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CIRED 2003 - Round Table on Magnetic Field Mitigation Techniques

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....

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  1. 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..............................................................................................................................1 P. Cruz Romero (Spain), Reduction of Magnetic Fields from Overhead Medium Voltage Lines......................................1 M. Chiampi (Italy), Some considerations about passive shielding.....................................................................................6 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).......................................................................................................................31 PRESENTATIONS P. Cruz Romero (Spain), Reduction of Magnetic Fields from Overhead J. Hoeffelman (Belgium), Introduction Medium Voltage Lines (ELIA, jean.hoeffelman@elia.be) (Universidad de Sevilla, plcruz@us.es) Round Table on Abstract Magnetic Field Mitigation Techniques In this contribution several topics concerning magnetic Chairman: Alain Robert fields and overhead medium voltage power lines are Coordinator: Jean Hoeffelman reviewed: simple formulation to assess the magnetic field 1. Reduction of magnetic fields from overhead MV lines (MF) level; characterization of magnetic fields generated by – P. Cruz Romero (ES) typical three-phase and one-phase primary distribution lines, 2. Some consideration about passive shielding with balanced and unbalanced current; and main mitigation – M. Chiampi (IT) techniques, analysed in relation with typical reduction level 3. Shielding of underground power cables obtained. Additional data concerning cost and performance – J. Hoeffelman (BE) of different solutions are also provided. 4. Field mitigation from secondary substations – E. Salinas (SE) Keywords: magnetic field mitigation, primary 5. Experiences in the mitigation of MV/LV substation magnetic distribution, compactness, tree wire, super-bundle, low- field emissions – O. Bottauscio (IT) reactance. 6. CESI-ENEL practical experience in reducing 50 Hz Magnetic Simplified magnetic field calculation fields – R. Conti (IT) The MF generated by a set of infinitely long, straight 7. Cases from Slovenian practices for reduction of 50 Hz EMF conductors can be formulated by a series decomposition of – B. Cestnik (SI) the Biot-Savart Law [1]. For points far from the line 8. Traction systems: generated magnetic field and its mitigation (several times the distance between conductors) only the – M. Tartaglia (IT) 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
  2. (1) where C : constant that depends on phase configuration (flat : C = √2 ; regular triangle : C = 1) d : clearance between adjacent phases µ 0 : 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 Fig. 1. Magnetic field profiles for different balanced 3-phase MV configurations (units in m) magnitude and phase in each circuit, the formula is the same, being I the total current of each phase. According to the conclusions previously obtained, the low- For low-reactance lines with equal current in magnitude and field configurations are the low reactance and the armless phase the resultant field lays 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 (2) height of calculation for this cases and the rest of simulations is 1 m above ground. where I is the RMS current in each circuit, and s the distance between both circuits. Other types of distribution systems, like unbalanced 4-wire 3-phase [4] and 2-wire 1-phase are also analysed. For the 4- For single-phase lines with metallic and ground return the wire 3-phase system several crossarm construction profiles approximated field is given respectively by 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 (3) 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 (4) observed. The negative effect of ground return current is explained observing eq. (4). An unbalanced system with From (1..4) relevant conclusions can be deduced: ground return can be decomposed into several current dipoles [5] (MF decay as 1/r2 ) and a homopolar component The MF generated by power lines is proportional to current (MF decay as 1/r) whose effect will be dominant at certain and distance between conductors distance from the line. In case of balanced single-circuit (current dipole) and super- bundle (SB) double-circuit three-phase lines, as well as one- Magnetic field reduction methods phase with metallic return MF decays as 1/r2 For the low-reactance (LR) configuration MF decays as 1/r3 In this section several methods to mitigate MF level from For the one-phase case with ground return MF decays as 1/r overhead MV lines [6,7] are reviewed. They can be classified as follows: Typical MF generated by overhead MV lines • Methods that try to reduce the load current of the line. If In figure 1 midspan magnetic field profiles generated by we can reduce the current, the MF will decrease typical primary distribution 3-wire, 3-phase configurations proportionately. Some possibilities are the following: with geometrical characteristics for 20 kV [2,3] are shown. - Increase the voltage level of the line It is assumed that the lowest conductor height at midspan is - Change one-phase lines to three-phase 6 m. • 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. CIRED 2003 - Special Report Session 2 - Power Quality & EMC 2/31
  3. - Increase the phase-ground clearance A last technique, passive loop, has also been considered. - Relocate the line The main drawback is that to obtain a reasonable reduction • Methods that try to compensate the power field with a of about 35 % it is needed to use for the loop a conductor of counteracting external field much lower resistance (about 0.12 Ω/km) than conventional - Passive loop. This technique consists on the installation for MV lines, with the additional cost that it implies. of a conductor loop near to the line, where a current is induced. This current creates a field that partially Table I. Main characteristics of different mitigation techniques cancels the original field. Global Effect of Mitigation Reduction Installation performance unbalanced In addition to these general methods, for net current lines technique level (%) cost over current conventional (e.g. multigrounded 4-wire 3-phase) it is needed simultaneously to take control of the ground return current Small 25-45 Low Lower Low levels. Therefore, specific actions must be done in this compactness sense: Crossarms → ∼ 60 Low/medium Lower Medium armless • Balance of phase currents by changing phase arrangement Tree wires ∼ 60 Medium Higher Medium of loads connected to a 3-phase line [8] or converting Spacer cable ∼ 80 High Higher High laterals single-phase to 3-phase lines ABC 100 Very high Higher High • Increase of neutral conductor size Underground ∼ 90 Very high Higher High line • Implementation of 5-wire system instead of 4-wire one [9] Circuit split 70-80 Medium Lower High Increase It is difficult to choose a particular method as the optimum. clearance to25-60 Low/medium Lower Low The selection of mitigation method is a case-by-case ground Compensation analysis, where different aspects must be considered: loop 35 Medium Lower Medium • New or already existent lines. If a low-field primary distribution line or set of lines must be projected, methods Conclusions that require a global system change could be feasible, like increase of voltage level, reduction of unbalance, etc. In this contribution major aspects related with • Local or whole system reduction. Some methods are characterization and mitigation of magnetic fields generated feasible for local application, but extremely costly for a by medium voltage overhead lines have been reviewed. The whole line or network. main mitigation techniques have been analysed, taking into • MF reduction level needed account mitigation effectiveness, installation cost, global • Cost of reduction method performance (reliability, aesthetic, maintenance, etc.) and • Other issues: safety and environmental aspects, sensibility to unbalanced current. If a reasonable MF maintenance, reliability, etc. mitigation is the unique objective to refurbish a section of an existing line the more feasible methods are the increase of The presentation is mainly devoted to analyse the more clearance to ground, and the low-to-medium compactness by feasible methods for local applications, although some of discrete reduction of phase-phase distance, replacement of them could be applied for global. crossarms by armless construction or reduction of swinging of string insulators. A summary of the methods is shown in table I, where the typical MF reduction levels at 10 m from the line are shown. References The highest mitigating methods are the ABC (Aerial Bundle [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. Cable), the underground line and the spacer cable [10]. Their 4, pp. 2082-2091, Oct. 1993. effectiveness is however conditioned by the absence of [2] W.F. Horton, S. Goldberg, Power Frequency Magnetic Fields and unbalance. Another main drawback of these methods is the Public Health. CRC Press, Boca Raton, 1995. cost. Their use is more feasible when other issues must be [3] POSTEMEL, S.L., Postes metálicos para líneas eléctricas de alta y baja tensión, Dic. 1990. satisfied (reduction of visual impact, reduction of outages). [4] H.L. Willis, Power Distribution Planning Reference Book. Marcel A method less costly could be the split of the line, but it is Dekker, New York, 1997. also strongly conditioned by the unbalanced current. Other [5] P. Pettersson, Simple Method for Characterization of Magnetic Fields from Balanced Three-phase Systems. Proceedings CIGRÉ Session, 1992, set of techniques (use of tree wires, armless construction and Paper 36-103. increase of ground clearance) are less mitigating-effective, [6] A.S. Farag, J. Bakhashwain, T.C. Chen, Y. Du, L. Hu, G. Zheng, D. but a significant reduction can be obtained, with the Penn, J. Thomson, Distribution Lines Electromagnetic Fields: Management advantage of a lower cost and an allowed higher unbalance and Design Guidelines. Proceedings CIGRÉ Session, 2000, Paper 36-105. [7] S. Rodick, P. Musser, Evaluation of Measures and Costs to Mitigate level, specially the increase to ground clearance. Eventually Magnetic Fields from Transmission and Distribution Lines. 37th IEEE Rural we can try to compact the line with no changes in the Electric Power Conference, Apr. 1993. conductor, like reducing the span length or replacing string [8] T. Chen, J. Cherng, Optimal Phase Arrangement of Distribution by post insulators. The reduction obtained is low, about 25- Transformers Connected to a Primary Feeder for System Unbalance Improvement and Loss Reduction Using Genetic Algorithm. IEEE 45 %, and the cost depends mainly of the original span Transactions on Power Systems, Vol.15, No. 3, Aug 2000, pp. 994 -1000. lengths of the line section to be mitigated. [9] D. J. Ward, J. F. Buch, T.M. Kulas, W.J. Ros, An Analysis of the Five- Wire Distribution System. IEEE Transactions on Power Delivery, Vol.18, No. 1, Jan 2003, pp. 295 -299. CIRED 2003 - Special Report Session 2 - Power Quality & EMC 3/31
  4. [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 CONTENTS 3-WIRE, 3-PHASE MV LINES •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 MAGNETIC FIELD GENERATED BY OH LINES 4-WIRE, 3-PHASE MV 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 MAGNETIC FIELD GENERATED BY OH LINES CIRED 2003 - Special Report Session 2 - Power Quality & EMC 4/31
  5. –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) : 6 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 •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 MAGNETIC FIELD REDUCTION METHODS Selection of mitigation technique •New or already existent project •MF exposition level allowed •Cost of reduction •Local or whole system reduction COMPACTNESS OF THE LINE (BALANCED) •Other issues –Safety and enviromental aspects Covered wires –Reliability –Insulation and electrical clearance requirements –Operation –Maintenance MAGNETIC FIELD REDUCTION METHODS Line compactness (balanced current) •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 COMPACTNESS OF THE LINE (BALANCED) •Posibilities of compactness CIRED 2003 - Special Report Session 2 - Power Quality & EMC 5/31
  6. Insulated wires USE OF PASSIVE CANCELLATION LOOP (BALANCED) •ABC (aerial bundle cable) •Dramatic MF decay with distance UNDERGROUND LINE (BALANCED) EFFECT OF UNBALANCED CURRENT •Reduction of mitigation effectiveness Ground return current = neutral wire 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 SUMMARY INCREASE CLEARANCE TO GROUND (BALANCED) •Increase poles height •Installation of new poles at midspan (long spans) •Reduction effectiveness close to the line M. Chiampi (Italy), Some considerations about passive shielding USE OF PASSIVE CANCELLATION LOOP (Politecnico di Torino, chiampi@polel1.polito.it) •More effective in flat configurations (horizontal, vertical) Some considerations about passive shielding •Increased reduction with compensation of inductance (capacitor) •Resistance of the loop conductor: much lower than typical MV O. Bottauscio (*), M. Chiampi (*), G. Crotti (°), conductor A. Manzin (°), M. Zucca (°) •Need to reinforce the poles (°) 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 CIRED 2003 - Special Report Session 2 - Power Quality & EMC 6/31
  7. experiments on a specific test apparatus and numerical computations using a 2D hybrid FEM/BEM model. 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 • Electrical resistivity: 13.9×10-8 ×Ωm Efficiency of magnetic plane shields • Non-Oriented Si – Fe 1.5% wt • Lamination thickness: 0.50 mm • Electrical resistivity: 27.9×10-8 ×Ωm • 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 Experimental set-up for tests 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 Magnetic flux in plane shields r.m.s. values of magnetic flux in the plane shield computated by a 2D hybrid FEM/BEM model Shielding configurations Efficiency of magnetic U-shaped shields CIRED 2003 - Special Report Session 2 - Power Quality & EMC 7/31
  8. Cired 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. 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 Air-gaps in the sheet corners 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. Effects of air-gaps in sheet corners Figure 1 shows, for both arrangements, and for three U-shaped screen FEM/BEM model different measurement positions above ground the decrease of the field with the distance to the axis of the cable layout. 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 1 kA. In the horizontal arrangement the distance between phases is 25 cm. trifoil arrangement 120-10 cm 100.00 10.00 h=0m µT 1.00 h = 1.5 m h=3m Efficiency of combined shields 0.10 0.01 0 5 10 15 20 25 30 distance to axis (m) horizontal arrangement :120-25 cm 100.00 10.00 h=0m µT 1.00 h = 1.5 m h=3m 0.10 0.01 J. Hoeffelman (Belgium), Shielding of 0 5 10 15 20 distance to axis (m) 25 30 underground power cables, From theory to practical implementation Figure 1: Comparison between trefoil arrangement (ELIA, jean.hoeffelman@elia.be) and horizontal arrangement Summary Metallic shielding This contribution is aimed at presenting the most recent When a trefoil arrangement cannot be applied or if a further achievements in shielding techniques for underground field reduction is required, a metallic shielding can reduce power cables. It is based on the work performed by Cigré- the field at the source. CIRED 2003 - Special Report Session 2 - Power Quality & EMC 8/31
  9. 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 2 b), sometimes called magnetostatic shielding, the field lines are absorbed by the low reluctance material, whereas in the second case (figure 2 c) they are repelled thanks to the eddy currents induced in the material. Figure 2: Shielding mechanisms: ferromagnetic material versus conductive material Shielding by ferromagnetic materials. Although theoretically more efficient at low frequency than conductive materials, ferromagnetic materials seem, in most Figure 3: Shielding by ferromagnetic material cases, to be less advantageous. The reasons are the following : Shielding by conductive materials. The effectiveness of conductive shields is more homogeneous in the space. Ferromagnetic materials are As far as conductive materials are concerned, two materials mainly effective nearby the shield, while conductive can be considered: copper and aluminium. materials are also effective at distance. Both materials have their own advantages and drawbacks: Good ferromagnetic materials like permalloy (“Mumetal”) Copper has a higher conductivity but also a higher cost than or transformer laminates are often expensive and highly aluminium. Although copper is easier to weld than sensitive to corrosion. Therefore they need a good protection aluminium, modern welding techniques under argon coating. atmosphere allow assembling aluminium plates on the yard. Ferromagnetic materials are more efficient when the Therefore, in the following sections, only shielding by magnetic circuit they offer to the flux lines is closed (no or aluminium plates will be considered. few gap). This particular layout is not often practically On the other hand, if some precautions are taken concerning achievable unless for shielding short cable lengths. the neutrality of the soil, corrosion problems should not arise neither with copper nor with aluminium. The possible Hence, the main example where a ferromagnetic shielding influence of stray currents needs however to be addressed. 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 Three main layouts will be taken into consideration: the flat shown in figure 3 taken from [ 0]. horizontal shield or plane shield, the U-shaped shield and the H-shaped shield. However, such a tubular shielding has also drawbacks: The maintenance or the repair of the cables is difficult. The Plane shield thermal behaviour of the cables is neither easy to manage, as the tube needs normally to be filled with concrete. A relatively simple way to mitigate the field produced by a 2 or 3 phases cable system is to install as close as possible On the other hand, the installation of a single tube allows a above the cable an horizontal plate. fast recovering of the trench, the cables being pulled-in by a Shield thickness single and fast operation afterwards. 2 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 9 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. CIRED 2003 - Special Report Session 2 - Power Quality & EMC 9/31
  10. To avoid this it is necessary to use a plate with a sufficient U-shaped shield width. Practically it is recommended that the ratio of the shield It has been very often written that a U-shape shield exhibits width to its distance to the conductors and to the distance better performances than a flat shield. In fact, as shown in between conductors remains larger than 4. For a maximum [ 0], for the same shielding area, it has not really a better effectiveness the plates need to be as close as possible to the effectiveness than a horizontal plane shield but it doesn’t cables but, if they are too close, the losses due to the induced necessitate to groove such a large trench as that required for eddy currents can become to high. Power capacity, however, an horizontal shield of the same total width before bending. is practically not influenced if the distance between cable One problem, however with U-shaped shields is that, sheets and shielding is not smaller than 5 to 7 cm [ 0]. contrary to what happens with plane shields, there is an Shield continuity absolute need to get a good contact between the vertical For manufacturing reasons, the shield is normally divided parts of each shielding element (assuming, of course, a non into smaller elements placed near each other with or without continuous shield). 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 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 Figure 5: Shielding by U-shaped conductive plates continuous shield with the same global thickness. It is important to note, here, that the quality of the electrical Nevertheless with a layout based on 2 mm aluminium plates contact between layers doesn’t play any part in the shielding of 100 cm (length) x 200 cm (width) bended to achieve effectiveness. vertical parts of 40 cm and bolded together in the Performances longitudinal direction, a shielding factor of 4 can be Figure 4 shows the comparison between calculation (2 D achieved up to 1.5 m above the shield. FEM-BEM model) and measurements for an aluminium The shielding effectiveness is also less dependent to the plate installed at 27 cm1 above the axis of a three phases height of measurement than with plane shields. system in flat configuration (distance between phases: 25 cm). The agreement is quite good although the calculation Another problem with U-shaped shields is the difficulty of refers to a 6 mm continue shield (99.5 % aluminium), installation. whereas the measurements are made on a double layer 3 mm For cooling reasons, power cables need to be embedded in a discontinue shield. controlled soil (dolomie…) that needs to be tamped. The presence of a U-shield layout makes the operation very Horizontal shielding - Comparison between calculation and difficult. measurement at 1 m above cables axis (73 cm above shield) For that reason, instead of using bended plates, it is easier to use an equivalent layout made of three plates: two vertical 16 and one horizontal. This layout is known as the H-layout. 14 12 H-layout shielding factor 10 3 mm 8 (calc) 6 mm 6 (calc) 6 mm In the H layout, two vertical plates are installed in the trench 4 (mes) before to fill it with a first layer of controlled soil. After the 2 cable laying and the second layer of controlled soil, the 0 horizontal plates are installed forming with the vertical plate 50 100 150 200 shielding width (cm) a H. Shield continuity Figure 4: Shielding by horizontal aluminium plates Contrary to what happens with the plane shield, and likewise the U-shaped shield2, a good continuity needs to be ensured between vertical plates. 2 This longitudinal continuity seems however to be less important for U- 1 This corresponds to the typical thickness of the dolomite layer above 2000 shaped shields because, in each individual element, thanks to the continuity mm2 alu power cables with the horizontal plate, the circuit is closed. CIRED 2003 - Special Report Session 2 - Power Quality & EMC 10/31
  11. Therefore, the electrical circuit formed by the vertical plates H-shaped shielding 100 cm width, 3 mm Al plates of 80 cm X 200 cm, needs to be closed at each extremity of the shielded area. Cables axis at 24 cm from trench bottom, 25 cm between conductors Horizontal plates at 16 cm above cables axis It has also been shown experimentally, at least for the flat Measurement distances from trench bottom cable configuration, that the longitudinal continuity of the 16 horizontal plates is not very important. 14 Shield thickness 12 Calculations show that increasing the shield thickness above shielding factor 10 150 cm 3 mm does not bring a important improvement in the 8 250 cm shielding effectiveness. On the other hand, for mechanical 6 300 cm and corrosion withstand reasons, it is not safe to use too thin 4 shields. Hence, the value of 3 mm seems to be a master 2 choice for this type of aluminium shielding. 0 -400 -300 -200 -100 0 100 200 300 400 Shielding effectiveness horizontal distance to cable central conductor (cm) Shielding factors up to 10 at 1.5 m above ground have been calculated with the same 2D model as for the plane shield. Figure 6 : Efficiency of a H shaped aluminium shielding However the continuity problems between elements being very important, there is real a necessity to make recourse to Actual implementation a 3D model for taking the discontinuities into consideration. This 3D model is still under development. The H layout described above has been implemented in Belgium on a new 30 km double circuit 150 kV link Laboratory results between the nuclear power plant of Tihange and the HV The measurements results presented on figure 6 have been substation of Avernas. achieved with the same cable layout as for the plane shield From the 30 km underground link, 6.5 km are shielded. (fig 4), i.e. a three phase flat configuration with 25 cm The link will be put into service at the end of this year; distance between phases laid 24 cm above the bottom of a hence measurement in situ has not yet been performed. trench of 150 cm depth. However, as an assembling technique by welding instead of The shield is built with 200 X 80 cm aluminium plates of 3 bolting has been used, better results than those extrapolated mm thickness. on basis of the laboratory tests (figure 7) are expected. Vertical plates are installed at a distance of 100 cm from each other, whereas the horizontal plates of 80 cm width are The per km cost increase of the link due this shielding is installed 40 cm above the bottom of the trench. estimated to be about 20 %. The vertical plates have an overlap of 8 cm and are fixed This estimation, however, doesn’t take into account the together with four M8 bolts. additional exploitation costs involved by the losses in the At both extremities of the shielded area a U-shaped shield. aluminium cover of the same thickness and width as the other plates ensures the necessary electrical link between Field produced by a double circuit 150 kV cables in flat configuration lateral plates (vertical right and left plate). spaced 25 cm - distance between axis of circuits: 2 m, depth: 1.25 m H-shaped aluminium shielding - Field at 1.5 m above ground for I = 1300 A On this figure, the important decrease of the shielding 25 effectiveness with lateral distance is partly due to the fact that the experimental model, being only 8 m length, gives 20 rise to important border effects at distance from the axis of 15 the cables. without shielding B (µT) with shielding The asymmetry in the curves is due to elliptical polarization 10 of the magnetic field and depends on the rotation order of 5 the three phases. 0 -30 -20 -10 0 10 20 30 Lateral distance (m ) Figure 7: Expected field in the Tihange-Avernas link References [ 0] Transmission Cable Magnetic Field Management Power Technologies Inc. EPRI TR-102003 – Project 7898-37 – Final Report June 1993 [ 0] On low frequency shielding of electromagnetic fields R.G. Olsen 10th International Symposium on High Voltage Engineering – Montréal – 1997 [ 0] Geometrical Aspects of Magnetic Shielding at Extremely Low Frequencies CIRED 2003 - Special Report Session 2 - Power Quality & EMC 11/31
  12. L. Hasselgren, J. Luomi IEEE Trans on EMC vol 37, No 3, August 1995 [ 0] Implementation of shielding principles for magnetic field management of power cables A.S. Farag et alii Electric Power System Research 48 (1999) pp 193-209 - Elsevier [ 0] Shielding Techniques to Reduce Magnetic Fields Associated with Underground Power Cables G. Bucea, H. Kent CIGRE Session 1998, paper 21-201 [ 0] Role of magnetic materials in power frequency shielding: numerical analysis and experiments O. Bottaauscio, M. Chiampi, D. Chiarabaglio, F. Fioillo, L. Rocchino, M. Zucca IEE Proc. Gener. Transm. Distr., Vol 148, No 2, March 2001 [ 0] Evaluation of different Analytical and Semi-Analytical Methods for the Shielding by ferromagnetic materials Design of ELF Magnetic Field Shields A. Canova, A. Manzin, M. Tartaglia Mainly effective neer the shield IEEE Trans. On Industry Applications, vol 38, no 3 May/June 2002 [ 0] A numerical Approach to the Design of Conducting Shields for ELF Expensive (Si steel, permalloy) Magnetic Field Reduction Corrosion protection O. Bottauscio, D. Chiarabaglio, M. Chiampi, M. Repetto No gap in magnetic circuit ETEP vol 12 No 2, March/April 2002 Steel tube : very efficient [ 0] Campi ellettrici e magnetici: possibilità offerte dagli elettrodotti in cavo A. Bolza, F. Donazzi, P. Maioli – Pirelli Caci e Sistemi 2000 Maintenance, repair Comments given at CIGRE 2002 Paris: Group 21, PS1, Q 4 Thermal behaviour of cable [ 0] Techniques for shielding underground power lines to minimize the Fast recovering of trench exposure to ELF magnetic field in residential areas A. Cipollone, A. Fabbri, E. Zendri EMC Europe – Sorrento – Sept 9-13, 2002 Shielding of underground power cables From theory to practical implementation Jean Hoeffelman ELIA General field mitigation techniques General field mitigation techniques Shielding by conductive materials Copper ­ Aluminium Copper more expensive but higher effectiveness Plates of about 2 to 4 mm Corrosion OK but AC corrosion ? Metallic shielding Continuity: not always needed Welding of aluminium OK Shapes : Plane U­shape H­shape Plane shield Copper ­ Aluminium Copper more expensive but higher effectiveness Plates of about 2 to 4 mm CIRED 2003 - Special Report Session 2 - Power Quality & EMC 12/31
  13. Corrosion OK but AC corrosion ? Continuity: not always needed Welding of aluminium OK Importance of shield width Plane shield performances H­shaped shield (2) U­shaped shield (1) Field attenuation more homogenous than with flat shield Practical implementation Difficulty of installation Shield continuity 2  sDouble 150 kV link 2000 mm alu sLength: 30 km s6.5 km shielded sPU cost increase: 20 %  U­shaped shield (2) Currents induced in the shield (a): real part (b): imaginary part H­shaped shield (1) CIRED 2003 - Special Report Session 2 - Power Quality & EMC 13/31
  14. effects of these fields. On the other hand PFMFs have the ability to interfere with electron-beam devices, which contributes to the awareness as regards EMC regulations. These two issues have induced efforts to study ways to mitigate these fields. Secondary substations represent the final stages of the electrical delivery system before reaching the customers. In Sweden and in other European countries it is not unusual, especially in neighbourhoods with a dense population, to locate secondary substations inside buildings (e.g. in cellars). Secondary substations are three-phase systems composed mainly of: transformers (10 kV/0.4 kV), cables, and switchboards containing busbars. As the voltage decreases the current and consequently the magnetic field increases. Unlike power lines or underground cables, the magnetic field originating from a secondary substation is rather complex as it corresponds to an intricate superposition of fields from various sources. Thus, in order to find out a cost- effective way to mitigate this type of field, a variety of techniques have to be explored. E. Salinas (GB/SE), Field Mitigation from Secondary Substations (South Bank University, London, salinae@sbu.ac.uk and Chalmers University of Technology, Gothenburg, ener.salinas@elteknik.chalmers.se) Fig. 1.1 Some typical magnetic field values (in microtesla) from a secondary substation situated in a cellar of a building. A usual configuration of major sources and distances is also shown. Summary 2. Strategy, Techniques and Methods This work aims to design cost-effective schemes for the mitigation of magnetic fields from secondary substations According to the Webster’s Encyclopedic Dictionary, and integrated solutions for larger systems. The most strategy is “the art of devising or employing plans or frequently adopted strategy consists of dealing with the stratagems to achieve a goal”. In the case of magnetic field mitigation operation at the origin of the field emission. The mitigation the goal is to search for cost-effective solutions. techniques applied are shielding, active compensation, and Independently of the technique to be used, there are two phase cancellation. Analytical, numerical (finite-element- possible strategies to follow. We can choose to apply the based), and experimental methods are used and improved for mitigation operation at the victim or to apply it at the the analysis of the electromagnetic fields. Finally, some sources. In Fig. 2.1 the field from various sources is examples of field mitigation in Sweden are presented. affecting a region (victim area) located at some distance above the sources. From the point of view of quantity of 1. Introduction material used to perform the mitigation, applying the mitigation operation (shielding, passive compensation or Epidemiological studies of people living nearby sources of active compensation) to the sources is often more cost- power frequency (50-60 Hz) magnetic fields (PFMFs) have effective than applying a similar operation at the victim area. opened a yet unresolved debate over suspected adverse CIRED 2003 - Special Report Session 2 - Power Quality & EMC 14/31
  15. As the field propagates along a distance it becomes weaker. Accordingly the area to deal with expands with a quadratic dependence on that distance. Fig. 2.1 Mitigating the field at the source is in most cases more cost-effective than mitigate at the victim. To investigate the field at an affected region it is customary to measure the field on a certain plane (usually the floor level) in that region and plot the results in a contour plot. It Fig. 3.1 Some of the complexities inherent to the use of finite/open shields. is then natural to ask: is it possible to discriminate in advance the magnitude of the contribution of each source by scanning the field values in the affected area? The answer is The field from busbars can be shielded by means of thin yes. There are two reasons for this: (1) analysis of the field conductive plates located in front of the busbars. The gradient on the scanned surface could suggest the possible induced eddy currents reduce the magnetic field in the far source behavior; (2) analysis of the variation of the field zone. Acceptable shielding efficiency is possible even when values with the distance perpendicular to the scanned the conducting shield is thinner than a skin depth. In cases surface could provide a 3-dimensional picture of the field when the shield does not form a closed surface, conducting decay, and subsequent source identification. Moreover, after shields are generally more efficient than ferromagnetic a simple inspection of the substation itself the problem can shields. This is in apparent contradiction with the concept of be resolved, and a mitigation technique can be suggested. skin effect, which gives for iron (σ = 1.07x107⋅ Sm-1, and ∝r = 250) a skin depth In secondary substations the techniques chosen to mitigate δ = (π f ∝0 ∝r σ)-1/2 ~ 0.14cm, the magnetic field depend upon the characteristics of the almost an order of magnitude less than for aluminium – sources. Busbars, transformers and cables are the sources consequently better efficiency would be expected. In fact the most often encountered in a substation and possible latter is correct right at the other side of the ferromagnetic techniques to apply are: phase cancellation, conductive and screen, where strong reduction is observed. However, at the ferromagnetic shielding, passive and active compensation. victim zone (about 4 m over the system) both properties of The methods used to carry out the mitigation techniques are iron, conductivity and magnetic permeability, manifest their analytical computations (e.g. using symbolic manipulation antagonism. As the first tries to cancel the field, the other codes), numerical modeling, e.g. finite element methods tries to attract the field lines. In fact, some of the field lines (FEM), and experimental setups. are pulled into the region where we want to mitigate the field. Using double-layer shielding, it was observed that the 3. Busbars order of the arrangement ironaluminum- coil provides a more effective shielding than the permuted case: aluminum- Busbars are the most efficient way to transport large iron- coil (Fig. 3.2). amounts of electrical energy within a reduced space such as a secondary substation. They are usually made of copper or aluminium covered by copper and should be able to withstand current values having several hundreds of amperes. Depending on their specific design they can have different lengths, geometrical arrangements, and cross sections. A typical system of busbars, where a shielding technique is being applied, is shown in Fig. 3.1. The complexity of the problem when the arrangement is modelled by numerical methods, such as finite elements (FEM), is evident when finite/open shields are utilized. Fig. 3.2 Double shield arrangement in front of a source, the field resultant of the permutation Iron-Aluminium is shown. Eddy currents circulating in a conductive plate due to the field of busbars give a suggestion about field mitigation by CIRED 2003 - Special Report Session 2 - Power Quality & EMC 15/31
  16. trying to imitate the behaviour of these currents. This can be achieved by passive compensation e.g. adding two rectangular short-circuited loops in front of the busbars. This was attempted with a system of square-shaped coils (N = 60 turns) but there was almost no attenuation, presumably the currents were two small to induce any field-counteracting effect. When the attempt was made with only N = 1, and thicker conductor, there was some non-negligible effect, although this occurred when the loops were placed very Fig. 4.2 Reduction of the magnetic field from a 3-phase transformer by phase rearrangement at the close (less than 1 cm) to the busbars. secondary side is a cost-effective mitigation method. Active compensation was also tested for a system of 2m- 5. Cables long busbars using an experimental arrangement. Two rectangular coils were placed in front of the busbars. Each One of the most frequent sources of high magnetic emission coil is 2m long and has a separation between phases of 0.25 (not only restricted to substations) is the non-cancellation of m. the feeding current is 1.67 A. The shielding efficiency the field from individual phases in cable bundling. This was measured at y = 1.5 m (parallel to the plane of the occurs due to poor phase arrangement in the cables. Phase busbars) as the distance between the coils and busbars varied re-arrangements in cables are often not a complicated task. between d = 10 and d = 20 cm. The result was a variation The phases are ordered in order to obtain an optimal between SE = 8.6 dB and SE = 15.4 dB. grouping with a minimum field emission. Fig. 5.1 shows an example of this procedure. The magnetic field of different 4. Transformers arrangements of three-phase conductors (carrying 100A) is plotted and compared. The optimal bundling (arrangement Measurement of the field around transformers usually yields 4) produced a good field reduction in comparison to the high values, especially if they are not encapsulated. Often arrangement 1. The guidelines to obtain a good phase these values do not decay with the distance as expected. arrangement are: (a) form bundles with mixed of phases, (b) Since the leaking field is due to core or coils, they should minimize distances between phases, and (c) twist the cable decay as ~ r–3. Therefore other must be the cause generating array to diminish longitudinal contributions to the field. these fields. A substation was study, where measured field Extra considerations have to be taken in order to avoid values were as high as 22 microtesla, right above the hindrances such as heating and mechanical stresses due to transformer on the next floor. The transformer was located eddy currents and induced forces. about 2 m below the measuring points; consequently the values were too high to be inherently originated from this source alone. An inspection of the interior of the substation made clear that the field was due to ill connections and poor field self-cancellation (Fig. 4.1-a). The solution adopted was to keep the phases mixed up to the point of connections with the transformer (Fig. 4.1-b). This solution provides a fairly good mitigation (17-18 dB), however it still leaves a residual field of around 3 microtesla. Additional mitigation is possible and various actions can be recommended, e.g. to change the paths of the connections to the ground level. Shielding the transformer with an aluminium metal box, which takes care of the remaining fields, can also be a complementary measure. Fig. 4.1 Mitigation of the field from transformers by rearrangement of connections at the secondary side (a) before: equal phases, (b) after: mixed phases. Fig. 5.1 Different phase arrangement in cables; the field contours are plotted in the interval [0.1-1] µT. CIRED 2003 - Special Report Session 2 - Power Quality & EMC 16/31
  17. 6. 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 (a) 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. (b) (c) Fig. 6.1 The magnetic field one floor above the library and its reduction at different stages. 7. 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 CIRED 2003 - Special Report Session 2 - Power Quality & EMC 17/31
  18. techniques and design methods in order to achieve field •Active compensation mitigation. The results of this investigation have been •Passive compensation applied to the renovation and building of secondary •Phase cancelation substations: Methods •Analytical computations Table 7.1 •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 2 - Power Quality & EMC 18/31
  19. 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 2 - Power Quality & EMC 19/31
  20. we intend to obtain: - Generalized mitigation around the substation - Mitigation in a specific area - Possible actions: O. Bottauscio (Italy), Experiences in the - Optimal layout of the substation (mainly for new installations) mitigation of MV/LV substation - Design of passive shielding (also for old installations) magnetic field emissions Model for magnetic field evaluation and reduction (IEN Galileo Ferraris, botta@ien.it) • The development of specific numerical tools for field modelling Experiences in the mitigation of must take into account the features of the problem: MV/LV substation magnetic field emissions -The domain under study is not limited -The behaviour of magnetic materials is not nonlinear O. Bottauscio (*), M. Chiampi (*), G. Crotti (°), -Complex electromagnetic phenomena arise inside shielding A. Manzin (°), M. Zucca (°) elements -Shielding elements are thin structures (°) Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino, Italy - The use of standard Finite Element formulations results to be (*) Dipartimento di Ingegneria Elettrica Industriale - Politecnico di inefficient Torino, Italy • A problem sometimes arises in presence of unknown sources -A preliminary evaluation of their contribution is required Introduction (inverse problem) - The presentation will be addressed to present the experiences Mathematical model: 3D hybrid FEM-BEM approach gained by the Turin group in the mitigation techniques for the reduction of the magnetic field produced by MV/LV substations. - Shielding elements are thin structures - The attention will be focused on the design of passive shielding to - The use of standard Finite Element formulations results to be be adopted in the case of old installations, where the modification inefficient, due to the aspect ratio of the problem of the layout is not always practicable. - 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 MV/LV substation for civil supply - Spatial nonuniformity due to the contribution of different sources 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: 3 m x 4 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 Validation of the mathematical model - The mitigation strategy is significantly dependent on the final goal CIRED 2003 - Special Report Session 2 - Power Quality & EMC 20/31