Cryogenics 52 (2012) 656–660

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Cryogenics

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High voltage insulation of bushing for HTS power equipment q

Woo-Jin Kim, Jae-Hyeong Choi, Sang-Hyun Kim ⇑Department of Electrical Engineering, Gyeongsang National University and ERI, Jinju, Gyeongnam 660-701, South Korea

a r t i c l e i n f o a b s t r a c t

Article history:Available online 16 May 2012

Keywords:Surface flashover characteristicsInsulating materialsHTS HV bushingAC and impulse voltage

0011-2275/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.cryogenics.2012.05.006

q Asian Conference on Applied Superconductivity a⇑ Corresponding author. Address: Department of El

sang National University and ERI, 900 Gazwa-dongSouth Korea. Tel.: +82 55 772 1714; fax: +82 55 772

E-mail address: shkim@gnu.ac.kr (S.-H. Kim).

For the operation of high temperature superconducting (HTS) power equipments, it is necessary todevelop insulating materials and high voltage (HV) insulation technology at cryogenic temperature ofbushing. Liquid nitrogen (LN2) is an attractive dielectric liquid. Also, the polymer insulating materialsare expected to be used as solid materials such as glass fiber reinforced plastic (GFRP), polytetra-fluoro-ethylene (PTFE, Teflon), Silicon (Si) rubber, aromatic polyamide (Nomex), EPDM/Silicon alloy compound(EPDM/Si).

In this paper, the surface flashover characteristics of various insulating materials in LN2 are studied.These results are studied at both AC and impulse voltage under a non-uniform field. The use of GFRPand Teflon as insulation body for HTS bushing should be much desirable. Especially, GFRP is excellentmaterial not only surface flashover characteristics but also mechanical characteristics at cryogenic tem-perature. The surface flashover is most serious problem for the shed design in LN2 and operation of super-conducting equipments.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, a superconducting technology has attracted a greatdeal of interest from the view point of energy. The HTS powerequipments are developing elsewhere in the world [1,2]. A bushingis one of the essential components for power equipments. It is usedfor connecting a cable or the winding of the transformer. In Korea,KERI and LS Cable Ltd. were developed 154 kV class HTS AC cable.However, the development of 154 kV class HTS AC bushing wascompletely not perfect. For the development of HTS bushing, it isnecessary to know a HV insulation and materials technology incryogenic environment. Especially, the insulation technology inelectrical problems must be solved for the long life, reliabilityand compact of HTS HV bushing.

So far, many papers have been published on the oil, gas and re-sin insulated bushing [3,4]. However, the research and develop-ment of HTS bushing have not been revealed satisfactorily untilnow, which make the insulation design of HTS power equipmentsvery difficult.

The insulating structure of HTS bushing is insulated with fourlayers of air, oil (or SF6), cryogenic gas nitrogen (GN2) and LN2.On HTS HV bushing, the conductor is wound with the insulatingmaterial such as paper or polymer. Also, the insulator with sheds

ll rights reserved.

nd Cryogenics 2011/P-118.ectrical Engineering, Gyeong-, Jinju, Gyeongnam 660-701,1719.

is made with the insulating material such as polymer or porcelain.Therefore, the insulating materials are very important part. Thespace between upper insulation body and insulator is filled withoil or gas. Especially, the lower insulation body is impregnatedwith cryogenic GN2 and LN2. HTS bushing must span a temperaturerange from 77 K to 300 K. Therefore, there are varieties of thermal,mechanical and electrical problems to be solved for such purpose.As electrical problems, we must reveal the characteristics of (1)breakdown of LN2, (2) breakdown of GN2 at cryogenic temperature,(3) breakdown of solid insulators at cryogenic temperature, (4)surface flashover on solid surface and liquid surface at cryogenictemperature, (5) degradation of electrical properties of solid insu-lator at cryogenic temperature, (6) surface discharge of solid insu-lator in oil and (7) surface discharge of solid insulator in air.

Until now, we have studied AC breakdown characteristics of154 kV class HTS cable and transformer in LN2 [5,6]. In supercon-ducting machines, various insulating materials such as polymersimpregnated with LN2 are used in many cases. However, the studyon various insulating materials have not revealed satisfactorily un-til now. In practical bushing, the surface flashover characteristics ofsolid materials at cryogenic temperature are most serious problemin insulation. Therefore, it is important to understand the surfaceflashover characteristics of solid insulating materials in LN2 at firststage.

In this paper, we will discuss mainly on the HV insulation ofGFRP, Si rubber, Nomex, EPDM/Si and Teflon in LN2. These resultsare studied at both AC and impulse voltages under a non-uniformfield.

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2. Experiment

2.1. Sample and electrode system

Fig. 1 shows the electrode system for surface discharge. To sim-ulate non-uniform high electric field, the electrode system for sur-face discharge is used stainless steel (SUS) triangle (tip angle: 60�)and planar configuration. To avoid the shift of distance in LN2 be-tween the electrodes, the electrodes were fixed by the polymerbolt on the specimen.

The samples used as solid insulating material was GFRP, Teflon,Si rubber, EPDM/Si (in 70:30 ratio) and Nomex board, which were0.5, 1 and 1.5 mm in thickness. And the surface flashover distanceswere 5, 10 and 15 mm, respectively. The samples with electrodeswere dried by using the drier to completely remove the moisture,and cleaned carefully with ethyl alcohol. The electrode system islocated by the horizontal arrangement of stainless steel electrodeson GFRP plate. The sample with electrodes was fixed with sampleholder. Also, the samples used as liquid were silicon oil and LN2.

Fig. 1. Electrode system for surface discharge.

Fig. 2. Experimental set up.

2.2. Experimental apparatus and method

Fig. 2 shows the experimental set up for AC and impulse break-down test. The cryostat made from SUS has dual vacuum layersoutside so as to stabilize LN2 thermally.

The sample holder was attached to the lower part of bushing onthe cryostat. After introducing the commercial-grade LN2, AC andimpulse voltage was applied between the electrodes of sample.AC power used in the experiment was 60 Hz frequency and100 kV maximum voltage, and its voltage was increased at 2 kV/sec speed manually. The lightning impulse power was 1.2/50 lsstandard waveform and 400 kV maximum voltage, and the exper-iment was carried out by 4 kV intervals until the breakdown volt-age was reached. The surface discharge voltage is obtained15 times at the same condition.

3. Results and discussion

In order to develop the HTS bushing, it is necessary to study thepuncture breakdown and the surface flashover characteristics ofvarious insulating materials in LN2. At first stage, we studied thesurface flashover characteristics of GFRP in LN2. Fig. 3 shows theflashover length dependence of surface flashover voltage of GFRPunder AC and impulse voltage. The surface flashover voltage ismeasured at the atmospheric LN2. When a negative or positivevoltage is applied to the SUS triangle electrode, we term this ‘‘neg-ative impulse’’ and ‘‘positive impulse’’, respectively. And the errorbar shows the maximum, average and minimum value of surfaceflashover voltage.

As seen in the figure, the AC surface flashover voltage of GFRPwas lower than the impulse surface flashover voltage, it increaseswith increasing of electrode length. The impulse surface flashovervoltage quickly increases with increasing of the electrode length.Also, the impulse surface flashover voltage of negative polarity isslightly higher than that of positive polarity. These characteristicswere interpreted in terms of the degradation, space charge underAC voltage and formation of positive and negative streamer underimpulse voltage. However, more detailed studies are now inprogress.

The ratio of negative/positive voltage is approximately 1.1.From above experiment, the relation between negative surfaceflashover voltage Vs and the electrode length (l) can be expressedby Vs = 14ln (kV) with n = 0.6. Note that the n-value for GFRP is0.6 (close to 1). It means that GFRP may be used as an insulatingmaterial for HTS bushing.

Fig. 3. Surface flashover characteristics of GFRP.

(a) Impulse (+)

(b) Impulse (-)

Fig. 5. Surface flashover characteristics of various insulating materials. (a) Impulse(+), (b) impulse (�).

658 W.-J. Kim et al. / Cryogenics 52 (2012) 656–660

The electrode length dependence of AC surface flashover volt-age in LN2 is shown in Fig. 4 with in comparison those of GFRP,EPDM/Si, Teflon, Nomex and Si rubber. As seen in the figure, theAC surface flashover voltage is higher in the order EPDM/Si, Teflon,GFRP, Nomex, Si rubber in every electrode length. However, thecharacteristics of AC surface flashover are similar in all the samplesbecause it is in the range of error. In fact, it can be found that the n-values for EPDM/Si, Teflon, Nomex and Si rubber are all close tothat for GFRP. Therefore, it is can be concluded that their electricalcharacteristics are in favor of HTS transformer bushing.

Fig. 5a and b shows the electrode length dependence of positiveand negative impulse surface flashover voltage under the same con-dition, respectively. As evident from Fig. 5a, the positive impulsesurface flashover voltage is higher in the order EPDM/Si, Teflon,GFRP, Nomex, Si rubber in every electrode length. Especially, thepositive impulse surface flashover voltage of Nomex is high remark-ably. It seems to be almost the same because it is in the range oferror. On the other hand, the negative impulse surface flashovervoltage is higher than that of positive. However, in positive andnegative impulse voltage, the characteristics of surface flashovervoltage seem to be almost the same. It should be noted that thecracks or broken parts are observed on the surface of material withEPDM/Si or Si rubber after surface flashover.

Fig. 6 shows the photographs of material surfaces obtained byusing the camera after many flashover events under impulse volt-age. A damage of surface can be observed by the eyes. As evidentfrom Fig. 6a, a discharge trace was observed on the surface of GFRP.It is not severely degraded by the surface discharge. However, itbecomes more severe with the increase of applied voltage. Espe-cially, in the case of impulse, the degradation of surface becomesmore severe. These phenomena at cryogenic temperature can beseen in Nomex and Teflon. However, in the case of Nomex, the sur-face becomes degraded severely according to break of the molecu-lar chain. On the other hand, the surfaces of Si rubber (Fig. 6b) andEPDM/Si are severely damaged with the cracks or broken partsafter surface flashover. It is clear from these results that at cryo-genic temperature Nomex, EPDM/Si and Si rubber cannot be usedthe as the insulating material for HTS bushing. These experimentalresults can be interpreted in terms of the discharge energy and theglass transition temperature (Tg) of materials [7].

Table 1 shows the Tg and mechanical characteristics of insulat-ing materials [8,9]. As is evident from table, Tg of EPDM/Si and Sirubber (�127 �C) are remarkably lower than that of others. It isclear from these result that at cryogenic temperature EPDM/Siand Si rubber cannot be used as the insulating material for HTSbushing. In conclusion, the use of GFRP and Teflon as the insulators

Fig. 4. AC surface flashover characteristics of various insulating materials.

(a) GFRP (b) Si rubber

Fig. 6. Photographs of material surface after surface discharge. (a) GFRP, (b) Sirubber.

for HTS bushing should be much desirable. Especially, GFRP isexcellent material not only electrical characteristics but alsomechanical characteristics at cryogenic temperature. However,manufactured and economic problems must study in the develop-ment of HTS bushing. A more detailed study on other insulatingmaterials for HTS bushing is now progress.

HTS bushing must span a temperature range from 77 K to 300 K.Therefore, it is necessary to study the temperature characteristicsof surface flashover voltage. Fig. 7a and b shows AC and impulsesurface flashover characteristics of GFRP in silicon oil (300 K) andLN2 (77 K). As evident from these figures, AC surface flashover volt-age is similar for both cases. The impulse surface flashover voltage

Table 1Tg value and mechanical characteristics of insulating materials.

Sample Tensile strength (MPa) Tg (�C)

GFRP 480 130Teflon 20–25 300Si rubber 7–45 �127EPDM/Si 15 �32Nomex 44 280

(a) AC

(b) Impulse(+)

Fig. 7. Surface flashover characteristics of GFRP in oil and LN2. (a) AC, (b) impulse(+).

Fig. 8. Breakdown strengths of LN2, oil, GFRP and surface flashover strength in LN2.

W.-J. Kim et al. / Cryogenics 52 (2012) 656–660 659

in LN2 is a little higher than that in silicon oil. For both cases, it isalmost similar, because it is in the range of error.

Fig. 8 shows AC breakdown strengths of LN2, silicon oil, GFRPand surface flashover strength of GFRP in LN2. The surface flashoverstrength (2–4 kV/mm) of GFRP in LN2 is much lower than that ofbreakdown strength (27–46 kV/mm) of LN2, silicon oil itself or so-lid insulating materials such as GFRP. It indicates that the surfaceflashover is most serious problem for the shed design in LN2 andoperation of superconducting machines. Details of the breakdownvoltage characteristics for various insulators will be reported in thenear future.

4. Conclusions

We have studied on the surface flashover characteristics of var-ious insulating materials under AC and impulse voltages. The re-sults may be summarized as follows:

(1) The impulse surface flashover voltage of negative polarity isslightly higher than that of positive polarity. The ratio ofnegative/positive voltage under impulse voltage is approxi-mately 1.1.

(2) The AC and impulse surface flashover voltage is higher in theorder EPDM/Si, Teflon, GFRP, Nomex, Si rubber in every elec-trode length. It seems to be almost the same because it is inthe range of error.

(3) However, it should be noted that the cracks or broken partsare observed on the surface of material with EPDM/Si or Sirubber after surface flashover. It can be interpreted in termsof Tg.

(4) The impulse surface flashover voltage in LN2 is a little higherthan that in silicon oil. Also, the AC surface flashover voltageis similar for both cases.

(5) GFRP and Teflon as the insulating material for HTS bushingare much more desirable than EPDM/Si, Teflon, Nomex andSi rubber. Especially, electrical as well as mechanical charac-teristics of GFRP are excellent at cryogenic temperature.However, manufactured and economic problems must studyin the development of HTS bushing.

(6) The surface flashover strength (2–4 kV/mm) of GFRP in LN2

is much lower than that of breakdown strength (27–46 kV/mm) of LN2, silicon oil itself or solid insulating material suchas GFRP. It indicates that the surface flashover is most seri-ous problem for the shed design in LN2 and operation ofsuperconducting machines.

Acknowledgments

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology (No. 2011-0013667).

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