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Aging characteristics of cryogenic insulator for development of HTS transformer
Cryogenics 45 (2005) 57–63 www.elsevier.com/locate/cryogenics
Aging characteristics of cryogenic insulator for development of HTS transformer Van-Dung Nguyen, Jong-Man Joung, Seung-Myeong Baek, Chang-Hwa Lee, Sang-Hyun Kim * Electrical Material Laboratory, Department of Electrical Engineering, (Engineering Research Institute), Gyeongsang National University, Jinju 660-701, Republic of Korea Received 16 December 2003; received in revised form 14 September 2004; accepted 14 September 2004
Abstract In the response to the demand for electrical energy, much effort aimed to develop and commercialize high temperature superconducting (HTS) power equipments has been made around the world. Especially, HTS transformer is one of the most promising devices. For the development of HTS transformer, the cryogenic insulation technology should be established. In this paper Vt characteristics of polyimide (Kapton) tape and GFRP used as turn-to-turn and structural insulations, respectively were studied. Moreover, breakdown hole site of GFRP after breakdown was also discussed. The experimental results show that the time to breakdown is conditioned on applied electric stress and the lifetime indices n of Kapton tape decrease slightly as the number of tape increases while the lifetime indices n of GFRP decrease strongly with increasing thickness. Furthermore, the breakdown holes of GFRP were not at the contact point, at which the electric field is maximum value, between sphere electrode and GFRP sample and its location depends on applied voltage as well as sphere diameter. 2004 Elsevier Ltd. All rights reserved. Keywords: Breakdown hole; Cryogenic insulation; HTS transformer; Vt characteristics
1. Introduction High temperature superconducting transformer has been developing in many countries because of its lighter weight, smaller volume and higher efficiency than those of the conventional transformer [1,2]. For designing electrical insulation of HTS transformer, it is very important to know the Vt and breakdown characteristics of liquid nitrogen (LN2)/polyimide (Kapton) and liquid nitrogen/ Glass fibre reinforced plastic (GFRP) composite insulation systems as well as the damage degree of insulation materials after breakdown [3–7]. Moreover, Vt characteristic is one of the important factors to establish the
testing level and estimate the lifetime of electrical insulation in superconducting transformer. However, the Vt and breakdown characteristics of these composite insulation systems have not been clarified yet. In this paper, we studied Vt characteristics of LN2/ Kapton, LN2/GFRP with considering of Weibull distribution and lifetime indices n on the condition of changing the number of Kapton layer and thickness of GFRP. We also investigated the breakdown hole position of GFRP sample after breakdown under different applied voltage and sphere diameter.
2. Experiment *
Corresponding author. Fax: +82 55 761 8620. E-mail address: [email protected] (S.-H. Kim).
0011-2275/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2004.09.002
Fig. 1 shows the electrode system for testing LN2/ Kapton tape composite insulation. Firstly, copper
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V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63
Kapton insulation
φ6 5
High voltage
A
A Copper electrodes Ground
Kapton-covered
A-A section
copper electrodes Fig. 1. Electrode configuration for testing Kapton tape.
High voltage High voltage
Sphere electrode
power supply
GFRP Outer dewar Inner dewar
Plane electrode Vacuum layer
Ground
LN 2
Fig. 2. Electrode configuration for testing GFRP.
Electrode system
electrode tapes with 0.3 mm · 4 mm cross-section were wound and overlapped helically by 0.03 mm · 10 mm cross section Kapton tape with 1, 2, 3 layers and then Kapton-covered copper electrodes were wound around 65 mm diameter disk solid insulators. Fig. 2 illustrates sphere-plane electrode configuration of LN2/GFRP composite insulation system. The GFRP sample having a dimension of 80 mm · 80 mm was laid between sphere and plane electrodes. The thickness of GFRP samples was varied from 1 mm to 2 mm. All electrodes made with stainless steel are 7 mm and 60 mm diameter for sphere and plane electrode, respectively. Fig. 3 shows the cryostat and experimental set-up. The electrode system was set in inner dewar flask of LN2. The outer dewar was filled with atmospheric liquid nitrogen (1 atm, 77 K) to prevent the temperature rise of the liquid nitrogen in the inner dewar. The power supply is ac dielectric strength set (60 Hz, 100 kV, 1 kVA). In order to investigate Vt characteristics, we first measured the breakdown voltage (breakdown means discharge in the solid insulators) and calculated 50% cumulative probability of breakdown voltage (BDV50) or breakdown strength (BDS50) from Weibull plot and then measured the time to breakdown with the applied voltage in the range of 100% to 60 % of BDV50 or BDS50. For measuring breakdown voltage, the 60 Hz ac high voltage was applied to the electrodes and increased with the rate of 1 kVrms/s until breakdown and
Fig. 3. Cryostat and experimental set-up.
ten samples were used. Kapton tape and GFRP sample were replaced with new one at each breakdown test. To determine the time to breakdown, ten samples were also used at each value of applied voltage and 50% cumulative probability of time to breakdown (t50) was computed from Weibull plot. In order to analyze the position of breakdown hole, we used ten samples of 2 mm thickness GFRP at each value of applied voltage. The applied voltages were set at high value (43 kV) and low value (31 kV) and three kinds of sphere electrode diameter were used as following 5, 7.0, and 9.5 mm. The high value takes 80% of average breakdown voltage of 2 mm thickness GFRP (54 kV) while the low value is 60% of the average breakdown voltage.
3. Results and discussion 3.1. Vt characteristics Fig. 4 shows the ac breakdown voltage (VB) in LN2 as a function of KaptonÕs layer number (N). As shown in the figure, the VB increases nonlinearly versus the num-
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63
Table 1 Parameters of Weibull plots for breakdown voltage of Kapton tape
20
Kapton 15
Breakdown Voltage [kV]
59
10
5
0 0
1
2
3
4
Number of layer [N] Fig. 4. ac breakdown characteristics of LN2/Kapton composite insulation.
No. of layer
m
V0 (kV)
1 2 3
32.5 28.5 27.5
11.3 14.4 16.5
of Kapton layer increases the number of impurities at the interfaces between layers and helical gaps between consecutive lapped tapes increases. These impurities have different shape and distribute randomly associated with the incidental forming bubbles in the helical gaps causing expansion of breakdown voltage distribution. Figs. 6 and 7 show Weibull plots of time to breakdown in LN2 for 2 layers and 3 layers, respectively. Table 2 lists Weibull scale parameter t0, shape parameter m and 50% probability breakdown time (t50) for 2 layers and 3 layers. It is cleared that the scale parameters t0 for 3 layers is higher than those of 2 layers but the shape parameter m is in inverse property. This result means
ber of layer N and the standard deviation of VB is nearly the same value as the number of layer increases. The relation between VB and N is expressed by ð1Þ
Fig. 5 shows Weibull plot of breakdown voltage in LN2 with 1 layer, 2 layers and 3 layers of Kapton tape. The breakdown data were fitted by two-parameter Weibull distribution P ðV Þ ¼ 1 exp½ðVVO Þm using the maximum likelihood method. Table 1 lists Weibull scale parameter V0 and shape parameter m estimated from the slope of Weibull plot. It is seen from this table that the scale parameter V0 increases from 11.3 to 16.5 kV when the number of layer increases from 1 layer to 3 layers. At the same time, the shape parameter m decreases from 32.5 to 27.5. It means that the higher the number of layer is, the larger the scattering of breakdown voltage is. The reason is considered that when the number
95 90 80 70 60 50 40 30
12 kV 13 kV 14 kV
95 90 80 70 60 50 40 30 20 10
5 3 2 1 0.01
0.10
1.00
10.00
100.00
1000.00
Time to breakdown [sec]
Fig. 6. Weibull plots of time to breakdown for 2- Kapton layers: solid lines, Weibull fit; Dotted lines, 95% confidence limits.
99
1 layer x2 2 layer x2 3 layer x2
Breakdown probability [%]
Breakdown probability [%]
99
Breakdown probability [%]
99
V B ¼ 11:3N 0:3
20 10 5 3 2
13 kV 14 kV 15 kV
95 90 80 70 60 50 40 30 20 10 5 3 2 1
1 9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5
17.5
Breakdown voltage [kV] Fig. 5. Weibull plots of breakdown voltage of Kapton tape in LN2: solid lines, Weibull fit; Dotted lines, 95% confidence limits.
0.0
0.1
1.0
10.0
100.0
1000.0
10000.0
Time to breakdown [sec]
Fig. 7. Weibull plots of time to breakdown for 3- Kapton layers: solid lines, Weibull fit; Dotted lines, 95% confidence limits.
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63
Table 2 Parameters of Weibull plots for time to breakdown of Kapton tape No. of layer
Two layers
Three layers
Applied voltage (kV)
12
13
14
13
14
15
m t0(s) t50(s)
1.7 565 455
0.8 84 54
1.4 4.3 3.3
0.9 1244 839
0.67 144.7 84
0.88 15.8 10.5
that the scattering of time to breakdown is directly proportional to the number of layers. The cause is considered that, as we have mentioned before, the increase of number of impurities at the interfaces between layers and the incidental forming bubbles in helical gaps between consecutive lapped tapes leads to be broader distribution of time to breakdown. Vt characteristics of Kapton tape for 2 layers and 3 layers are shown in Fig. 8 basing on the data of Table 2. It is seen that the time to breakdown t50 increases as the applied voltage decreases and the relation between E (kV/mm) and time to breakdown t50(s) for 2 layers and 3 layers Kapton tape is shown by 1=32:3
E ¼ 121:6t50
1=30:6
E ¼ 90t50
ðfor 2 layersÞ
ð2Þ
ðfor 3 layersÞ
ð3Þ
This figure shows that the lifetime indices n decrease from 32.3 for 2 layers to 30.6 for 3 layers. Because of increasing the number of layers, the number of impurities at the interfaces between layers and the incidental forming bubbles in helical gaps between consecutive lapped tapes increases, which results in decreasing the time to breakdown faster as the applied electric stress increases. Fig. 9 indicates the Weibull plots of the breakdown strength of GFRP with 1 mm and 2 mm thickness. Although the scale parameter of 1 mm thickness (E0 = 29 kV/mm) is slightly higher than that of 2 mm thickness (E0 = 27 kV/mm), the shape parameter of 1 mm thickness (m = 46.8) is higher than that of 2 mm
99
Breakdown probability [%]
60
95 90 80 70 60 50 40 30
2mm 1mm
20 10 5 3 2 1 22
Breakdown Strength [kV/mm]
3 layer
n =32.3 120
80
n =30.6 40
0 1.E+00
26
27
28
29
30
thickness (m = 31). It is obvious that the scattering of breakdown strength of 2 mm thickness is larger than that of 1 mm thickness because of thickness increase, which leads to increase the numbers of contaminants, voids at the interface between fibers and matrix resin in the bulk. Unfortunately, these voids and contaminants have different shapes and distribute randomly leading to wider distribution of breakdown strength. The Weibull plots of time to breakdown for 1 mm thickness and 2 mm thickness GFRP were shown in Fig. 10 and Table 3 lists Weibull scale parameter t0, shape parameter m and 50% probability breakdown time (t50) for 1 mm thickness and 2 mm thickness. In this table the shape parameters m decreases as the applied electric field stress increases and the decline of m for 2 mm thickness is larger than that of 1 mm thickness. It means that the scattering of time to breakdown is proportional to the raise of thickness and applied electric field stress. The Vt characteristics of GFRP are shown in Fig. 11 basing on the data of Table 3 and the expression of E (kV/mm) as a function of time to breakdown t50(s) for 1 mm and 2 mm thickness are
1=5:3
160
25
Fig. 9. Weibull plots of breakdown strength of GFRP: solid lines, Weibull fit; Dotted lines, 95% confidence limits.
1=28:8
2 layer
24
Breakdown Strength [kV/mm]
E ¼ 29:4t50 200
23
E ¼ 44t50
ðfor 1 mm thicknessÞ
ðfor 2 mm thicknessÞ
ð4Þ ð5Þ
This figure indicates that the lifetime indices n of Vt characteristics reduce quickly from 28.8 to 5.3 for 1 mm thickness and 2 mm thickness, respectively. The reason is considered that the thickness increase will make the raise of the number of contaminants and voids in the bulk, leading to decrease in the time to breakdown more rapidly as the applied electric stress increases. 3.2. Breakdown hole position
1.E+01
1.E+02
1.E+03
Time to breakdown [sec] Fig. 8. Vt characteristics of Kapton tape.
1.E+04
Fig. 12 illustrates the pictures of 2 mm thickness GFRP sample after breakdown for 31 kV and 43 kV-
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63 Table 3 Parameters of Weibull plots for time to breakdown of GFRP
Breakdown probability [%]
99 95 90 80 70 60 50 40 30
23 kV/mm 25 kV/mm 27 kV/mm 29 kV/mm
Thickness (mm)
Applied electric field stress (kV/mm)
m
t0(s)
t50(s)
1
23 25 27 29 15.5 17.5 19.5 21.5 23.5 27
2.5 1.3 1.6 1.65 4 4.4 4.3 3.7 2.2 1.9
1452 1063 11.4 2.7 265.8 138.6 81 65 30 14.6
1254 80.4 9.0 2.2 242.8 127.5 74.4 58.8 25.5 12
20 10
2
5 3 2 1 0.1
100.0
1000.0
Time to breakdown [sec]
95 90 80 70 60 50 40 30
15.5 kV/mm 17.5 kV/mm 19.5 kV/mm 21.5 kV/mm 23.5 kV/mm 27 kV/mm
40
20 10 5 3 2 1 1
(b)
10
100
1000
Time to breakdown [sec]
Fig. 10. Weibull plots of time to breakdown of GFRP: solid lines, Weibull fit; Dotted lines, 95% confidence limits. (a) 1 mm thickness and (b) 2 mm thickness.
applied voltages. In these pictures, we can see that the eroded area has circular shape with the cracking along the interface between glass fibre and epoxy on the surface of GFRP sample and the breakdown hole is at the eroded area, which is far from the contact point (CTP) between sphere electrode and GFRP sample. Moreover, the GFRP has no damage around the CTP. The reason is that the occurrence of partial discharges
Breakdown strength [kV/mm]
99
Breakdown probablitity [%]
10.0
1.0
(a)
61
30
n =28.8
20
n =5.3 10 1mm-thickness 2mm-thickness 0 1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
Time to breakdown [sec] Fig. 11. Vt characteristics of GFRP.
(PDs) around the CTP at the small radius protrusions on the surface of sphere electrode. The PDs generate local heating, erode the insulation, make it more brittle and finally create cracks at the interfaces between glass fiber and epoxy on the surface of insulation. At the same time, the PDs also occur in voids in the bulk of
Fig. 12. Pictures of GFRP sample after breakdown. a: contact point, b: eroded area, c: breakdown hole, and d: non- eroded area. (a) 31 kV and (b) 43 kV.
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V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63 6
1E + 08
Breakdown hole site [mm]
AC 31 kV LN 2
9E + 07
Electric field (V/m)
8E + 07 7E + 07 6E + 07 5E + 07 4E + 07
Contact point
3E + 07
Breakdown hole position
2E + 07
4
2
0
1E + 07 0 0
2
4
6
8
7
9.5
Sphere diameter [mm]
10
Distance from contact point (mm)
5
Fig. 15. Breakdown hole site as a function of sphere electrode diameter.
Fig. 13. Electric field of sphere-plane electrode.
insulation and leads to the formation of electrical trees. When the applied voltage increases, these cracks become longer and deeper associated with the development of electrical trees leading to electric puncture locally. The electric field of sphere-plane electrode on the upper surface of GFRP sample with 31 kV applied voltage is shown in Fig. 13. This electric field was calculated with FEM method based on Maxwell program simulation. The origin of co-ordinate was set at the contact point (CTP). The abscissa represents position from the CTP and the ordinate denotes electric field intensity on the surface of GFRP. In this figure, the electric field gets maximum value at the contact point between sphere electrode and sample and decreases quickly as the distance from the CTP increases. Especially, the electric field value at the breakdown hole is very low in compare to that of CTP and takes 12% and 14% of the maximum value for 31 kV and 43 kV-applied voltage, respectively. The reason is that, as we have discussed before, the generation of deep cracks on the surface of insulation associated with the development of electrical trees in the voids due to PDs lead to breakdown at very low electric field point.
Firstly, we analyze the location of breakdown hole as the applied voltage change. Fig. 14 shows the distribution of breakdown hole with the 31 kV and 43 kV-applied voltage of 2 mm-thickness GFRP. From these figure, it is cleared that the breakdown hole site is around the boundary of eroded area and the scattering of breakdown hole position decrease as applied voltage increase. Secondly, we establish the relation between the breakdown-hole site and sphere diameter. Fig. 15 illustrates the position of breakdown hole with 5 mm, 7 mm, and 9.5 mm sphere diameter. This figure indicates that the position of breakdown hole increases when the sphere diameter increases due to the generation of PDs at the larger surface area of sphere electrode because of increasing stressed electrode area [8].
4. Conclusion In this paper, we investigated the breakdown and Vt characteristics of LN2/Kapton and LN2/GFRP as well as the breakdown hole site as a function of applied
Fig. 14. Distribution of breakdown hole of 2 mm-thickness GFRP. (a) 31 kV and (b) 43 kV.
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63
voltage and sphere diameter. The main results are summarized as follow: The breakdown voltage increases when the number of Kapton layer increases but the shape parameter m decreases from 32.5 for 1 layer to 27.5 for 3 layers so the scattering of breakdown voltage is proportional with the layerÕs number. Lifetime indices n decrease from 32.3 for 2 layers to 30.6 for 3 layers. Thus the slope of Vt characteristics increases slightly as the number of layer is higher. The reasons are considered that the increasing of the number of impurities at the interfaces between layers and incidental forming bubbles in helical gaps between consecutive lapped tapes. The breakdown strength decreases slightly when the thickness of GFRP increases, but the shape parameter m decreases largely from 46.8 for 1 mm thickness to 31 for 2 mm thickness. Lifetime indices n decrease quickly from 28.8 for 1 mm thickness to 5.3 for 2 mm thickness so the slope of Vt characteristics increases quickly when the thickness increases. The reason for these results above is the increase of the number of contaminants and voids in the bulk. Due to the generation of partial discharges on the surface of sphere electrode and in the voids, the breakdown hole (BDH) locates around the boundary of eroded area, which is far from the contact point (CTP), and the electric field at the BDH is about 10% of the maximum electric filed at CTP. Moreover the scattering of breakdown hole site is lower when the applied voltage is higher from 31 kV to 43 kV and the breakdown hole site increases with increasing of sphere diameter from 5 mm to 7.0 mm to 9.5 mm diameter because of increasing stressed electrode area.
63
Acknowledgment This research was supported by a grant from Center for Applied Superconductivity Technology of the 21st Century Frontier R & D Program funded by the Ministry of Science and Technology, Republic of Korea.
References [1] Sykulski JK et al. Prospects for large high-temperature superconducting power transformers: conclusions from a design study. IEEE Electr Power Appl 1999;146(1):41–52. [2] McConnell BW, Mehta SP, Walker MS. HTS transformers. IEEE Power Eng Rev 2000;20(6):7–11. [3] Hazeyama M, Hayakawa N, Honjo S, Masuda T, Okubo H. Partial discharge inception Vt characteristics for pressurized liquid nitrogen/PPLP composite system. in: Proceeding of 14th International Conference on Dielectric Liquids (ICDL, 2002), Austria, July 7–12, 2002. p. 123–6. [4] Kosaki M, Nagao M, Shimizu N, Mizuno Y. Solid insulation and its deterioration. Cryogenics 1998;38(11):1095–104. [5] Kim SH, Kim HH, Chung SY, Choi HS, Cho KW. Surface flashover discharge and partial discharge of polymer film in liquid helium. J KIEE Mater Eng 1996;9(9):942–8. [6] Suzuki T, Kishi K, Uozumi T, Yatsuka K, Yashuda N, Fukui T. Study on V–t characteristics for XLPE cable. in: Proceeding of the 1994 IEEE Power Engineering Society, April 1994. p. 192–9. [7] Okubo H, Hikita M, Goshima H, Sakakibara H, Hayakawa N. High voltage insulation performance of cryogenic liquids for superconducting power apparatus. IEEE Trans Power Delivery 1996;11(3):1400–6. [8] Goshima H, Sakakibara H, Hayakawa N, Hikita M, Uchida K, Okubo H. High voltage insulation based on area and volume effects of cryogenic liquids for superconducting power apparatus. IEE Jpn Trans 1996;116-B(7):812–8.
Aging characteristics of cryogenic insulator for development of HTS transformer Van-Dung Nguyen, Jong-Man Joung, Seung-Myeong Baek, Chang-Hwa Lee, Sang-Hyun Kim * Electrical Material Laboratory, Department of Electrical Engineering, (Engineering Research Institute), Gyeongsang National University, Jinju 660-701, Republic of Korea Received 16 December 2003; received in revised form 14 September 2004; accepted 14 September 2004
Abstract In the response to the demand for electrical energy, much effort aimed to develop and commercialize high temperature superconducting (HTS) power equipments has been made around the world. Especially, HTS transformer is one of the most promising devices. For the development of HTS transformer, the cryogenic insulation technology should be established. In this paper Vt characteristics of polyimide (Kapton) tape and GFRP used as turn-to-turn and structural insulations, respectively were studied. Moreover, breakdown hole site of GFRP after breakdown was also discussed. The experimental results show that the time to breakdown is conditioned on applied electric stress and the lifetime indices n of Kapton tape decrease slightly as the number of tape increases while the lifetime indices n of GFRP decrease strongly with increasing thickness. Furthermore, the breakdown holes of GFRP were not at the contact point, at which the electric field is maximum value, between sphere electrode and GFRP sample and its location depends on applied voltage as well as sphere diameter. 2004 Elsevier Ltd. All rights reserved. Keywords: Breakdown hole; Cryogenic insulation; HTS transformer; Vt characteristics
1. Introduction High temperature superconducting transformer has been developing in many countries because of its lighter weight, smaller volume and higher efficiency than those of the conventional transformer [1,2]. For designing electrical insulation of HTS transformer, it is very important to know the Vt and breakdown characteristics of liquid nitrogen (LN2)/polyimide (Kapton) and liquid nitrogen/ Glass fibre reinforced plastic (GFRP) composite insulation systems as well as the damage degree of insulation materials after breakdown [3–7]. Moreover, Vt characteristic is one of the important factors to establish the
testing level and estimate the lifetime of electrical insulation in superconducting transformer. However, the Vt and breakdown characteristics of these composite insulation systems have not been clarified yet. In this paper, we studied Vt characteristics of LN2/ Kapton, LN2/GFRP with considering of Weibull distribution and lifetime indices n on the condition of changing the number of Kapton layer and thickness of GFRP. We also investigated the breakdown hole position of GFRP sample after breakdown under different applied voltage and sphere diameter.
2. Experiment *
Corresponding author. Fax: +82 55 761 8620. E-mail address: [email protected] (S.-H. Kim).
0011-2275/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2004.09.002
Fig. 1 shows the electrode system for testing LN2/ Kapton tape composite insulation. Firstly, copper
58
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63
Kapton insulation
φ6 5
High voltage
A
A Copper electrodes Ground
Kapton-covered
A-A section
copper electrodes Fig. 1. Electrode configuration for testing Kapton tape.
High voltage High voltage
Sphere electrode
power supply
GFRP Outer dewar Inner dewar
Plane electrode Vacuum layer
Ground
LN 2
Fig. 2. Electrode configuration for testing GFRP.
Electrode system
electrode tapes with 0.3 mm · 4 mm cross-section were wound and overlapped helically by 0.03 mm · 10 mm cross section Kapton tape with 1, 2, 3 layers and then Kapton-covered copper electrodes were wound around 65 mm diameter disk solid insulators. Fig. 2 illustrates sphere-plane electrode configuration of LN2/GFRP composite insulation system. The GFRP sample having a dimension of 80 mm · 80 mm was laid between sphere and plane electrodes. The thickness of GFRP samples was varied from 1 mm to 2 mm. All electrodes made with stainless steel are 7 mm and 60 mm diameter for sphere and plane electrode, respectively. Fig. 3 shows the cryostat and experimental set-up. The electrode system was set in inner dewar flask of LN2. The outer dewar was filled with atmospheric liquid nitrogen (1 atm, 77 K) to prevent the temperature rise of the liquid nitrogen in the inner dewar. The power supply is ac dielectric strength set (60 Hz, 100 kV, 1 kVA). In order to investigate Vt characteristics, we first measured the breakdown voltage (breakdown means discharge in the solid insulators) and calculated 50% cumulative probability of breakdown voltage (BDV50) or breakdown strength (BDS50) from Weibull plot and then measured the time to breakdown with the applied voltage in the range of 100% to 60 % of BDV50 or BDS50. For measuring breakdown voltage, the 60 Hz ac high voltage was applied to the electrodes and increased with the rate of 1 kVrms/s until breakdown and
Fig. 3. Cryostat and experimental set-up.
ten samples were used. Kapton tape and GFRP sample were replaced with new one at each breakdown test. To determine the time to breakdown, ten samples were also used at each value of applied voltage and 50% cumulative probability of time to breakdown (t50) was computed from Weibull plot. In order to analyze the position of breakdown hole, we used ten samples of 2 mm thickness GFRP at each value of applied voltage. The applied voltages were set at high value (43 kV) and low value (31 kV) and three kinds of sphere electrode diameter were used as following 5, 7.0, and 9.5 mm. The high value takes 80% of average breakdown voltage of 2 mm thickness GFRP (54 kV) while the low value is 60% of the average breakdown voltage.
3. Results and discussion 3.1. Vt characteristics Fig. 4 shows the ac breakdown voltage (VB) in LN2 as a function of KaptonÕs layer number (N). As shown in the figure, the VB increases nonlinearly versus the num-
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63
Table 1 Parameters of Weibull plots for breakdown voltage of Kapton tape
20
Kapton 15
Breakdown Voltage [kV]
59
10
5
0 0
1
2
3
4
Number of layer [N] Fig. 4. ac breakdown characteristics of LN2/Kapton composite insulation.
No. of layer
m
V0 (kV)
1 2 3
32.5 28.5 27.5
11.3 14.4 16.5
of Kapton layer increases the number of impurities at the interfaces between layers and helical gaps between consecutive lapped tapes increases. These impurities have different shape and distribute randomly associated with the incidental forming bubbles in the helical gaps causing expansion of breakdown voltage distribution. Figs. 6 and 7 show Weibull plots of time to breakdown in LN2 for 2 layers and 3 layers, respectively. Table 2 lists Weibull scale parameter t0, shape parameter m and 50% probability breakdown time (t50) for 2 layers and 3 layers. It is cleared that the scale parameters t0 for 3 layers is higher than those of 2 layers but the shape parameter m is in inverse property. This result means
ber of layer N and the standard deviation of VB is nearly the same value as the number of layer increases. The relation between VB and N is expressed by ð1Þ
Fig. 5 shows Weibull plot of breakdown voltage in LN2 with 1 layer, 2 layers and 3 layers of Kapton tape. The breakdown data were fitted by two-parameter Weibull distribution P ðV Þ ¼ 1 exp½ðVVO Þm using the maximum likelihood method. Table 1 lists Weibull scale parameter V0 and shape parameter m estimated from the slope of Weibull plot. It is seen from this table that the scale parameter V0 increases from 11.3 to 16.5 kV when the number of layer increases from 1 layer to 3 layers. At the same time, the shape parameter m decreases from 32.5 to 27.5. It means that the higher the number of layer is, the larger the scattering of breakdown voltage is. The reason is considered that when the number
95 90 80 70 60 50 40 30
12 kV 13 kV 14 kV
95 90 80 70 60 50 40 30 20 10
5 3 2 1 0.01
0.10
1.00
10.00
100.00
1000.00
Time to breakdown [sec]
Fig. 6. Weibull plots of time to breakdown for 2- Kapton layers: solid lines, Weibull fit; Dotted lines, 95% confidence limits.
99
1 layer x2 2 layer x2 3 layer x2
Breakdown probability [%]
Breakdown probability [%]
99
Breakdown probability [%]
99
V B ¼ 11:3N 0:3
20 10 5 3 2
13 kV 14 kV 15 kV
95 90 80 70 60 50 40 30 20 10 5 3 2 1
1 9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5
17.5
Breakdown voltage [kV] Fig. 5. Weibull plots of breakdown voltage of Kapton tape in LN2: solid lines, Weibull fit; Dotted lines, 95% confidence limits.
0.0
0.1
1.0
10.0
100.0
1000.0
10000.0
Time to breakdown [sec]
Fig. 7. Weibull plots of time to breakdown for 3- Kapton layers: solid lines, Weibull fit; Dotted lines, 95% confidence limits.
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63
Table 2 Parameters of Weibull plots for time to breakdown of Kapton tape No. of layer
Two layers
Three layers
Applied voltage (kV)
12
13
14
13
14
15
m t0(s) t50(s)
1.7 565 455
0.8 84 54
1.4 4.3 3.3
0.9 1244 839
0.67 144.7 84
0.88 15.8 10.5
that the scattering of time to breakdown is directly proportional to the number of layers. The cause is considered that, as we have mentioned before, the increase of number of impurities at the interfaces between layers and the incidental forming bubbles in helical gaps between consecutive lapped tapes leads to be broader distribution of time to breakdown. Vt characteristics of Kapton tape for 2 layers and 3 layers are shown in Fig. 8 basing on the data of Table 2. It is seen that the time to breakdown t50 increases as the applied voltage decreases and the relation between E (kV/mm) and time to breakdown t50(s) for 2 layers and 3 layers Kapton tape is shown by 1=32:3
E ¼ 121:6t50
1=30:6
E ¼ 90t50
ðfor 2 layersÞ
ð2Þ
ðfor 3 layersÞ
ð3Þ
This figure shows that the lifetime indices n decrease from 32.3 for 2 layers to 30.6 for 3 layers. Because of increasing the number of layers, the number of impurities at the interfaces between layers and the incidental forming bubbles in helical gaps between consecutive lapped tapes increases, which results in decreasing the time to breakdown faster as the applied electric stress increases. Fig. 9 indicates the Weibull plots of the breakdown strength of GFRP with 1 mm and 2 mm thickness. Although the scale parameter of 1 mm thickness (E0 = 29 kV/mm) is slightly higher than that of 2 mm thickness (E0 = 27 kV/mm), the shape parameter of 1 mm thickness (m = 46.8) is higher than that of 2 mm
99
Breakdown probability [%]
60
95 90 80 70 60 50 40 30
2mm 1mm
20 10 5 3 2 1 22
Breakdown Strength [kV/mm]
3 layer
n =32.3 120
80
n =30.6 40
0 1.E+00
26
27
28
29
30
thickness (m = 31). It is obvious that the scattering of breakdown strength of 2 mm thickness is larger than that of 1 mm thickness because of thickness increase, which leads to increase the numbers of contaminants, voids at the interface between fibers and matrix resin in the bulk. Unfortunately, these voids and contaminants have different shapes and distribute randomly leading to wider distribution of breakdown strength. The Weibull plots of time to breakdown for 1 mm thickness and 2 mm thickness GFRP were shown in Fig. 10 and Table 3 lists Weibull scale parameter t0, shape parameter m and 50% probability breakdown time (t50) for 1 mm thickness and 2 mm thickness. In this table the shape parameters m decreases as the applied electric field stress increases and the decline of m for 2 mm thickness is larger than that of 1 mm thickness. It means that the scattering of time to breakdown is proportional to the raise of thickness and applied electric field stress. The Vt characteristics of GFRP are shown in Fig. 11 basing on the data of Table 3 and the expression of E (kV/mm) as a function of time to breakdown t50(s) for 1 mm and 2 mm thickness are
1=5:3
160
25
Fig. 9. Weibull plots of breakdown strength of GFRP: solid lines, Weibull fit; Dotted lines, 95% confidence limits.
1=28:8
2 layer
24
Breakdown Strength [kV/mm]
E ¼ 29:4t50 200
23
E ¼ 44t50
ðfor 1 mm thicknessÞ
ðfor 2 mm thicknessÞ
ð4Þ ð5Þ
This figure indicates that the lifetime indices n of Vt characteristics reduce quickly from 28.8 to 5.3 for 1 mm thickness and 2 mm thickness, respectively. The reason is considered that the thickness increase will make the raise of the number of contaminants and voids in the bulk, leading to decrease in the time to breakdown more rapidly as the applied electric stress increases. 3.2. Breakdown hole position
1.E+01
1.E+02
1.E+03
Time to breakdown [sec] Fig. 8. Vt characteristics of Kapton tape.
1.E+04
Fig. 12 illustrates the pictures of 2 mm thickness GFRP sample after breakdown for 31 kV and 43 kV-
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63 Table 3 Parameters of Weibull plots for time to breakdown of GFRP
Breakdown probability [%]
99 95 90 80 70 60 50 40 30
23 kV/mm 25 kV/mm 27 kV/mm 29 kV/mm
Thickness (mm)
Applied electric field stress (kV/mm)
m
t0(s)
t50(s)
1
23 25 27 29 15.5 17.5 19.5 21.5 23.5 27
2.5 1.3 1.6 1.65 4 4.4 4.3 3.7 2.2 1.9
1452 1063 11.4 2.7 265.8 138.6 81 65 30 14.6
1254 80.4 9.0 2.2 242.8 127.5 74.4 58.8 25.5 12
20 10
2
5 3 2 1 0.1
100.0
1000.0
Time to breakdown [sec]
95 90 80 70 60 50 40 30
15.5 kV/mm 17.5 kV/mm 19.5 kV/mm 21.5 kV/mm 23.5 kV/mm 27 kV/mm
40
20 10 5 3 2 1 1
(b)
10
100
1000
Time to breakdown [sec]
Fig. 10. Weibull plots of time to breakdown of GFRP: solid lines, Weibull fit; Dotted lines, 95% confidence limits. (a) 1 mm thickness and (b) 2 mm thickness.
applied voltages. In these pictures, we can see that the eroded area has circular shape with the cracking along the interface between glass fibre and epoxy on the surface of GFRP sample and the breakdown hole is at the eroded area, which is far from the contact point (CTP) between sphere electrode and GFRP sample. Moreover, the GFRP has no damage around the CTP. The reason is that the occurrence of partial discharges
Breakdown strength [kV/mm]
99
Breakdown probablitity [%]
10.0
1.0
(a)
61
30
n =28.8
20
n =5.3 10 1mm-thickness 2mm-thickness 0 1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
Time to breakdown [sec] Fig. 11. Vt characteristics of GFRP.
(PDs) around the CTP at the small radius protrusions on the surface of sphere electrode. The PDs generate local heating, erode the insulation, make it more brittle and finally create cracks at the interfaces between glass fiber and epoxy on the surface of insulation. At the same time, the PDs also occur in voids in the bulk of
Fig. 12. Pictures of GFRP sample after breakdown. a: contact point, b: eroded area, c: breakdown hole, and d: non- eroded area. (a) 31 kV and (b) 43 kV.
62
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63 6
1E + 08
Breakdown hole site [mm]
AC 31 kV LN 2
9E + 07
Electric field (V/m)
8E + 07 7E + 07 6E + 07 5E + 07 4E + 07
Contact point
3E + 07
Breakdown hole position
2E + 07
4
2
0
1E + 07 0 0
2
4
6
8
7
9.5
Sphere diameter [mm]
10
Distance from contact point (mm)
5
Fig. 15. Breakdown hole site as a function of sphere electrode diameter.
Fig. 13. Electric field of sphere-plane electrode.
insulation and leads to the formation of electrical trees. When the applied voltage increases, these cracks become longer and deeper associated with the development of electrical trees leading to electric puncture locally. The electric field of sphere-plane electrode on the upper surface of GFRP sample with 31 kV applied voltage is shown in Fig. 13. This electric field was calculated with FEM method based on Maxwell program simulation. The origin of co-ordinate was set at the contact point (CTP). The abscissa represents position from the CTP and the ordinate denotes electric field intensity on the surface of GFRP. In this figure, the electric field gets maximum value at the contact point between sphere electrode and sample and decreases quickly as the distance from the CTP increases. Especially, the electric field value at the breakdown hole is very low in compare to that of CTP and takes 12% and 14% of the maximum value for 31 kV and 43 kV-applied voltage, respectively. The reason is that, as we have discussed before, the generation of deep cracks on the surface of insulation associated with the development of electrical trees in the voids due to PDs lead to breakdown at very low electric field point.
Firstly, we analyze the location of breakdown hole as the applied voltage change. Fig. 14 shows the distribution of breakdown hole with the 31 kV and 43 kV-applied voltage of 2 mm-thickness GFRP. From these figure, it is cleared that the breakdown hole site is around the boundary of eroded area and the scattering of breakdown hole position decrease as applied voltage increase. Secondly, we establish the relation between the breakdown-hole site and sphere diameter. Fig. 15 illustrates the position of breakdown hole with 5 mm, 7 mm, and 9.5 mm sphere diameter. This figure indicates that the position of breakdown hole increases when the sphere diameter increases due to the generation of PDs at the larger surface area of sphere electrode because of increasing stressed electrode area [8].
4. Conclusion In this paper, we investigated the breakdown and Vt characteristics of LN2/Kapton and LN2/GFRP as well as the breakdown hole site as a function of applied
Fig. 14. Distribution of breakdown hole of 2 mm-thickness GFRP. (a) 31 kV and (b) 43 kV.
V.-D. Nguyen et al. / Cryogenics 45 (2005) 57–63
voltage and sphere diameter. The main results are summarized as follow: The breakdown voltage increases when the number of Kapton layer increases but the shape parameter m decreases from 32.5 for 1 layer to 27.5 for 3 layers so the scattering of breakdown voltage is proportional with the layerÕs number. Lifetime indices n decrease from 32.3 for 2 layers to 30.6 for 3 layers. Thus the slope of Vt characteristics increases slightly as the number of layer is higher. The reasons are considered that the increasing of the number of impurities at the interfaces between layers and incidental forming bubbles in helical gaps between consecutive lapped tapes. The breakdown strength decreases slightly when the thickness of GFRP increases, but the shape parameter m decreases largely from 46.8 for 1 mm thickness to 31 for 2 mm thickness. Lifetime indices n decrease quickly from 28.8 for 1 mm thickness to 5.3 for 2 mm thickness so the slope of Vt characteristics increases quickly when the thickness increases. The reason for these results above is the increase of the number of contaminants and voids in the bulk. Due to the generation of partial discharges on the surface of sphere electrode and in the voids, the breakdown hole (BDH) locates around the boundary of eroded area, which is far from the contact point (CTP), and the electric field at the BDH is about 10% of the maximum electric filed at CTP. Moreover the scattering of breakdown hole site is lower when the applied voltage is higher from 31 kV to 43 kV and the breakdown hole site increases with increasing of sphere diameter from 5 mm to 7.0 mm to 9.5 mm diameter because of increasing stressed electrode area.
63
Acknowledgment This research was supported by a grant from Center for Applied Superconductivity Technology of the 21st Century Frontier R & D Program funded by the Ministry of Science and Technology, Republic of Korea.
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