Cryogenics 45 (2005) 45–50

www.elsevier.com/locate/cryogenics

Electrical characteristics of high-Tc superconductingmini-model cable under mechanical stresses in liquid nitrogen

H.J. Kim a, D.S. Kwag b, Y.S. Kim c, S.H. Kim b,*

a Applied Superconducting Group, KERI, 28–1 Seongju-dong, Changwon 641-120, Republic of Koreab Dept. of Electrical Engineering, Gyeongsang National University, 900 Gazwa, Jinju 660-701, Republic of Korea

c Electrical Safety Research Institute, KESCO, 27 Gapyeong-gun, Gyeonggi-do 477–814, Republic of Korea

Received 16 December 2003; received in revised form 14 September 2004; accepted 14 September 2004

Abstract

To develop 22.9kV class high-Tc superconducting (HTS) cable in Korea, we have been studying electrical insulation properties of

dielectric paper, such as breakdown voltage, partial discharge, which is one of the HTS cable structure elements. However, the

research on the mechanical stress of dielectric paper compared to breakdown properties of dielectric paper is insufficient. A cracking

and variation of the electrical insulation due to mechanical stresses during cooling and bending of HTS cables in cryogenic temper-

ature is a serious problem. Thus, we investigated tensile stress and breakdown stress of dielectric paper under mechanical stress.

Moreover, we manufactured mini-model cables investigated breakdown stress under bending stress to design a cable drum for con-

veyance. In the AC, impulse and partial discharge properties, all test results showed a similar tendency, and the suitable bending

radius ratio R/r was decided to be more than 25.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: HTS cable; Mechanical stress; Breakdown stress; Dielectric paper

1. Introduction

The application of superconducting cable using high-

Tc superconductors (HTS) has been studied and devel-oped because of the advantage of achieving large power

delivery with negligible AC loss compared with conven-

tional power cables [1–3]. The Korea Electrotechnology

Research Institute (KERI) and LG cable are developing

a 22.9kV class HTS cable in one of the 21st century

superconducting frontier projects in Korea [4].

The HTS cable is composed of a conductor, pipe for

cryogenic temperature, electrical insulation and so on.Also, the composite insulation of the liquid nitrogen

and synthetic polypropylene laminated paper (PPLP)

0011-2275/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cryogenics.2004.09.001

* Corresponding author. Fax: +82 55 761 8620.

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

has been used for the HTS cable insulating materials

[5,6]. Until now, we have investigated AC breakdown,

impulse breakdown and partial discharge of PPLP in

liquid nitrogen. Based on these results, the parametersof the electrical insulation for the 22.9kV class HTS

cable were designed [7].

Mechanical properties and electrical properties of

dielectric paper in cryogenic temperature and room tem-

perature are expected to be different because the HTS

cable system is operated in liquid nitrogen. The HTS

cable is exposed to mechanical stresses consisted of

residual internal stress generated during initial cooling,external stress due to the cable bend during the installa-

tion and for the conveyance and thermo-mechanical

stress due to the differential between thermal expansion

of the conductor and the insulation material. It is gener-

ally known that mechanical stresses have some influence

on the electrical properties and degradation of dielectric

Carbon paper

Butt gap PPLP

0.119 mm

25mm

Carbon paper

Butt gap PPLP

0.119 mm

25mm

Fig. 2. Cross section drawing of the insulation of the mini-model

cable.

46 H.J. Kim et al. / Cryogenics 45 (2005) 45–50

paper [8,9]. However, the studies of electrical properties

of dielectric paper that are used in an HTS cable have

not been investigated in liquid nitrogen though long-

term operation of HTS cables for development of the

realization. Furthermore, a cable drum is fabricated

more than 10m of length because it is necessary to con-vey a HTS cable place [10].

In order to develop the 22.9kV class HTS cable,

firstly, the electrical and mechanical stress characteristics

of PPLP were investigated in liquid nitrogen. Secondly,

using these data, we designed and manufactured mini-

model HTS cables. Finally, we studied AC breakdown,

impulse breakdown and partial discharge of the cable in

liquid nitrogen under mechanical stress that was gener-ated in cable bending.

2. Sample and manufacture of the mini-model cable

The dielectric paper of the mini-model cable was

PPLP that had a thickness of 0.119mm and a density

of 0.89g/cm2. Fig. 1(a) and (b) shows the shape of thesample and the electrode on an experiment for mechan-

ical and electrical test. In Fig. 1(a), we made two types of

sample (i) MD type and (ii) CD type. Here, the machine

direction (MD) and cross direction (CD) represent a

30

3070

Adhesive tape

Adhesive tape

Gau

ge le

ngth

i) MD type

(a) (b)

ii) CD type

Jig

2510

SUS

electrode

φ φ

Fig. 1. Shape of the sample and electrode system: (a) shape of the

sample; (b) electrode system.

Fig. 3. Shape of the mini-model cable by bending stress:

horizontal direction and a vertical direction of PPLP.

The size of sample has a width of 30mm and a gauge

length of 70mm. The both ends of the sample were rein-

forced the adhesive tapes to prevent a break of the jig

part. It is effective to prevent break of jig part. Moisture

in the sample did not suppress 0.1% by dryings around105 �C for 1h. The sample was fixed with metallic frame

jigs as shown in Fig. 1(b), and then sandwiched with two

electrodes for breakdown measurements.

Fig. 2 shows an insulation composition of a mini-

model cable. A flexible stainless former of the cable

was wrapped by carbon paper. Starting from the first

layer, the PPLP tape was wound spirally, overlapping

at 30% between each layout of the PPLP. Consequently,the cable had a thickness of 1mm. The stress cone was

made with PPLP to prevent surface flashover on the

cable terminal. The manufactured mini-model cables

were bent by using a cable drum that had a bending ra-

dius. The manufactured cables are shown in Fig 3(a) and

(b).

The bending radius ratio is calculated by Eqs. (1) and

(2) as follows:

W þ 2G ¼ ðRþ 2rÞh ð1Þ

W ¼ Rh ð2ÞThis equation is classified as follows:

Rr¼ W

Gð3Þ

Where, G is the length of the butt-gap, W is the width ofthe PPLP, R is the radius of the cable drum and r is the

radius of the cable with a value of 15mm that includes

the conductor and the insulation. h is the bending angle.

The bending radius ratio R/r of the manufactured

mini-model cable was five cases (R/r = 10, 15, 20, 25,

(a) bending radius of cable; (b) shape of the cable.

H.J. Kim et al. / Cryogenics 45 (2005) 45–50 47

straight). As the bending radius ratio decreased, the butt

gap length of the out side of the cable was extended.

3. Experimental method

Fig. 4 shows a schematic for the breakdown test and

the tensile test of samples. The sample was elongated

uniaxially at 300K and 77K. The crosshead speed of

the test machine was 5mm/min. The tensile stress was

applied parallel to the sample surface. The tensile strain

of both directions is expressed by the ratio of elongation

as follows:

L� L0

L0

� 100 ½%� ð4Þ

where L0 and L represent the gauge length before and

after the elongation, respectively.

5

5 m5 mm/min

H.H.V4

1

2

3

6 7

8

1. Cryostat 2. Jig

3. Electrode system 4. Load cell

5. LN2 dewar 6. Pressure gauge

7. Vacuum gauge 8. Controller

Fig. 4. The schematic drawing of the experimental apparatus.

Fig. 5. The tensile stress–strain curves of PPLP at differen

After the elongation, the sample measurement was

performed after 10 min because there were the relaxa-

tion phenomena of internal stress. Also, measurements

were done 2 and 4 times in order to obtain reliable val-

ues. High voltage was applied by using a high voltage

apparatus (Max 100kV, Baur Co.), after power of thetensile machine (Lloyd) was turned off to protect it from

high voltage.

On the order hand, the breakdown test of the mini-

model cable under bending stress was performed in liq-

uid nitrogen. Impulse voltage was applied by using an

impulse apparatus (1.2 · 50ls waveform, maximum

300kV). The PD inception stress was measured by using

a partial discharge detector(IEC6027, NihonkesokiCo.).

4. Results and discussion

Fig. 5(a) and (b) shows tensile-stress strain curves at

300K in the air and at 77K in liquid nitrogen. In Fig.

5(a), the tensile stresses at breaking point of MD andCD were 87.5MPa, 36.9MPa, respectively. The tensile

stress of MD was higher than one of CD, but the tensile

strain of CD was larger than that of MD. In Fig. 5(b),

the tensile stresses at breaking point of MD and CD

were 100.8MPa, 68MPa, respectively. These values were

higher than these of the air, but the tensile strains were

decreased sharply. Therefore, it was known that the ten-

sile stress was high and tensile strain was declined as thetemperature went down. The PPLP consisted of two lay-

ers of kraft paper with thickness of 25lm and rough

polypropylene film with a thickness of 69lm. The for-

mer has high tensile stress and the latter has high tensile

strain. According as the temperature decreases, the mol-

ecule activity of PPLP is gradually weak and tensile

stress is high by effect of kraft paper.

Photographs of broken PPLP of CD at 300K and at77K are shown in Fig. 6(a) and (b), respectively. They

showed the different break shape slightly as the each

case. In the case of the air as shown in Fig. 6(a), the type

of broken PPLP had many cracks by strain. In the case

t temperatures: (a) in air and (b) in liquid nitrogen.

Fig. 6. Photograph of the broken PPLP in the direction of CD: (a) in air and (b) in liquid nitrogen. (a) 300K (b) 77K.

Fig. 7. Tensile stress dependence of dielectric breakdown stress of

PPLP.

Fig. 9. After tensile stress at the air, dielectric breakdown stress of

PPLP.

48 H.J. Kim et al. / Cryogenics 45 (2005) 45–50

of liquid nitrogen as shown in Fig. 6(b), however, the

cracks were not observed and the side of the broken

PPLP was rough.

Fig. 7 shows tensile stress dependence of breakdown

stress of the PPLP. In this figure, the x- and y-axis rep-

resent tensile stress and breakdown stress respectively

and the breakdown value is an average value. Accordingas tensile stress increase, breakdown stress of PPLP is

not influenced to 20 MPa tensile stress. However, the

breakdown stress is somewhat decreased around frac-

ture of PPLP as shown in Fig. 5(b).

Fig. 8 shows a photograph of broken PPLP after

breakdown at 81.6MPa. As tensile stress increased,

weak points such as micro-cracks of PPLP were gener-

ated. Simultaneously with distractions of PPLP it isthought that the breakdown occurred.

Fig. 8. Photograph of breakdown under tensile stress at 81.6MPa.

Fig. 9 shows breakdown stress of the PPLP. This test

was carried out in liquid nitrogen after elongation in the

air. As well as results shown in the Fig. 7, the break-

down stress was decreased around the fracture. Break-

down stresses in air were somewhat lower than that of

Fig. 7. Since tensile strain in air was relatively higher

than that in liquid nitrogen, the weak points such as

micro-cracks or wrinkle cracks were easily exposed in

Fig. 10. AC breakdown voltage of the mini-model cables according to

bending radius ratio.

Fig. 11. Photograph of breakdown of the mini-model cable: (a) straight and cable (b) in case of R/r = 10.

Mini- odel cablem77 K

PD extinction

PD inception

10 20 30

Bending radius multiple R / rStraight0

8

6

4

2

0

Part

ial d

isch

arge

str

ess

[kV/

mm

]

Fig. 13. Partial discharge stress of the mini-model cables that were

bended at various bending radius ratio (R/r).

H.J. Kim et al. / Cryogenics 45 (2005) 45–50 49

the air. Therefore, in case of manufacturing the insula-

tion of the HTS cable, the tensile stress is suitable under

19.4MPa. It does not influence to electrical degradation

in manufacture process of HTS cable insulation.Fig. 10 shows the AC breakdown voltage of the mini-

model cable for the bending radius ratio. Breakdown

stresses were almost the same in the case of straight

and the R/r of 25 and were sharply decreased in the case

of R/r of 10. The reason is considered is that the butt gap

has more faults according to mechanical stresses.

Fig. 11 shows the breakdown photograph of the

straight and bended mini-model cable. In the case ofthe straight cable, the butt gap was a starting point of

the electric discharge. However, in the case of R/

r = 10, the PPLP was cracked by wrinkles (white part

of the surface in the photo) and the electric discharge

started at the crack.

If AC high voltage was applied, the intensity of elec-

tric stress was divided to two parts of liquid nitrogen

and PPLP. The breakdown happened in liquid nitrogenfirstly because the liquid nitrogen has lower permittivity

(e = 1.432). However, PPLP of the mini-model cable was

cracked by the bending and the breakdown was pro-

duced around this cracked part.

Fig. 12 shows the impulse breakdown voltage of

mini-model cables that were bended at various bending

Fig. 12. Impulse breakdown voltage of the mini-model cables that

were bended at various bending radius ratio (R/r).

radius ratios. Also, breakdown voltages were almost the

same in the case of straight and the R/r of 25 and were

sharply decreased in the case of R/r of 10.

Fig. 13 shows the inception and extinction stress of

partial discharge by the bending radius ratio of mini-model cable. Also, partial discharge characteristics indi-

cate tendency similar to AC and impulse breakdown

characteristics.

5. Conclusion

The PPLP had a high tensile stress in liquid nitrogen,but low tensile strain. As tensile stress increased the

breakdown stress of PPLP was somewhat decreased

because of the microcrack occurrence. Based on these

results, the tensile stress of PPLP was calculated to

be suitable value in HTS cable manufacture.

We manufactured the mini-model cable using these

data. Also, the bending radius ratio (R/r) of bended

mini-model cables has five cases (R/r = 10, 15, 20, 25,straight). In the AC, impulse and partial discharge prop-

erties, all test results showed a similar tendency, and the

suitable bending radius ratio (R/r) was decided to be

more than 25.

50 H.J. Kim et al. / Cryogenics 45 (2005) 45–50

References

[1] Steve N et al. High temperature superconducting cable field

demonstration at Detroit Edison. Physica C 2001;354:49.

[2] Nassi M. HTS prototype for power transmission cables:recent results

and future programmes. Supercond Sci Technol 2000;13:460–3.

[3] Honjo S, Takahashi Y. Outline of verification tests on a

superconducting cable system for practical use. Cryogenic Eng

In Japan 2001;36:242–8.

[4] Chul KC et al. Development of 22.9kV Class Superconducting

Cable, Center for Applied Superconductivity Technology Report,

2002.

[5] Okubo H et al. Partial discharge inception V-t characteristics for

pressurized liquid nitrogen/PPLP composite insulation system.

Dielectric Liquids, ICDL, Proceedings of 2002 IEEE 14th

International Conference on, p. 123, 2002.

[6] Suzuki H et al. Dielectric insulation characteristics of liquid

nitrogen impregnated laminated paper insulated cable. IEEE

Trans Power Delivery 1992;7(4):1677.

[7] Kim SH et al. Dielectric Characteristics of Insulating papers for

HTS Cable. Korea-japan joint Workshop 2002 on Applied

Superconductivity and Cryogenics, p. 9, 2002.

[8] Kim SH et al. Mechanical and electrical properties of insulating

materials at cryogenic temperature. The Journal of the Korean

Institute of Electrical and Electronic Material Engineering

1996;9(10):1033–9.

[9] Nishijima S, Hara M. Mechanical influence on long-term dielec-

tric performance of insulants. Cryogenics 1998;38:1105–13.

[10] Mukoyma S et al. Design and production of high-tc supercon-

ducting power transmission cable. IEEE Trans Appl Supercond

2001;11(1).