Study on electric insulation properties for development of conduction-cooled HTS SMES

Cryogenics 47 (2007) 397–401 www.elsevier.com/locate/cryogenics

Study on electric insulation properties for development of conduction-cooled HTS SMES Jae-Hyeong Choi a, Hyeon-Gweon Cheon a, Dong-Soon Kwag a, Hae-Jong Kim b, Ki-Chul Seong b, Mun-Soo Yun b, Sang-Hyun Kim a,* a

Department of Electrical Engineering, Gyeongsang National University and Engineering Research Institute, 900 Jinju, Gyeongnam 660-701, Republic of Korea b Applied Superconducting Group, KERI, 28-1 Seongju-dong, Changwon 641-120, Republic of Korea Received 12 April 2006; received in revised form 28 September 2006; accepted 25 April 2007

Abstract The conduction-cooled HTS SMES magnet is operated in high vacuum and cryogenic condition. Thus, electric insulation design at high vacuum and cryogenic temperature is a key and important element that should be established to accomplish compact design that is a big advantage of HTS SMES. However, the performance of insulators under cryogenic conditions in air or vacuum is virtually unknown. Therefore, we need active research and development of insulation concerning application of the conduction-cooled HTS SMES. Specially, this paper will present the results of high vacuum and cryogenic temperature breakdown and flashover discharge characteristics between cryocooler and magnet-coil. The breakdown and surface flashover discharge characteristics are studied at cryogenic temperature and vacuum. Also, some materials such as aluminum nitride (AlN), Al2O3 that have high thermal conduction are taken up and the flashover discharge characteristics are investigated. From the results, we confirmed that basic database on the discharge between cryocooler and magnet-coil was established for the electric insulation design.  2007 Elsevier Ltd. All rights reserved. Keywords: Flashover; Breakdown; Insulation design; Conduction-cooled HTS SMES

1. Introduction The conduction-cooled high temperature superconductor (HTS) superconducting magnetic energy storage (SMES) magnet is driven in cryogenic and vacuum condition. The need to reduce the size and weight of the system has led to the consideration of the cryogenic vacuum [1]. Thus, for the development of the magnet, cryogenic insulation design should be established to accomplish compact system that is a big advantage of SMES [2]. In order to assess the adequacy of the insulation, it is necessary to have some understanding of the breakdown mechanism, especially in vacuum and solids which are normally used in combination. Although our understanding about these is *

Corresponding author. E-mail address: [email protected] (S.-H. Kim).

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incomplete, the characteristics or trends were studied by variety tests using model electrode system. Particularly, the characteristics of surface electrical discharge creep on the surface of a solid insulator in a cryogenic vacuum must be understood, because the surface flashover discharge on the solid insulator is lower than that without one [3]. The general structure of a conduction-cooled SMES system in vacuum consists of cryostat, cryocooler, current leads, magnet, cryogenic interface of the magnet and instrumentation that are shown in Fig. 1. The HTS magnet is the most important part, which can store electromagnetic energy without changing the energy to chemical or mechanical. A power conversion module is also very important [4]. Depending on size and application, the magnet could be a solenoid or a toroid [5]. The magnet is cooled by cryocooler. Here, a material of joint part that connect magnet and cryocooler must have high thermal

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Fig. 1. A sketch of the conduction-cooled magnet and insulating compositions.

conductivity and high dielectric strength. This point is very important for the insulation of magnet in terms of stability and reliability. However, according to the winding form, the insulation components become some different. Practically, the electrical and thermal insulations between coil-to-cryocooler, lead-to-flange and lead-to-lead are very important in a conduction-cooled HTS SMES [6]. These insulations must have thermal and electrical stability at the same time. These insulation techniques are very difficult because heat and electrical conduction are in an inverse relation. In this paper, we present and discuss very first results in this field, focused on materials such as AlN, Al2O3 that have high thermal conduction; and we investigate the flashover and breakdown characteristics and flashover characteristics dependence on the materials at cryogenic temperature and in vacuum. From the results, we confirm that our research established basic information for the insulation design of the magnet.

cryocooler, cryostat, electrodes and voltage source are used. The apparatus was used for measurement of breakdown and flashover voltage in cryogenic vacuum condition. The cryostat was made of stainless steel with 520 mm inner diameter and 1000 mm height. The electrical test apparatus was on ac dielectric strength test set, of which capacity was 100 kV, 1 kVA under 60 Hz. A slow ac ramp rate of 1 kV/sec was applied to the test samples until the breakdown or flashover occurred. The experiments were repeated 10 times for each sample to obtain an average of discharge voltage.

H. V

Sphere electrode d

2. Experimental

Plane electrode

Cooling head

Fig. 2 shows the established insulation test technique for conduction-cooled HTS SMES. Some equipments such as

Fig. 3. Electrode system for the breakdown characteristics.

Fig. 2. Experimental apparatus: (a) schematic diagram and (b) photograph.

J.-H. Choi et al. / Cryogenics 47 (2007) 397–401

399

H.V V H .V d

Triangle electrode

Sample Sample Plane electrode

Plane electrode

Sample

Cooling heading

Cooling head

Fig. 4. Electrode system for the flashover discharge: (a) for the layer-to-layer insulation and (b) for the magnet coil-to-cooling head insulation.

known that the breakdown characteristics at 300 K display U shape against degree of vacuum. This result shows that the voltage rises up when the pressure becomes low, following Paschen’s law. Fig. 6 shows the breakdown voltage as a function of temperature. The breakdown voltage slowly increased as the temperature decrease. 30 2×10-6 torr, SUS electrode, Sphere-planer 25 Breakdown Voltage (kV)

The electrode system for the breakdown characteristics is shown in Fig. 3. The diameter of upper sphere electrode was 10 mm and the diameter of lower plane electrode was 40 mm. Both were made of stainless steel. The lower electrode was contacted to cooling head. The gap was changed at 3 and 5 mm. Fig. 4a shows the electrode system for the surface flashover discharge characteristics. Multi-layer Al2O3 insulation spacer was placed between plane and plane electrodes. The diameter of plane electrode was 40 mm. The electrodes were made of stainless steel, too. Fig. 4b shows the electrode system for the characteristics of flashover discharge creep on the surface of a material. The tip angle of triangle electrode was 45. Also, the curvature radius of plane electrode was 10 mm to prevent the concentration of electric field at edge. The electrodes were made of aluminum tape. The electrode systems were cooled by the cryocooler after the electrode was set inside. The vacuum and temperature inside cryostat were down to 2 · 10 6 torr and 45 K from atmospheric, respectively.

d = 5mm 5 d = 3mm 3 0

100

200 Temperature (K)

300

400

Fig. 6. Breakdown characteristics dependence on the temperature.

25

25

Surface Flashover Voltage (kV)

300K, SUS electrode, Sphere-plane

Breakdown Voltage (kV)

10

0

Fig. 5 shows the breakdown voltage when the vacuum was changed at 300 K. The breakdown voltage sharply increased around the pressure of 2 · 10 6 torr. It is well

20 d=5mm d=3mm

10 5 0 1×10-6

15

5

3. Results and discussion

15

20

1×10-4

1×10-2

1×100

1×102

1×104

Vacuum (torr) Fig. 5. Breakdown characteristics dependence on the vacuum.

300K, d=4.95mm, Plane-plane(SUS) 20

15 10

5 0 1×10-6

1×10-4

1×10-2 1×100 Pressure (torr)

1×102

1×104

Fig. 7. Flashover characteristics dependence on the vacuum.

J.-H. Choi et al. / Cryogenics 47 (2007) 397–401

From the results of Figs. 5 and 6, it is clear that the high vacuum strongly affects on the breakdown voltage, but the cryogenic temperature has a little affect on the breakdown voltage. Fig. 7 shows the flashover voltage with spacer when the vacuum was changed at 300 K. As shown in Fig. 7, the flashover voltage with spacer is low in the pressure range from atmospheric to 2 · 10 6 torr. However, the flashover voltage abruptly increased around the pressure of 2 · 10 6 torr. It shows the similar tendency with the breakdown characteristics of Fig. 5. The flashover voltage at the vacuum of 2 · 10 6 torr is over double of that at atmospheric pressure. Fig. 8 shows flashover voltage dependence on the temperature. In the same condition, the flashover voltage slowly increases as the temperature decrease. The flashover voltage at 300 K and 45 K were 17 and 21 kV, respectively. In other words, there is little effect of temperature change on the flashover voltage. Surface flashover discharge volt-

15

Surface Flashover Voltage (kV)

400

10

d=6mm, 2×10-6 torr, 45K

5

AIN Al2O3 0

0

10

20

30

Number (N) Fig. 10. Flashover discharge characteristics at cryogenic and vacuum.

15

Surface Flashover Voltage (kV)

AlN Al2O3

40 Surface Flashover Voltage (kV)

-6

2×10 torr, d=4.95mm, Plane-plane(SUS) 30

20

d=6mm, 45K

10

5

0

10

0

5

10

15

Number (N) Fig. 11. Flashover discharge characteristics of without interval time of supply voltage.

0 0

100

200 Temperature (K)

300

400

Fig. 8. Flashover characteristics dependence on the temperature.

12 Surface Flashover Voltage (kV)

Al2O3

Triangle-planed electrode, d = 6mm, AC, in air

10 8 6

AlN Al2O3 FRP Sapphire Kapton

4 2 0 0

10

20 Number (N)

Fig. 9. Flashover characteristics in air.

30

age decreases gradually as the temperature rises. This tendency is same as the breakdown property shown in Fig. 6. Fig. 9 shows the flashover characteristics of various insulation materials in air. As shown in Fig. 9, the flashover voltage of Al2O3 is the highest and we obtained the ensuing results in the order of AlN, FRP, sapphire and Kapton. Moreover, Figs. 10 and 11 show the flashover characteristics in cryogenic vacuum. These materials such as Al2O3 and AlN have dielectric voltage of about 10 kV. Fig. 11 shows the flashover voltage when the voltage applied consecutively at the high vacuum and cryogenic. The flashover voltage slowly decreases. The flashover discharge will be one factor to make vacuum worse. 4. Conclusions In this paper, we have been construct insulating composition of the conduction-cooled HTS SMES that consists of seven parts. And we investigated the flashover and breakdown characteristics and the dependence of flashover

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behavior on the materials at cryogenic temperature and in vacuum. The main results are summarized as follows: (1) Breakdown and surface flashover voltages are very low in the range of atmospheric to 2 · 10 6 torr, but they rise rapidly and reach about two times larger level in high vacuum more than 2 · 10 6 torr. Also, surface flashover voltage slowly increases as temperature decreases. (2) Flashover voltage of Al2O3 is the highest among the materials tested and we obtained the ensuing results in the order of AlN, sapphire, FRP and Kapton. Therefore, from the view point of thermal conduction and high insulation properties, Al2O3 and AlN are thought to be suitable as HTS SMES magnet insulation materials. (3) If the operation condition of conduction-cooled HTS SMES is 10 4–10 5 torr and temperature is about 20–40 K, the breakdown and flashover voltages for insulation design of the HTS SMES must be considered carefully, because vacuum of the system is not enough.

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Acknowledgement This work was supported by Electric Power Industry Technology Evaluation and Planning. References [1] Allred DB, Benson JD, Cohen HA, Raitt WJ, Burt DA, Katz I, et al. The SPEAR-1 experiment: high voltage effects on space charging in the ionosphere. Nucl Sci, IEEE Trans on 1988;35(6):1386–93. [2] Cheon HG, Baek SM, Seong KC, Kim HJ, Kim SH. Insulation characteristics for a conduction-cooled HTS SMES. J Korea Inst Appl Supercond Cryogenics 2005;7(2):39–43. [3] Gerhold J. Properties of cryogenic insulants. Cryogenics 1998;38: 1063–81. [4] Nylund K, Schuler R. Insulation systems for synchronous machines, Proceeding of the international conference SM 100, Zurich, part 1, 1991; p. 182–8. [5] Kustom RL, Skiles JJ, Wang J, Klontz K, Ise T, Ko K, et al. Research on power conditioning systems for superconductive magnetic energy storage (SMES). IEEE Trans Magnetics 1991;27(2):2320–3. [6] Kasahara H, Akita S, Tasaki K, Tomioka A, Hase T, Ohata K, et al. Basic characteristic evaluation of cryocooler-cooled HTS coils. IEEE Trans Appl Supercond 2002;12(1):766–9.