload. rms values (excessive overheating) and to the v Disturbances induced by low current systems frequency spectrum (vibration and mechanical (remote control, telecommunications, hi-fi stress) of voltages and currents. systems, computer screens, television sets). The effects always have an economic impact v Abnormal vibrations and acoustic noise resulting from the additional costs linked to: (LV switchboards, motors, transformers). c degradation in the energy efficiency of the v Destruction of capacitors by thermal overload installation (energy loss), If the actual frequency of the upstream c oversizing of equipment, capacitor-network system is similar to a c loss of productivity (accelerated ageing of harmonic order, this causes resonance and equipment, unwanted tripping). amplification of the corresponding harmonic. v Malfunctions are probable with a harmonic Loss of accuracy of measurement instruments distortion factor of greater than 8 % of the A class 2 induction energy meter will produce in voltage. Between 5 and 8 %, malfunctions are current and voltage, a 0.3 % additional error in possible. the presence of 5 % of harmonic 5. c Instantaneous or short term effects c Long term effects v Unwanted operation of protective devices: Current overload produces excessive overheating harmonics have a harmful influence mainly on and leads to premature ageing of equipment: thermal control devices. Indeed, when protective v Overheating of sources: transformers, devices of this type calculate the rms value of alternators (through increased joule and iron the current from the peak value, there is a risk of losses).

Cahier Technique Schneider Electric no. 199 / p.13 v Mechanical stress (pulse torque in installation earth, including the metal structures asynchronous machines). of the building. Third harmonic (and multiples v Overheating of equipment: phase and neutral of 3) currents will flow through these circuits conductors through increased joule and and produce variations in potential with the dielectric losses. Capacitors are especially following results: sensitive to harmonics as their impedance v corrosion of metal parts, decreases in proportion to the harmonic order. v overcurrent in the telecommunication links v Destruction of equipment (capacitors, circuit between the exposed-conductive-part of two breakers, etc.). devices (for example, printer and computer), v Overload and excessive overheating of the electromagnetic radiation causing screen neutral conductor may result from the presence disturbance (computers, laboratory apparatus). of third harmonic (and multiples of 3) currents in The table in figure 5 summarises the main effects the phase conductors which add in the neutral. of harmonics and the normal permitted levels. The TNC neutral uses the Interharmonics affect remotely-controlled same conductor for neutral and protection devices and produce a phenomenon known as purposes. This conductor interconnects the flicker.

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 MV or 8 hrs/days at LV Motors Losses and excessive overheating. HVF i 2% Reduction of capacity for use at full load. for usual asynchronous Pulse torque (vibrations, mechanical stress) motors Noise 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 % i (especially in the neutral conductor if third harmonic Uh / U1 7% currents present).

Computers Operating problems. Uh / U1 i 5% Power Problems related to waveform electronics (commutation, synchronisation).

13 = 2 HVF∑ Uh h (Harmonic Variation Factor according to IEC892) h=2 Fig. 5: Effects of harmonics and practical limits.

3.3 Overvoltages

The consequences are extremely varied company, loss of production for industrial according to the period of application, repetitivity, companies). magnitude, mode (common or differential), c Disturbance in control system and low current gradient and frequency: communication circuits (see "Cahier Technique" c Dielectric breakdown, causing significant no. 187). permanent damage to equipment (electronic c Electrodynamic and thermal stress (fire) components, etc.). caused by: c Degradation of equipment through ageing v Lightning (usually) (repetitive rather than destructive overvoltages). Overhead networks are most vulnerable to c Long interruptions caused by the destruction of lightning, but installations supplied by equipment (loss of sales for distribution underground networks may also be affected by

Cahier Technique Schneider Electric no. 199 / p.14 stress due to high voltage if lightning strikes considerably higher than that of lightning, with close to the site. a longer duration. v Switching overvoltages: these are repetitive They can lead to degradation as serious as that and their probability of occurrence is caused by lightning.

3.4 Voltage variations and fluctuations

As fluctuations have a magnitude no greater the composition of the spectrum and the than ± 10 %, most equipment is not affected. duration of the disturbance. The main effect of voltage fluctuations is a There is however a perceptibility threshold (the fluctuation in the luminance of lamps (flicker). amplitude as a function of the variation The physiological strain (visual and nervous frequency) defined by the IEC below which fatigue) depends on the magnitude of the flicker is no longer visible. fluctuations, the repetition rate of the variations,

3.5 Unbalance

The main effect is the overheating of 3-phase differ considerably. This increases the asynchronous machines. overheating of the phase(s) which the highest In fact, the zero sequence reactance of an current flows through and reduces the operating asynchronous machine is equivalent to its life of the machine. reactance during the start-up phase. The current In practice, a voltage unbalance factor of 1 % unbalance factor will thus be several times that over a long period, and 1.5 % over a few minutes of the supply voltage. Phase currents can thus is acceptable.

3.6 Summary

Equipment Sensitivity to disturbance Voltage dips Overvoltages Harmonics Unbalance Voltage < 0.5 s > 0.5 s fluctuation c Asynchronous motor c Synchronous motor c Actuator c Speed drive c Data processing load, numerical control c Induction furnace c Lighting c Capacitor bank c Transformer c Inverter c Circuit breaker c Cable

Cahier Technique Schneider Electric no. 199 / p.15 4 Level of Power Quality

4.1 Evaluation methodology

Contractual application c Examination of the installation The contract must state: This phase is sometimes sufficient for quickly c Its duration. determining the origin of the malfunction. c The parameters to be measured. Environmental conditions such as humidity, dust c and temperature must not be overlooked. The contractual values. The installation, especially the wiring, circuit c The measurement point(s). breakers and fuses, have to be checked. c The voltages measured: these voltages c Monitor the installation (between phases and/or between phase and neutral) must be the equipment supply voltages. This step consists in equipping the site with measurement apparatus to detect and record the c For each parameter measured the choice of event where the problem originated. It may be measurement method, the time interval, the necessary to place instruments at several points measurement period (e.g. 10 minutes and 1 year in the installation, especially (where possible) for the voltage amplitude) and the reference close to the equipment subject to disturbance. values; for voltage dips and interruptions, for example, the reference voltage, detection The apparatus detects events when the thresholds and the distinction between long and thresholds of the parameters used to measure short interruptions must be defined. the Power Quality are exceeded, and records the data characterising the event (for example c The measurement accuracy. date, time, depth of voltage dip, THD). The c The method of determining penalties in the waveforms just before, during and after the event of one party failing to honour the terms of disturbance can also be recorded. The threshold the contract. settings must match the sensitivity of the c Clauses in the event of disagreement equipment. concerning the interpretation of the When using portable apparatus, the duration of measurements (intervention of third parties, etc.). the measurements must be representative of the c Data access and confidentiality. operating cycle of the factory in question (e.g. one week). It must always be assumed that the Corrective maintenance disturbance will recur. This is generally the consequence of incidents or Fixed apparatus can be used for continuous malfunctions during operation requiring monitoring of the installation. If the apparatus troubleshooting in order to apply corrective settings are correct, it will carry out prevention measures. and detection by recording each occurrence of The usual steps are: disturbance. The data can be displayed locally or c Data collection remotely via an Intranet or Internet connection. This can be used to diagnose events as well as This involves the collection of information such to anticipate problems (preventive maintenance). as the type of load, the age of the network This is the case with apparatus in the Power components and the single-line diagram. Logic System range (Circuit Monitor - Power c Search for symptoms Meter), Digipact and the latest generation of This involves identifying and locating the equipment Masterpact circuit breakers fitted with subject to disturbance, determining the time and Micrologic P trip release (see fig. 6). date (fixed or random) when the problem occurred, Records of disturbance from the distributor’s any correlation with particular meteorological power system which have caused damage conditions (strong winds, rain, storm) or recent (destruction of equipment, production losses, modification of the installation (installation of new etc.) may also prove useful when negotiating machines, modification of the power system). compensation claims.

Cahier Technique Schneider Electric no. 199 / p.16 c Identification of origin v locating potential economies, The signature (waveform, profile of rms value) of v managing peaks in consumption (load the disturbance can in general be used by shedding, standalone sources), experts to locate and identify the source of the v optimising the power contract (reduction in problem (fault, motor starting, capacitor bank subscribed power demand), energisation, etc.). v improving the (reduction in The simultaneous recognition of the signature for reactive power). the voltage and the current can be used to c Ensuring the Power Quality: determine if the disturbance is sourced upstream or downstream of the measurement point. The v displaying and monitoring the measurement disturbance may come from either the parameters for Power Quality, installation or the distribution power system. v detecting problems in advance (monitoring of harmonics and neutral current, etc.) for c Definition and choice of mitigation solutions preventive maintenance purposes. A list of solutions and costings is prepared. The c choice of solution is often made by comparing Ensuring continuity of service: the cost with the potential lost earnings in the v optimising maintenance and operation, event of disturbance. v becoming acquainted with the network in real time, After implementing a solution, it is important to v monitoring the protection plan, verify, via measurement, that it is effective. v diagnosing faults, v Optimising the operation of electrical reconfiguring a network following a fault, installations v ensuring an automatic source transfer. The operation of electrical installations can be Software tools are used for the control-command optimised through three complementary actions: and monitoring of the installation. They can be used for example to detect and archive events, c Saving energy and reducing energy bills: monitor circuit breakers and protection relays in v making users aware of costs, real time, control circuit breakers remotely, and v assigning costs internally (by site, department generally make use of the possibilities of or product line), communicating devices (see fig. 6).

Circuit Monitor Digipact measurement power meter and control device

Sepam

Digipact DC150 Compact NS data circuit breaker concentrator

Masterpact circuit breaker

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

Cahier Technique Schneider Electric no. 199 / p.17 4.2 EMC and planning levels

Electromagnetic compatibility (EMC) based on a statistical calculation over a given Electromagnetic compatibility is the ability of an measurement period. For example, for the equipment or system to fonction satisfactorily in its harmonic voltage the measurement period is one electromagnetic environment without introducing week: 95 % of the rms values calculated over intolerable electromagnetic disturbances to successive periods of 10 minutes must not anything in that environment (IEC 60050-161). exceed the specified limits. The aim of electromagnetic compatibility is to Planning levels ensure that: These are the internal quality objectives c The emission of disturbances from each specified by the network operator which are separate source is such that the combined used to evaluate the impact of all disturbance- emission from all sources does not exceed the producing loads on the network. They are expected levels of disturbance in the environment. usually equal to or below the compatibility c The immunity level of the equipment gives the levels. appropriate level of performance for the expected disturbance in three classes of Summary environment (see fig. 7). Figure 8 summarises the relations between the Note that the environment is also determined by various levels of disturbance. the characteristics specific to the user installation (single-line diagram, types of load, etc.) and by the characteristics of the supply voltage. One way of ensuring compatibility levels is to Disturbance level specify the emission limits of user installations with Susceptibility a sufficient margin below the compatibility level. In of equipment practice this is possible for large installations (IEC Immunity level 61000-3-6, IEC 61000-3-7). For other installations (specified test value) (e.g. LV) the "product" standards specify emission Voltage limits for families of equipment (e.g. standard IEC characteristic 61000-3-2 imposes emission limits on current harmonics for loads under 16 A). Compatibility level (conventional value) In certain cases, technical solutions must be applied to keep the emission levels below the Planning level prescribed levels. Emission level Voltage characteristics Probability density The method used to evaluate the actual voltage characteristics at a given point on the network Fig. 8: Relations between the various levels of and to compare them to the predefined limits is disturbance.

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 Ð 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.

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

Cahier Technique Schneider Electric no. 199 / p.18 5 Solutions for improving PQ

A degradation of quality may lead to a change in disturbance to be prevented (e.g. remedies may behaviour, performance or even the destruction differ depending on the duration of an of equipment and dependent processes with interruption). This determines the effectiveness possible consequences for the safety of of the chosen solution. The definition, choice, personnel and additional economic costs. implementation and maintenance (to ensure This assumes three elements: long-term effectiveness) of solutions must also c one or more generators of disturbance, be carried out by specialists. The value of the choice and implementation of a c one or more loads sensitive to the disturbance, solution depends on: c a channel for the disturbance to be propagated c between them. The required level of performance The solutions consist in taking action with regard Malfunction is not permitted if it would put lives at to all or part of the three elements, either globally risk (e.g. in hospitals, airport lighting systems, (the installation) or locally (one or more loads). lighting and safety systems in public buildings, The solutions can be implemented to: auxiliary plant for power stations, etc.). c c correct a malfunction in an installation, The financial consequences of malfunction c take preventive action when polluting loads are Any unprogrammed stop, even when very short, to be connected, of certain processes (manufacture of semi- conductors, steelworks, petrochemicals, etc.) c ensure the installation conforms to a standard results in loss or non-quality of production or or to the power distributor’s recommendations, even restarting of production facilities. c reduce energy bills (reduction of subscribed c power in kVA, reduction in consumption). The time required for a return on the investment Loads are not sensitive to the same disturbance This is the ratio of financial losses (raw and have different levels of sensitivity, the materials, production losses, etc.) caused by the solution adopted, as well as being the best from non-quality of electrical power and the cost a technical and economic point of view, must (research, implementation, operation, ensure an appropriate level of PQ which meets maintenance) of the solution. actual requirements. Other criteria such as practices, regulation and It is vital that specialists carry out a prior the limits on disturbance imposed by the diagnosis to determine the nature of the distributor must also be taken into account.

5.1 Voltage dips and interruptions

The network architecture, automated power Reducing the number of voltage dips and restart systems, the reliability of equipment, the interruptions presence of a control-command system and Distributors can take certain measures such as maintenance policy all play an important role in making their infrastructure more reliable the reduction and elimination of interruptions. (targeted preventive maintenance, Correct diagnosis is vital before choosing an modernisation, underground installation) or effective solution. For example, at the point of restructuring power systems (shortening common coupling (the customer’s electricity feeders). For impedance earthed neutral power input), it is important to determine whether the systems, they can also replace auto-reclosing voltage dip is coming from the customer’s circuit breakers with shunt circuit breakers which installation (with a corresponding increase in present the major advantage of not causing current) or from the distribution power system (no interruptions on a damaged feeder in the event increase in current). of a transient earth fault (reducing the number of Different types of solution exist. short interruptions).

Cahier Technique Schneider Electric no. 199 / p.19 These circuit breakers allow the extinction of regulation systems and those for motors and transient earth faults by cancelling the voltage to large power consumers. Indeed, a voltage dip or the fault terminals for at least 300 ms by earthing interruption (even of short duration) may be the single faulty phase at the substation . sufficient to open all of the contactors whose This does not alter the voltage between phases coils are supplied by the power circuit. Loads supplying the customer. controlled by the contactors are thus no longer supplied when the voltage is restored. Reducing the duration and depth of voltage dips Increasing immunity of the control system c At power system level The increase of immunity of a process is in v Increasing the possibilities of ring connections general based on providing immunity to the (new substations, ring closing ) control system. v Improving the performance of electrical In general, the control system is not of high protective devices (selectivity, automatic power power and is thus extremely sensitive to restart, remote control devices on the network, disturbances. It is therefore often more remote management, replacement of spark gaps economical to immunise only the control system with surge arresters, etc.) rather than the equipment power supply. v Increasing the network short-circuit power Maintaining control of machines assumes: c c At equipment level There will be no risk to the safety of personnel or equipment when the voltage is restored. Decrease the power consumed by the switched c large loads with real time reactive compensators The loads and processes tolerate a short and soft starters which limit current peaks (and interruption in the power circuit (high inertia or mechanical stress). slowdown is tolerated) and can be restarted on the fly when the voltage is restored. Increasing immunity of industrial and service c The source can ensure that all of the installations equipment can be supplied simultaneously (in The general principle for ensuring that equipment the case of a replacement source) and provide is immune to voltage dips and interruptions is to the inrush current caused by the simultaneous compensate for a lack of power with an energy restart of several motors. storage device between the distribution power The solutions consist in powering all of the system and the installation. The availability of the contactor coils from a reliable auxiliary source storage device has to be greater than the duration (battery or rotating set with flywheel), or in using of the disturbances to which the system has to be an off-delay relay, or in using a rectifier and a immune to. capacitor connected in parallel with the coil. The information required when choosing mitigation solutions is: Increasing immunity of the equipment power c the quality of the source (maximum level of supply existing disturbances), Certain loads cannot withstand the expected c the load requirements (voltage sag ride- disturbance levels, i.e. neither voltage dips nor through capability in the duration-depth scale). interruptions. This is the case for "priority" loads Only by careful analysis of the process and of the such as computers, lighting and safety systems technical and financial consequences of (hospitals, airport lighting systems, public disturbances can these two elements be buildings) and continuous production lines reconciled. There are various possible solutions (manufacture of semi-conductors, data to provide immunity depending on the power processing centres, cement works, water required by the installation and the duration of treatment, materials handling, paper industry, the voltage dip or interruption. It may well be steelworks, petrochemicals, etc.). helpful to study solutions by making a distinction The following different technical solutions are between power supplies for control systems and possible depending on the power required by the

Cahier Technique Schneider Electric no. 199 / p.20 installation and the duration of the voltage dip or v Off-line (or stand-by) technology interruption. This is used for applications of no more than a few kVA. During normal operation, power is c Solid state uninterruptible power supply (UPS) supplied from the network. In the event of loss of A UPS consists of three main elements: network power or if the voltage exceeds the v a rectifier-charger, powered from the main prescribed tolerances, use is transferred to the supply, to convert AC voltage to DC, UPS. The changeover causes an interruption of v a flywheel and/or battery (kept charged) which 2 to 10 ms. on interruption provide the necessary power for c Sources transfer the load via the inverter, A device is used to control transfer between the v a DC-AC inverter. main source and a replacement source (and vice Two technologies are currently in use: on-line versa) for supply to priority loads and if and off-line. necessary orders the shedding of non-priority v On-line technology loads. During normal operation, power is supplied There are three types of transfer depending on continuously via the inverter without drawing on the duration of transfer (∆t): the battery. This, for example, is the case for v synchronous (∆t = 0), MGE-UPS brand Comet and Galaxy UPS units. v delayed (∆t = 0.2 to 30 s), They ensure continuity (no changeover delays) v pseudo-synchronous (0.1 s < ∆t < 0.3 s). and quality (voltage and frequency regulation) The devices require special precautions (see of the power supply for sensitive loads ranging "Cahier Technique" no. 161). For example, if from a few hundred to several thousand kVA. there are several motors in the installation, Several UPS can be connected in parallel to simultaneous restart may produce a voltage obtain more power or to provide redundancy. drop which could prevent restart or lead to In the event of overload, power is provided by excessively long restarts (with the risk of the static contactor (see fig. 9) from network 2 overheating). It is therefore prudent to install a (which may be combined with network 1). PLC which will restart the priority motors at Power is maintained without interruption via a intervals, especially with a replacement maintenance by-pass. (backup) source with a low short-circuit power.

Manual maintenance by-pass (NO)

Static contactor

Network 2 Switch Supply network (NC) feeders Rectifier / charger Inverter Sensitive equipment Network 1 Switch Switch Battery or (NC) circuit breaker (NC) NO : normally Circuit breaker open (NC) Battery NC : normally closed

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

Cahier Technique Schneider Electric no. 199 / p.21 This solution is selected where the installation example the real time reactive compensator cannot withstand a long interruption of more than compensates the reactive power in real time and a few minutes, and/or requires a large amount of is especially well suited to loads with rapid, large power. It can also be used in conjunction with a variations (welding machines, lifts, presses, UPS. crushers, motor starting, etc.). c Zero-time set Clean stop In certain installations, the autonomy required in If a stoppage is acceptable, it is especially the event of interruption makes it necessary to advisable to prevent uncontrolled restarting if an install a generating set (large batteries would be unwanted restart would present a risk for the too expensive, or cause technical or installation machine operator (circular saws, rotating electrical problems). Here, in the event of any loss of machines) or for the equipment (compression power supply, the battery or flywheel is used to chambers while still under pressure, staggered provide sufficient time for starting and running up restarts of air-conditioning compressors, heating the stand-by engine generator, load shedding (if pumps or refrigeration units) or for the application necessary) and interruption-free coupling by (necessity of controlling production restart). The means of an automatic source changeover. process may be automatically restarted by a c Electronic conditioners PLC using a predetermined restart sequence These are modern electronic devices to when conditions return to normal. compensate voltage dips and interruptions to a certain extent with a short response time: for Summary (see table below)

Installation Duration (indicative values) Immunisation solution power and technical requirements 0 to 100 ms 400 ms 1 s 1 min > 3 min 100 ms to 400 ms to 1 s to 1 min to 3 min A few VA Time-delayed contactors contacteurs. 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 storage battery capacity < 500 kVA Transfer time to a backup source may cause a short Rotating set with flywheel and interruption thermal motor or backup source < 500 kVA Between 15 minutes and several hours depending on DC motor connected to battery battery capacity and alternator < 1 MVA (up to Between 10 minutes (standard) and several hours UPS 4800 kVA with depending on battery capacity several UPS in parallel)

Effective mitigation solution Ineffective mitigation solution

Cahier Technique Schneider Electric no. 199 / p.22 5.2 Harmonics

There are three possible ways of suppressing or v Derate equipment at least reducing the influence of harmonics. One v Segregate polluting loads section will examine the question of protective As a first step, the sensitive equipment must be devices. connected as close as possible to the power c Reducing generated harmonic currents supply source. v Line choke Next, the polluting loads must be identified and separated from the sensitive loads, for example A 3-phase choke is connected in series with the by powering them from separate sources or from power supply (or integrated into the DC bus for dedicated transformers. These solutions involve frequency inverters). It reduces the line current work on the structure of the installation and are, harmonics (especially high number harmonics) and of course, usually difficult and costly. therefore the rms value of the current consumption v and the distortion at the inverter connection point. Protective devices and oversizing of capacitors It is possible to install the choke without affecting The choice of solution depends on the the harmonics generator and to use chokes for installation characteristics. A simple rule is used several drives. to choose the type of equipment where Gh is the v Using 12-phase rectifiers apparent power of all generators of harmonics supplied from the same busbar system as the Here, by combining currents, low-order capacitors, and Sn is the apparent power of the harmonics such as 5 and 7 are eliminated upstream transformer(s): upstream (these often cause the most i disturbance owing to their large amplitude). This - If Gh/Sn 15 %, standard equipment is suitable solution requires a transformer with two - If Gh/Sn > 15 %, there are two possible secondary windings (star and delta), and only solutions. generates harmonics numbered 12 k ± 1. 1- For polluted networks v Sinewave input current devices (15 % < Gh/Sn i 25 %): the current rating of the (see "Cahier Technique" no. 183) and in-series links must be oversized, This method consists in using static converters as must the voltage rating of the capacitors. where the rectifier uses PWM switching to 2- For very polluted networks absorb a sinusoidal current. (25 % < Gh/Sn i 60 %): anti-harmonic chokes c Modifying the installation must be connected to the capacitors and set to a frequency lower than the frequency of the lowest v Immunise sensitive loads with filters harmonic (for example, 215 Hz for a 50 Hz v Increase the short-circuit power of the network) (see fig. 10). This eliminates any risk of installation resonance and helps to reduce harmonics.

Z (Ω)

Network only with capacitor with anti-harmonic choke

f (Hz) fr far Zone where harmonics are present

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

Cahier Technique Schneider Electric no. 199 / p.23 c Filtering v Active filtering (see "Cahier Technique" no. 183) Where Gh/Sn > 60%, specialists must calculate This consists in neutralising the harmonics and install the harmonics filter (see fig. 11). induced by the load. First an analysis of the v Passive filtering (see "Cahier Technique" current identifies them in amplitude and phase. no. 152) Then the same but opposite harmonics are produced by the active filter. It is possible to This involves connecting a low impedance by- connect several active filters in parallel. An pass to the frequencies to be attenuated using active filter may for example be connected to a passive components (inductor, capacitor, UPS to reduce harmonics which have been resistor). Several passive filters connected in injected upstream. parallel may be necessary to eliminate several components. Careful attention must be paid to v Hybrid filtering the sizing of harmonic filters: a poorly designed This consists of an active filter and a passive passive filter may lead to resonance and amplify filter set to the order of the dominant harmonic frequencies which did not cause disturbance (e.g. 5) which supplies the necessary reactive before installation of the filter. power.

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. Network Polluting c Elimination of one or more harmonic orders load(s) (generally 5, 7, 11). One filter for one or two Passive orders to be compensated. filter(s) c Risk of amplification of harmonics in the event of 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" filtering harmonics created by the load. (local). c Filtering on a wide frequency band (elimination of harmonic orders 2 to 25). Load(s) c Self-adapting: Network v network modification has no effect, v adapts to all variations in load and harmonic spectrum, v open-ended, flexible solution for each type of load. c Simple engineering study. Active filter

Hybrid Offers the advantages of passive and active Network filtering solutions and covers a wide range of power and performance: Load(s) c filtering on a wide frequency band (elimination of harmonics numbered 2 to 25), Active filter c compensation of reactive power, Hybrid Passive filter c large capacity for current filtering, filter c good technical-economic solution for "network" filtering.

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

Cahier Technique Schneider Electric no. 199 / p.24 c Special case: circuit breakers calculate the current being carried may present a (see "Cahier Technique" no. 182) risk of unwanted tripping and care must therefore Harmonics may cause unwanted tripping of be taken when choosing these devices that the protective devices: care must be taken when true rms value of the current is measured. These choosing protective devices to avoid this. devices have the advantage of being better able Circuit breakers can be fitted with two types of to track changes in the temperature of cables, trip device, thermal-magnetic or electronic. particularly in the case of cyclical loads, as their The heat sensors of thermal-magnetic circuit thermal memory is superior to that of indirectly breakers are particularly sensitive to harmonics heated bimetallic strips. and can identify the actual load on the conductors c Derating caused by the presence of harmonics. They are This solution is applicable to some equipment thus well suited to use on low current circuits, and is a simple and frequently adequate essentially in domestic or industrial applications. response to disturbance caused by harmonics. The method used by electronic circuit breakers to

5.3 Overvoltages

Correct insulation co-ordination involves with a high capacitance to earth (capacitive ensuring the protection of personnel and filters connected to earth, extended cable equipment against overvoltages, with the best networks, etc.) which flow through the network balance between technical and economic downstream of the RCD (residual current considerations. device) via the network capacitance to earth. This requires (see "Cahier Technique" no. 151): Lightning overvoltages c knowledge of the level and energy of the c overvoltages which may occur on the network, Primary protection c selection of the level of overvoltage This protects the building and its structure withstand of the power system components to from direct lightning strikes (lightning meet constraints, conductors, Faraday cages, overhead earth wire/earthing wire). c use of protective devices where necessary, c in fact, the appropriate solutions depend on Secondary protection the type of overvoltage encountered. This protects equipment against the overvoltages which follow lightning. Temporary overvoltages Surge arresters (spark gaps are now used less c Switch off all or some of the capacitors and less) are installed on the particularly during periods of low load. exposed points of HV and MV networks and at c Avoid configurations susceptible to the input to MV/LV installations (see "Cahier ferroresonance or introduce losses (reducing Technique" no. 151). resistors) to damp the phenomenon (see On LV installations, they are installed as far "Cahier Technique" no. 190). upstream as possible (to offer maximum protection) and as close as possible to the Switching overvoltages load. It is sometimes necessary to cascade c Limit the capacitors energisation transients surge arresters: one at the head of the by installing a fixed reactor and pre-insertion installation, and one close to the load (see resistors. Static automatic reactive "Cahier Technique" no. 179). An LV surge compensators which control closing instant are arrester is always connected to a especially suitable for LV applications which disconnection device. In addition, the use of cannot withstand transient overvoltages main residual current circuit breakers of (PLCs, computer systems). discriminatory type on LV installations avoids any current flow to earth via the surge arrester c Connect line chokes upstream of the tripping the circuit breaker at the head of the frequency inverters to limit the effects of installation, which would be incompatible with transient overvoltages. some equipment (freezers, controllers, etc.). c Use main residual current circuit breakers of Note that overvoltages can be propagated to discriminatory type (type "S") for LV and circuit the equipment by other routes than the breakers of type "si" (I∆n = 30 mA and 300 mA). electrical power supply: telephone lines Their use avoids unwanted tripping due to (telephone, fax), coaxial cables (computer transient leakage currents : lightning and links, TV aerials). Suitable protective devices switching overvoltages, energisation of circuits are commercially available.

Cahier Technique Schneider Electric no. 199 / p.25 5.4 Voltage fluctuations

Fluctuations produced by industrial loads may c Modify the network affect a large number of consumers supplied from v Increase the short-circuit power by connecting the same source. The fluctuation magnitude lighting circuits to the nearest power supply depends on the ratio between the impedance of point. the device generating the disturbance and the v Increase the "electrical distance" between the impedance of the power supply. The solutions are: disturbance-generating load and lighting circuits c Changing the type of lighting by powering the disturbance-generating load Fluorescent lamps are less sensitive than from an independent transformer. incandescent lamps. c Use a reactive compensator c Installing an uninterruptible power supply This device provides real time reactive This may be a cost-effective solution if users subject compensation for each phase. Flicker can be to disturbance are identified and grouped together. reduced from 25 % to 50 %. c Modify the device generating the disturbance c Connect a reactance in series Changing the starting mode of motors which By reducing the inrush current, a reactance have to start frequently, for example, can reduce downstream from the connection point of an arc overcurrents. furnace can reduce flicker by 30 %.

5.5 Unbalance

The solutions are: c fitting the appropriate protective device for the c balancing single phase loads on all three phases, machines, c reducing the power system impedance c using carefully connected LC loads (Steinmetz upstream of the devices causing the unbalance by connection). increasing the transformer rated power and the cable cross-section,

5.6 Summary

Type of Origins Consequences Examples of mitigation solutions disturbance (special equipment and modifications) Voltage Large load variations (welding Fluctuation in the luminance of lamps Electromechanical reactive power compensator, real variations machines, arc furnaces, etc.). (flicker). time reactive compensator, series electronic and conditioner, tap changer. fluctuations Voltage Short-circuit, switching of Disturbance or shutdown of process: UPS, real time reactive compensator, dynamic dips large loads (motor starting, loss of data, incorrect data, opening of electronic voltage regulator, soft starter, series etc.). contactors, locking of drives, slowdown electronic conditioner. or stalling of motors, extinguishing of Increase the short-circuit power (Scc). discharge lamps. Modify the discrimination of protective devices. Interruptions Short-circuit, overloads, UPS, mechanical source transfer, static transfer maintenance, unwanted swtich, zero-time set, shunt circuit breaker, remote tripping. management. Harmonics Non-linear loads (adjustable Overloads (of neutral conductor, Anti-harmonic choke, passive or active filter, hybrid speed drives, arc furnaces, sources, etc.), unwanted tripping, filter, line choke. welding machines, discharge accelerated ageing, degradation of Increase the Scc. lamps, fluorescent tubes, etc.). energy efficiency, loss of productivity. Contain polluting loads. Derate the equipment. Inter- Fluctuating loads (arc Interruption of metering signals, flicker. Series reactance. harmonics furnaces, welding machines, etc.), frequency inverters. Transient Operation of switchgear and Locking of drives, unwanted tripping, Surge arrester, surge diverter, controlled switching, overvoltages capacitors, lightning. destruction of switchgear, fire, operating pre-insertion resistor, line chokes, static automatic losses. compensator. Voltage Unbalanced loads (large Inverse motor torque (vibration) and Balance the loads. unbalance single-phase loads, etc.). overheating of asynchronous machines. Shunt electronic compensator, dynamic electronic voltage regulator. Increase the Scc.

Cahier Technique Schneider Electric no. 199 / p.26 6 Case studies

6.1 Hybrid filtering

Description of the installation After commissioning, measurements show that Ski-lifts are powered by an MV/LV transformer the device reduces the magnitude of the (800 kVA). harmonics over a wide frequency spectrum in The connected loads are the chair lifts together both current and voltage (see fig. 14) and with other loads such as payment booths, ski- reduced the voltage distortion factor from 12.6 % pass validation systems, the official timing to 4.47 %. It also increased the power factor of system for competitions and a telephone the installation from 0.67 to 0.87. This solution network. solved all of the problems as no malfunction was subsequently detected. Problems encountered When the chair lifts are running, the low voltage network powered by the MV/LV transformer is subject to disturbance. The measures taken at the site pinpointed a high pre-existing harmonic distortion factor in the voltage (THD ≈ 9 %) from the MV power system as well as harmonic pollution from the chair-lift feeder. The resulting distortion of the supply voltage (THD ≈ 12 %) disturbed sensitive equipment (payment booths, timing system, etc.). Solutions The aim of the device is to ensure the simultaneous reactive compensation when harmonics are present and neutralisation of harmonics likely to disturb the installation. Fig. 13: Rectiphase hybrid filter device (Merlin Gerin The solution chosen (see fig. 12) was to install a brand). hybrid filter (see fig. 13) consisting of a passive filter tuned to the order of the dominant harmonic (H5) a % which provides the required reactive power 14 (188 kvar), and an active filter rated at 20 A is No filter 12 dedicated to the elimination of all other harmonics. H5 filter 10 Hybrid filter 8 6 4 2 SCC 0 THD H5 H7 H11 H17 H23 b % MV/LV transformer 40 No filter 35 H5 Filter Hybrid filter 30 25 5th order 20 passive 15 filter 10 Harmonics Active 5 generator filter 0 THD H5 H7 H11 H17 H23 Linear loads Hybrid filter Fig 14: Spectrums showing the effectiveness of a Fig. 12: Implementation of the solution. hybrid filter: [a] in voltage [b] in current.

Cahier Technique Schneider Electric no. 199 / p.27 6.2 Real time reactive compensation

Description of the installation The problems clearly stemmed from voltage A car equipment manufacturer’s plant in Concord fluctuations caused by the operation of welders (Ontario - Canada) is supplied by a transformer with loads which vary rapidly and frequently and rated at 2000 kVA - 27.6 kV / 600 V - Yy - which consume significant reactive power. Ucc = 5.23 %. A voltage dip of 6 % produces a reduction of It manufactures exhaust assemblies from steel 12 % (1-0.942) in the power available for plate using spot welders and seam welders. welding. This was the reason for the large number of defective welds. Problems encountered Standard devices for reactive power c Visual and nervous fatigue in personnel, due to compensation use electromechanical the fluctuation in brightness of lamps (flicker) contactors which cannot achieve the required when welding equipment was in operation. response times; the operation of capacitor steps c Noise pollution and premature mechanical is deliberately time delayed to reduce the number ageing of equipment caused by vibrations mainly of operations and avoid reducing the service life in the transformer and the main switchgear when of the contactors through premature wear, as welding equipment was in operation. well as to enable the capacitors to discharge. c Inability to add equipment for fear that the The solution chosen was to connect a real time installation would be overloaded (peak currents reactive compensator (see fig. 16). This when welders were fired were greater that the innovative device offers: nominal current of the main circuit breaker). c ultra-rapid compensation of the variations in Expansion of the installation would thus require reactive power within one cycle (16.6 ms at substantial investment, either to upgrade the 60 Hz), which is especially suitable for loads existing installation or to build a new power with rapid, large variations (welding machines, supply facility. lifts, presses, crushers, motor starting, etc.); c Annual penalties of 5 kE for exceeding the c transient-free switch through controlled reactive power consumption limit (0.75 power switching, which is especially useful with loads factor). which cannot withstand transient overvoltages c Defective parts caused by welding faults (PLCs, computer systems, etc.); appeared at the end of the manufacturing c increased service life of capacitors and process when the tubes are bent into shape. contactors owing to the absence of moving All these factors reduced company productivity. mechanical parts and overvoltages With compensation of 1200 kvar it would be Solutions possible to minimise voltage dips, but 800 kvar The measures taken during the operation of the was deemed sufficient to maintain the voltage at welding equipment showed a nominal voltage of 584 V, voltage dips of 5.8 %, current peaks of 2000 A, and reactive power peaks of 1200 kvar a b (see fig. 15). L1 L2 L3

Before After Voltage (V) 584 599 Voltage dip c Depth (%): 5.8 3.2 c Duration (cycle) 20 to 25 10 to 15 Current c Average 1000 550 c Peak 2000 1250 Reactive power (kvar) 600 0 to 1200 to 300 Power factor 0.75 > 0.92

Fig. 15: Improvements due to the real time reactive Fig. 16: Real time reactive compensator [a] principle, compensator. [b] practical implementation.

Cahier Technique Schneider Electric no. 199 / p.28 an acceptable level for all processes in the plant to over 0.92, thus avoiding power factor under all load conditions. penalties; The results of implementing the solution are c an increase in the nominal voltage to 599 V (see fig. 17): and a reduction in voltage dips to 3.2 % (see c a reduction in current peaks to 1250 A and the fig. 16). This is a consequence of the increase in addition of loads without modification of the the power factor and reduction in the current installation, with improved installation efficiency amplitude (see fig. 18). Visual and nervous through reduction of joule losses; fatigue in personnel due to the flicker was also c a reduction in reactive power peaks to eliminated. Welding quality improved, as did 300 kvar and an increase in the power factor productivity.

a 340

Voltage 335 (V) 330

1500 Current (A) 1000 1000

kvar 500

d = 3 s

b 350 Voltage (V) 340 1250

Current 750 (A)

500 kvar 0

d = 1.5 s

Phase 1 Phase 2 Phase 3 Average Fig. 17: Measurement of current, voltage and reactive power: [a] without compensation [b] with compensation.

∆V ∆V ∆V

VS VS VS Reactive VL VL VL Welder compensator

With Without Fig. 18: Reduction in voltage drop obtained using a real time reactive compensator.

Cahier Technique Schneider Electric no. 199 / p.29 6.3 Protection against lightning

Description of the installation The site consists of offices (computer hardware, lighting and heating unit), a security post (fire alarm, burglar alarm, access control, video surveillance) and three buildings for the manufacturing process on 10 hectares in the Avignon region of France (probability of lightning is 2 strikes per km2 per year). There are trees and metal structures (pylons) in Main LV Main LV distribution board the vicinity of the site. All of the buildings are Building 1 Building 2 fitted with lightning conductors. The MV and LV power supplies are underground. 3L 3L N N Problems encountered

A storm struck the site, destroying the LV PF65 PF65 installation in the security post and causing 36.5 kE of operating losses. The presence of lightning conductors prevented the structure from catching fire, but the electrical equipment which was destroyed was not protected by surge arresters, contrary to the recommendation in standards UTE C-15443 and IEC 61024.

Solutions After analysing equipotentiality and earthing of L L the power system, followed by verification of N N the installation of lightning conductors and checking of the values of the earth electrodes, the decision was taken to install surge PF8 PF8 arresters. Secondary Secondary Surge arresters were installed at the head of the distribution board distribution board installation (main LV distribution board) and in Building 1 Building 1 cascade in each manufacturing building (see fig. 19). As the neutral point connection was Fig. 19: Installation diagram for several surge arresters TNC, protection would only be provided in in cascade. common mode (between phases and PEN). In conformity with guide UTE C-15443 regarding operation in the presence of lightning conductors, the characteristics of the Merlin Gerin PF65 and PF8 surge arresters (see fig. 20) are as follows: c At the head of the installation In=20kA Ð Imax = 65 kA Ð Up=2kV PF65 c In cascade (at least 10 m apart) In=2kA Ð Imax = 8 kA Ð Up = 1.5 kV In cascade, good protection is provided for the secondary distribution boards (offices and security post). As the neutral point connection was converted to TNS, protection had to be provided in common mode (between phase and PE) and differential PF8 mode (between phases and neutral). The Fig. 20: Low voltage surge arresters (Merlin Gerin disconnection devices in this case are circuit PF65 and PF8). breakers with a breaking capacity of 22 kA.

Cahier Technique Schneider Electric no. 199 / p.30 7 Conclusion

Electrical disturbance may originate in the mean that the quality of electricity has become a distribution power system, in the installation of strategic issue for electricity companies, the the user who is subject to disturbance or in the operating, maintenance and management installation of a nearby user. personnel of service sector and industrial sites, The consequences of the disturbance vary as well as for equipment manufacturers. according to the economic context and the area However, problems of disturbance should not be of application: from inconvenience to shutdown regarded as insurmountable, as solutions do of production facilities - it can even put lives at exist. Employing specialists to define, implement risk. and maintain these solutions while observing The search to improve company competitiveness best practice will provide users with the right and the deregulation of the electricity market quality of power supply for their requirements.

Cahier Technique Schneider Electric no. 199 / p.31 Bibliography

Standards c Harmonic disturbances in networks, and their treatment. c IEC 61000-X-X Ð Electromagnetic compatibility (EMC): C. COLLOMBET, J.-M. LUPIN, J. SCHONEK, Cahier Technique no. 152. v 2-1: Description of the environment. c Inverters and harmonics (case studies of non-linear v 2-2: Compatibility levels (public low-voltage power loads). supply systems). J.-N. FIORINA, Cahier Technique no. 159. v 2-4: Compatibility levels in industrial plants for low- c Harmonics upstream of rectifiers in UPS. frequency conducted disturbances. J.-N. FIORINA, Cahier Technique no. 160. v 2-5: Classification of electromagnetic environments. c Automatic transferring of power supplies in HV and v 3-2: Limits for harmonic current emissions LV networks. (equipment input current i 16A per phase). G. THOMASSET, Cahier Technique no. 161. v 3-3: Limitation of voltage fluctuations and flicker in c HV industrial network design. low-voltage supply systems for equipment with rated G. THOMASSET, Cahier Technique no. 169. current i 16 A. c v 3-5: Limitation of voltage fluctuations and flicker in Earthing systems in LV low-voltage supply systems for equipment with rated B. LACROIX, R. CALVAS, Cahier Technique no. 172. current > 16 A. c Earthing systems worldwide and evolutions v 3-6: Assessment of emission limits for distorting B. LACROIX, R. CALVAS, Cahier Technique no. 173. loads in MV and HV power systems. c The IT earthing system (unearthed neutral). v 3-7: Assessment of emission limits for fluctuating F. JULLIEN, I. HERITIER, Cahier Technique no. 178. loads in MV and HV power systems. c LV surges and surge arresters. v 4-7: Harmonics and interharmonics measurements LV insulation co-ordination. v 4-11: Voltage dips, short interruptions and voltage Ch. SERAUDIE, Cahier Technique no. 179. variations immunity tests. c LV circuit-breakers confronted with harmonic, v 4-12: Oscillatory waves immunity test. transient and cyclic currents. v 4-15: Flickermeter. M. COLLOMBET, B. LACROIX, c Other standards and laws Cahier Technique no. 182. c v European Union "Council Directive 85/374 on the Active harmonic conditioners and unity power factor approximation of the laws of the Member States rectifiers. relating to the liability for defective products", Official E. BETTEGA, J.-N. FIORINA, Cahier Technique no. 183. Journal (07.08.1985). c Cohabitation of high and low currents. v EN 50160 Characteristics of electricity supplied by R. CALVAS, J. DELABALLE, public distribution systems (07-1994). Cahier Technique no. 187. v Application Guide to the European Standard c Ferroresonance. EN 50160 - July 1995 - UNIPEDE. Ph. FERRACCI, Cahier Technique no. 190. v IEEE Std 1159-1995: Recommended Practice for Monitoring Quality. Other publications v IEEE Std 1000-1992: IEEE Recommended Practice c Guide de l’ingénierie électrique des réseaux internes for Powering and Grounding Sensitive Electronic d’usines - Collection ELECTRA. Equipment. c Method of symmetrical co-ordinates applied to the v IEC 60071-1: Insulation coordination. solution of polyphase networks - Trans. Amer. Inst. v IEC 60050-161: International Electrotechnical Electr. Engrs, June, 1918 - C.L. FORTESCUE. Vocabulary. c Guide to quality of electrical supply for industrial installations Part 2: voltage dips and short interruptions Cahiers Techniques Schneider Electric Working Group UIE Power Quality 1996. c Residual current devices. c Supply Quality Issues at the Interphase between R. CALVAS, Cahier Technique no. 114. Power System and Industrial Consumers - PQA 1998, c Electrical disturbances in LV. A. ROBERT. R. CALVAS, Cahier Technique no. 141. c Real time reactive compensation systems for welding c Switching MV capacitor banks. applications - PQ 1998, R. WODRICH. D. KOCH, Cahier Technique no. 142. c Low voltage hybrid harmonic filters, technical & c EMC: Electromagnetic compatibility. economic analysis - PQ 1999, J. SCHONEK. F. VAILLANT, Cahier Technique no. 149. c Overvoltages and insulation coordination in MV and HV. D. FULCHIRON, Cahier Technique no. 151.

Cahier Technique Schneider Electric no. 199 / p.32

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