The present work aims to propose a methodology for evaluating the performance of low-frequency magnetic shields used for the shielding of complex sources such as MV/LV substations. Today there is no standard procedure for testing shields based on the use of a single-coil for generating artificial MF (MF: Magnetic Field) as is the case for high-frequency anechoic chambers. In the case of shielding for industrial frequency sources, these are generally open shields with sources of various types: 3D transformers, switch-gear, and power lines. These kinds of sources and shields are better checked if the artificial MF has all three components in space. The work proposes a methodology based on the use of a tri-axial source, capable of generating a rotating magnetic field. The supports of the coils are made using a 3D printing process. The paper presents the design steps, the analytical evaluation of the performance of the source and as an application a test of a magnetic shielding plate was presented

A. Canova

L. Giaccone

M. Quercio

English

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

08 November 2022POLITECNICO DI TORINORepository ISTITUZIONALEA proposal for performance evaluation of low frequency shielding efficiency / Canova, A.; Giaccone, L.; Quercio, M.. -ELETTRONICO. - (2021), pp. 935-939. ((Intervento presentato al convegno CIRED2021-The 26th InternationalConference & Exhibition on Electricity Distribution tenutosi a Online nel 20-23 September 2021 [10.1049/icp.2021.1634].OriginalA proposal for performance evaluation of low frequency shielding efficiencyIEEE postprint/Author's Accepted ManuscriptPublisher:PublishedDOI:10.1049/icp.2021.1634Terms of use:openAccessPublisher copyright©2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in anycurrent or future media, including reprinting/republishing this material for advertising or promotional purposes, creatingnew collecting works, for resale or lists, or reuse of any copyrighted component of this work in other works.(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2953372 since: 2022-01-26T11:35:25ZIEEECIRED 2021 Conference Geneva, 20– 23 September 2021Paper XXXXA proposal for performance evaluation of low frequencyshielding efficiencyAldo Canova1* , Luca Giaccone1, Michele Quercio11Energy Department "G.Ferraris",Politecnico di Torino, Torino, Italy* aldo.canova@polito.itKeywords: Magnetic Shielding,Low Frequency,3D Magnetic SourceAbstractThe present work aims to propose a methodology for evaluating the performance of low-frequency magnetic shields used forthe shielding of complex sources such as MV/LV substations. Today there is no standard procedure for testing shields based onthe use of a single-coil for generating artificial MF(MF: Magnetic Field) as is the case for high-frequency anechoic chambers.In the case of shielding for industrial frequency sources, these are generally open shields with sources of various types: 3Dtransformers, switch-gear, and power lines. These kinds of sources and shields are better checked if the artificial MF has all threecomponents in space. The work proposes a methodology based on the use of a tri-axial source, capable of generating a rotatingmagnetic field. The supports of the coils are made using a 3D printing process. The paper presents the design steps, the analyticalperformance evaluation of the source and the application to the test of a magnetic shield plate.1 IntroductionThe issue of evaluating the performance of shielding for elec-tromagnetic fields at industrial frequency is currently an openproblem. While there are guidelines for the characterizationof shielding materials[1], nothing is defined for final shield-ing. A definition of some basic principles that guide a testerof a shielding system in his activity is therefore important andof great practical utility. In high frequency, some standardsdefine how to measure the performance of a shielding room.The methodologies proposed by these standards are based onthe use of two loops antennas, one source and one receiver,arranged in correspondence with two opposite faces of theshield [2,3]. The measurement can therefore be carried out atdifferent points to verify the shielding efficiency of the entireshield. The validity of this procedure is based on the fact thatthe shield is of the closed type and the area to be protected isinside the shield. The situation is very different in the case oflow-frequency magnetic fields. In this case, the shield is oftenplaced close to the source and is very often of the open type.These two aspects lead us to think that performance evaluationmust take into account aspects such as:1. type of source: extension and position of the sources withrespect to the shield.2. geometry of the area to be protected: extension and positionof the field points with respect to the source and the shield.Correct testing can therefore only be carried out when thesource has been completely installed and is sufficiently loaded(eg 25-30% of the rated load). On the contrary, to obviate thewait for the source to be put into operation, we often see testsin which, with the excuse of imitating what is proposed forradio frequencies, a field is generated with a small coil placedat a few tens of centimeters behind the screen and the magneticfield is measured with a probe placed a few centimeters on theother side of the screen. This type of test does not verify theshielding but gives us back the performance of the material asif the shield had infinite dimensions to the source. Therefore,if it is not possible to test the shielding by directly supplyingthe real source (eg panels, lines, and transformer of an MV/ LV substation[4,5,6]) it would be advisable to simulate thesources through magnetic field generators placed at a sufficientdistance from the screen, for example in the positions wherethe main components of the real source are located. In addi-tion to this, a further approximation concerns the directionalityof the magnetic field. In the case of a single-coil, the directionof the magnetic field is fixed in space and the module varieswith a sinusoidal trend. In reality, magnetic fields have a three-dimensionality that changes over time, i.e. at a certain point, themagnetic field vector rotates in space. The three-dimensionalityof the magnetic field must also be considered as the materialsof the screens, depending on whether they are conductive orferromagnetic, have different behavior depending on the angleof incidence of the magnetic field with the shielding surface. Tosimulate this phenomenon, it is, therefore, necessary to createa source capable of generating a rotating magnetic field. In thepresent work, we want to present a test methodology based onan artificial source capable of generating a rotating magneticfield. As an example, Fig.1 shows the artificial source madeby a 3D printing process. The first comparisons between val-ues calculated and measured on a single axis show an excellentresponse. The results obtained by feeding the source with threeout of phase currents capable of generating a rotating field willbe presented and the test methodology of a real shield will bepresented.1CIRED 2021 Conference Geneva, 20– 23 September 2021Paper XXXXFig. 1. 3D printing model of the artificial source2 System descriptionThe triaxial coil support system was designed in 3D throughthe use of Solidworks software and subsequently realized witha 3D printer. The sizing of the apparatus was carried out takinginto consideration the magnetic field values to be generated.The procedure followed is illustrated below.2.1 Numerical model of the magnetic fieldThe magnetic field generator is simulated by arranging thethree coils in 3D space. Each coil was approximated withsegments, which represent filiform conductors. The coils arewound along the perimeter of the support and are consideredpositioned side by side to the other in the available space. Fig-ure 2 show this concept and the final result.Fig. 2. Three axial coil simulationThe simulation of the magnetic field generated by the threecoils was carried out using MATLAB software. The magneticfield was calculated for a single instant of time in a definedregion of space and a series of field points. The waveformsof the three components of the magnetic field over time wereanalyzed, on a single field point. To identify the points ofthe magnetic field, a region has been defined centered on theorigin of the axes ( figure 3) and considering the center ofthe structure. The origin of the system is placed at the cen-ter of the three coils The magnetic induction B was calculatedFig. 3 Example of the magnetic induction trend calculated bypowering the xy coil with unitary current.through the superposition principle of the analytical closedform expression of the magnetic flux density produced by astraight segment(1)[6]. If P1, P2, are respectively the startpoint, the end point of the segment and PQ is the field point,the formulation is:B =∫P2P1µ0 × i4π× dl× rr3(1)The function allows to calculate the magnetic field, gener-ated by filiform conductors crossed by current i , in a series ofpoints at a distance R. For evaluation in a single instant of time,the field is calculated considering the contributions of each coilindividually. The total field is obtained through superpositiontheorem.2.2 Coil sizingThe magnetic field strength is proportional to the number ofturns of each coil and so, to maximize the field, it needs tomaximize the product N × Imax. The choice of the number ofturns and the maximum current is based on the considerationof some power constraints. The supply system is based on asignal generator that controls a power amplifier. This allows togenerate an arbitrary waveform or to control the frequency of asinusoidal current in a range from some hertz to some kilohertz.Each coil is a load of each channel of the power amplifier andto exploit the maximum power the coil impedance should beadapted to a value of 2 or 4 ohm. For this reason, is importantto preliminary evaluate the inductance of each coil. The eval-uation of the impedance Z has been calculated for each of thethree rectangular rings because they have different dimensions.2CIRED 2021 Conference Geneva, 20– 23 September 2021Paper XXXXThen the inductance L was calculated for each loop, using theformula:Ls =µ0π× [−2× (W +H) + 2×√(H2 +W 2)−H ×lnH +√(H2 +W 2)W−W × ln(W +√(H2 +W 2)H) +H × ln2×Hd2+W × ln2×Wd2Where:• W is the longitudinal dimension of the coil.• H is the transversal dimension of the coil.• µ0 is the permeability magnetic vacuum.• d is the cable diameter(or equivalent one in the case ofrectangular cross section)3 Experimental activity3.1 MeasurementsIn a first test analysis three groups of measures have been cre-ated, alternately feeding coil 1(white), coil 2 (red) and finallyboth simultaneously.• Coil 1The measurements were made in three different points of thestructure (figure 4). The acquisitions have been made with theoscilloscope on an electrical period. Coil 1 was powered witha current equal to:I =√2× 0.5× sin(ωt) f=45 HzFig. 4. Some typical points of measurement• Coil 2The procedure is the same as the previous one. The frequencyof 45 Hz was chosen in order to avoid the disturbances at 50Hz. The value of the current in the coil 2 was:I =√2× 0.9× sin(ωt) f=45 HzIn this case, the expected main induction direction is alongthe x-axis. Considering however the transformation into globalcoordinates, the measurement results are more affected by acoordinate error. In particular, the measure doesn’t show a rel-evant component in the z-axis. The presence of the componentin the z-axis is justifiable by the curvature that the field linesundergo to that distance from the structure since they tend toclose.• Coil 1 and Coil 2The measurements were carried out at the center of the struc-ture and externally, in order to evaluate the different intensityof the generated field.I1 =√2× 0.5× sin(ωt) I2 =√2× 0.9× sin(ωt) f=45 HzTest 8 was carried out in the center of the structure with twoequal-sized currents, offset by 60◦I1 =√2× 0.9× sin(ωt) I2 =√2× 0.9× sin(ωt) f=45 Hz3.2 Numerical and experimental comparisonA comparison between the experimental measurements andthe results of Matlab simulations had been made consider-ing the RMS value of the waveforms resulting from magneticinduction. In Fig. 5 are indicated the values of the three mag-netic induction components, both for the experimental mea-surements and for Matlab simulations. The overall RMS wasobtained with the vector sum of the three components:Brms =√(Bx2 +By2 +Bz2)Fig. 5. Data resultsIt is observed that the intensity of the field outside the struc-ture is lower than inside with values that are between 50 µTfor field points close to the coils (test 1) and 35 µT for fieldpoints spaced 20 cm from the structure (tests 6-7). Compar-ing the measurements made at the central point of the structure(tests 2, 5, 8) it is evident that the value of induction in test8 is greater than test 2 and test 5. This happens because twocoils are fed, and the overall magnetic induction is obtainedfrom the superposition of individual effects. The sum of thecontribution of coil 1 and coil 2 fed at the same time it is notthe same as the contribution of the individual coils fed indi-vidually, because part of the effect of one coil is compensatedby the effect of the other: consequently test 8 has a value notmuch greater than in test 5, even though both coils are poweredwith 0.9 A RMS. To characterize the device, further measureswere carried out for completeness. In particular, the maximum3CIRED 2021 Conference Geneva, 20– 23 September 2021Paper XXXXdistance at which the device provides an acceptable magneticfield value was evaluated. The simulation results reflect the realones Fig.6Fig. 6. Comparison simulation-real results4 Methodology for the testing of a shield plateThe set-up for the testing of a magnetic field shield is schema-tized in Fig.7 and show in Fig.8Fig. 7. Set-up configurationsThe goal of this test is to characterize a shield sample undera magnetic field with different vector components. The shieldsamples under test have a multi-layer structure based on con-ductive and ferromagnetic material Al and FeGo respectively:aluminum (Al) and grain-oriented silicon iron(FeGo)[7,8].Two different plate configurations have been analyzed with twodifferent aluminum thicknesses(2,7 and 4 mm) and the samenumber of FeGo sheets (2). The procedure consists of a firstFig. 8. Test shielding systemstep in which one coil at a time is powered and the magneticfield values are measured with and without the shield. To eval-uate the shielding coefficient for each axis component. Table 1highlights the supply coils configurations.Table 1 Supply coil configurationsTest case Prevalent component Coil fed1 Z White2 Y Red3 X Green4-5 Y-Z Red-White6-7 X-Y Green-Red8-9 X-Z Green-White10-11-12 X-Y-Z Green-Red-WhiteFig. 9. SF results4CIRED 2021 Conference Geneva, 20– 23 September 2021Paper XXXXIn Fig.8 are reported the two set-up configurations regard-ing the orientation of the shield concerning the source and theprobe. Conf.A put the Al to the side of the probe and the FeGoto the side of the source; Conf. B is the opposite. The resultsobtained for the two configurations in terms of shielding fac-tor (SF= ratio between the magnetic induction component withand without the shield) are reported in Fig.9. Some generalconclusion can be done:• The aluminum thickness increase significantly the SF inconf. A and when the MF has a z-component (componentnormal to the shield)• In conf. A the plate with higher Al thickness has alwaysbetter performances, this is not true for conf. B• Conf. A and B have similar SF values (between 50 and65); The only configurations that show very different perfor-mances are those when z component is present(1-5-9-12).5 ConclusionIn this paper, an innovative MF source for the testing oflow-frequency magnetic shielding is presented. The proposedsource is composed of three coils for the generation of rota-tional MF in space. The presence of all the axial componentsof the magnetic field allows to better test magnetic shield per-formances independent from the shield material: conductive,ferromagnetic, or mixed. The source has been applied duringthe test procedure of a sample plate but it can applied alsoduring the test of a real shield.6 AcknowledgmentWe want to thank the BEShielding SRL company(WWW.BESHIELDING.IT) for providing the sample of mate-rial on which the tests were carried out.7 References[1] ASTM A698 / A698M – 20 Standard Test Method forMagnetic Shield Efficiency in Attenuating AlternatingMagnetic Fields[2] EN 50147-1 Anechoic chambers – Part 1: shield attenua-tion measurements[3] IEC 61000-4-8: ‘Electromagnetic compatibility (EMC) –Part 4–8: testing and measurement techniques – powerfrequency magnetic field immunity test’, 2009[4] Juan Carlos del-Pino-López et al."Design of active loopsfor magnetic field mitigation in MV/LV substation sur-roundings",Electric Power Systems Research Volume119, February 2015, Pages 337-344[5] Mitigation techniques of power frequency magnetic fieldsoriginated from electric power systems, Tech. Rep. Work-ing group C4.204, CIGRE’, 2009[6] Canova et al. "Description of Power Lines By EquivalentSource System",COMPEL Vol. 24, No. 3, 2005, pp. 893-905[7] Bavastro, D.et al.: ‘Numerical and experimental devel-opment of multilayer magnetic shields’, Electric PowerSyst. Res., 2014, 116, pp. 374–380[8] Canova A.,"Multilayer magnetic shielding: an innova-tive overlapping structure", October 2017CIRED - OpenAccess Proceedings Journal 2017(1):780-783; DOI:10.1049/oap-cired.2017.04875

A proposal for performance evaluation of low frequency shielding efficiency

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