Insulation Resistance under DC Stress and Electrification Characteristics of GISEpoxy Insulator

SHUHEI KANEKO,1 SHIGEMITSU OKABE,1 TAKAYUKI KOBAYASHI,1 KENICHI NOJIMA,2

MASAFUMI TAKEI,2 and TAKETOSHI MIYAMOTO21Tokyo Electric Power Company, Japan

2Toshiba Corporation, Japan

SUMMARY

Gas-insulated switchgear (GIS) has widely been usedfor AC power distribution because of its high reliability andcompactness. Recently, DC GIS has been developed withvarious investigations for dielectric breakdown charac-teristics of DC gas insulation. GIS insulation is composedof SF6 gas and solid spacers, and it has been recognized thatthe dielectric performance of DC GIS is mainly influencedby solid spacers. Under DC stress, the electric field isdirected one way, the effect of electrification for charges tobe accumulated in the spacer must be taken into accountand also the effect exists in AC GIS because the switchingoperations may leave the remnant DC charge on the AC GISspacer. This paper first describes the effective resistivity(the bulk or the surface) of the solid spacer under the DCstress from the experimental investigation, and the criticalfactor on the solid spacer that causes reduced dielectricperformance of the GIS insulation is studied. Second, thepresent paper deals with the electrification on the GIS withvarious levels of surface roughness of the epoxy insulatorand metallic electrode. Finally, the DC insulation charac-teristics of GIS insulator are investigated based on theexperimental results. © 2009 Wiley Periodicals, Inc. ElectrEng Jpn, 168(4): 6–13, 2009; Published online in WileyInterScience (www.interscience.wiley.com). DOI10.1002/eej.20788

Key words: DC; GIS; epoxy insulator; spacer; di-electric breakdown characteristics; remnant DC charge;electrification; SF6 gas.

1. Introduction

AC gas-insulated switchgear (GIS) is conceived ofand designed for a variety of insulation duties with respectto insulation performance for ordinary commercial fre-quency voltages and different surge-like overvoltages [1].However, with respect to insulation duties for direct current,there are no items for testing resistance voltage in theconventional standards, and consideration for the topic isinadequate at present. In recent years, the development ofDC GIS has advanced [2], and considerable research hasbeen performed on the behavior [3] of foreign matter in DCGIS and on the design [4] of spacers for DC GIS with theeffects of electrification taken into consideration. Even inAC GIS, DC residual voltage sometimes occurs as a resultof switching equipment in circuits separated from the maincircuit, and the effect on insulation characteristics must beconsidered. With respect to GIS in which specificationshave been streamlined (shrunk) due to the use of UHVtechnology in recent years, it is clear that the insulationperformance must be taken into consideration as regardsthis DC residual voltage.

The mechanism behind the deterioration of insulationcharacteristics in insulation spacers when there is a residualDC voltage in a busbar in a GIS as a result of GIS operationinvolves a concentration of electric fields or electrification,or a combination of the two, resulting from the DC voltageon the surface or in the bulk of the insulation spacer. Theconcentration of an electric field occurring on the surfaceof an insulator is thought to have a greater impact oninsulation characteristics, and so is important. Factors in-volved in the concentration of an electric field on the surfaceof the insulation spacer include effects resulting from anonuniform resistivity for the surface layer, in addition tothe presence of minute protrusions or foreign materialattached to the surface. When DC voltage remains in an ACGIS, the electric potential distribution in the insulation

© 2009 Wiley Periodicals, Inc.

Electrical Engineering in Japan, Vol. 168, No. 4, 2009Translated from Denki Gakkai Ronbunshi, Vol. 127-B, No. 9, September 2007, pp. 1009–1015

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spacer shifts from an AC distribution to a DC distributionwith the passage of time, and the electric field changes froma capacitive field to a resistance field [5]. The time constantvaries depending on the volume resistance of the insulationspacer, but because it is thought to be in the range of severalhours to several hundred hours, when evaluating the electricfield distribution accompanying a residual DC voltage, along time domain must be taken into consideration. On theother hand, evaluating the charge source is also importantas regards factors related to the electrification of the insu-lation spacer. The effects of dust and other added materialhave been indicated as an electrification source, but thepossibility of charge being supplied from the electrodesurface or the surface of the insulator has also been sug-gested, and must be considered. In particular, it has beensuggested [6] that when the surface roughness of the insu-lator is high, the electric field at the tips of surface irregu-larities rises, leading to minute charge being formed underconditions in which streamers do not occur, and this maytravel along the electric power line and reach the surface ofthe insulator, resulting in electrification.

With respect to the insulation spacer when a DCvoltage remains in the GIS, in this research, the authorsfocus on the surface or the layer nearest it, and then meas-ured the resistance characteristics of the surface layer of theinsulation spacer linked to the concentration of the electricfield while taking into consideration the DC resistance ofthe hardened material. They also performed measurementsin order to confirm the effect of electrode surface andinsulator surface roughness on the electrification of theinsulation spacer. In all of the experiments in this researchthe authors used an epoxy resin for the GIS spacer withgreater heat resistance than a representative insulator resin.

2. Dominant Component of the Insulation SpacerLeak Resistance

When analyzing the electric potential distributionand the electric field distribution where a DC voltage isapplied to a GIS insulation spacer, evaluating the main flowpaths and resistance value for the leakage current thatdetermines the electric potential distribution is vital. Ingeneral in the surface of an insulator, the molecular chainsof polymers that comprise the insulator are thought to besevered or the packing is thought to peel, and so resistancecharacteristics different from those of the inside of theinsulator may be found. The leakage current and resistancecharacteristics in the surface layer of the insulating spacerare thought to vary depending on the material that theinsulator is made of, and so when dirt or other materialadheres to the surface, its effects must be taken into consid-eration as well. Affixed material or protrusions in the sur-face layer may have an effect on surface resistance, and

differences in the structure of the material included in theresin may influence the bulk resistance (volume resistance).In this section the authors measure the leakage current andexamine the dominant component of the resistance on thesurface layer of the insulation spacer.

2.1 Experimental methods and experimentalcircuit

The authors performed an experiment to see if thedominant component of the resistance was the bulk resis-tance or the surface resistance based on measurements ofthe leakage current when a DC voltage was applied to theepoxy resin. Figure 1 shows the experimental circuit. Acircular epoxy resin was used as the test object (TP), andthe leakage current was measured by placing it between astainless-steel high-voltage electrode with a mirror finishand an electrode for measuring current. The leakage currentwas found as the output of the average value at 200 ms, andthe effects of the ripple in the DC power source (repeatingfrequency of 36.5 kHz) were ignored. In order to avoid asmuch as possible electric field emission current from thehigh-voltage electrode during measurement, the electrodeused to measure the current had the same diameter as thetest object. Moreover, a grounding sheet that insulated themeasurement electrode with a 1-mm gap around the elec-trode used to measure the current was set up. The bindingsurface between the TP and the electrode was lightly coatedwith grease to ensure the binding area was precise. Theelectrode system was set up in a tank with SF6 gas at 0.5MPa sealed in it, and then the experiment was performed.Although moisture in the tank was thought to influence thesurface resistance of the resin, Zeorum was placed in thetank, and the moisture was adjusted to 20 to 30 ppm.

The bulk resistance Rb and the surface resistance Rsof a circular insulator are in general given by Eqs. (1) and(2), where r is the radius, d is the thickness, ρb is the volume

Fig. 1. Experimental setup for measuring the leakagecurrent.

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resistivity and ρs is the surface resistivity. Moreover, whenEqs. (1) and (2) are converted to logarithms on both sides,Eqs. (3) and (4) respectively result.

In other words, whether the bulk resistance or thesurface resistance is dominant can be ascertained based onthe radius dependency with respect to the resistance of thetest material in question. Here, the experiment was per-formed for resin 4 mm thick at φ 30 mm, φ 60 mm, and φ105 mm.

2.2 Results of the experiment

The authors applied a DC voltage to a TP at φ 30 mm,φ 60 mm, or φ 105 mm, and measured the current waveform.The voltage applied was 10 kV (2.5 kV/mm), 20 kV (5kV/mm), or 32 kV (8 kV/mm). Figure 2 shows the relation-ship between the voltage and current at 10,000 seconds afterthe voltage was applied, and Fig. 3 shows an expanded viewof φ 30 mm alone. At φ 60 mm and φ 105 mm, the currentvalue is clearly almost proportional to the applied voltage.However, for φ 30 mm, the relationship between the currentand the applied voltage is nonlinear, and a tendency inwhich the resistance varies depending on the applied volt-age can be seen. The authors calculated the resistance value(= value of applied voltage/current value) based on thecurrent value at 10,000 seconds after the voltage was ap-plied. Figure 4(a) shows the TP radius dependencies for the

resistance value. In Fig. 4(a), the resistance value decreasesas the radius of the resin increases. Figure 4(b) shows theTP radius dependencies of the resistance value plotted aslogarithmic values. In Fig. 4(b), the trend for the radiusdependencies for the resistance values is close to –2 for anyapplied voltage. Given this, the bulk resistance can be takento be dominant in the leakage current in SF6 gas in the resin.

(1)

(2)

(3)

(4)

Fig. 3. Leakage current at various DC voltages (t = 10,000 s, φ 30).

Fig. 4. Resistance at various TP radiuses (t = 10,000 s).Fig. 2. Leakage current at various DC voltages

(t = 10,000 s).

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In Fig. 3, the V–I characteristics are linear, and the effect ofthe surface resistance in a test object with a relatively smallradius is thought to be greater. Whether this is simply dueto the effect of the surface resistance or due to some othereffect will require further analysis.

3. Effect of the Hardening Agent on the ResistanceCharacteristics of the Insulation Spacer

In the evaluation in the previous section the authorsshowed that the bulk resistance was dominant in the epoxyresin for the most part used in GIS. One factor that has aneffect on the resistance characteristics of the bulk materialis thought to be the sublimation of the hardening agent inthe surface layer of the insulation spacer. When the insula-tion spacer is formed using a multistep injection process,sublimation of the hardening agent is normally preventedby a mold releasing agent. When sublimation of the hard-ening agent occurs for some reason, however, the volumeresistivity in the surface layer of the insulator may change.When this layer becomes nonuniform and a DC voltage isapplied, this nonuniformity may become a factor behind theconcentration of the electric field in the insulator surface.In this section the authors perform experiments and anevaluation of the effects on the volume resistivity when thehardening agent proportion is varied.

3.1 Experimental methods and experimentalcircuit

The authors prepared three types of TP by varying theamount of the hardening agent to 35, 25, or 15 PHR (PHR:percentage where the epoxy resin weight is 100) and withthe resin kept at a constant packing level. They then evalu-ated the volume resistivity by measuring the leakage currentwhen a voltage was applied to each TP. Figure 5 shows theexperimental circuit. The electrode on the high-voltage sideand the electrode used to measure the current on the ground-ing side were created using gold deposition. The test sam-ples were approximately 2 mm thick. The material inquestion was confirmed as having a low electric field de-pendency up to a 5 kV/mm field. With respect to the domainfor the electric field in real equipment, the applied voltagewas adjusted so that the applied electric field inside thematerial was 5 kV/mm, matching the assumed severalkilovolts per millimeter. The electrodes were set up in thesealed container filled with SF6 gas at atmospheric pressure.

3.2 Experimental results

Figure 6 shows the time dependency of the volumeresistivity found based on the measured current. Theauthors performed the experiment for 10 TP with the

Fig. 6. Volume resistivity at various PHR of hardeningagent.

Fig. 5. Experimental setup for the measurement ofvolume resistivity at various PHR of hardening agent.

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amount of hardening agent at 35, 25, or 15 PHR. A trendwas seen in which the TP volume resistivity rose with thepassage of time after the voltage was applied, and thentended to increase gently when the specific weight of thehardening agent was low. TP1, TP2, TP3, TP7, and TP8 forthe specific weight of the hardening agent was 25 PHR, aswell as TP1 and TP8 when this was 15 PHR had a lowervolume resistivity, particularly over a longer time period,as compared to the other TP or TP for a hardening agentwith a specific weight of 35 PHR.

When an absorption current waveform for thegrounding material is obtained, then the quantity of thepolarization component by the material can be evaluatedbased on the calculation of the complex relative dielectricconstant and a plot using the real and imaginary portion ofthis value. Figure 7 shows a comparison of this plot forvarious TP. The complex relative dielectric constant εr* isgiven in Eq. (5), and the real part ε′ and the imaginary partε″ are referred to as the relative dielectric constant and therelative dielectric loss. Because the arc relationship on acomplex plane holds based on the relationship in Eq. (6)holding, this is known as a Cole-Cole’s circular arc law. Thegreater the polarization component, the larger εr0 becomes,and as a result the radius of the plot increases in size. TP1,TP2, TP3, TP7, and TP8 in Fig. 6(b) and TP1 and TP8 inFig. 6(c) showing a low volume resistivity have a larger

radius in the plot in Fig. 7. This is thought to show that thepolarization component increases depending on the in-crease in the unreactive components in the hardening agent.

εr0: the dielectric constant under direct current; εr∞: thedielectric constant at high frequency.

The increase in the uncreative component is thoughtto affect the volume resistivity as well. A characteristic ofthe volume resistivity of TP is that the dispersion in theresults of measuring the volume resistivity rises. This trendis particularly visible in Fig. 6(b). Whether or not theincrease in dispersion affects the quantity of the unreactivecomponent in the hardening agent will require furtheranalysis. Among sample materials with a low level ofhardening agent there are sample materials showing a highvolume resistivity. On the other hand, there are some witha volume resistivity that is lower by an order of magnitude.This suggests that there may be a complex effect with otherfactors such as moisture in the unreactive component. Theresults of measurements show that the amount of hardeningagent affects the resistivity. The effect of the amount ofhardening agent must be taken into consideration as a factorin the concentration of the electric field on the surface layerof the spacers.

4. Electrification Phenomenon in the InsulationSpacers

The authors analyzed the factors behind electrifica-tion in the insulation spacers. A phenomenon in whichcharge accumulates on the surface of the insulator is knownto occur when a DC voltage is applied to GIS. The cause ofthis electrification charge is thought to be primarily thelocal concentration of an electric field. Except for whenthere are metal foreign particles in the space in a GIS tank,the site where the electric field concentrates is thought tobe the triple junction consisting of the electrode surface orthe insulation material surface, or the electrode, insulationmaterial, and SF6 gas. The electric field at the triple junctionis generally suppressed in an insulation spacer. In thisinstance, the state of the surface of the electrode and theinsulating material is assumed to affect the generation ofelectrification charge. Specifically, when the surface rough-ness of the electrode or the insulator is high, the supply ofcharge as a result of charge production due to the greaterelectric field at the tips of small protrusions on the surfaceis thought to be a factor in electrification. In this section theauthors attempt to evaluate this relationship between sur-face roughness of the electrode and the insulator and elec-

(5)

(6)

Fig. 7. Complex relative dielectric constants at variousPHR of hardening agent (εr′: real part; εr″: imaginary

part).

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trification by comparing the damping time for the leakagecurrent.

4.1 Experimental methods

Figure 8 shows the measurement system using thesample insulator and a gap in the insulator. The authors useda highly heat-resistant epoxy resin used in real GIS equip-ment as the insulator. An insulator/insulator gap was createdby attaching two test insulators with embedded electrodescoated with epoxy resin 1 mm thick on the top and bottom,and then having them face each other so that the gap lengthbetween the test insulator surfaces was 2 mm. The leakagecurrent was measured by applying a DC voltage to the testgap. During measurement the test gap was placed inside atank filled with SF6 gas at 0.6 MPa. A guard electrode forthe grounding potential was set up around the electrode onthe grounding side attached to the ammeter, making itdifficult for charge emitted from the side current or upperor lower electrode to flow to the ammeter. The spacebetween the guard electrode and the electrode on thegrounding side was insulated with a polyethylene terphtha-late (PET) shield. By setting up an embedded electrode inthe support insulator and the test insulator, the electric fieldat the triple junction consisting of the insulator, and the areain contact with the upper and lower electrodes was eased.The effects of surface roughness were compared by varyingthe average surface roughness Rz between 2.8, 11.5, and27.6 µm for the opposite test insulator in the insulation gapexperiment. In the experiment, electrode pairs coated withinsulation having the same surface roughness were set inopposition to each other. The authors then confirmed thatthe leakage current was due to electrification, and after

applying a DC voltage confirmed the electrification char-acteristics using toner.

Moreover, the authors performed similar measure-ments for a metal/insulator gap by replacing the test insu-lator on the high-voltage electrode side with an aluminumelectrode. In this instance, the surface roughness of the testinsulator on the grounding side was made consistent at 2.8µm in order to see the effects of surface roughness in thealuminum electrode alone. The surface roughness of thealuminum electrode was compared at 0.9 µm and 22.2 µm.

4.2 Results of the experiment

The electric field expected in a real spacer is thoughtto be above approximately 8 kV/mm. In the experimenthere, the authors focused on an insulator/insulator gap andmetal/insulator gap, and performed an analysis for condi-tions in which a voltage of 16 or 32 kV was applied to a2-mm gap. Table 1 summarizes the damping time when avoltage of 16 kV DC was applied to a high-voltage elec-trode. When the average surface roughness was low, thedamping time for the leak current was under 3 minutes forboth the insulator/insulator gap and the metal/insulator gap,and the current at 10 minutes after the voltage was appliedwas damped to less than 0.1 pA. Table 1 also shows the casein which the metal surface between the metal and insulationgap was 22.2 µm. When the metal surface was rough, acurrent component with a longer damping time was ob-served, but results in which this was under 0.1 pA at 1 hourafter voltage was applied were obtained.

The authors also performed an experiment for anapplied voltage of 32 kV. Figure 9 shows the current wave-form observed in the insulator/insulator gap. The authorsconfirmed that as the surface becomes rougher, there tendsto be an increase in the current component with a longdamping time in the time range of 100 s to 1000 s aftervoltage application. Because an electric field is applied tothe insulators that enclose the gap, the measured currenttakes a current waveform with superimposed current corre-sponding to the charge moving across the gap and thecharging current in the insulator. The increase in the current

Fig. 8. Experimental setup for measuring theelectrification characteristics of the epoxy insulator.

Table 1. Damping time at various electrodeconfigurations (electric field strength at the gap is

approximately 8 kV/mm)

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with a long damping time appearing after the chargingcurrent component immediately after voltage is appliedcorresponds to an increase in the charge moving across theinsulator/insulator gap. Table 2 summarizes the dampingtimes for the current waveforms obtained for the insula-tor/insulator gap and the metal/insulator gap for 32 kV ofvoltage. The authors performed experiments with threeconditions for the insulator/insulator gap and with twoconditions for the metal/insulator gap, and found that com-pared to when surface roughness was low for both, thedamping time increased as the surface roughness rose. Thistendency was particularly strong when the average surfaceroughness Rz rose above 20 µm. Moreover, the dampingtime for electrification generated in the metal/insulator gapwas greater than for between the insulator/insulator gap.

Based on the results of the experiments above, it isclear that the damping time for electrification in a spacerinsulator varies depending on the state of the surface, andthat the time may increase as the surface becomes rougher.However, the damping time was relatively short at a maxi-mum of 2 hours.

5. Conclusion

Based on experimental measurements in a series ofevaluations of the insulation characteristics of insulatingspacers where a DC voltage remains in GIS, the authorsevaluated various spacer electrification phenomena result-ing from electric field emissions as well as factors thatdetermine the resistivity of the insulator surface layer. Theresults obtained are summarized below.

(1) Using a highly heat-resistant epoxy resin widelyemployed as a material for GIS insulation spacers, andbased on the radius dependency for the resistance of the testmaterials, the authors found the main pathway for theleakage current when a DC voltage is applied. Based on theresults of an analysis, the authors obtained experimentalresults in which the bulk resistance was dominant comparedto the surface resistance.

(2) The authors experimentally evaluated the effectsof sublimation of the hardening agent when molding insu-lation spacers as a factor that has an effect on resistivity inbulk materials for the layers near the surface of the insula-tion spacer. For an epoxy resin with low levels of thehardening agent, the volume resistivity over a long timeperiod after DC voltage is applied tends to decrease, andthe dispersion in the volume resistivity between the testmaterials tends to increase. Moreover, based on a compari-son of the changes in the complex relative dielectric con-stant when the amount of the hardening agent is varied, thepolarization component thought to correspond to the unre-active component of the hardening agent tends to be largerin test materials with lower levels of the hardening agent.

(3) The authors evaluated electrification phenomenain the insulation spacers using surface roughness as a pa-rameter. The current component for a slow damping time issuperimposed on the current waveform when a DC voltageis applied to the electrode in opposition to the insulator/in-sulator or the metal/insulator. This tends to increase as thesurface roughness rises, and becomes significant when thesurface roughness exceeds 20 µm. The time for electrifica-tion is approximately 2 hours at a maximum, with themetal/insulator gap being greater than the insulator/insula-tor gap.

The authors plan to continue their evaluations towardthe end of establishing an evaluation method for the per-formance of DC resistance obtained in AC GIS by conduct-ing evaluations of the effects of the surface state of epoxyinsulation materials on GIS DC voltage resistance usingexperiments and overvoltage electric field analysis.

REFERENCES

1. Gas Insulated Switchgear. JEC-2350-2005. (in Japa-nese)

Fig. 9. Current at an insulator/insulator gap condition.

Table 2. Damping time at various electrodeconfigurations (electric field strength at the gap is

approximately 16 kV/mm)

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2. For example, Yoshida Y, Hashimoto T, Sampei M,Aoyagi H, Nishiwaki S, Kobayashi A, Iida T. Con-ception of insulation design for DC 500 kV GIS. ProcSixth Annual Conference of Power & Energy Soci-ety, IEE Japan, Vol. B, p 733–734, 1995. (in Japanese)

3. For example, Rokunohe T, Endo F, Yoshida Y, Naka-goe Y, Hatano M. Fundamental investigation for fire-fly phenomenon of particle under DC voltage in SF6

gas. IEEJ Trans PE 2004;124:1154–1160. (in Japa-nese)

4. Hasegawa T, Yamaji K, Sampei M, Kobayashi A,Aoyagi H, Ishikawa M. Development of DC 500 kV

GIS insulating spacer. Proc Sixth Annual Conferenceof Power & Energy Society, IEE Japan, Vol. B, p741–742, 1995. (in Japanese)

5. DC insulation in gas insulated switchgear. TechnicalReport of the Institute Electrcal Engineers of Japan(II), No. 397, 1991.

6. Fujinami H, Takuma T, Yashima M, Kawamoto T.Mechanism of the charge accumulation and insula-tion characteristics of gas insulated spacer under DCstress. Trans IEE Japan 1988;108-B:297–304. (inJapanese)

AUTHORS (from left to right)

Shuhei Kaneko (member) completed a master’s at Keio University in 2000 and joined the Tokyo Electric Power Company.At present he is with the High Voltage and Insulation Technology Group in the Technology Development Laboratory. He isprimarily pursuing research related to gas-insulated switchgear.

Shigemitsu Okabe (senior member) completed a doctorate at the University of Tokyo in 1986 and joined the Tokyo ElectricPower Company. He became a researcher at the Munich Institute of Technology in 1992. At present he is the group manger ofthe High Voltage and Insulation Technology Group in the Technology Development Laboratory. He holds a D.Eng. degree, andis a member of IEEE.

Takayuki Kobayashi (senior member) graduated from the Department of Electrical Engineering at Yokohama NationalUniversity in 1984 and joined the Tokyo Electric Power Company. Since then he has been working on the design anddevelopment of technology for transformers. At present he is in charge of transformer technology in the Engineering Division.

Kenichi Nojima (member) completed a master’s at Osaka University in 1982 and joined Toshiba Corporation. At presenthe is with the Transmission and Electrical Device Development Division at the Power and Society Systems TechnologyDevelopment Center. He is primarily working on the development of insulation technology for gas-insulated switchgear. Heholds a D.Eng. degree, and is a member of IEEE.

Masafumi Takei (nonmember) has been working on the development of insulation materials and the design anddevelopment of gas-insulated switchgear. At present he is the head of the Switchgear Department at the Hamakawasaki Factory.

Taketoshi Miyamoto (member) completed a master’s at Tohoku University in 1992 and joined Toshiba Corporation. Sincethen he has been working on engineering for power transformers. At present he is the head of the Transformer TechnologyGroup in the Power Transformer Technology Division.

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