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Research on the electrical insulation design of a bushing for a 154 kV class HTS transformer
Physica C 463–465 (2007) 1213–1217 www.elsevier.com/locate/physc
Research on the electrical insulation design of a bushing for a 154 kV class HTS transformer D.S. Kwag, H.G. Cheon, J.H. Choi, S.H. Kim
*
Department of Electrical Engineering, Gyeongsang National University and ERI, Jinju, Gyeongnam 660-701, Republic of Korea Accepted 19 March 2007 Available online 2 June 2007
Abstract The cryogenic high voltage bushing for a 154 kV/100 MVA high temperatures superconducting (HTS) transformer is described. The bushing is energized with the line-to-ground voltage between the coaxial center and outer surrounding conductors, in the axial direction there is a temperature difference between ambient and 77 K. Therefore, it has to endure for electrical insulation as well as the thermal contraction as well. In this research, the electrical insulation characteristics of GFRP to achieve high durability in the cryogenic environment were surveyed in the air and LN2. Moreover, the insulation constructions of the commercial condenser type bushing were studied. Based on these data, the electrical insulation design of the cryogenic condenser type bushing for a 154 kV class HTS transformer was performed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 85.25.Am; 77.22.Jp Keywords: Cryogenic bushing; Insulation design; HTS transformer
1. Introduction The issue which emerged in many fields of cryogenics technology of the high temperature superconducting (HTS) power equipment is applying the high voltage in the cryogenic temperature part of the superconducting equipment. In particular, the high voltage bushing for a HTS transformer provides the electrical insulation of the conductor from room temperature to cryogenic environment that such a situation permits the high voltage while having great difficulties to flow down the current from the air to the cryogenics equipment. In other words, the bushing for a HTS transformer is operated in LN2 and air that it has to endure for electrical insulation as well as the thermal contraction as well [1,2]. Accordingly, the displacement of the insulation has to be consistent, completely
*
Corresponding author. Tel.: +82 55 751 5345; fax: +82 55 761 8820. E-mail address: [email protected] (S.H. Kim).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.483
sealed off on the atmosphere, and thermally stabilized under the worst possible operation condition. The structure of a commercial bushing has the conductor temperature higher than the external temperature that it has to be designed in a structure to easily release the heat from the conductor, and it is deemed as difficulty to directly apply in the structure like the HTS transformer system that has to cut off the heat penetration. Therefore, the structure of a commercial bushing has to be studies, and make re-design to fit into the HTS transformer with the reference to the existing commercial bushing. In general, the structure of the high voltage bushing can be divided into the non-condenser type and the condenser type [3]. Fig. 1 shows the equi-potential distribution of the non-condenser type without foils and condenser type with foils. For the non-condenser type, it has the biggest advantage in having simple structure, and since the structure is simple, it may use various kinds of insulation material as well. However, due to the electrical stress concentration problem, the non-condenser type is broadly
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D.S. Kwag et al. / Physica C 463–465 (2007) 1213–1217
High voltage
Bushing body (insulator) A
Transformer tank (top flange) D C
B
Conductor (current lead) Fig. 1. Equi-potential distribution of the high voltage bushing; (a) noncondenser type without foils, and (b) condenser type with foils.
Fig. 2. Insulation constructions of the condenser type bushing for a HTS transformer.
used in the equipment insulation of 25 kV class or less, but it is difficult to apply on the above class of high voltage power equipment. On the other hand, the condenser type is a method to arrange the metal foil such as aluminium foil with certain interval on the inside of stress cone of bushing, and it can reduce the electrical stress concentration by arranging the metal foil for the consistent voltage sharing. The shared voltage for each insulation body is determined by the number of cylinder condenser, and the compact design is available through the consistent electrical stress sharing [4]. Therefore, the high voltage bushing for a 154 kV class HTS transformer is emerged with the condenser type, and it displayed the insulation construction of the bushing for a HTS transformer in Fig. 2. The insulation configuration of the condenser type bushing can be divided into the air-end clearance (A) from the air in the insulation body, the LN2-end clearance (B) from the LN2 of the insulation body, the radial diameters (C) from the insulation thickness of the insulation body and the condenser cone (D) from the arrangement of metal foils in the insulation body. The air-end clearance can apply the existing commercial bushing insulation technology [5]. LN2-end clearance is designed through the surface flashover of the insulation material from LN2, and the radial diameters are designed through the characteristics of the electrical breakdown strength of the insulation material. The condenser cone is designed by adjusting the condenser interval and width to have the consistent capacitance for each condenser. The insulation material of the bushing for a HTS transformer has to be able to sustain the contraction and expansion from the temperature difference of the internal current lead, and minimize the possibility of failure of confidentiality by
having a gap or crack on the contacting surface. Therefore, by adjusting the contents of the glass fabric, it uses the GFRP that is easy to control the thermal contraction and thermal expansion, and has outstanding electrical and mechanical characteristics. 2. Electrical insulation design of the bushing 2.1. Air-end clearance Fig. 3 shows the schematic diagram of the air-end clearance, LN2-end clearance and cross-section for a HTS transformer bushing. The design of the surface flashover distance from the air, the upper part of the bushing, may facilitate the insulation data of the existing system [6]. According to Table 1, the creapage distance, the total surface flashover distance, in the air of the insulation body requires 25 mm per ac 1 kV, and considering the wrinkles of the bushing, the length of the upper insulation body is taken for impulse voltage of 5.5 kV per 1 cm. Therefore, the creapage distance of the air-end clearance is multiplied for 25 mm on the ac 170 kV, the highest voltage of the 154 kV class transformer to design for 4250 mm, and the length of the upper insulation body is designed for 1400 mm each by dividing the impulse withstand voltage of 750 kV with 5.5 kV. 2.2. LN2-end clearance The surface flashover distance from LN2, the lower part of the bushing, is designed through the experiment, and it displayed the Weibull probability plot on impulse surface flashover strength of GFRP from LN2 in Fig. 4. From
D.S. Kwag et al. / Physica C 463–465 (2007) 1213–1217
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Fig. 3. Schematic diagrams of the air-end clearance, LN2-end clearance and cross-section for a HTS transformer bushing.
Table 1 Design of the air-end clearance for a 154 kV class bushing
Standard
Design
Rated voltage (kV)
BIL (kV)
Creapage distance
Straight length
170
750
25 mm/kV (for power frequency)
2.8 kV/cm (for power frequency) 5.5 kV/cm (for 1.2/50 ls impulse voltage) 1400 mm
4250 mm
clearance has to be designed for 750 mm to endure the impulse withstand voltage of 750 kV. 2.3. Radial diameters The external diameter of the insulation body is determined by the electric stress calculation. The maximum radial stress occurs from the conductor surface (x = rc), and the value is as follows: Emax ¼
LN2 of GFRP, the optimal strength value of the surface flashover was surveyed for approximately 1.7 kV/mm, and when considering the spare value, 1 kV/mm was applied. Hence, the 154 kV class bushing with LN2-end
Vx Vx ; R0 ¼ Te rc ln rc
T e ¼ rc ln
R0 rc
ð1Þ
Here, the external radius of conductor for rc and the external radius of insulation for R0 with the random radius x, the radial stress Ex and electric potential Vx. In general the value is referred to the equivalent insulation thickness. In accordance with the above calculation, the external diameter of the insulation body can be calculated. In Fig. 4, the maximum breakdown strength of the GFRP is shown as 18 kV/mm. The withstand voltage of the sold insulation material is influenced by the shape, thickness, area and others of electrode that the maximum breakdown strength has to consider the spare value to determine for 10 kV/mm. Therefore, the insulation thickness is the value that subtracted rc from R0 and is calculated for approximately 100 mm. 2.4. Condenser cone
Fig. 4. Weibull probability plot on impulse surface flashover and breakdown strength of GFRP in LN2.
The metal foil for the equi-potential distribution in the condenser type bushing is inserted for approximately 10– 20 in general, and in designing of condenser for the bushing of this 154 kV class HTS transformer, 10 condensers are
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D.S. Kwag et al. / Physica C 463–465 (2007) 1213–1217
L1 L2 r0 r1 r2 r3 r4 r5 r6 r7 r8 r9 r10
L3 L4 L5 L6 L7 L8 L9 L10
C2 C4 C6 C8 C10
Radius (mm)
Width (mm)
r1
33
L1
1800
r2
43
L2
1320
r3
53
L3
1040
r4
63
L4
860
r5
73
L5
730
r6
83
L6
640
r7
93
L7
570
r8
103
L8
510
r9
113
L9
460
L10
420
r10 123
Creapage distance (4,250mm)
Current lead Insulator (GFRP)
Al foils
C1 C3 C5 C7 C9 Fig. 5. Condenser cone design of the bushing for a 154 kV class HTS transformer.
Mounting flange
LN2
46mm 246mm
Fig. 6. Detailed drawing of the condenser type bushing for a 154 kV class HTS transformer.
inserted with the consideration of the difficulty in work process. The number of condenser inserted could be modificatory through the later studies. In the case of inserting 10 condensers, the voltage for each condenser to provide is 75 kV, and the distance between each condenser is calculates for 10 mm. The determination of the condenser width has to be determined within the length of the entire bushing, and it could be sought under the condition to make the capacitance between each condenser consistent. The capacitance of each condenser and the width of condenser are sought by the following (2). 2pe Ln Ln1 rn Cn ¼ ; Ln ¼ ln ð2Þ rn ln rn1 rn1 ln rrn1 n2 Fig. 5 shows the radius (rn) and width (Ln) of each condenser designed on the bushing for a 154 kV class HTS transformer. The radius of the conductor (r0) is 23 mm, and the width of the first condenser (L1) was 1800 mm considering the length of entire bushing. 3. Design analysis of the bushing Based on the above insulation design data, the detailed drawing of the bushing for a 154 kV class transformer displayed in Fig. 6. The creapage distance of air-end clearance
Fig. 7. Equi-potential distribution of the designed bushing for a HTS transformer.
D.S. Kwag et al. / Physica C 463–465 (2007) 1213–1217
is designed for 4250 mm, the length of upper insulation body is for 1400 mm, and the LN2-end clearance is for 750 mm. In addition, the insulation thickness of the insulation body is 100 mm, and it designed the radius and width of condensers to have the voltage burden of 75 kV each by inserting 10 condensers in 10 mm interval. Fig. 7 shows the equi-potential distribution of the designed bushing for a HTS transformer. FLUX 2D analyzing program was used for simulation. The simulation model gives a value of 1000 V for the current lead, 0 V for the transformer tank and top flange. Also, the relative dielectric constant of the GFRP and LN2 were 4.432 and 1.431, respectively. As the simulation result, the equipotential distribution shows that is formed through condenser in the bushing insulator.
4. Conclusion In this study, the electrical insulation designs of the high voltage bushing for a 154 kV class HTS transformer that are developing in Korea are undertaken. The bushing for a HTS transformer has the condenser type structure to make the equi-potential distribution on the insulation body. The insulation material of the bushing for a HTS transformer has to be able to sustain the contraction and expansion from the temperature difference of the internal current lead, and minimize the possibility of failure of confidentiality by having a gap or crack on the contacting surface. Therefore, by adjusting the contents of the glass fabric, it uses the GFRP that is easy to control the thermal
1217
contraction and thermal expansion, and has outstanding electrical and mechanical characteristics. In the case of the electrical insulation design, it has the condenser type structure that used the GFRP insulation material. The creapage distance of air-end clearance is designed for 4250 mm, the length of upper insulation body is for 1400 mm, and the LN2-end clearance is for 750 mm. In addition, the insulation thickness of the insulation body is 100 mm, and it designed the radius and width of condensers to have the voltage burden of 75 kV each by inserting 10 condensers in 10 mm interval. Acknowledgement 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] F. Schauer, Cryogenics 24 (1984) 90. [2] V.A. Glukhikh, S.A. Egorov, O.G. Filatov, V.E. Korsunsky, E.A. Lamzin, S.E. Sychevsky, IEEE Trans. Appl. Supercond. 10 (2000) 1477. [3] E. Kuffel, W.S. Zaengl, J. Kuffel, High Voltage Engineering: Fundamentals, Newnes, Oxford, 2001, pp. 235–241. [4] J.V. Champion, S.J. Dodd, in: Proceedings of the IEEE 7th International Conference Solid Dielectrics, 2001, p. 329. [5] J.Y. Koo, H.S. Lee, K.B. Lee, IEEE Annual Report, Electrical Insulation and Dielectric Phenomena, Arlington, 1994, pp. 512–517. [6] High-voltage alternating-current circuit-breakers, IEC 62271-100, 2006.
Research on the electrical insulation design of a bushing for a 154 kV class HTS transformer D.S. Kwag, H.G. Cheon, J.H. Choi, S.H. Kim
*
Department of Electrical Engineering, Gyeongsang National University and ERI, Jinju, Gyeongnam 660-701, Republic of Korea Accepted 19 March 2007 Available online 2 June 2007
Abstract The cryogenic high voltage bushing for a 154 kV/100 MVA high temperatures superconducting (HTS) transformer is described. The bushing is energized with the line-to-ground voltage between the coaxial center and outer surrounding conductors, in the axial direction there is a temperature difference between ambient and 77 K. Therefore, it has to endure for electrical insulation as well as the thermal contraction as well. In this research, the electrical insulation characteristics of GFRP to achieve high durability in the cryogenic environment were surveyed in the air and LN2. Moreover, the insulation constructions of the commercial condenser type bushing were studied. Based on these data, the electrical insulation design of the cryogenic condenser type bushing for a 154 kV class HTS transformer was performed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 85.25.Am; 77.22.Jp Keywords: Cryogenic bushing; Insulation design; HTS transformer
1. Introduction The issue which emerged in many fields of cryogenics technology of the high temperature superconducting (HTS) power equipment is applying the high voltage in the cryogenic temperature part of the superconducting equipment. In particular, the high voltage bushing for a HTS transformer provides the electrical insulation of the conductor from room temperature to cryogenic environment that such a situation permits the high voltage while having great difficulties to flow down the current from the air to the cryogenics equipment. In other words, the bushing for a HTS transformer is operated in LN2 and air that it has to endure for electrical insulation as well as the thermal contraction as well [1,2]. Accordingly, the displacement of the insulation has to be consistent, completely
*
Corresponding author. Tel.: +82 55 751 5345; fax: +82 55 761 8820. E-mail address: [email protected] (S.H. Kim).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.483
sealed off on the atmosphere, and thermally stabilized under the worst possible operation condition. The structure of a commercial bushing has the conductor temperature higher than the external temperature that it has to be designed in a structure to easily release the heat from the conductor, and it is deemed as difficulty to directly apply in the structure like the HTS transformer system that has to cut off the heat penetration. Therefore, the structure of a commercial bushing has to be studies, and make re-design to fit into the HTS transformer with the reference to the existing commercial bushing. In general, the structure of the high voltage bushing can be divided into the non-condenser type and the condenser type [3]. Fig. 1 shows the equi-potential distribution of the non-condenser type without foils and condenser type with foils. For the non-condenser type, it has the biggest advantage in having simple structure, and since the structure is simple, it may use various kinds of insulation material as well. However, due to the electrical stress concentration problem, the non-condenser type is broadly
1214
D.S. Kwag et al. / Physica C 463–465 (2007) 1213–1217
High voltage
Bushing body (insulator) A
Transformer tank (top flange) D C
B
Conductor (current lead) Fig. 1. Equi-potential distribution of the high voltage bushing; (a) noncondenser type without foils, and (b) condenser type with foils.
Fig. 2. Insulation constructions of the condenser type bushing for a HTS transformer.
used in the equipment insulation of 25 kV class or less, but it is difficult to apply on the above class of high voltage power equipment. On the other hand, the condenser type is a method to arrange the metal foil such as aluminium foil with certain interval on the inside of stress cone of bushing, and it can reduce the electrical stress concentration by arranging the metal foil for the consistent voltage sharing. The shared voltage for each insulation body is determined by the number of cylinder condenser, and the compact design is available through the consistent electrical stress sharing [4]. Therefore, the high voltage bushing for a 154 kV class HTS transformer is emerged with the condenser type, and it displayed the insulation construction of the bushing for a HTS transformer in Fig. 2. The insulation configuration of the condenser type bushing can be divided into the air-end clearance (A) from the air in the insulation body, the LN2-end clearance (B) from the LN2 of the insulation body, the radial diameters (C) from the insulation thickness of the insulation body and the condenser cone (D) from the arrangement of metal foils in the insulation body. The air-end clearance can apply the existing commercial bushing insulation technology [5]. LN2-end clearance is designed through the surface flashover of the insulation material from LN2, and the radial diameters are designed through the characteristics of the electrical breakdown strength of the insulation material. The condenser cone is designed by adjusting the condenser interval and width to have the consistent capacitance for each condenser. The insulation material of the bushing for a HTS transformer has to be able to sustain the contraction and expansion from the temperature difference of the internal current lead, and minimize the possibility of failure of confidentiality by
having a gap or crack on the contacting surface. Therefore, by adjusting the contents of the glass fabric, it uses the GFRP that is easy to control the thermal contraction and thermal expansion, and has outstanding electrical and mechanical characteristics. 2. Electrical insulation design of the bushing 2.1. Air-end clearance Fig. 3 shows the schematic diagram of the air-end clearance, LN2-end clearance and cross-section for a HTS transformer bushing. The design of the surface flashover distance from the air, the upper part of the bushing, may facilitate the insulation data of the existing system [6]. According to Table 1, the creapage distance, the total surface flashover distance, in the air of the insulation body requires 25 mm per ac 1 kV, and considering the wrinkles of the bushing, the length of the upper insulation body is taken for impulse voltage of 5.5 kV per 1 cm. Therefore, the creapage distance of the air-end clearance is multiplied for 25 mm on the ac 170 kV, the highest voltage of the 154 kV class transformer to design for 4250 mm, and the length of the upper insulation body is designed for 1400 mm each by dividing the impulse withstand voltage of 750 kV with 5.5 kV. 2.2. LN2-end clearance The surface flashover distance from LN2, the lower part of the bushing, is designed through the experiment, and it displayed the Weibull probability plot on impulse surface flashover strength of GFRP from LN2 in Fig. 4. From
D.S. Kwag et al. / Physica C 463–465 (2007) 1213–1217
1215
Fig. 3. Schematic diagrams of the air-end clearance, LN2-end clearance and cross-section for a HTS transformer bushing.
Table 1 Design of the air-end clearance for a 154 kV class bushing
Standard
Design
Rated voltage (kV)
BIL (kV)
Creapage distance
Straight length
170
750
25 mm/kV (for power frequency)
2.8 kV/cm (for power frequency) 5.5 kV/cm (for 1.2/50 ls impulse voltage) 1400 mm
4250 mm
clearance has to be designed for 750 mm to endure the impulse withstand voltage of 750 kV. 2.3. Radial diameters The external diameter of the insulation body is determined by the electric stress calculation. The maximum radial stress occurs from the conductor surface (x = rc), and the value is as follows: Emax ¼
LN2 of GFRP, the optimal strength value of the surface flashover was surveyed for approximately 1.7 kV/mm, and when considering the spare value, 1 kV/mm was applied. Hence, the 154 kV class bushing with LN2-end
Vx Vx ; R0 ¼ Te rc ln rc
T e ¼ rc ln
R0 rc
ð1Þ
Here, the external radius of conductor for rc and the external radius of insulation for R0 with the random radius x, the radial stress Ex and electric potential Vx. In general the value is referred to the equivalent insulation thickness. In accordance with the above calculation, the external diameter of the insulation body can be calculated. In Fig. 4, the maximum breakdown strength of the GFRP is shown as 18 kV/mm. The withstand voltage of the sold insulation material is influenced by the shape, thickness, area and others of electrode that the maximum breakdown strength has to consider the spare value to determine for 10 kV/mm. Therefore, the insulation thickness is the value that subtracted rc from R0 and is calculated for approximately 100 mm. 2.4. Condenser cone
Fig. 4. Weibull probability plot on impulse surface flashover and breakdown strength of GFRP in LN2.
The metal foil for the equi-potential distribution in the condenser type bushing is inserted for approximately 10– 20 in general, and in designing of condenser for the bushing of this 154 kV class HTS transformer, 10 condensers are
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D.S. Kwag et al. / Physica C 463–465 (2007) 1213–1217
L1 L2 r0 r1 r2 r3 r4 r5 r6 r7 r8 r9 r10
L3 L4 L5 L6 L7 L8 L9 L10
C2 C4 C6 C8 C10
Radius (mm)
Width (mm)
r1
33
L1
1800
r2
43
L2
1320
r3
53
L3
1040
r4
63
L4
860
r5
73
L5
730
r6
83
L6
640
r7
93
L7
570
r8
103
L8
510
r9
113
L9
460
L10
420
r10 123
Creapage distance (4,250mm)
Current lead Insulator (GFRP)
Al foils
C1 C3 C5 C7 C9 Fig. 5. Condenser cone design of the bushing for a 154 kV class HTS transformer.
Mounting flange
LN2
46mm 246mm
Fig. 6. Detailed drawing of the condenser type bushing for a 154 kV class HTS transformer.
inserted with the consideration of the difficulty in work process. The number of condenser inserted could be modificatory through the later studies. In the case of inserting 10 condensers, the voltage for each condenser to provide is 75 kV, and the distance between each condenser is calculates for 10 mm. The determination of the condenser width has to be determined within the length of the entire bushing, and it could be sought under the condition to make the capacitance between each condenser consistent. The capacitance of each condenser and the width of condenser are sought by the following (2). 2pe Ln Ln1 rn Cn ¼ ; Ln ¼ ln ð2Þ rn ln rn1 rn1 ln rrn1 n2 Fig. 5 shows the radius (rn) and width (Ln) of each condenser designed on the bushing for a 154 kV class HTS transformer. The radius of the conductor (r0) is 23 mm, and the width of the first condenser (L1) was 1800 mm considering the length of entire bushing. 3. Design analysis of the bushing Based on the above insulation design data, the detailed drawing of the bushing for a 154 kV class transformer displayed in Fig. 6. The creapage distance of air-end clearance
Fig. 7. Equi-potential distribution of the designed bushing for a HTS transformer.
D.S. Kwag et al. / Physica C 463–465 (2007) 1213–1217
is designed for 4250 mm, the length of upper insulation body is for 1400 mm, and the LN2-end clearance is for 750 mm. In addition, the insulation thickness of the insulation body is 100 mm, and it designed the radius and width of condensers to have the voltage burden of 75 kV each by inserting 10 condensers in 10 mm interval. Fig. 7 shows the equi-potential distribution of the designed bushing for a HTS transformer. FLUX 2D analyzing program was used for simulation. The simulation model gives a value of 1000 V for the current lead, 0 V for the transformer tank and top flange. Also, the relative dielectric constant of the GFRP and LN2 were 4.432 and 1.431, respectively. As the simulation result, the equipotential distribution shows that is formed through condenser in the bushing insulator.
4. Conclusion In this study, the electrical insulation designs of the high voltage bushing for a 154 kV class HTS transformer that are developing in Korea are undertaken. The bushing for a HTS transformer has the condenser type structure to make the equi-potential distribution on the insulation body. The insulation material of the bushing for a HTS transformer has to be able to sustain the contraction and expansion from the temperature difference of the internal current lead, and minimize the possibility of failure of confidentiality by having a gap or crack on the contacting surface. Therefore, by adjusting the contents of the glass fabric, it uses the GFRP that is easy to control the thermal
1217
contraction and thermal expansion, and has outstanding electrical and mechanical characteristics. In the case of the electrical insulation design, it has the condenser type structure that used the GFRP insulation material. The creapage distance of air-end clearance is designed for 4250 mm, the length of upper insulation body is for 1400 mm, and the LN2-end clearance is for 750 mm. In addition, the insulation thickness of the insulation body is 100 mm, and it designed the radius and width of condensers to have the voltage burden of 75 kV each by inserting 10 condensers in 10 mm interval. Acknowledgement 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] F. Schauer, Cryogenics 24 (1984) 90. [2] V.A. Glukhikh, S.A. Egorov, O.G. Filatov, V.E. Korsunsky, E.A. Lamzin, S.E. Sychevsky, IEEE Trans. Appl. Supercond. 10 (2000) 1477. [3] E. Kuffel, W.S. Zaengl, J. Kuffel, High Voltage Engineering: Fundamentals, Newnes, Oxford, 2001, pp. 235–241. [4] J.V. Champion, S.J. Dodd, in: Proceedings of the IEEE 7th International Conference Solid Dielectrics, 2001, p. 329. [5] J.Y. Koo, H.S. Lee, K.B. Lee, IEEE Annual Report, Electrical Insulation and Dielectric Phenomena, Arlington, 1994, pp. 512–517. [6] High-voltage alternating-current circuit-breakers, IEC 62271-100, 2006.