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In what cases could this good EMC (and safety) practice not be applied? robust reporting and audit framework for quality of service information (see

    th17 International Conference on Electricity Distribution Barcelona, 12-15 May 2003 C I R E D C I R E D

    Special Report Technical Theme 2

    POWER QUALITY & EMC

    A. Robert (Chairman), E. De Jaeger and J. Hoeffelman (Rapporteurs), Belgium

    CONTENTS Introduction Call for Contributions ..........................................1 Each block will be divided in two main parts : I EMI, EMF and Safety ........................................................1 1) a few presentations by key note speakers or authors, Electromagnetic Interferences (EMI) ......................................2 2) discussion (prepared contributions and free discussion). Lightning ..............................................................................2 Ground Potential Rise and neutral grounding ..........................3 The aim of this special report is : Electromagnetic fields ...........................................................3 1) to present a synthesis of the present concerns in each of the II Connection of disturbing installations ................................4 four sections, mainly based on the selected papers, Harmonics and distorting loads ..............................................4 2) to call for prepared contributions on particular points which Harmonics and flicker: industrial case studies and mitigation appear in the papers or which are not covered by them, techniques .............................................................................6 3) to stimulate the free discussion. Power Quality measurement methods .....................................7 III Voltage dips and disturbances in customers installations ....8 Call for prepared contributions. Prepared contributions will Measurement and characterization of voltage dips and short preferably aim at answering the questions of the Special Report. interruptions ..........................................................................8 However, other kinds of contributions will be welcome : Immunization techniques ..................................................... 10 - fresh information on particular points which appear in the papers Techno-economical assessment ............................................ 11 or which are not covered by them ; Voltage dip related industrial power quality problems ........... 12 - case studies (outstanding disturbance experiences, causes, IV Power Quality in the competitive market ......................... 12 solutions...) ; Context ............................................................................... 12 - comments on a particular paper (“I agree/disagree with that Benchmarking utilities ......................................................... 13 result/conclusion”, "My own practical experience in the same field Obtaining and analysing reliability data ................................ 14 is...") ; Power Quality monitoring .................................................... 16 - just plain questions to the author of a paper. Customer relationships ........................................................ 17 Power Quality related Papers in other Sessions ......................... 19 According to the successful experiment since CIRED 1997, all List of Papers .......................................................................... 21 prepared contributions will be made available to attendees at the

    entrance of the conference room. Furthermore, some of the most relevant ones will be selected for a verbal presentation (second part of each section). INTRODUCTION CALL FOR CONTRIBUTIONS General guidelines for authors of prepared contributions : - language: preferably English for the written document ; The scope of Session 2 has been defined as follows by the - starting with: title, name of author(s), affiliation, country, Session Advisory Group : number of the relevant question in the special report or number of ? Power Quality, i.e. voltage continuity (often referred to as the commented paper ; supply reliability - problem of outages) and voltage quality (LF - font: Arial or Times New Roman, size: 10, margins: 2.5 cm top disturbances, ? 9 kHz, reaching equipment through the electricity and bottom, 1.8 cm left and right, preferably two columns with supply) ; 0.5 cm gap ; ? EMI, EMF and Safety : HF disturbances on the electricity - maximum length: 2 pages, including both text and illustrations supply and all disturbances - HF or LF - reaching equipment other (this allows for a lot of information if a 2-column presentation is than through the electricity supply ; some safety and resistibility chosen) ; concerns (Electromagnetic fields - overvoltages - step, touch and - if you wish to use a Power Point slide show, please send the transferred voltages...) are also considered. Power Point file also (only ppt files received in advance will be available in the computer on the platform) ; N.B. The concept of Quality of Supply is a little broader than - deadline : 29 April 2003 (possibly 5 May at the very latest) ; Power Quality. In addition to Voltage Continuity and Voltage - addresses : alain.robert@elia.be ; jean.hoeffelman@elia.be ; Quality, it includes the Commercial Quality (quality of response emmanuel.dejaeger@laborelec.be ; info@cired2003.be. to telephone calls, etc.).

     The 2003 session will be divided in four blocks of 90 minutes:

    1) EMI, EMF, Safety, I EMI, EMF AND SAFETY 2) Connection of disturbing installations (emission limits for harmonics, flicker or unbalance ; filters or compensators, etc.) and Four different topics will be addressed in this first part of monitoring methods, the session: Pure EMI subjects dealing with immunity and 3) Voltage dips and disturbances in customers installations emission, lightning related questions, Ground Potential (immunity levels, remedial measures, etc), 1Rises (GPR) and neutral grounding practices with their 4) Power quality as seen by the different players in the competitive

    market (system operator, regulator, customers, etc). influence on power quality and on safety and, finally, EMF (Electromagnetic fields).

     1 In the following we will use the american terminology “ground” and “grounding” as equivalent to “earth” and “earthing” CIRED 2003 - Special Report Session 2 - Power Quality & EMC 1/24

    Electromagnetic Interferences (EMI) to face? Has an international agreement on the

     emission limits finally been reached? As already stated during the previous sessions, pure EMI problems (i.e. interferences between equipment or systems) are not very often discussed in CIRED, probably because Lightning they are considered as too specific and left to more specialised forums. Lightning overvoltages (OV) and lightning protection Four papers, however, address directly EMI questions. The remain one of the most important concerns system first, paper 2.4 (JP), highlights experimentally the well-operators have to face. Indeed, not less than 8 papers known problem of common impedance coupling (under presented at this session address this topic: lightning surge conditions) on the auxiliary cabling of HV Paper 2.23 (ES) presents a warning system based on the substations. Also related to HV substations, paper 2.13 (ES) use of an electric “field mill”. Paper 2.1 (BR) develops a presents a detailed analysis of the disturbances produced by model for the statistical analysis of the OV and the switching of disconnectors. Many papers have already been necessary related insulation coordination. Starting from a published on that question, mainly in CIGRE and IEEE, case study, paper 2.2 (ES) analyses the interruptions due to but the link with the standardised assessment methods and lightning and the OV protection of a mixed cable-overhead the need to improve them is seldom highlighted. The network. Another case study related to the protection of a influence of power lines on long structures like pipelines small hydroelectric plant connected to a network with and telecommunication lines cables is often analysed in isolated neutral is proposed in paper 2.7 (ES). A more CIRED and CIGRE papers but less frequently the inductive systematic approach based on Monte Carlo simulations and coupling of power cables on nearby telecommunication on the use of fuzzy logic techniques is applied in paper links. This problem is discussed in paper 2.18 (CN) with a 2.22 (ES) for the protection scheme of mixed networks. special attention on the cable screen grounding. The special case of the protection of covered conductors is A completely different and quite new topic is introduced in addressed in paper 2.19 (RU) which makes recourse to an paper 2.11 (ES) dealing with broadband power line carrier original antenna-type arrester, whereas paper 2.20 (NL-MK) (PLC). Emission problems related to PLC will probably dealing with radio base stations on HV towers makes the become more stringent in the future if this new technology link with power frequency ground potential rise problems really succeeds in challenging the other classical and with transfer of potentials from HV to LV networks. telecommunication media (like ADSL). This latter report shows clearly the difference in behaviour

     of grounding systems depending on the frequency spectrum

    involved (cf. figure 1) Question 1 1.1 Common impedance coupling is the price to pay 100when single point and independent grounding Towerpractices are abandoned in favour of meshed grounding80Earthstructures and equipotential bonding. It is wiresimportant to highlight these risks but are there 60reasons to believe that the general cabling

    practices presented in CIGRE guide 124 and in 40IEC guide 61000-6-5 should be amended Magniturrent (%) of Cude20accordingly? MV/LV1.2 The need to improve the IEC 61000-4-12 transient cables0oscillatory wave test (and the related 60255-22-1 10010001e+610000010000Frequency (Hz)standard) has been admitted by IEC. This basic (a)standard, however, is seldom used outside the Figure 1: Current distribution between earth wires, tower 100power industry. Therefore, is there a sufficient grounding and metal shields of MV/LV cable Towermarked need to justify the revision of this groundingfor ? = 100 ?m 80standard? In absence of this revision can the IEC Earth61000-4-4 fast transient test be considered as 60wiresequivalent ? Question 2 1.3 It is well known that coupling between power 40MV/LV2.1 When can it be recommended to link the automatic cablescables and telecommunication cables is highly disconnection of an installation to the use of a 20Magnent (%)f Currtude oireduced when the cable screens are earthed at lightning warning system? both ends and when cross bonding is applied. In 2.2 What are the advantages and drawbacks of using 0what cases could this good EMC (and safety) 10010001e+610000010000covered conductors with respect to classical Frequency (Hz)practice not be applied? Are there other related overhead lines? Is this technique sometimes used to (b)experiences? reduce EMF? Up to what rated voltage can it be 1.4 As stated in paper 2.11 broadband PLC will applied? probably take an increasing market place in the 2.3 Many formulas have been published concerning the future. Is it correct to state that EMC is one of the statistics of direct and indirect lightning OV (cf most important problems this new technology has paper 2.1 and 2.2). Are these formulas in good

    CIRED 2003 - Special Report Session 2 - Power Quality & EMC 2/24

     agreement with the modern computation

    techniques and with the measurement data? 500

    2.4 The calculation of the best location for surge 400arresters needs not only powerful tools but also the 300input of all the characteristics of the analysed 200network. How far can such tools be applied for Ut (V/kA)practical cases? Are codes like Matlab, with all 100

    their modules, becoming more powerful than 0dedicated programs like EMTP? Isn‟t it possible to urban rural draw, from a set of simulations, some practical Figure 2 : Touch voltages in LV installations during a guidelines? phase-to-neutral fault on overhead MV lines 2.5 Most protection schemes are based on good insulation coordination and the use of well located MO arresters. However, particularly when the soil Question 3 resistivity is high, shouldn‟t it be useful to pay more 3.1 Standards like HD 637 S1 or IEC 61936 propose attention to the equipotential bonding ? safety limits for touch voltages in HV installations. 2.6 The protection of radio base station installed on HV These limits are generally higher than the limits tower has led to a close collaboration between Cigre, adopted in the different countries for LV Cired and UIT-T and, hence, to a draft Kbsp installations. Although, as stated in paper 2.10, no recommendation. Do the conclusions of paper 2.20 accident in LV installations due to faults in HV modify the protection scheme proposed by UIT-T ? installations seems to have ever been reported, are What are the practical consequences of the use of there countries where, in case of propagation of LV cables without metal sheat ? potential, the HV safety limits have been accepted in LV installations? If not what are the common practices? Ground Potential Rise and neutral grounding

    3.2 Low impedance limitation of zero sequence current

    seems to be one of the best practices for Ground potential rises (GPR) can be produced by lightning

    (underground) MV networks. What are, in that or by fault current. In the first case they lead mainly to

    respect, the pro and contra of reactors versus insulation coordination problems and therefore have been

    resistors? Up to which rated voltage is it safe to addressed together with the other lightning related

    apply this type of neutral grounding? How far problems. In the second case they lead to temporary

    could the conclusions of paper 2.17 (to limit at 500 overvoltages (TOV) and, depending on the neutral

    A instead of 2000 A) be transposed to a public distribution scheme, to possible step and touch voltages.

    distribution network? GPR problems are also tightly linked to the way the neutral

    3.3 On basis of the papers presented, two different is grounded, to the protection scheme used and finally to

    trends seem to coexist in MV networks: Evolution the power quality.

    towards resonant grounding or evolution towards Seven papers address these topics:

    low impedance grounding. What are, besides the Paper 2.10 (CA) and 2.21 (NL) propose, for low resistance

    ratio underground/overhead and the weight of the or direct grounding networks, in situ assessment procedures

    past, the main rationales for choosing one or the based on the injection of currents at frequencies slightly

    other grounding scheme? different from the power frequency. Paper 2.10 highlights

     also the differences that exist between a rural network and

     a urban network, this latter taking benefit of the global

    earthing (cf. figure 2). Paper 2.14 (CH) focuses on the risk Electromagnetic fields of resonance induced overvoltages in 110 kV networks grounded through reactors. Paper 2.3 (DK) points out the Severe regulation or (uncontrolled) recourse to the facts that harmonics are not compensated in MV networks precautionary principle often lead to the application of grounded through an arc suppression coil (Petersen) and, mitigation techniques in order to reduce EMF in the hence, could produce voltages exceeding the allowed limits. vicinity of power installations. Switzerland (see figure 3) Paper 2.17 (DE) gives a interesting analysis of the possible and Slovenia are two countries where much more severe solutions to improve the quality of supply of an industrial limits than those recommended by ICNIRP are legally MV network by letting the neutral grounding evolve from applied. Paper 6 (SI) shows even that very restrictive limits insulated to low resistance. Alternatively, paper 2.5 (ES) on the E field can oblige the utilities to take more care of shows how the evolution from a solidly grounded network this latter than of the magnetic field ! Paper 8 (CH) shows, to a resonant grounded system can be a good solution in a on the other hand, that the costs involved for ensuring the mixed network. Paper 2.16 (DE), on the other hand, compliance with the regulation could be much higher for proposes an original way to adopt different neutral existing installations than for new ones. Dealing with the grounding schemes in the different parts of a network assessment of installations, paper 15 (FR) points out the depending of the proportion of overhead lines involved. difficulties related to the share of responsibilities between

    CIRED 2003 - Special Report Session 2 - Power Quality & EMC 3/24

    II CONNECTION OF DISTURBING manufacturer and contractor, to the influence of the cabling INSTALLATIONS and to the definition of the rated conditions. A completely

    different sound is found in paper 2.12 (ES) that highlights (EMISSION LIMITS FOR HARMONICS,

    the rationales of a judgement stating that exposure to EMF FLICKER OR UNBALANCE; FILTERS OR of less than 100 µT has no adverse effects on public health ! COMPENSATORS)

    Harmonics and distorting loads

    Among LF disturbing phenomena, harmonics are still site of sensitive usesite of general use

    receiving much attention from energy service companies ILVELVand grid operators. Harmonics are basically produced by 1 T100 Tdistorting loads (non linear loads) that may be found > 4 h / dayeverywhere, at all voltage levels, from LV distribution > 800 h / year(household appliances using switching mode power supply time limit for renovation: ELV until January 2005 ILVsuch as TV sets, personal computers, compact fluorescent until January 2007lamps etc) to MV or HV levels, where big industrial Figure 3 : Installation Limit Values and Exposure Limit Values consumers or dispersed generation units are connected according to the Swiss regulation. The dashed line means a (power electronics interfaces, adjustable speed drives, minimum measuring distance of 0.2 m welding machines, arc or induction furnaces and so forth). The work reported about harmonics in this session covers: ? Large scale measurement campaigns in distribution or Question 4 transmission networks, 4.1 Authorities too often set limits without specifying ? Case studies of connection of distorting loads, including the way to assess the compliance of installations. the difficult problem of the assessment of individual Are the limits for “rated” conditions or “normal” emission levels, conditions? (But what is the meaning of “rated” or ? Solutions to industrial harmonics problems and “normal” conditions?) Are they simply maximum mitigation techniques (active filters). values or values not exceeded during a given percentage of the time? Is there a minimum Paper 2.31 (FR) discusses the major conclusions from a distance to the source like that suggested in paper harmonic survey on French LV networks since 2000. 2.8 (but apparently not present in the Ordinance)? thE.g. 5 harmonic voltages, in 2001, on a sample of 16 4.2 The Swiss Ordinance seems to open a door and to typical LV networks (Figure 4): allow derogations when the owner of the ? the 95% level measured over one year was between 4 installation can prove that he has taken appropriate and 6% for more than 60% of the networks; (and economically affordable) mitigation measures. ? about 20% of the networks did not comply with How far is this of application? In the framework of standard EN 50160. the EU, the legality of Swiss regulation could probably be fought because it applies some

    discrimination (e.g. the limits are not the same for

    railways installations as for power installations).

    Are there other known examples of discriminating

    regulation?

    4.3 Paper 2.8 mentions costs as high as ? 35 000 for

    ensuring the compliance of existing MV/LV

    substations. Are there other evaluations available?

    4.4 An important judgement in Spain seems to be

    considered as a legal “precedent”. What are the

    similar experiences in other countries? Could the

    fact that WHO and IARC statements are not taken

    as reference in the Spanish judgement be considered as a weakness for the jurisprudence?

     Figure 4 : 5th harmonic yearly measurement results

    (sample of 16 typical French LV networks in 2001)

    In order to observe the time evolution, EDF will continue

    the harmonic survey on the same sample of typical LV

    networks for several years. According to the authors, if the

    increase of harmonic levels is confirmed, the situation on

    the French distribution networks will be critical in the

    medium term. In this case, more efficient solutions will CIRED 2003 - Special Report Session 2 - Power Quality & EMC 4/24

    the conclusions, it appears that single-phase conductors have to be applied, and, in particular, more severe emission reduce the voltage THD at the load node near the end of the limits for equipment and installations. feeder. Additional measurements on typical LV networks in 2002 rd New families of power electronics converters, using pulse showed important harmonic voltage differences for the 3thand 9 harmonic voltages along the feeders (Figure 5). For width modulation (PWM), are more and more widely used these harmonic orders, although the levels are generally today. One of the most growing applications is the low in MV/LV substations, the compatibility levels may be connection of non-conventional power generators to MV or exceeded at the end of typical LV feeders (the same even LV distribution grids (variable speed windmills, conclusion is also pointed out in Paper 2.38 (BE)). micro-turbines, photovoltaic generators etc.). Two reports investigate the harmonic disturbances

    produced by such devices (Papers 2.36 (FR) and 2.40

    (DE)).

    Due to their working principle, PWM converters are well

    known to produce frequency components in the range of

    several kHz (typically between 2 to 16 kHz). Paper 2.36

    (FR) shows that a quick evaluation of these components is

    possible, taking the main influencing parameters into

    account. Measurements were also carried out (see e.g.

    Figure 6) showing a good agreement with predictions.

     rdFigure 5: time evolution of the 3 harmonic voltage

    over one week: 10-minute average values

Paper 2.77 (NL) summarizes the findings of harmonics

    monitoring programs conducted by NUON in the

    Netherlands during the second half of the nineties, at

    various voltage levels.

    No significant increase in harmonics can be discerned from

    6 years of measurement. According to the authors, harmonic problems are not due to Figure 6: Example of measured current components excessive emission but rather the consequence of resonance. in the range 2 to 9 kHz (the characteristic components Local harmonic problems are usually caused by a parallel appear to have frequencies surrounding multiples resonance for a specific frequency while Power Quality of the switching frequency of the converter) problems over a large area are the consequence of series resonance. Paper 2.40 (DE) describes another approach to modelling Taking into account the loads, generations, grid power electronic devices and power distribution networks configuration and operation of reactive power in the frequency domain, using frequency-coupling compensation, the realistic prediction of resonance in the matrices. An example is treated with two back-to-back grid seems to be quite a challenge for the future. PWM converters feeding a simple L/R load. It is shown, that the characteristic and non-characteristic The massive penetration of electronically controlled harmonics and interharmonics result from three main devices and equipment in low voltage distribution networks mechanisms: (the "digital society") could be responsible for the ? Harmonics generated by the switching of one converter worsening of power quality problems. only, resulting in characteristic harmonics. These Combinations of such loads may result in serious non-harmonics may have significant amplitudes, depending on symmetrical loading of distribution networks, with the modulation index. overloading of the neutral conductor and increase of the ? Non-characteristic harmonics and interharmonics losses. The authors of Paper 2.34 (RO) propose an generated by one converter as a result of the switching of approach to correctly incorporate in a mathematical model, the second converter. This mechanism leads to non-the impact of the most significant disturbances, i.e. non-characteristic harmonics if both converters operate at the symmetrical loading combined with non-sinusoidal same switching frequency and to interharmonics if they operating regime. Quantitative assessments are given. operate at different switching frequencies. This leads to distortions with generally small amplitudes. Harmonics propagation mechanisms in distribution ? Non-characteristic harmonics and interharmonics due to networks and influencing factors are thoroughly AC side impedance unbalance. Depending on the degree of investigated in Paper 2.38 (BE). Differences in distribution unbalance this mechanism leads to distortions that may system design and simulation approach are studied. Among have significant amplitudes. CIRED 2003 - Special Report Session 2 - Power Quality & EMC 5/24

Besides controlling the emission limits, as suggested in

    Paper 2.31, some technical solutions could be investigated

    to mitigate the effects of harmonics in a centralized manner.

    Paper 2.33 (DE) is describing a new method to determine

    the optimal placement and the mutual static and dynamic

    interaction of active filters for harmonics in power systems.

    It provides an easy way to derive statements about the

    mutual influence of two or more active filters or

    compensators from general network parameters. Using the

    original distortion levels and the power system’s

    impedance matrix, an analytical solution for optimal placement of active filters can be calculated. Figure 7: Example of the determined respective impacts of the power supply system and a particular consumer load The assessment of harmonic emission level from a (trolleybus traction substation) in 13-th harmonic voltage particular distorting load is dealt with in Papers 2.32 (FR) and 2.37 (RU). Harmonic current flowing at an installation’s metering Question 5 point results from harmonics originating in this installation 5.1 Is there a general trend towards the increase of and also in other installations connected to the same power harmonics levels in distribution networks? system. 5.2 Are there practical measurement experiences of The basic idea developed in Paper 2.32 (FR) is to scrutinize the harmonic emission level from a particular sudden harmonic variations at the point of common distorting industrial consumer or load? How is this ???ealΔVhΔIh. An iterative coupling using the sign of problem usually dealt with, from a practical point method makes it possible to follow the estimates of of view, for contractual purposes? harmonic impedances and currents over the time. A non-5.3 Are there any practical experiences of applying the linear regression method is used to complete the IEC recommendations for the assessment of assessment for periods with no significant harmonic harmonic emission limits for big distorting loads at variation,. LV level (IEC 61000-3-4 and IEC 61000-3-12)? The authors trust that the method will gain sufficient What are the major difficulties linked to their precision in the medium term to allow for its use within a application? Are other methods or approaches contractual framework application. considered? 5.4 Is the central harmonic compensation with active Paper 2.37 (RU) presents the application of a similar filters already used? Is there actually a need to do method for estimating the influence of voltage distortion this? Are there projects towards this solution? sources at the border between two subsystems. What are the major advantages and drawbacks? The method is based on the results of continuous long 5.5 Are there any practical experiences involving the duration measurements (more than 24 hours) from which disturbances generated by PWM converters in the the correlation factor between the controlled harmonic and range 2 to 16 kHz? What are the EMC problems the load of the subsystem is estimated, as well as the to be feared with such components? distortion power sign. According to this criterion the so-5.6 Besides harmonics, are there any recent called dominant subsystem is determined. experiences or interesting cases-studies with other Measurements performed in an actual power supply system distorting-type disturbances (e.g. interharmonics, confirm the practical usefulness of the suggested method transients)? (see the example given in Figure 7)

    Harmonics and flicker: industrial case studies

    and mitigation techniques

    Paper 2.24 (SE) explains how the London underground is

    connected to the public grid in order to limit the

    disturbances. A total of five Static Var Compensators (SVC)

    and ten harmonic filters have been installed in critical

    points of the LUL 22 kV distribution grid. Due to the

    scarceness of space and vicinity to underground stations,

    special measures had to be taken to lay out the hardware in

    a compact way as well as ensure adequate confinement of

    noise and magnetic fields.

CIRED 2003 - Special Report Session 2 - Power Quality & EMC 6/24

    3rd and 5th Harmonics measuredAnother interesting industrial application is described in at both ends of the cablePaper 2.30 (CH). A new SVC was designed and installed 5HD [%]for CERN’s Super Proton Synchrotron (SPS) accelerator. 3rd clamps4Due to the sensitive nature of the converter of the SPS 5th clamps3rd stationmagnets, very strict requirements were imposed on the 5th station3stabilization of the 18 kV bus voltage and its harmonic

    distortion. The adopted solution comprises a 150 2Mvar TCR and eight harmonic filters with a total power of

    130 Mvar. 1

    Problems frequently encountered with capacitor banks in S/Snom [%]0020406080100industrial premises are listed in Paper 2.35 (ES). While actual problems in MV utilities are related to Figure 8: Welding device - Harmonic content of the voltage capacitor switching, and less frequently to harmonics or measured at the clamps and at the distribution substation other PQ issues, a very complete list of problems can be found within industrial sites. Harmonic resonance is currently appearing, as expected, but also damages due to rdFigure 8 shows that the 3 harmonic damping along the high order harmonics or interharmonics, or due to high cable is nearly constant over the whole power range, while frequency overvoltages and even EMC problems. ththe 5 damping varies. This is because of the interaction In contrast to utilities, the nominal power of capacitor thbetween the 5 harmonic component present in the supply banks in some industries can be higher than the transformer voltage and the component introduced by the welder. power. Some loads consume more reactive power than It is interesting to see that the maximum distortion injected active power. Various factors affect the harmonic contents by this machine into the supply voltage lies near the point or give rise to addition laws different from those generally rdof 40-50% nominal power and consists mainly of the 3 used in distribution networks (closer to arithmetic harmonic. Its propagation is blocked by the delta-star stepsummation in industries). down transformers. However, careful attention should be Mostly the owner of the capacitor bank suffers himself the paid to the neutral cable, which can be overloaded in the effects of disturbances. Nevertheless, capacitor banks case of multiple devices. directly connected at the Point of Common Coupling (PCC) are cases that must be considered carefully. Harmonics can affect the whole MV system, especially when there are

    Question 6 changes in the network impedance. Another problem of

    6.1 What are the most remarkable harmful effects of this configuration is the unexpected presence of high

    harmonics (and interharmonics) as encountered in frequency currents that can cause damages to the breaker of

    industrial practice? Are they different in public the utility’s capacitor bank.

    distribution networks? What are the most

    frequently used mitigation devices or techniques? The problem of compensating electric arc furnaces flicker

    6.2 Are there any other interesting industrial is addressed in Paper 2.28 (UK). This paper develops a

    experiences with modern flicker compensating transient model for 11-level cascade converters-based

    devices such as the SVC or the STATCOM? STATCOM. A proposed new control strategy has the

    6.3 Are there any experiences in LV (such as e.g. the benefits of faster response and more accuracy.

    compensation of welding devices)?

     Paper 2.41 (DE) investigates the disturbances emanated

     from welding devices. An experimental approach, based on

    a measurement method involving synchronous Power Quality measurement methods measurements both at the device’s clamps and at the substation bus, is described (Figure 8). A mathematical Paper 2.25 (PL) reports on comparative tests of model is presented and discussed. flickermeters designed according to the requirements of IEC 61000-4-15 standard. Measurements were carried out

    at one Polish steelwork (seven days). Ten various IEC

    61000-4-15 compliant flickermeters have been used for

    these measurements. These measurements and further

    laboratory tests showed that the standard requirements are

    not enough precisely formulated, leaving designers too

    much freedom.

    Paper 2.29 (ES) presents a system for real time

    identification of different types of power quality

    disturbances in power systems (voltage dips, short

    interruptions and transient or temporary overvoltages). The

    CIRED 2003 - Special Report Session 2 - Power Quality & EMC 7/24

    play a crucial role to make the optimum choice of different proposed method estimates the magnitude of the three-solutions (Paper 2.55 (PL)). phase voltage supply in real time in order to determine when a voltage event appears, making use of three Kalman filters. Measurement and characterization of voltage Using this method, the start of a voltage event can be

    dips and short interruptions detected accurately with detection times ranging from less

     than 1 millisecond to about 8 milliseconds, depending on

    Voltage dips at the terminals of sensitive equipment are the magnitude of the voltage event and the point on the

    often due to faults occurring at a much higher voltage level. wave when it starts.

    Even though the load current is small compared to the fault

    current, the changes in load current during and after the Question 7 fault still strongly influence the voltage at the equipment 7.1 Is the recently published IEC standard 61000-4-30 terminals. Paper 2.47 (SE) discusses the changes in shape (Power Quality measurement methods) well and magnitude of the voltage dip during its propagation known? Are there already measuring devices, from the faulted voltage level down to the equipment compliant with this standard? Are there any terminals. Both balanced and unbalanced voltage dips are difficulties to implement the methods described in discussed. Studies of voltage-dip propagation due to the standard? symmetrical and non-symmetrical faults can be simplified

    7.2 What are the difficulties related to the using the related dip characteristics - “characteristic

    measurement transducers (voltage and current voltage” and “PN-factor”. transformers)? Up to which frequency is it The conclusions for induction-motor loads are: practically realistic to perform accurate ? for short-duration dips the positive-sequence voltage measurements? increases from the faulted level towards the load. This 7.3 What are the new trends in Power Quality increase is due to the motor-contribution to the fault and it assessment methods? Are there any recent becomes less after a few cycles. experiences using advanced techniques such as ? for long-duration dips and large amounts of motor load, wavelet analysis or similar? the motor load will take a larger current when its speed

    drops, thus leading to a drop in voltage which will be larger towards the load. ? the negative-sequence voltage becomes less when III VOLTAGE DIPS AND DISTURBANCES IN moving from the fault to the load. It is constant during the CUSTOMERS INSTALLATIONS fault. (IMMUNITY LEVELS, REMEDIAL MEASURES, From a qualitative analysis it is concluded that the behavior

    of other load is similar to that of induction-motor load. ETC) These conclusions are confirmed by measurements. Solving power quality problems within industrial Power quality monitors are being installed in transmission consumers premises is a rather complex task involving, and distribution networks all over Europe to collect generally speaking, the following steps, as discussed in statistics on voltage dips. Drawing up valuable voltage dips Paper 2.45 (ES): statistics is not as easy as it seems at first sight : the ? Power Quality monitoring in order to characterize the equipment of the network user is connected at a different main involved disturbances as well as the actual immunity voltage level, often behind a Dy transformer. The zero-level of the installation, sequence component is filtered out and the line and phase ? Analyzing the sensitive processes and identifying the voltages are shifted. As a consequence the network user critical parts, does not experience the same dips as recorded by the ? Choosing the adequate immunization techniques power-quality monitors in the network. ? Costs estimation: balance between the costs caused by the disturbances and the investment and costs related to the In Paper 2.50 (BE), a method is described to obtain immunization project valuable information on voltage dips with the commonly ? Decision of implementing the solution used monitors that do only record the r.m.s. voltages during the dip. The dip type and voltage phasors are derived, Paper 2.58 (MY) stresses that close cooperation between making it possible to take into account the way the dip the utility and the customer is mandatory for such operation propagates through the HV/MV transformer (see also Paper to succeed since both the customer and the utility engineers 2.59). will have to act as a team in the process of understanding The approach can be used for statistical purposes, e.g. to the power system dynamic and equipment behaviour. obtain dips statistics for the phase-to-phase voltages from Power Quality in its complexity involves grid operators, phase-to-ground measurements or to remove the zero-energy service companies, end users, equipment sequence out of the measurements. The algorithm can also manufacturers and finally manufacturers of technical be applied to compare the measurement information solutions and measuring equipment. Electrical designers obtained from monitors installed at different voltage levels

    CIRED 2003 - Special Report Session 2 - Power Quality & EMC 8/24

     or with different connections (star and delta) and to obtain

    information of the dip as experienced by customer

    equipment.

    The limitations of the described characterization algorithms

    should be taken into account when choosing the connection

    mode of the monitor :

    ? It is not possible to derive the phase voltages from the

    line voltages (the zero-sequence voltage is missing).

    Therefore, it is recommended to connect the monitor

    equipment in star in directly grounded systems (to record the phase-to-ground voltages). Figure 9: Functional steps for the identification process ? In impedance grounded systems, the zero-sequence impedance differs strongly from the positive and negative-It is possible to train the classification tool (knowledge-sequence impedances and the results from the algorithm based system). It helps to determine the location and become questionable for two-phase-to-ground faults. A possible cause of the voltage dip, addressing the restoration connection in delta might then be preferred (although two-plan and preventing some kinds of events in the future. phase-to-ground faults are only a minority). Table 1: Summary of the three-phase unbalanced dips In a similar way, Paper 2.62 (BR) analyses two methods to depending on fault type and location estimate the so-called short duration voltage variations (SDVV) in electric power distribution systems. Fault Type Dip Location One can distinguish two different cases : I I II III ? Systems in which monitoring sites are located in many Dy 3-phase A A A II 3-phase-to-ground A A A network busses (transmission and subtransmission systems). Dy 2-phase-to-ground E F G The main parameter for estimation is then the voltage. III 2-phase C D C ? Systems in which the monitored site is located at the 1-phase-to-ground B C D supply bus (primary feeders, PQ-meters located in the Type A: substation). In this case, both voltage and current are important parameters for the SDVV estimation. Three-phase fault The developed estimators allow for the evaluation of SDVV indices in any given location, from a limited Type B: number of monitored sites. Single-phase fault Moreover, the fault location algorithms provide useful

    information for operation crews, which can considerably Type C: improve the overall quality of service by reducing restoring Two-phase fault times. (or secondary type voltage dip, e.g. single-phase fault as seen behind a Yd or yD transfo) Type D: Secondary type voltage dip, e.g. two-Question 8 phase fault as seen behind a Yd or yD 8.1 Is the classical characterisation and classification transfo. method for voltage dips (in terms of depth / Type E: duration) satisfying? Should other characteri- sation techniques such as described in Papers Two-phase-to-ground fault

    2.50 or 2.60 be preferred, among other things in terms of predicting equipment sensitivity? Type F: Secondary type voltage dip, e.g. two-8.2 What are the actual practices of counting voltage phase-to-ground fault as seen behind a dips and short interruptions for contractual Yd or yD transfo. purposes? Are there any comments or significant Type G: experiences concerning the way of defining Secondary type voltage dip, e.g. two-„events‟, the aggregation procedures and dealing phase-to-ground fault as seen behind a YNy, Yyn, Yy or Dd transfo with complex events? 8.3 On the other hand, what are the actual practices

    for system benchmarking purposes? Is the use of

    global indices (such as SARFI) common practice? x

    What kind of statistical processing is usually performed for system characterisation regarding Paper 2.60 (ES) presents an interesting approach for the voltage dips and short interruptions? characterization of voltage dips, based on the extraction of significant features. The process is composed of five steps (Figure 9).

    CIRED 2003 - Special Report Session 2 - Power Quality & EMC 9/24

    Immunization techniques can be considered very locally Immunization techniques (i.e. close to the key sensitive devices) or more globally (immunization of an entire process or plant). Paper 2.45 (ES) explains that, basically, there are two Paper 2.46 (ES) gives a thorough description of the immunization philosophies: leaving the plant with a classical ASD (adjustable speed drive), being very largely controlled stop trying to restart after the dip, or keeping the used in the industry. plant working during the dip. In both cases, the “brains” The sensitivity of these devices to voltage dips and short (control systems) of the plant must go on working. As the interruptions is clearly investigated and some local total associated power is usually low, the UPS costs are immunization techniques are reviewed: very low as well, especially if there is a separated feeding ? methods with additional energy storage, system for controls in the plant. ? use of a boost-converter, To implement any one of the two immunization ? active front-end rectifier. philosophies, there is a need to combine a set of tools: ? Temporized undervoltage relays Paper 2.44 (IR) reviews the available technology for global ? UPS immunization. FACTS equipment can be used for solving ? Time and level protection sets stability or reliability problems in transmission system, but ? Special programs in “brains” also offer some opportunities for utilities to provide ? “Ride through” features in static converters. adequate voltage quality and supply reliability, and for Most of them have a very short payback time, but it is customers who can ensure adequate power quality inside important to take care of voltage dips since the beginning the plant (Figure 10) of the plant project. The ongoing deregulation in electrical energy transmission and distribution should push the development and the

    application of these new technologies.

    Name Preferred Tasks

    Topology Transmission Distribution Transmission Distribution

     voltage control flicker ??STATCOM DSTATCOM compensation oscillation ? damping reactive power ? (Static ( Distribution compensation reactive power ?Synchronous STATCOM ) regulation Compensator ) harmonic filter ? ESS: Energy Storage System power flow dip/swell ??SSSC DVR compensation transient ? (Static stability Synchronous (Dynamic oscillation ?Series Voltage Restorer) damping Compensator )

     STATCOM under voltage / ??UPFC UPQC and SSSC over voltage advantages compensation (Unified Power (Unified Power Flow Controller ) Quality DSTATCOM ?Controller) and DVR advantages

     power flow coupling of ??Direct Current Transmission remote loads or reactive power ?& remote energy control in HVDC Light System sources connected AC networks optimization of ? energy cost interconnecting ? through asynchronous coupling of networks bus bars or system parts

     Figure 10: FACTS equipment in transmission and distribution networks

An original proposal of global immunization is described in of kW) that can be interconnected to the distribution

    Paper 2.49 (IT), mainly for non-industrial utilities. It starts network. Some of these, such as photovoltaic arrays and

    from the fact that many users choose to enhance Power fuel cells, generate DC current; others, such as micro-

    Quality by using uninterruptible power supply (UPS) units, turbines, need an interface converter with a DC stage for

    generally provided with a DC section containing a storage their connection to the network.

    system. In addition, recent technological developments The increasingly widespread use of DC current, both in the

    made smaller generators available (in the order of the tens generators and in the electrical equipments, leads to the

    CIRED 2003 - Special Report Session 2 - Power Quality & EMC 10/24

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