Distribution Transformers and EMC

Transcription

1 Distribution Trnsformers nd EMC Specil reprint Author: MBA, Dipl.-Eng. (FH) Bernd Hofmnn, Hed of Technology & Innovtion Answers for energy.

2 Distribution Trnsformers nd EMC Since electromgnetic comptibility (EMC) is governed not only by technicl specifictions but by legl regultions too, questions frequently rise. Wht evidence of EMC hs to be provided for medium-voltge equipment? Wht certifiction of conformity is required? Do we need CE mrk? To wht extent does emissions-control legisltion pply? The nswers concern mnu fcturers nd users like, for both re jointly responsible for electromgnetic compti bility. The electromgnetic comptibility (EMC) of trnsformers is determined by their stry mgnetic fields. These re influenced, bove ll, by trnsformer s rting nd lod nd by its proximity to other items of equipment. When plnning instlltions, however, it is importnt to consider not only the trnsformer s stry field but lso the stry field generted by the low-voltge trnsmission system. Tsk definition In n industril society, our living nd work environments re incresingly influenced by the use of electronic equipment. These include dt processing systems with computers nd monitors s well s dt nd mesured-vlue cquisition systems. At the sme time, power supply instlltions move closer to the centres of consumption, i.e. lso nerer to people s working environment. Power distribution systems cn ct s source of interference cusing disturbnces in electronic equipment. Trnsformers re n importnt element of supplies to lod centre. This rticle explins the chrcteristics of trnsformers which generte electric nd mgnetic fields nd ct s source of interference in terms of electromgnetic comptibility (EMC). Legl nd technicl rules Electromgnetic comptibility is bound up with both technicl nd legl regultions. Legisltion dels with the effect of equipment on the generl public, in wide rnge of contexts. Technicl spects include description of the electromgnetic phenomen involved; the tretment thereof is covered by stndrds contining the relevnt design- relted nd test specifictions. Since certin equipment gives off electromgnetic fields into the environment, the Germn Federl Emissions Control Ordinnce (26. BImSchV) imposes limits in order to prevent impermissible effects on people. Germn EMC legisltion (derived from the Europen EMC directive) pursues different objective. To ensure free trde (without EMC problems), the lw specifies EMC protection trgets nd governs the topic of conformity rting, i.e. wht products should ber CE mrk. EMC legisltion nd the CE mrk Do distribution trnsformers hve to ber CE mrk? This is frequently sked question, nd sometimes the CE mrk is mistken for kind of qulity symbol. The Europen directive on electromgnetic comptibility [1] sttes tht equip ment should be constructed nd operted such tht it neither cuses interference to other devices, nor is subject to ny interference itself. Tht effectively sums up the ide behind the Germn lw on the electromgnetic comptibility of devices (EMVG) [2]. According to EN nd IEC Chpter 12, power trnsformers constitute pssive elements in terms of electromgnetic interference emission nd interference immunity. Pssive elements re declred to be not cpble of cusing electromgnetic interference, nd their operting chrcteristics to be not ffected by such interference. A further spect is conformity rting ( sttement nd evidence tht product complies with the protection specifictions). 2

3 Distribution trnsformers require no CE mrk Devices hve to meet the EMC protection trgets. A device in the sense of the EMC directive mens ny electric or electronic system, network or item of equipment. But not ll devices require CE mrk or declrtion of conformity. The reson is comprised in the very sense of the EMC directive. Its intention is to prevent everydy use of commonplce electricl devices from interfering with rdio trnsmission or telecommunictions, nd to void the risk of such devices hving ny mutully dverse effect on ech other. In contrst, distribution trnsformers re not commonly vilble, but re used for instlltion in sttionry electricl systems. In ccordnce with Section 12 of the EMC-Sttute, declrtion of conformity or CE mrk is therefore unnecessry for distribution trnsformers. 26th Germn Federl Emissions Control Ordinnce Distribution trnsformers produce electric nd mgnetic fields (t system frequency) tht cn spred into the surrounding re. Since this surrounding re my well be ccessible to the public, ny risk to humns (resulting from such fields) must be ruled out. The 26th Germn Federl Emissions Control Ordinnce (26. BImSchV) [3] therefore imposes limits on electric nd mgnetic fields. Complince with these limits is cogent under lw; the ordinnce tkes precedence over VDE nd other stndrds. Scope of vlidity The ordinnce pplies to sttionry systems for the trnsformtion nd trnsmission of electricity: n Overhed power lines nd buried cbles with frequency of 50 Hz nd voltge of 1000 V. n Overhed power lines nd overhed contct lines, including the trction substtions nd switchger, with frequency of 16 2/3 Hz or 50 Hz. n Trnsformer substtions, including switchger, with frequency of 50 Hz nd high voltge of 1000 V. The limits for electric field intensity nd mgnetic flux density re: n 5 kv/m nd 100 µt t 50 Hz, n 10 kv/m nd 300 µt t 16 2/3 Hz. These figures pply to continuous opertion nd highest system opertionl lod. The defined limits pply in the re ffected by complete systems of vrying design depending on customer requirements nd sptil conditions. The electric nd mgnetic field intensities for prticulr instlltion therefore cnnot be trnsposed onto nother. Specific evidence relting to n overll system must ultimtely be produced. 26. BImSchV permits both mesurementbsed nd theoreticlly clculted evidence of complince with the relevnt limits. EN contins instructions for the mesurement or clcultion of electromgnetic fields. 3

4 35 Mesurement mv Bottom edge of coil Centre of coil Probe Top edge of coil 1.7 T 20 U rms 1.7 T T 1.5 T T 1.5 T 5 0 B Fig. 1: Voltge induced by no-lod stry field t the surfce of 630 kva cst-resin-encpsulted trnsformer; prmeters: limb induction 1.5 T to 1.7 T (mesurement probe with 2900 turns, A eff = 1.53 cm 2 nd distnce from surfce of the winding 20 cm) Fields occurring in the proximity of trnsformers Electric nd mgnetic fields which occur in the frequency rnge of the public supply network 16 2/3 nd 50 Hz cn be decoupled nd therefore considered seprtely [4]. Higher frequencies which emit electromgnetic fields occur only in conjunction with prtil-dischrge processes in nd round the trnsformer. These prtil-dischrge pulses cn contin frequencies of 0.1 to 50 MHz. Distribution trnsformers do not generlly produce prtil dischrges t supply voltge, however. In such cses, it is merely necessry to design the high- voltge connections so tht the field intensity in ir which is criticl for prtil dischrge inception is not exceeded t ny point. High-frequency electro mgnetic fields re therefore of only secondry importnce in the cse of trnsformers nd will not be discussed here. Electric fields occur between live conductors nd between the conductors nd erth. The mgnitude of the electric fields depends on the supply voltge, distnces nd electrode geometry. A field intensity of round 10 kv/mm my be present in the internl structure of n oil-immersed trnsformer, with intensities rnging from 0.5 kv/mm to 3 kv/mm inside dry-type trnsformers. Trnsformer tnks, for exmple, which ct s Frdy cges, shield the electric field of enclosed fluid-filled or dry-type trnsformers. Drytype trnsformers without enclosures re generlly instlled in comprtments without wlls nd ceilings of electric insulting mteril. These comprtments therefore lso represent Frdy cges in first pproximtion. The sme pplies to the electric fields generted by connections nd incoming cbles which re thus virtully inctive inside buildings. Mgnetic fields, by comprison, re fr greter source of disturbnces. These re normlly not shielded by building wlls, not even steel-lined concrete wlls. This rticle therefore concentrtes on these fields. Stry mgnetic fields in trnsformers lt is necessry to nlyze different sources of stry mgnetic fields in trnsformers. These sources hve the following scending order of importnce: n Stry field in no-lod opertion; n stry field from the terminl leds under lod (prticulrly on the lowvoltge side becuse of the greter current) nd n stry field from the current-crrying windings. These three-dimensionl fields re complicted, becuse they re generted in time nd spce by the three-phse system nd re ffected by the iron contined in the enclosing structurl elements. The mgnetomotive force which the no-lod current genertes in the primry winding produces the stry field in no-lod opertion. The mgnitude nd wveshpe of no-lod current depend on the mgnetiztion requirement of the iron core. Becuse of the curved shpe of the mgnetiztion chrcteristic, the no-lod current contins hrmonic components which re consequently lso present in the stry field. The effective vlue of the voltge induced in mesurement coil ws therefore plotted in Fig. 1 s function of the mesurement loction on cst-resin-encpsulted trnsformer. The no-lod field is emitted mostly t the top nd bottom end of the primry winding nd from the trnsition points of the iron core. However, the no-lod field is smller by fctor of bout 10 thn the stry field generted by the lod-currentcrrying windings so it cn generlly be disregrded. 4

5 Iron core z y x No. B -mgnetic field µt µt µt 4 1 µt 5 2 µt 6 10 µt 7 20 µt µt µt µt Lowvoltge winding Highvoltge winding x z y 10 m 15 Fig. 2: Bsic stry field of trnsformer under lod Fig. 3: Sptil clcultion of stry field for 630 kva GEAFOL trnsformer As with every current-crrying conductor, leds such s connecting cbles nd busbrs lso generte stry field [5], which, t every point in spce, is function of the vectoril sum of field intensities generted t tht sme point by the current-crrying conductors. In conductor configurtions where the sum of currents equls zero, s in the supply nd return lines of n AC circuit, or in three-phse system, for exmple, the more tightly these conductors re bunched, the smller the resultnt field will be. The trnsformer s lod-current-crrying primry nd secondry windings generte stry field which is emitted from the spce between the windings (Fig. 2). This stry field cuses interferences minly in the vicinity of the trnsformer. Lekge from the low-voltge leds, prticulrly where these re not tightly bunched, is n dditionl fctor. Stry field of the trnsformer under lod Fig. 3 shows the results of model clcultion performed to scertin the stry field of rted current-crrying 630 kva resin-encpsulted trnsformer in the event of short circuit. The stry field spreds outwrds in virtully hemisphericl form t even reltively close distnce. When distnce is pproximtely 10 m, the short-line function roughly proportionlly follows 1/ 3 [6]. In the ner field, i.e. from 0 m to 1 m, field density tends to be proportionl to 1/ 2 [6]. Up to bout 10 m, this produces function of between 1/ 2 nd 1 3. The mesured vlues for 630 kv trnsformer nd the computed vlues for busbr system re both plotted in Fig. 4. If we ssume tht the stry field mesured t distnce of 3 m is B 3, we obtin the following eqution for the ctul trnsformer (without leds): B() = B 3. (3 m/) x where 2 < x < 3 Using the sme mesured vlues, for the distnce rnge between 1 m nd 10 m we obtin: B() B 3 (3 m/) 2.8 For the distnce rnge bove 10 m the eqution is s follows: B() = B 10 (10 m/) 3 By comprison, stry field induction in the busbr system vries only s the squre of the distnce, nd therefore hs more powerful effect under certin circumstnces thn the trnsformer stry field. The stry field generted by the busbr system is lso function of conductor proximity, i.e. it decreses with incresing proximity. The trnsformer stry field is influenced by n the trnsformer s lod fctor k T (directly proportionl to k T = I/I r ), n the design of the trnsformer nd n the design dt. 5

6 10 4 µt Geometry of three-phse busbr system for stry field clcultion B kva GEAFOL trnsformer with u z = 6% Three-phse busbr system Low-voltge-rted current for trnsformers I r = 909 A m 10 2 Fig. 4: Stry field mesurement for 630 kva GEAFOL trnsformer nd stry field clcultion for busbr system In the cse of 630 kva cst-resin trnsformer where u z = 6%, pek vlue of B 3 = 5 µt must be expected t rted lod. This vlue is reduced by fctor of 2 to 3 in the cse of oil-immersed distribution trnsformers nd lso sheet-steel-enclosed, dry-type trnsformers becuse of the shielding effect of the trnsformer tnk. The stry field cn be considered in first pproximtion s function of the trnsformer s lekge flux. This in turn determines the reltive impednce voltge u z of the trnsformer with the result tht the stry field is roughly proportionl to the impednce voltge. From the lws of propgtion pplying to the trnsformer, it cn be inferred tht lekge flux is function of pprent trnsformer power S r rrived t vi root function. From this we obtin: B ~ B 0 u z S r B = B 0 u z u z0 0 S r S r0 x k T or 0 x k T where B 0 denotes induction t distnce 0 ; the exponent is 2 < x 3. Relted to the vilble mesured vlues for the 630 kva trnsformer, we obtin the following eqution for rted lod (k T = 1) B = 5 µt u z 6% S r 630 kva 3 m 2 to 3 or for the rnge = 1 m to 10 m B = 5 µt u z 6% S r 630 kva 3 m 2.8 The stry field is thus product of the trnsformer s technicl specifictions nd is virtully impossible to lter without chnging those specifictions. Stry Field in the Trnsformer during Short Circuit or Connection to the Supply The short-circuit stte is rre but extreme loding stte of the trnsformer. The stry fields resulting from such short circuits re likewise proportionl to the current. The short-circuit current corresponds to function of impednce voltge: I c = I r /u z 100 or, ccording to [11]: A short circuit my therefore briefly produce stry fields which cn exceed the rted vlues by mximum fctor of = 45 for u z = 4 % nd impulseto-ac-strength rtio K = 1.8, or by fctor of = 30 for u z = 6 %. By comprison, virtully the entire useful flux of the trnsformer my occur in the ir spce inside the primry winding t the moment the trnsformer is energized due to sturtion of the iron core [7]. However, this mximum vlue is only obtined if the trnsformer is connected to the supply t the zero crossing of the voltge wve. The rtio of useful flux to lekge flux is 1/u z, i.e. vlues for u z = 4 % or 6 % would be 25 times or 16.7 times higher respectively thn nominl lekge flux. Mgnifiction of the stry field is fr less pronounced, however, due to dmping of the inrush current s result of lekge impednce nd ohmic resistnce in the winding. The stry field is therefore likely to ttin vlues bout 15 to 10 times higher thn the rted stry field during the initil cycles of the inrush current. I cmx = I c K 6

7 Mesuring the stry field There re vrious methods of mesuring the stry fields of trnsformers including Hll probes or mesurement coils. In the ltter exmple, the voltges induced in the coil by the stry fields re recorded with n oscilloscope nd their wveform is nlyzed. The mximum vlue or direction of the field cn be clerly determined nd recorded by rotting the mesurement coil. If the lod current, for exmple, hs hrmonic component, it is lso possible to scertin the hrmonic content of the field using hrmonics nlyzer. lt is lso possible to tke the frequency response of the mesurement coil (U ~ Bf) into ccount nd to distinguish this from the frequency spectr of other sources of interference. By mesuring stry field induction t different intervls it is possible to plot n induction curve versus the distnce from the source of interference. Limits The EMC limits to be observed depend on the sensitivity of the instruments which cn be ffected by these stry mgnetic fields. In medicl pplictions, DIN VDE limits the mximum permissible redings in the vicinity of ptient to 0.4 µt for electrocrdiogrms (ECGs) nd 0.2 µt for electroencephlogrms (EEGs)[8]. These re pek-to-pek vlues. The electromgnetic susceptibility of disply screens becomes importnt in technicl pplictions. Screen mnufcturers specify n interference immunity of round 1 µt [9] with field strength limits s low s 0.1 µt for electron microscopes. Protective Mesures Since field strength declines roughly in proportion to the cube of the distnce from the trnsformer, the most importnt mesure is to site the trnsformer sufficiently fr wy from the nticipted source of interference. In medicl pplictions, DIN VDE , for exmple, considers distnce of 6 m to be sufficient. The dt nd curves presented here cn be used to clculte or select cceptble distnces for certin field intensity limit. This simple mesure should be tken into ccount t the plnning stge [10]. Subsequent dmping mesures, such s lying low-voltge leds in bunched configurtion or use of shielding covers, re usully expensive nd demnd detiled investigtions. Monitor screens which offer protection ginst stry mgnetic fields re lredy vilble s fctoryssembled products. With these it is essentilly possible to reduce interference resulting by stry mgnetic fields even s subsequent refinement. Literture [1] Council Directive 2004/108/EEC of 15 Dec [2] Gesetz über die elektromgnetische Verträglichkeit von Geräten (EMVG) vom [3] EMVU-Verordnung. Sechsundzwnzigste Verordnung zur Durchführung des Bundes- Immissionsschutzgesetzes (Verordnung über elektromgnetische Felder 26. BImSchV) vom 16. Dezember [4] EN 50413, section 4.3 [5] Hdrin, W.: Leitungen, Trnsformtoren und Schltnlgen ls Störquellen. Elektrotech. und Msch.-bu 101 (1983) vol. 1, p [6] EN 50413, section A [7] Schäfer, W.: Trnsformtoren. Smmlung Göschen Vol. 952/952. Berlin: Wlter de Gruyter, p [8] DIN VDE Errichten von Niederspnnungsnlgen Anforderungen für Betriebsstätten, Räume und Anlgen besonderer Art [9] Kohling, A: Zimmer, G.: Beeinflussung von Bildschirmrbeitsplätzen durch Mgnetfelder. etz Elektrotech. Z 114 (1993) vol. 12, p [10] Kohling, A.: EMV Plnung für Krnkenhusneubuten. etz Elektrotechn. Z. 106 (1985) vol. 9, p [11] EN : 2006 Power trnsformers Ability to withstnd short-circuit 7

8 Published by nd copyright 2012: Siemens AG Energy Sector Freyeslebenstrsse Erlngen, Germny Trnsformtorenwerk Kirchheim Power Trnsmission Division Trnsformers Hegelstrsse Kirchheim/Teck, Germny Phone: +49 (0) Fx: +49 (0) For more informtion, plese contct our Customer Support Center. Phone: / Fx: / (Chrges depending on provider) E-mil: support.energy@siemens.com Power Trnsmission Division Order No. E50001-G640-A132-V2-4A00 Printed in Germny Dispo c4bs No TH BR SD Printed on elementry chlorine-free bleched pper. All rights reserved. Trdemrks mentioned in this document re the property of Siemens AG, its ffilites, or their respective owners. Subject to chnge without prior notice. The informtion in this document contins generl descriptions of the technicl options vilble, which my not pply in ll cses. The required technicl options should therefore be specified in the contrct.