Tài liệu CIRED 2003 - Round Table on Magnetic Field Mitigation Techniques ppt

CIRED 2003 Round Table on Magnetic Field Mitigation Techniques A Robert (Chairman), J Hoeffelman (Coordinator), Belgium (alain.robert@elia.be, jean.hoeffelman@elia.be) www.cired-s2.org CONTENTS PRESENTATIONS .1 J Hoeffelman (Belgium), Introduction P Cruz Romero (Spain), Reduction of Magnetic Fields from Overhead Medium Voltage Lines M Chiampi (Italy), Some considerations about passive shielding J Hoeffelman (Belgium), Shielding of underground power cables, From theory to practical implementation .8 E Salinas (GB/SE), Field Mitigation from Secondary Substations 14 O Bottauscio (Italy), Experiences in the mitigation of MV/LV substation magnetic field emissions 20 R Conti (Italy), CESI-ENEL Practical Experience in Reducing 50 Hz Magnetic Fields 22 B Cestnik (Slovenia), Cases from Slovenian practice for reduction of 50 Hz electric and magnetic fields (high voltage overhead lines and underground cables) 26 M Tartaglia (Italy), Traction Systems: generated magnetic field and its mitigation 28 DISCUSSION (summary by J Hoeffelman) 30 PRESENTATIONS J Hoeffelman (Belgium), Introduction P Cruz Romero (Spain), Reduction of Magnetic Fields from Overhead Medium Voltage Lines (ELIA, jean.hoeffelman@elia.be) (Universidad de Sevilla, plcruz@us.es) Round Table on Magnetic Field Mitigation Techniques Chairman: Alain Robert Coordinator: Jean Hoeffelman Reduction of magnetic fields from overhead MV lines – P Cruz Romero (ES) Some consideration about passive shielding – M Chiampi (IT) Shielding of underground power cables – J Hoeffelman (BE) Field mitigation from secondary substations – E Salinas (SE) Experiences in the mitigation of MV/LV substation magnetic field emissions – O Bottauscio (IT) CESI-ENEL practical experience in reducing 50 Hz Magnetic fields – R Conti (IT) Cases from Slovenian practices for reduction of 50 Hz EMF – B Cestnik (SI) Traction systems: generated magnetic field and its mitigation – M Tartaglia (IT) Abstract In this contribution several topics concerning magnetic fields and overhead medium voltage power lines are reviewed: simple formulation to assess the magnetic field (MF) level; characterization of magnetic fields generated by typical three-phase and one-phase primary distribution lines, with balanced and unbalanced current; and main mitigation techniques, analysed in relation with typical reduction level obtained Additional data concerning cost and performance of different solutions are also provided Keywords: magnetic field mitigation, primary distribution, compactness, tree wire, super-bundle, lowreactance Simplified magnetic field calculation The MF generated by a set of infinitely long, straight conductors can be formulated by a series decomposition of the Biot-Savart Law [1] For points far from the line (several times the distance between conductors) only the first non-zero term is needed For a single-circuit three-phase line with balanced current the resultant magnetic field is given by CIRED 2003 - Round Table on Magnetic Field Mitigation Methods - Thursday 15 May 2003 - updated 21/05/2003 1/31 (1) where C : constant that depends on phase configuration (flat : C = √2 ; regular triangle : C = 1) d : clearance between adjacent phases µ0 : magnetic permeability of vacuum r : distance from center-of-mass of conductors to calculation point I : phase current For super-bundle double-circuit lines with equal current in magnitude and phase in each circuit, the formula is the same, being I the total current of each phase For low-reactance lines with equal current in magnitude and phase the resultant field lays (2) where I is the RMS current in each circuit, and s the distance between both circuits For single-phase lines with metallic and ground return the approximated field is given respectively by (3) (4) From (1 4) relevant conclusions can be deduced: The MF generated by power lines is proportional to current and distance between conductors In case of balanced single-circuit (current dipole) and super-bundle (SB) double-circuit three-phase lines, as well as one-phase with metallic return MF decays as 1/r2 For the low-reactance (LR) configuration MF decays as 1/r3 For the one-phase case with ground return MF decays as 1/r Typical MF generated by overhead MV lines In figure midspan magnetic field profiles generated by typical primary distribution 3-wire, 3-phase configurations with geometrical characteristics for 20 kV [2,3] are shown It is assumed that the lowest conductor height at midspan is m CIRED 2003 - Special Report Session - Power Quality & EMC Fig Magnetic field profiles for different balanced 3-phase MV configurations (units in m) According to the conclusions previously obtained, the lowfield configurations are the low reactance and the armless ones We can also observe that in the conventional crossarm constructions the better behaviour of delta configuration is compensated by the higher phase-phase distance The height of calculation for this cases and the rest of simulations is m above ground Other types of distribution systems, like unbalanced 4-wire 3-phase [4] and 2-wire 1-phase are also analysed For the 4-wire 3-phase system several crossarm construction profiles with different unbalance levels are comparised, concluding that with no ground return the field increases with unbalance level, in a higher or lesser extend depending on relative location of neutral conductor in relation with phase conductors, and that with ground return the MF increase is even higher, and growing with ground current percentage For the 2-wire one-phase system a similar behaviour is observed The negative effect of ground return current is explained observing eq (4) An unbalanced system with ground return can be decomposed into several current dipoles [5] (MF decay as 1/r2 ) and a homopolar component (MF decay as 1/r) whose effect will be dominant at certain distance from the line Magnetic field reduction methods In this section several methods to mitigate MF level from overhead MV lines [6,7] are reviewed They can be classified as follows: • Methods that try to reduce the load current of the line If we can reduce the current, the MF will decrease proportionately Some possibilities are the following: - Increase the voltage level of the line - Change one-phase lines to three-phase • Methods that try to compact the line The aim is reduce the phase-phase distance - Change from crossarm to armless poles construction - Use covered or insulated cables (overhead or underground) - Split the line • Methods that try to move away the phase conductors from the interest area Due to the decay with distance, the MF influence will be weaker 2/31 - Increase the phase-ground clearance - Relocate the line • Methods that try to compensate the power field with a counteracting external field - Passive loop This technique consists on the installation of a conductor loop near to the line, where a current is induced This current creates a field that partially cancels the original field A last technique, passive loop, has also been considered The main drawback is that to obtain a reasonable reduction of about 35 % it is needed to use for the loop a conductor of much lower resistance (about 0.12 Ω/km) than conventional for MV lines, with the additional cost that it implies Table I Main characteristics of different mitigation techniques In addition to these general methods, for net current lines (e.g multigrounded 4-wire 3-phase) it is needed simultaneously to take control of the ground return current levels Therefore, specific actions must be done in this sense: • Balance of phase currents by changing phase arrangement of loads connected to a 3-phase line [8] or converting laterals single-phase to 3-phase lines • Increase of neutral conductor size • Implementation of 5-wire system instead of 4-wire one [9] It is difficult to choose a particular method as the optimum The selection of mitigation method is a case-by-case analysis, where different aspects must be considered: • New or already existent lines If a low-field primary distribution line or set of lines must be projected, methods that require a global system change could be feasible, like increase of voltage level, reduction of unbalance, etc • Local or whole system reduction Some methods are feasible for local application, but extremely costly for a whole line or network • MF reduction level needed • Cost of reduction method • Other issues: safety and environmental aspects, maintenance, reliability, etc The presentation is mainly devoted to analyse the more feasible methods for local applications, although some of them could be applied for global A summary of the methods is shown in table I, where the typical MF reduction levels at 10 m from the line are shown The highest mitigating methods are the ABC (Aerial Bundle Cable), the underground line and the spacer cable [10] Their effectiveness is however conditioned by the absence of unbalance Another main drawback of these methods is the cost Their use is more feasible when other issues must be satisfied (reduction of visual impact, reduction of outages) A method less costly could be the split of the line, but it is also strongly conditioned by the unbalanced current Other set of techniques (use of tree wires, armless construction and increase of ground clearance) are less mitigating-effective, but a significant reduction can be obtained, with the advantage of a lower cost and an allowed higher unbalance level, specially the increase to ground clearance Eventually we can try to compact the line with no changes in the conductor, like reducing the span length or replacing string by post insulators The reduction obtained is low, about 25-45 %, and the cost depends mainly of the original span lengths of the line section to be mitigated CIRED 2003 - Special Report Session - Power Quality & EMC Mitigation technique Global Reduction Installation performance level (%) cost over conventional Small 25-45 compactness Crossarms → ∼ 60 armless Tree wires ∼ 60 Spacer cable ∼ 80 ABC 100 Underground ∼ 90 line Circuit split 70-80 Increase clearance to25-60 ground Compensation 35 loop Low Lower Effect of unbalanced current Low Low/medium Lower Medium Medium High Very high Higher Higher Higher Medium High High Very high Higher High Medium Lower High Low/medium Lower Low Medium Medium Lower Conclusions In this contribution major aspects related with characterization and mitigation of magnetic fields generated by medium voltage overhead lines have been reviewed The main mitigation techniques have been analysed, taking into account mitigation effectiveness, installation cost, global performance (reliability, aesthetic, maintenance, etc.) and sensibility to unbalanced current If a reasonable MF mitigation is the unique objective to refurbish a section of an existing line the more feasible methods are the increase of clearance to ground, and the low-to-medium compactness by discrete reduction of phase-phase distance, replacement of crossarms by armless construction or reduction of swinging of string insulators References [1] W.T Kaune, L.E Zaffanella, Analysis of Magnetic Fields Produced Far from Electric Power Lines IEEE Trans on Power Delivery Vol 7, No 4, pp 2082-2091, Oct 1993 [2] W.F Horton, S Goldberg, Power Frequency Magnetic Fields and Public Health CRC Press, Boca Raton, 1995 [3] POSTEMEL, S.L., Postes metálicos para líneas eléctricas de alta y baja tensión, Dic 1990 [4] H.L Willis, Power Distribution Planning Reference Book Marcel Dekker, New York, 1997 [5] P Pettersson, Simple Method for Characterization of Magnetic Fields from Balanced Three-phase Systems Proceedings CIGRÉ Session, 1992, Paper 36-103 [6] A.S Farag, J Bakhashwain, T.C Chen, Y Du, L Hu, G Zheng, D Penn, J Thomson, Distribution Lines Electromagnetic Fields: Management and Design Guidelines Proceedings CIGRÉ Session, 2000, Paper 36-105 [7] S Rodick, P Musser, Evaluation of Measures and Costs to Mitigate Magnetic Fields from Transmission and Distribution Lines 37th IEEE Rural Electric Power Conference, Apr 1993 [8] T Chen, J Cherng, Optimal Phase Arrangement of Distribution Transformers Connected to a Primary Feeder for System Unbalance Improvement and Loss Reduction Using Genetic Algorithm IEEE Transactions on Power Systems, Vol.15, No 3, Aug 2000, pp 994 -1000 3/31 [9] D J Ward, J F Buch, T.M Kulas, W.J Ros, An Analysis of the FiveWire Distribution System IEEE Transactions on Power Delivery, Vol.18, No 1, Jan 2003, pp 295 -299 [10] T.J.Orban, Spacer Cable Revisited Transmission and Distribution World, Dec 2002 Reduction of Magnetic Fields from Overhead Medium Voltage Lines Pedro Cruz Universidad de Sevilla OBJECTIVES •Revision of main aspects related with magnetic fields and overhead medium voltage lines • Comparison between different magnetic field mitigation methods MAGNETIC FIELD GENERATED BY OH LINES CONTENTS •Magnetic field generated by OH MV lines •3-wire 3-phase •4-wire 3-phase •2-wire 1-phase •Magnetic field reduction methods •Selection of mitigation technique •Line compactness •Split of the line •Increase clearance to ground •Use of passive cancellation loops •Effect of unbalanced current •Summary 3-WIRE, 3-PHASE MV LINES MAGNETIC FIELD GENERATED BY OH LINES MAGNETIC FIELD GENERATED BY OH LINES 4-WIRE, 3-PHASE MV LINES MAGNETIC FIELD GENERATED BY OH LINES 4-WIRE, 3-PHASE MV LINES CIRED 2003 - Special Report Session - Power Quality & EMC 4/31 •Local or whole system reduction •Other issues –Safety and enviromental aspects –Reliability –Insulation and electrical clearance requirements –Operation –Maintenance MAGNETIC FIELD REDUCTION METHODS Line compactness (balanced current) 4-WIRE, 3-PHASE MV LINES •Effectiveness: depends on initial arrangement •Lesser visual impact •To keep clearance requirements: often needed to insert a midspan pole •Large-scale compacness: reduction of inductance •Change live-line maintenance practices •Posibilities of compactness –Minor changes –Crossarms -> armless –Covered wires –Insulated wires COMPACTNESS OF THE LINE (BALANCED) Minor changes 2-WIRE, 1-PHASE MV LINES •Existence at primary and secondary distribution system •p-g clearance (midspan) : m COMPACTNESS OF THE LINE (BALANCED) Crossarms → Armless MAGNETIC FIELD REDUCTION METHODS Shorter spans: ~ 50 m Reduction: ~ 60 % COMPACTNESS OF THE LINE (BALANCED) Covered wires MAGNETIC FIELD REDUCTION METHODS •Avoid Tree treeming •Reduction of operating costs •Greater reliability and quality of service (reduction of outages) •Suitable for complete new lines or upgrade of old ones •Types –Tree wire ( PAS, BLX) –Spacer cable Selection of mitigation technique •New or already existent project •MF exposition level allowed •Cost of reduction CIRED 2003 - Special Report Session - Power Quality & EMC 5/31 COMPACTNESS OF THE LINE (BALANCED) Covered wires USE OF PASSIVE CANCELLATION LOOP COMPACTNESS OF THE LINE (BALANCED) •More effective in flat configurations (horizontal, vertical) •Increased reduction with compensation of inductance (capacitor) •Resistance of the loop conductor: much lower than typical MV conductor •Need to reinforce the poles Insulated wires •ABC (aerial bundle cable) •Dramatic MF decay with distance USE OF PASSIVE CANCELLATION LOOP (BALANCED) UNDERGROUND LINE (BALANCED) EFFECT OF UNBALANCED CURRENT SPLIT OF THE LINE (BALANCED) •Existing DC Super-Bundle → Low-reactance •Existing SC lines and mitigation in few spans: special pole SC → DC •Conversion of SC pole to DC pole: increase of height/strength •Need to have equal loading between circuits •Reduction of mitigation effectiveness Ground return current = neutral wire current SUMMARY INCREASE CLEARANCE TO GROUND (BALANCED) •Increase poles height •Installation of new poles at midspan (long spans) •Reduction effectiveness close to the line CIRED 2003 - Special Report Session - Power Quality & EMC 6/31 Experimental set-up for tests M Chiampi (Italy), Some considerations about passive shielding The set-up is constituted by a 180 cm X 60 cm X 60 cm wooden frame 60 cm X 60 cm magnetic sheets can be disposed on the frame Two external busbars are supplied by a 50 Hz single-phase system with currents of some hundred amperes (Politecnico di Torino, chiampi@polel1.polito.it) Some considerations about passive shielding O Bottauscio (*), M Chiampi (*), G Crotti (°), A Manzin (°), M Zucca (°) (°) Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino, Italy (*) Dipartimento di Ingegneria Elettrica Industriale - Politecnico di Torino, Italy Aim of the presentation • The presentation is addressed to analyse the shielding capabilities of different low cost magnetic materials • The study has been developed in the Turin Unit by means of both experiments on a specific test apparatus and numerical computations using a 2D hybrid FEM/BEM model Shielding configurations Outline of the presentation • Magnetic materials for shielding in industrial and civil application • Influence of shape and building of passive shields • Experimental and computational results Magnetic Laminations for shields • Low-Carbon Steel (Si < 1% wt) • Lamination thickness: 0.80 mm ã Electrical resistivity: 13.9ì10-8 ìm ã Non-Oriented Si Fe 1.5% wt • Lamination thickness: 0.50 mm • Electrical resistivity: 27.9ì10-8 ìm Efficiency of magnetic plane shields ã Grain-Oriented Si – Fe 3.0% wt • Lamination thickness: 0.30 mm • Electrical resistivity: 48.0×10-8 ×Ωm Magnetic Characteristic of the Shielding Materials Magnetic flux density in plane shields r.m.s values of magnetic flux density in the plane shield: Lines: computations by a 2D hybrid FEM/BEM model Points: measures by test coils CIRED 2003 - Special Report Session - Power Quality & EMC 7/31 Efficiency of combined shields Magnetic flux in plane shields r.m.s values of magnetic flux in the plane shield computated by a 2D hybrid FEM/BEM model Efficiency of magnetic U-shaped shields J Hoeffelman (Belgium), Shielding of underground power cables, From theory to practical implementation (ELIA, jean.hoeffelman@elia.be) Summary This contribution is aimed at presenting the most recent achievements in shielding techniques for underground power cables It is based on the work performed by CigréCired JTF C4-04-02 and focuses mainly on the use of aluminium shields, which have been applied on an important 150 kV link in Belgium Air-gaps in the sheet corners Effects of air-gaps in sheet corners U-shaped screen FEM/BEM model CIRED 2003 - Special Report Session - Power Quality & EMC General field mitigating techniques The reduction of ELF magnetic fields produced by power cables can become an important concern due to the fact that they are sometimes laid very close to inhabited areas As for overhead lines, the magnetic field due to underground cables is inversely proportional to the distance between conductors Therefore the easiest mitigation technique remains, of course, to install the cables in a trefoil arrangement However, when a very high load capacity is required, it is not always possible to install the cables in trefoil Horizontal layouts with distances of several tens of cm between conductors are sometimes needed In that case the magnetic field strength above the conductors, at ground level, can be higher than that produced by an equivalent overhead line and can require some mitigation method Figure shows, for both arrangements, and for three different measurement positions above ground the decrease of the field with the distance to the axis of the cable layout 8/31 In both cases, the cables are buried at a depth of 120 cm, have a diameter of 10 cm and are carrying a current of kA In the horizontal arrangement the distance between phases is 25 cm trifoil arrangement 120-10 cm 100.00 10.00 µT h=0m 1.00 h = 1.5 m h=3m 0.10 0.01 10 15 20 25 30 distance to axis (m) horizontal arrangement :120-25 cm 100.00 10.00 µT h=0m 1.00 h = 1.5 m h=3m 0.10 0.01 10 15 20 25 30 distance to axis (m) Figure 1: Comparison between trefoil arrangement and horizontal arrangement Metallic shielding Shielding by ferromagnetic materials Although theoretically more efficient at low frequency than conductive materials, ferromagnetic materials seem, in most cases, to be less advantageous The reasons are the following : The effectiveness of conductive shields is more homogeneous in the space Ferromagnetic materials are mainly effective nearby the shield, while conductive materials are also effective at distance Good ferromagnetic materials like permalloy (“Mumetal”) or transformer laminates are often expensive and highly sensitive to corrosion Therefore they need a good protection coating Ferromagnetic materials are more efficient when the magnetic circuit they offer to the flux lines is closed (no or few gap) This particular layout is not often practically achievable unless for shielding short cable lengths Hence, the main example where a ferromagnetic shielding seems to be superior to a conductive one is the steel tube In this case a shielding factor up to 50 can be achieved as shown in figure taken from [ 0] However, such a tubular shielding has also drawbacks: The maintenance or the repair of the cables is difficult The thermal behaviour of the cables is neither easy to manage, as the tube needs normally to be filled with concrete On the other hand, the installation of a single tube allows a fast recovering of the trench, the cables being pulled-in by a single and fast operation afterwards When a trefoil arrangement cannot be applied or if a further field reduction is required, a metallic shielding can reduce the field at the source As stated in [ 0] to [ 0], ferromagnetic material as well as good conducting material are used At low frequencies the physical mechanisms involved by both materials are completely different: In the first case (figure b), sometimes called magnetostatic shielding, the field lines are absorbed by the low reluctance material, whereas in the second case (figure c) they are repelled thanks to the eddy currents induced in the material Figure 2: Shielding mechanisms: ferromagnetic material versus conductive material CIRED 2003 - Special Report Session - Power Quality & EMC Figure 3: Shielding by ferromagnetic material 9/31 Shielding by conductive materials As far as conductive materials are concerned, two materials can be considered: copper and aluminium Both materials have their own advantages and drawbacks: Copper has a higher conductivity but also a higher cost than aluminium Although copper is easier to weld than aluminium, modern welding techniques under argon atmosphere allow assembling aluminium plates on the yard Therefore, in the following sections, only shielding by aluminium plates will be considered On the other hand, if some precautions are taken concerning the neutrality of the soil, corrosion problems should not arise neither with copper nor with aluminium The possible influence of stray currents needs however to be addressed Three main layouts will be taken into consideration: the flat horizontal shield or plane shield, the U-shaped shield and the H-shaped shield result achieved with a continuous shield is to use a double layer of metallic plates, each layer being shifted by half the length of one plate with respect to the other layer, like the bricks of a wall In that case the resulting effectiveness is close to that of a single continuous shield with the same global thickness It is important to note, here, that the quality of the electrical contact between layers doesn’t play any part in the shielding effectiveness Performances Figure shows the comparison between calculation (2 D FEM-BEM model) and measurements for an aluminium plate installed at 27 cm above the axis of a three phases system in flat configuration (distance between phases: 25 cm) The agreement is quite good although the calculation refers to a mm continue shield (99.5 % aluminium), whereas the measurements are made on a double layer mm discontinue shield Horizontal shielding - Comparison between calculation and measurement at m above cables axis (73 cm above shield) A relatively simple way to mitigate the field produced by a or phases cable system is to install as close as possible above the cable an horizontal plate Shield thickness mm plates give already fair results but the effectiveness clearly increases with the thickness as far as this latter remains smaller than the skin effect (about 12 mm for aluminium and mm for copper) Shield width The main problem with plane shields is that the shielding effectiveness usually strongly decreases with the distance to the centre of the plate with, as result, that the shielded field presents two peaks in the vicinity of the edges of the plate To avoid this it is necessary to use a plate with a sufficient width Practically it is recommended that the ratio of the shield width to its distance to the conductors and to the distance between conductors remains larger than For a maximum effectiveness the plates need to be as close as possible to the cables but, if they are too close, the losses due to the induced eddy currents can become to high Power capacity, however, is practically not influenced if the distance between cable sheets and shielding is not smaller than to cm [ 0] Shield continuity For manufacturing reasons, the shield is normally divided into smaller elements placed near each other with or without air gaps It can been shown [ 0] that the shielding continuity between the different elements is not absolutely necessary The presence of gaps reduces in fact the eddy currents and the global shield effectiveness, but this effect decreases with the observation distance On the contrary, near the boundaries of the gaps, due to the fact that the eddy currents are flowing in opposite direction, there is a strong enhancement of the field that behaves a little bit like as a compressed fluid leaking through the gaps A good way to avoid this enhancement and to approach the theoretical CIRED 2003 - Special Report Session - Power Quality & EMC shielding factor Plane shield 16 14 12 10 50 mm (calc) mm (calc) mm (mes) 100 150 200 shielding width (cm) Figure 4: Shielding by horizontal aluminium plates U-shaped shield It has been very often written that a U-shape shield exhibits better performances than a flat shield In fact, as shown in [ 0], for the same shielding area, it has not really a better effectiveness than a horizontal plane shield but it doesn’t necessitate to groove such a large trench as that required for an horizontal shield of the same total width before bending One problem, however with U-shaped shields is that, contrary to what happens with plane shields, there is an absolute need to get a good contact between the vertical parts of each shielding element (assuming, of course, a non continuous shield) This corresponds to the typical thickness of the dolomite layer above 2000 mm2 alu power cables 10/31 and (c) twist the cable array to diminish longitudinal contributions to the field Extra considerations have to be taken in order to avoid hindrances such as heating and mechanical stresses due to eddy currents and induced forces (a) (b) Fig 5.1 Different phase arrangement in cables; the field contours are plotted in the interval [0.1-1] µT Example Here a study is presented of magnetic fields originating from a secondary substation placed in the cellar of the Gothenburg City Library This public library is located in the centre of Gothenburg and is surrounded by other public and urban buildings About 190 persons work in this building and it receives around 3,000 visits each day The electricity supply to the library and nearby public buildings consists of a secondary 10/0.4 kV substation (two 800 kVA, three phase transformers) Rather high magnetic field values (Fig 6.1-a) were registered at the floor above the substation and these field values propagated over a rather extended area outside the substation Stray currents were discovered to be the cause of the anomaly Various mitigation operations were carried out, taking advantage of renovation procedures Among them, phase cancellation via cable management, shielding of busbars, replacements by low emission transformers, as well as laminated-ferromagnetic field reducers for cancellation of stray currents, were used The final field values (Fig 6.1-c) were reduced in average to acceptable sub-microtesla levels CIRED 2003 - Special Report Session - Power Quality & EMC (c) Fig 6.1 The magnetic field one floor above the library and its reduction at different stages Conclusion Given a source, belonging to a substation, which is producing relatively high magnetic field values (e.g in excess of the microtesla level) on a certain area of interest, this report has shown various possible ways to mitigate these values The emphasis was to use simple and costeffective methods For this reason it was preferable to apply the mitigation to take place at the sources instead of dealing with shielding of the affected areas Various techniques were applied according to the specific characteristics of each source Table 7.1 shows the different 17/31 techniques and design methods in order to achieve field mitigation The results of this investigation have been applied to the renovation and building of secondary substations: Table 7.1 •Active compensation •Passive compensation •Phase cancelation Methods •Analytical computations •Numerical simulations (Finite elements) •Experimental setups •Optimization methods Main sources •Busbars •Transformers •Cables Some of the complexities inherent to the use of finite/open shields Ener Salinas, Anders Bondeson Summary •The study aims to develop techniques that can be used for the reduction of magnetic fields from secondary substations • The goals are to develop cost-effective methods for field reduction and to develop integrated solutions for larger systems FEM formulation for busbars Pure conductive vs ferromagnetic shielding Mitigating the field at the source is often more cost-effective than mitigating at the victim Double shield arrangement in front of a source The field corresponds to the permutation Iron-Aluminium Strategy: Mitigate the fields at the source instead of at the victim area Techniques: •Shielding CIRED 2003 - Special Report Session - Power Quality & EMC 18/31 Mitigation of the field from transformers by rearrangement of connections at the secondary side (a) before: equal phases, (b) after: mixed phases Reduction of the magnetic field from a 3-phase transformer by phase rearrangement at the secondary side is a cost-effective mitigation method The magnetic field of a shielded and unshielded transformer at 3m over the floor of the substation The shield consists of an aluminium box (5 mm thick); otherwise the transformers have similar sizes and connections The magnetic field one floor above the library and its reduction at different stages Different phase arrangement in cables The field contours are plotted in the interval [0.1-1] μT Conclusions •The mitigation operation was performed at the sources rather than at the affected areas •Magnetic fields and parameters of mitigation techniques were modelled using modern methods (e.g Symbolic computation, 3D finite element codes) •Magnetic field was mitigated from transformers, busbars and cables •Cost-effective solutions were obtained CIRED 2003 - Special Report Session - Power Quality & EMC 19/31 O Bottauscio (Italy), Experiences in the mitigation of MV/LV substation magnetic field emissions (IEN Galileo Ferraris, botta@ien.it) Experiences in the mitigation of MV/LV substation magnetic field emissions O Bottauscio (*), M Chiampi (*), G Crotti (°), A Manzin (°), M Zucca (°) (°) Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino, Italy (*) Dipartimento di Ingegneria Elettrica Industriale - Politecnico di Torino, Italy Introduction - The presentation will be addressed to present the experiences gained by the Turin group in the mitigation techniques for the reduction of the magnetic field produced by MV/LV substations - The attention will be focused on the design of passive shielding to be adopted in the case of old installations, where the modification of the layout is not always practicable - Generalized mitigation around the substation - Mitigation in a specific area - Possible actions: - Optimal layout of the substation (mainly for new installations) - Design of passive shielding (also for old installations) Model for magnetic field evaluation and reduction • The development of specific numerical tools for field modelling must take into account the features of the problem: -The domain under study is not limited -The behaviour of magnetic materials is not nonlinear -Complex electromagnetic phenomena arise inside shielding elements -Shielding elements are thin structures - The use of standard Finite Element formulations results to be inefficient • A problem sometimes arises in presence of unknown sources -A preliminary evaluation of their contribution is required (inverse problem) Mathematical model: 3D hybrid FEM-BEM approach - Shielding elements are thin structures - The use of standard Finite Element formulations results to be inefficient, due to the aspect ratio of the problem - A coupling of separate approaches, addressing a particular geometrical scale, is required: Outline of the presentation - Characteristics of the magnetic field emissions of MV/LV substations and mitigation strategy - Shielding efficiency of materials employed for passive shielding - Modelling aspects (sources and shield structures) - Application to substations for industrial and civil supply Characteristics of MV/LV field emissions - Spatial nonuniformity due to the contribution of different sources MV/LV substation for civil supply Medium voltage: 22 kV; Low voltage: 400 V; Power of the transformer: 400 kVA ; Maximum MV busbar current: 210 A; MV board insulation: air; Geometrical dimensions: m x m, h = 2.9 m - Necessity of a model to evaluate the contributes of the different sources Characteristics of MV/LV field emissions Model of the substation - Variability due to the substation layout - Time variability due to the fluctuation of the load Mitigation solutions - The mitigation strategy is significantly dependent on the final goal we intend to obtain: CIRED 2003 - Special Report Session - Power Quality & EMC Validation of the mathematical model LV board wall 20/31 Comparison between shields Analysis of emissions LV board wall Solution achieved mm thick aluminium alloy Analysis of emissions MV board wall Result of the proposed solution: LV board Analysis of emissions Ceiling Result of the proposed solution: MV board Shield configurations MV/LV substation for industrial supply CIRED 2003 - Special Report Session - Power Quality & EMC 21/31 Medium voltage: 22 kV; Low voltage: 400 V; Power of the transformers: MVA ; MV board insulation: air; Geometrical dimensions: 20 m x m, h = m R Conti (Italy), CESI-ENEL Practical Experience in Reducing 50 Hz Magnetic Fields (CESI, rconti@cesi.it) Due to some epidemiological studies that have suggested a weak association between exposure to power frequency magnetic fields and certain diseases, especially childhood leukaemia, some local and central authorities have shown themselves inclined to adopt precautionary approaches and develop policies aimed at reducing, when possible, human exposure to magnetic fields produced by power lines In addition, from the point of view of electromagnetic compatibility, low-level low frequency magnetic field may cause interference with electronic apparatuses and VDU Analysis of field emissions Solution achieved Due to these reasons, remarkable efforts have been devoted to research activities aiming at evaluating the magnetic field levels generated by electrical installations and at studying possible techniques to reduce their emissions The paper illustrates the direct experience acquired by CESI and ENEL in studying and designing technical solutions to reduce the magnetic field generated by HV and EHV lines (overhead lines and underground cables) and by MV/LV substations In particular, the paper is focused on the main techniques for possible application to existing installations, since they represent the real challenge an electricity company may encounter Some case studies are also presented of interventions performed to reduce the magnetic field from power lines, cable and substations, describing both the reasons that led to the decision to reduce the magnetic field and the technical solution adopted Interventions on overhead lines in the vicinity of schools Result of the proposed solution Three interventions have been carried out on HV overhead lines, on the basis of memorandum of understanding stipulated with the town administrators involved, who have contributed totally or partially, to the expenses supported by TERNA, the transmission company of the ENEL's Group The following table summarises the reasons that led to the mitigation of the magnetic field generated by the lines in question and describes the technical solution adopted The facts The primary school Francesco Petrarca of Mirano CIRED 2003 - Special Report Session - Power Quality & EMC The solution To obtain such a B-field level, ENEL reduced the 22/31 (VE) had been built in the vicinity of a 132 kV line The town administration asked ENEL to carry out the necessary works to keep the magnetic field inside the school at a level not exceeding that of 0.5 T, as recommended by some local health authorities conductor-to-conductor clearance of the two spans passing over the school, by replacing the existing insulator strings with others of the "V" type Nevertheless, a group of pupils' parents appealed to the regional administrative court (TAR) of Veneto against the planned transfer of their children in the new building in question The TAR agreed to accept the appeal and forced the administration not to allow the utilisation of the school in presence of magnetic fields higher than 0.2 T Among the considered hypotheses, the splitting of the phases was the solution that showed the best performances from both the technical and economical points of view, This was obtained by replacing the head of two masts in order to realise a double circuit line in antisymmetrical configuration The Pistelli kindergarten is located in Livorno in proximity of a 132 kV double-circuit line It was agreed to perform the transposition of the phase conductors of one of the two lines and increase the conductor-to-ground clearance by means of the installation, at about half span, of a new tubular support with insulating arms After some measurements carried out in the kindergarten had shown that the B-field ranged from 0.14 to 0.7 µT, and under the pressure of local health and environmental authorities, the Livorno administration decided to find out, in collaboration with the Regional Agency for Environmental Protection, TERNA and the Italian National Grid Operator, a technical solution to reduce the field levels The kindergarten of Castelnuovo dei Sabbioni, (Commune di Cavriglia - AR) is located in proximity of a 220 kV line Measurements indicated that the magnetic field ranged from 0.1 to 0.48 T inside the kindergarten and from 0.56 to 1.76 T in the relevant garden As in the previously described "case studies", an intervention to reduce those levels of magnetic field was agreed by the involved parties Shielding options considered for 132 – 150 kV Underground cables The following solutions have been considered: - acting on laying geometry and laying depth; - introduction of compensating loops; - adoption of solid bonding connection of the metallic sheaths; - shielding with conductive and/or ferromagnetic materials Independently from the shielding efficiency of each of the above mentioned solutions, it was understood that the best solution strongly depends on whether the intervention must be carried out on an existing cable already in operation or on a new cable still to be laid down 2.1 Interventions operation on existing cables already The adopted solution consisted in the enhancement of the conductors in the line section adjacent to kindergarten and in the splitting of the phases obtained by the installation, at about half span, of a new double circuit support A magnetic field reduction of about 70% was obtained The new support, and relevant split.phase conductors, were also designed to avoid any corona effect [inception of audible noise (AN) and television interference (TVI)] possibly deriving from the new arrangement Indeed, besides offering a satisfactory reduction of the magnetic field, it is technically feasible and rather simple, and allows to keep the excavation works and the application of metallic screens to a minimum, thus limiting both the risk of damaging the cables and the impact of the works on the daily life of the citizens Experience showed that conductive material are more effective than analogous ferromagnetic plates 2.2 Installation of new, low-magnetic-field cables In this case, laying the cables in a trefoil arrangement, instead of the conventional flat configuration with an interphase distance of 0.25 m, represents the best solution for the majority of practical situations in Shielding the cables by means of metallic screens of open shape was considered the best solution for this kind of interventions CIRED 2003 - Special Report Session - Power Quality & EMC This solution was adopted on the basis of a study performed by CESI that showed a magnetic field reduction of the order of 35÷40%, when the currents in the two lines flow in opposite direction, and of 55÷60% when the current flows are in the same direction Indeed, in spite of a modest reduction in cables transmission capacity, the trefoil arrangement allows, in general, to keep the magnetic field levels sufficiently low, at the same time offering good performances also from 23/31 both the economic and environmental viewpoints, given the fact that it requires the narrowest trench In case it would be necessary to obtain a more drastic reduction of the magnetic field, the best solution seems to be that of enclosing the cables inside a tube of proper ferromagnetic material Indeed, tests performed on an experimental line, placed inside a pipeline made of steel and fed with three phase symmetric currents of up to 3000 A, have demonstrated that this shielding method can reduce the original field of up to two orders of magnitude The “pipe-cable” solution has been adopted in the town of Genova to realise a 132 kV circuit (about km long) in place of an existing cable that had to be removed for town planning reasons Such a choice was forced by the existence of a regional law establishing that magnetic fields produced by new transmission and distribution plants must not exceed the level ("quality objective") of 0.2 µT in areas where people may stay for prolonged periods of time Shielding options considered for MV/LV substations Studies and experimental investigations have been carried out considering the following options - acting on the lay-out of equipment and on the arrangement of their electrical connections to the transformer; - shielding equipment and/or electrical connections with conductive and/or ferromagnetic materials CIRED 2003 - Special Report Session - Power Quality & EMC 24/31 CIRED 2003 - Special Report Session - Power Quality & EMC 25/31 B Cestnik (Slovenia), Cases from Slovenian practice for reduction of 50 Hz electric and magnetic fields (high voltage overhead lines and underground cables) (Electric power research institute Milan Vidmar, breda.cestnik@eimv.si, co-authors K Grabner, karol.grabner@elektro-ljubljana.si, M Ramovs, marko.ramovs@ibe.si) Continuos research of possible health hazardous effects of magnetic field and interference with sensitive electronic devices and VDU's have stimulated the research of CIRED 2003 - Special Report Session - Power Quality & EMC 26/31 technically feasible mitigation techniques worldwide In Slovenia, in addition also precautionary policy within the national environmental legislation is supporting application of technical solutions with lower EMF emissions, when constructing new electric power facilities or reconstructing the existing ones On the areas with high EMF protection degree new facilities have been subjugated to the limit values of power-frequency magnetic and electric field emissions of 0,5 kV/m and 10 T on the assessment locations For some planned new distribution double-system 110 kV high-voltage overhead lines and power cable lines the possibilities to reduce EMF emissions in the environment were analyzed to be taken into account as an input guideline for the design phase Within the existing technologies and set starting-points the reduction measures such as optimal phase configuration, reduction of the distance between phases, selection of advantageous geometrical disposition of the conductors, increase of the distance to the conductors (vertical or horizontal) were defined and checked regarding its feasibility In our contribution case analysis for the two planned high-voltage overhead lines from Slovenian practice are shown (namely x 110 kV Polje – Vič and x 110 kV Grosuplje – Trebnje), taking into account the foreseen two three-phase system underground cable on a segment of densely populated area Comparisons of the field diagrams show the efficiency of the mentioned field reduction measures The field diagrams were aimed to be used in the design phase of the transmission line route for the predictive determination of the clearances (ground clearances, distances from the conductors to objects) that assure the legally imposed permissive emission values These estimated clearances are smaller when the reduction measures such as optimal phase configuration are applied, which consequently results in easier and less expensive design solutions and environmental siting of the line CASES FROM SLOVENIAN PRACTICE FOR REDUCTION OF 50 Hz ELECTRIC AND MAGNETIC FIELDS HIGH VOLTAGE OVERHEAD LINES AND UNDERGROUND CABLES B Cestnik(1), K Grabner(2), M Ramovš(3) (1) Electric Power Research Institute Milan Vidmar, Ljubljana, Slovenija (2) Elektro-Ljubljana, Ljubljana, Slovenija (3) IBE, Ljubljana, Slovenija Reasons to analyze and implement measures for reduction of EMF? Options to reduce E and B Increase of the distance to the conductors Case analysis New planned HV objects: a) x 110 kV Polje – Vič (segment of HVTL + segment of KB) b) x 110 kV Grosuplje – Trebnje (HVTL) Starting points for HVTL: - standard towers (double circuit semi-vertical configuration NC73/h, NC74/h, ZC73/h, ZC74/h) - Al/Fe 240/40 mm2 conductors - max current and voltage value for calculation: 400 A and 121 kV Starting points for KB: - geometry defined in advance (in earth and in pipe system) - max phase current value for calculation: 400 A Towers with diferent phase-to-phase distances National EMF legislation regarding new facilities of the nominal voltage above kV: - is supporting technical solutions with lower EMF emissions; - limit values: E =0,5 kV/m and B=10 µT on the assessment locations of the areas with high EMF protection degree; CIRED 2003 - Special Report Session - Power Quality & EMC Simetrical phase configuration 27/31 Towers with diferent phase-to-phase distances Use of optimal phase disposition - B Simetrical phase configuration NC 73/h Towers with diferent phase-to-phase distances Two 110 kV kable systems Optimal phase configuration Two 110 kV kable systems Towers with diferent phase-to-phase distances Optimal phase configuration Use of optimal phase disposition – E NC 73/h M Tartaglia (Italy), Traction Systems: generated magnetic field and its mitigation (Politecnico di Torino, michele.tartaglia@polito.it) Electric traction systems are essential to reduce air pollution of internal combustion engine vehicles in medium and large towns but it is also necessary to evaluate the CIRED 2003 - Special Report Session - Power Quality & EMC 28/31 consequent magnetic fields A general method to evaluate magnetic fields produced by d c traction systems (like tramways and underground railways) will be presented The model must simulate the behaviour of the electrical circuit constituted by supplies and vehicles considering their movement laws and it predicts the magnetic field generated by the currents flowing in the conductors In the neighbourhood of aerial line, cables and rails in the tramway system of Torino (Italy) it was predicted and experimentally verified that the magnetic field assumes values of hundreds of µT These values are lower than limits indicated by ICNIRP but they could cause disturbances to electrical or electronic apparatus Field mitigation in the case of dc magnetic field is also quite difficult and an accurate analysis is necessary to avoid useless or harmful effects Traction Systems: generated magnetic field and its mitigation O Bottauscio, G Crotti, G Farina, A Manzin Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino (Italy) A Canova, M Tartaglia Dipartimento di Ingegneria Elettrica Industriale, Politecnico di Torino (Italy) The position of the vehicle varies along the cell Behaviour of the magnetic flux density versus coordinate y in an elementary cell (x=5m and z= 5m) Traction Systems (in Italy) DC traction systems: –Tramways (rated supply voltage 600 V) –Subways (rated supply voltage 750 V) –Railways (rated supply voltage 3.000 V) AC traction systems: –High speed systems (rated supply voltage 25 kV) Amplitude of the magnetic flux density along the elementary cell of length L at x=5m and z=5m far from perturbed zones Tramway network structure Amplitude of the magnetic flux density versus x for different distances X of the buried cable from the rail Scheme of a sub-plant Scheme of the network modification due to the vehicle motion An example of numerical simulation •Coloured maps give an immediate, impression of instantaneous magnetic field spatial distribution In the figures the field at m far from ground plane is represented when two trams are present in the considered sub-plant (/) Mechanical and electric characteristic of a vehicle supplied by a chopper Simplified Approach Elementary Cell CIRED 2003 - Special Report Session - Power Quality & EMC 29/31 Source shielding -> U-shaped shield in vicinity of the buried cable •A cross section of the coloured map is useful to individuate the most severe regions In the figure two overhead lines and two tracks are considered with a buried cable on the left part of the figure Shielding of a localized device The efficiency of the plane shield is negligible Time evolution of the magnetic flux density in a point near the plant Shielding of the entire area Typical time evolution of the magnetic flux density amplitude measured near a tramway network Shielding of the entire area: effect of a window Field mitigation analysis •shielding of the source -> buried cable •shielding of the “victim” -> area in vicinity of overhead line conductors Sheet materials •low carbon steel (LCS) -> mr=200; thickness=2mm •grain oriented (GO) FeSi -> mr=15000; thickness=0.6mm Position of conductors and shielded regions DISCUSSION (summary by J Hoeffelman) This Round Table has highlighted most of the possible mitigation techniques than can be used for reducing the magnetic fields produced by MV/LV substations, overhead lines, underground cables and other possible sources like traction systems The practical cases presented have shown CIRED 2003 - Special Report Session - Power Quality & EMC 30/31 that there is not an unique way to solve the problems Each case is particular and needs an individual approach The main questions and answers raised after the presentations by the authors are summarised here after: Q1: How is the field assessed for overhead lines with unbalanced currents and ground return? A1: They are calculated assuming a return path with a depth of about 600 to 900 m, depending of the ground conductivity Q2: Field produced by localised sources like transformers decrease very rapidly with the distance to the source (1/r law) What kind of mitigation technique can be applied when the “victim” is very close to the source? A2: Best solution is to apply shielding techniques However most time, with oil insulated power transformers, the source is not the stray field of the transformer but the current in the cables at the lowest voltage side In that case a better conductor arrangement can sometimes solve the problem Stray fields, however, are usually much more important with dry type transformers Q3: Is it possible to shield the E field produced by MV lines with an earth cable under the phase conductors? A3: This is indeed theoretically achievable, as any object acts as a good shielding (house, tree…) However this solution should not be required for MV lines because their E field remain always quite low Q4: How important are the losses in case of shielding power cables with good conductive materials? Isn’t there a risk of overheating the shield or the cable? A4: If the shield is held at least at 10 cm from the cables, the losses remain acceptable There is normally no risk of CIRED 2003 - Special Report Session - Power Quality & EMC overheating The shielding could even help for cooling the cable Q5: What about the junctions boxes of underground cables? A5: As the distances between phases is the highest at the junctions there is usually a need to shield them as well Q6: What about fields at harmonic frequencies? A6: The higher the frequency, the better the efficiency of the shielding; hence harmonics should not be a problem The question is somewhat different for conductor arrangements where the effect on harmonics should be analysed with care Q7: The limits of 0.2 or 0.5 µT to which the lines had to comply with, according to local Italian authorities, are they referring to the maximum, the rated or the mean field values ? A7: These questions haven’t received a clear answer yet A risk exists, in absence of good measurement protocols and good definitions of the limits that somebody performs a measurement at any place or anytime and compares the result with the given limit Q8: Split line techniques can be applied on overhead lines designed for a single circuit What about double circuit lines of which the second circuit is not yet built? A8: Splitting the first circuit on the line side dedicated to the second circuit can temporarily solve the field problem but the power capacity of the line is then seriously reduced Hence this technique is mainly intended to be applied for single circuit lines 31/31 ... GENERATED BY OH LINES CONTENTS ? ?Magnetic field generated by OH MV lines •3-wire 3-phase •4-wire 3-phase •2-wire 1-phase ? ?Magnetic field reduction methods •Selection of mitigation technique •Line compactness... section to be mitigated CIRED 2003 - Special Report Session - Power Quality & EMC Mitigation technique Global Reduction Installation performance level (%) cost over conventional Small 2 5-4 5 compactness... old installations, where the modification of the layout is not always practicable - Generalized mitigation around the substation - Mitigation in a specific area - Possible actions: - Optimal layout