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A study on insulation characteristics according to cooling methods of the HTS SMES
Physica C 470 (2010) 1703–1706
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
A study on insulation characteristics according to cooling methods of the HTS SMES J.H. Choi a, H.G. Cheon a, J.W. Choi a, H.J. Kim b, K.C. Seong b, S.H. Kim a,* a b
Department of Electrical Engineering, Gyeongsang National University and ERI, Jinju, Gyeongnam 660-701, Republic of Korea Superconductivity Research Center, KERI, Changwon, Gyeongnam 641-120, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 16 May 2010 Keywords: HTS SMES Breakdown Insulation Cooling method
a b s t r a c t The high temperature superconducting magnetic energy storage (HTS SMES) stores electric power in the form of magnetic energy, and then converts it to electric energy. For the operation, the HTS SMES must have a cryogenic temperature. The cooling methods for a cryogenic temperature are divided into an immersed method and a conduction cooled method. The immersed method is a direct cooling method that immerses the superconducting magnet into a cryogen. On the other hand, the conduction cooled method is an indirect cooling method that cools a superconductor through thermal conduction with a cryocooler. This paper classified the structures of insulation according to cooling methods, and studied the insulation characteristics of each insulation factor. Ó 2010 Published by Elsevier B.V.
1. Introduction Recently, high temperature superconductor (HTS) wires were improved to be high in critical currents and excellent in mechanical properties, and various magnets using this are designed and manufactured [1]. HTS has superior magnetic properties and higher critical temperature than low temperature superconductor (LTS), so it seems possible to reduce the size and capacity of a magnet [2]. Due to this advantage, development and research on power equipment using a HTS magnet make progress globally actively [3,4]. The superconducting magnet energy storage (SMES) system is one of them. The HTS SMES is being developed to stabilize a power system, compensate changes in load and voltage, level weekday load, etc. [5]. The SMES requires a cryogenic device by all means. Methods of making a cryogenic temperature can be largely divided into two kinds. One is a method of immersing superconducting coils in cryogen, and the other is a method of conduction cooling using a cryocooler. The immersed method has an advantage of excellent thermal stability. However, it has a disadvantage of presence of heat loss due to cryogen storage and difficulty in miniaturization and lightweight of systems. On the other hand, the conduction method does not use cryogen, so there is no heat loss by using cryogen. Besides, this has an advantage that a flexible configuration is possible
* Corresponding author Address: Department of Electrical Engineering, Gyeongsang National University, 900 Gazwa-dong, Jinju, Gyeongnam 660-701, Republic of Korea Tel.: +82 55 751 5345; fax: +82 55 761 8820. E-mail address: [email protected] (S.H. Kim). 0921-4534/$ - see front matter Ó 2010 Published by Elsevier B.V. doi:10.1016/j.physc.2010.05.190
regardless of its installation position or angle. However, this has a disadvantage that thermal stability is low because there is a big influence of exothermic characteristics of cryocooler and magnet. The SMES using a different cooling method like this are different in insulation elements and properties as well. Therefore, researches on insulation properties should be performed considering the insulation configuration of each SMES. This paper analyzed major insulation elements of SMES of immersed type and conduction type in a practical point of view. For each element, electrode systems were simulated and insulation properties were researched depending on cooling methods.
2. Cooling method and insulation element Fig. 1 shows the structures and properties of each insulation element depending on cooling methods. Insulation elements are largely divided into five kinds, each of which has the following properties. The turn-to-turn insulation (part A) is dependent on the thickness and winding number of Kapton films wrapping a conductor at the cryogenic temperature. The layer-to-layer insulation (part B) is determined according to the creeping discharge properties of glass fiber reinforced plastics (GFRP). The cryocooler-to-coil insulation (part C) is dependent on the insulated thickness of Al2O3 coatings on a metallic magnet bobbin. This element applies to the conduction method only, being a necessary element to protect the cryocooler. The coil-to-coil insulation (part D) applies to the immersed method only, being dependent on the creeping discharge properties of a GFRP spacer. This is an element requiring higher insulation
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insulated Superconducting with Kapton
coated bobbin with Al 2O3
C
A
B
D
GFRP spacer
GFRP spacer
E vacuum or LN2
cooling head Fig. 1. The structures of insulation elements.
80
Break down Voltage (kV)
capability than the layer-to-layer insulation. The ground insulation (part E) has different properties depending on the interior insulation material of a cryostat. The immersed method is dependent on the puncture breakdown properties of cryogen, and the conduction method is determined by the puncture breakdown properties of a vacuum. For an immersed method, liquid nitrogen (LN2) was used as cryogen. For a conduction method, a cryocooler was used, and the inside of a cryostat is a vacuum. The immersed method and the conduction method are similar to each other in insulation elements, but are different from each other in insulation properties. This research selected proper insulating materials depending on the insulation structures and discharge properties of each element, fabricated an electrode system simulating the insulation structures of each element, and then investigated its electric properties.
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40
20
0
0
1
2
3
Winding Number (sheets) 3. Experimental equipment and method Experimental equipment to research the insulation properties in LN2 and the electric insulation properties in a cryogenic temperature and a high vacuum is largely comprised of a cryocooler, vacuum pump, cryostat, power supply source, etc. The cryocooler is a Gifford–McMahon (GM) refrigerator (Cryomech, Co., AL300) with 40 K maximum cooling temperature of the cooling head. The maximum degree of vacuum of the vacuum pump is 1.3 10 6 Torr, which is measured using a vacuum sensor (PKR-251) of Balzer’s company. The cryostat was fabricated of a stainless steel vessel with 900 mm height and 300 mm inner diameter, and is comprised of a vacuum thermal insulation layer to prevent heat invasion. The maximum output voltage of high voltage power is DC 100 kV. The experiments for insulation elements of an immersed method investigated insulation properties by filling a cryostat with LN2 and installing a simulated electrode system for each element. The insulation elements of a conduction method were cooled in a conduction method up to 40 K by operating a cryocooler after exhausting the inside of a cryostat provided with a simulated electrode system into a high vacuum less than 2 10 6 Torr. High voltage for each insulation test increased at a speed of 2 kV/s.
4. Experiment result and consideration 4.1. Turn-to-turn insulation Kapton films (0.025 mm thickness, 10 mm width) were wound on a copper wire (0.3 mm thickness, 4 mm width), with 30% overlapping, in order to simulate a turn-to-turn insulation model. The thickness of Kapton films wound once on a wire is 0.05 mm. Two insulated wires were joined, and high voltage and grounding were connected to each opposite side. Therefore, the total thickness of Kapton wound once on a turn-to-turn electrode system is 0.1 mm.
Fig. 2. Puncture breakdown properties depending on the winding number of Kapton films.
Fig. 2 shows a DC puncture breakdown properties depending on the winding number of Kapton films for turn-to-turn insulation. As winding number increases, breakdown voltage increases, and electric strength decreases. However, considering that turn-to-turn voltage of magnets for SMES is less than a maximum of several tens to several hundreds volts, it can be seen that enough insulation can be secured by one winding of Kapton films. Nevertheless, the thickness of a film is very thin, so it is judged that care is required to avoid mechanical damage while winding an insulated superconducting wire. Besides, it is thought that 50% overlapping will be appropriate to complement defects, etc. considering the mechanical property of insulating film.
4.2. Layer-to-layer insulation The layer-to-layer insulation of SMES is dependent on the creeping discharge property of GFRP. GFRP is excellent in electric insulation and mechanical properties and low in contraction rate at a cryogenic temperature. Accordingly, this is widely used as a cryogenic insulating material and structure. Creeping discharge has diverse properties due to kinds and shapes of insulating materials, surface roughness of electrodes, types of applied voltage, surrounding medium and environments, etc. Fig. 3 shows the creeping discharge properties of GFRP in LN2 and in a vacuum. Experiments were performed by attaching a triangular-plane electrode to the surface of GFRP. The acute angle of the triangular electrode corner is 60°, its curvature radius is 25 lm, the length of the plane electrode is 30 mm, and the curvature radius of the corner is 10 mm. Breakdown voltage was measured
1705
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40
4
30
20
10 vacuum
Break down Voltage (kV)’’
Surface Flash over Voltage (kV)
J.H. Choi et al. / Physica C 470 (2010) 1703–1706
3
2
1
LN2 0 0
2
4
6
0
Surface Length (mm)
0
0.02
0.04
0.06
0.08
Thickness (mm)
Fig. 3. Creeping discharge properties of GFRP in LN2 and in a vacuum. Fig. 4. Puncture breakdown properties of Al2O3 in a vacuum.
4.3. Cryocooler-to-coil insulation The cryocooler-to-coil insulation is an element applied to the SMES of the conduction method. The cooling head of a cryocooler and the coil are combined for conduction cooling. Insulation is needed for a connection to protect the cryocooler from high voltage in the coil. The insulating materials used for a connection should have both high thermal conductivity and excellent insulation properties. However, general insulating materials are good in insulation properties but low in thermal conductivity. This research used Al2O3 that has electrically excellent insulation properties as well as maximizes cooling effects. If an aluminium bobbin is oxidized in oxygen, the surface of the bobbin becomes Al2O3. The bobbin insulated with Al2O3 enables both efficient conduction cooling and electrical insulation. This type of insulation is dependent on the puncture breakdown property of Al2O3 coated on the bobbin, so insulation properties were investigated by changing its thickness. Fig. 4 shows the puncture breakdown properties of Al2O3 in a vacuum. Puncture breakdown properties were investigated by changing the thickness of Al2O3 coated on a aluminium plate into 25–66 lm. As shown in the figure, as the thickness increases, puncture breakdown voltage also increases. Besides, it can be seen that errors in breakdown voltage are very large. It is thought that such a big error in experimental results is due to an error in the thickness of Al2O3. Therefore, it is considered that accurate and exact production technology should be backed in the production process of a bobbin. Besides, it is necessary to research mechanical properties such as cracks and breakage which are properties of a ceramic insulating material. 4.4. Coil-to-coil insulation The coil-to-coil insulation applies to insulation structure of an immersed method. This is dependent on the creeping discharge properties of GFRP, used as a structure between a coil and a coil, in LN2. A toroidal magnet in which pancake coils are connected in series is an insulation element to which maximum voltage of the equipment is applied.
Fig. 5 shows the creeping discharge properties of GFRP in LN2. A circular GFRP spacer was inserted between two pancake coils in which copper wires of 4 mm width are wound on a GFRP disc of 50 mm diameter and 4 mm thickness. Two pancake coils were connected to high voltage and grounding respectively. Creeping discharge distance was changed by applying the diameter of a circular GFRP spacer differently. As shown in the figure, as length increases, creeping discharge voltage increases. Besides, it can be seen that electric strength per unit length is higher than the creeping discharge properties of GFRP in layer-to-layer insulation. It is thought that this is because the electric potential gradient of a non-uniform electric field is slower than a triangular-plane simulated electrode system used in layer-to-layer insulation. Besides, the layer-to-layer insulation has a one-dimensional discharge route in the longitudinal direction, but the coil-to-coil insulation has a two-dimensional discharge route in the longitudinal direction and in the collar direction. For the creeping discharge of the same length, the twodimensional route shows a little higher voltage than the onedimensional route even though there is difference depending on the ratio of length and collars. 4.5. Ground insulation A cryostat is generally fabricated of metals, having ground potential. The insulation of a cryostat from the coil inside the SMES system and the high voltage in the current lead is defined ground 50
Surface Flash over Voltage (kV)
by changing the electrode gap in LN2 and in a vacuum by 1 – 6 mm respectively. Both tests show that as the electrode gap increases, breakdown voltage increases, but electric strength decreases. Besides, it can be seen that the insulation properties of LN2 is similar to the insulation properties of a vacuum.
40
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10
0 0
2
4
6
8
Collor Length (mm) Fig. 5. Creeping discharge properties of GFRP in LN2.
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J.H. Choi et al. / Physica C 470 (2010) 1703–1706
60
Break down Voltage (kV)
50
40
30
20
10
vacuum LN2 0
0
0.2
0.4
0.6
0.8
1
Gap Distance (mm) Fig. 6. Puncture breakdown properties of LN2 and a vacuum.
insulation. The inside of a cryostat in SMES was simulated to determine the insulation distance of ground insulation. Puncture breakdown properties of LN2 and a vacuum were investigated respectively by fabricating a spherical stainless steel electrode of 10 mm diameter and a plane electrode system of 40 mm diameter, 15 mm height and 10 mm end curvature radius. Fig. 6 shows puncture breakdown properties of LN2 and a vacuum. As shown in the figure, the puncture breakdown properties of ground insulation is very excellent in electric strength per unit length. Several ten kilovolt of breakdown voltage is provided by only several hundred micrometers of insulation distance. However, this is a quantitatively measured value by a simulation experiment. Therefore, an insulation distance considering a margin should be determined by considering the mechanical properties such as shocks, oscillations, etc. outside the cryostat during actual operation, the shape of a non-uniform electric field in the internal devices. 5. Conclusions This paper classified it into an immersed method and a conduction method depending on the cooling method of SMES. It
was divided into turn-to-turn insulation, layer-to-layer insulation, cryocooler-to-coil insulation, coil-to-coil insulation, ground insulation, etc. depending on the configuration of each method. For insulating materials for each element, the insulation properties were investigated in LN2 and in a vacuum. For turn-to-turn insulation, its puncture breakdown properties were investigated using a wire on which Kapton films of 25 lm thickness were wound with 30% overlapping. For layer-to-layer insulation, its creeping discharge properties were tested by attaching a triangular-plane electrode to the surface of GFRP plate. For cryocooler-to-coil insulation, its puncture breakdown properties of Al2O3 coated with oxidized aluminium were investigated. For coil-to-coil insulation, a creeping discharge tests for the GFRP disc inserted between pancake coils were performed. For ground insulation, puncture breakdown properties of LN2 and a vacuum were investigated respectively. It is expected that the experiment results in this paper can be used as basic data for the insulation design of SMES of an immersed method and a conduction method under planning in the future. Besides, the fabrication and insulation performance test of a mini model based on the insulation design of each SMES are under planning. Acknowledgement This work was supported by Electric Power Industry Technology Evaluation & Planning. References [1] K. Tasaki, Y. Sumiyoshi, M. Tezuka, H. Hayashi, K. Tsutsumi, K. Funaki, M. Iwakuma, Physica C 357–360 (2001) 1332. [2] P. Tixador, B. Bellin, M. Deleglise, J.C. Vallier, C.E. Bruzek, S. Pavard, J.M. Saugrain, IEEE Trans. Appl. Supercond. 15 (2005) 1907. [3] M. Ono, S. Hanai, K. Tasaki, M. Hiragishi, K. Koyanagi, C. Noma, T. Yazawa, Y. Otani, T. Kuriyama, Y. Sumiyoshi, S. Nomura, Y. Dozono, H. Maeda, T. Hikata, K. Hayashi, H. Takei, K. Sato, M. Kimura, T. Masui, IEEE Trans. Appl. Supercond. 10 (2000) 499. [4] K. Funaki, M. Iwakuma, K. Kajikawa, M. Takeo, J. Suehiro, M. Hara, K. Yamafuji, M. Konno, Y. Kasagawa, K. Okubo, Y. Yasukawa, S. Nose, M. Ueyyama, K. Hayashi, K. Sato, Cryogenics 38 (1998) 211. [5] H. Hayashi, K. Tsutsumi, K. Funaki, M. Iwakuma, K. Tasaki, Y. Sumiyoshi, M. Tezuka, Physica C 357–360 (2001) 1327.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
A study on insulation characteristics according to cooling methods of the HTS SMES J.H. Choi a, H.G. Cheon a, J.W. Choi a, H.J. Kim b, K.C. Seong b, S.H. Kim a,* a b
Department of Electrical Engineering, Gyeongsang National University and ERI, Jinju, Gyeongnam 660-701, Republic of Korea Superconductivity Research Center, KERI, Changwon, Gyeongnam 641-120, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 16 May 2010 Keywords: HTS SMES Breakdown Insulation Cooling method
a b s t r a c t The high temperature superconducting magnetic energy storage (HTS SMES) stores electric power in the form of magnetic energy, and then converts it to electric energy. For the operation, the HTS SMES must have a cryogenic temperature. The cooling methods for a cryogenic temperature are divided into an immersed method and a conduction cooled method. The immersed method is a direct cooling method that immerses the superconducting magnet into a cryogen. On the other hand, the conduction cooled method is an indirect cooling method that cools a superconductor through thermal conduction with a cryocooler. This paper classified the structures of insulation according to cooling methods, and studied the insulation characteristics of each insulation factor. Ó 2010 Published by Elsevier B.V.
1. Introduction Recently, high temperature superconductor (HTS) wires were improved to be high in critical currents and excellent in mechanical properties, and various magnets using this are designed and manufactured [1]. HTS has superior magnetic properties and higher critical temperature than low temperature superconductor (LTS), so it seems possible to reduce the size and capacity of a magnet [2]. Due to this advantage, development and research on power equipment using a HTS magnet make progress globally actively [3,4]. The superconducting magnet energy storage (SMES) system is one of them. The HTS SMES is being developed to stabilize a power system, compensate changes in load and voltage, level weekday load, etc. [5]. The SMES requires a cryogenic device by all means. Methods of making a cryogenic temperature can be largely divided into two kinds. One is a method of immersing superconducting coils in cryogen, and the other is a method of conduction cooling using a cryocooler. The immersed method has an advantage of excellent thermal stability. However, it has a disadvantage of presence of heat loss due to cryogen storage and difficulty in miniaturization and lightweight of systems. On the other hand, the conduction method does not use cryogen, so there is no heat loss by using cryogen. Besides, this has an advantage that a flexible configuration is possible
* Corresponding author Address: Department of Electrical Engineering, Gyeongsang National University, 900 Gazwa-dong, Jinju, Gyeongnam 660-701, Republic of Korea Tel.: +82 55 751 5345; fax: +82 55 761 8820. E-mail address: [email protected] (S.H. Kim). 0921-4534/$ - see front matter Ó 2010 Published by Elsevier B.V. doi:10.1016/j.physc.2010.05.190
regardless of its installation position or angle. However, this has a disadvantage that thermal stability is low because there is a big influence of exothermic characteristics of cryocooler and magnet. The SMES using a different cooling method like this are different in insulation elements and properties as well. Therefore, researches on insulation properties should be performed considering the insulation configuration of each SMES. This paper analyzed major insulation elements of SMES of immersed type and conduction type in a practical point of view. For each element, electrode systems were simulated and insulation properties were researched depending on cooling methods.
2. Cooling method and insulation element Fig. 1 shows the structures and properties of each insulation element depending on cooling methods. Insulation elements are largely divided into five kinds, each of which has the following properties. The turn-to-turn insulation (part A) is dependent on the thickness and winding number of Kapton films wrapping a conductor at the cryogenic temperature. The layer-to-layer insulation (part B) is determined according to the creeping discharge properties of glass fiber reinforced plastics (GFRP). The cryocooler-to-coil insulation (part C) is dependent on the insulated thickness of Al2O3 coatings on a metallic magnet bobbin. This element applies to the conduction method only, being a necessary element to protect the cryocooler. The coil-to-coil insulation (part D) applies to the immersed method only, being dependent on the creeping discharge properties of a GFRP spacer. This is an element requiring higher insulation
1704
J.H. Choi et al. / Physica C 470 (2010) 1703–1706
insulated Superconducting with Kapton
coated bobbin with Al 2O3
C
A
B
D
GFRP spacer
GFRP spacer
E vacuum or LN2
cooling head Fig. 1. The structures of insulation elements.
80
Break down Voltage (kV)
capability than the layer-to-layer insulation. The ground insulation (part E) has different properties depending on the interior insulation material of a cryostat. The immersed method is dependent on the puncture breakdown properties of cryogen, and the conduction method is determined by the puncture breakdown properties of a vacuum. For an immersed method, liquid nitrogen (LN2) was used as cryogen. For a conduction method, a cryocooler was used, and the inside of a cryostat is a vacuum. The immersed method and the conduction method are similar to each other in insulation elements, but are different from each other in insulation properties. This research selected proper insulating materials depending on the insulation structures and discharge properties of each element, fabricated an electrode system simulating the insulation structures of each element, and then investigated its electric properties.
60
40
20
0
0
1
2
3
Winding Number (sheets) 3. Experimental equipment and method Experimental equipment to research the insulation properties in LN2 and the electric insulation properties in a cryogenic temperature and a high vacuum is largely comprised of a cryocooler, vacuum pump, cryostat, power supply source, etc. The cryocooler is a Gifford–McMahon (GM) refrigerator (Cryomech, Co., AL300) with 40 K maximum cooling temperature of the cooling head. The maximum degree of vacuum of the vacuum pump is 1.3 10 6 Torr, which is measured using a vacuum sensor (PKR-251) of Balzer’s company. The cryostat was fabricated of a stainless steel vessel with 900 mm height and 300 mm inner diameter, and is comprised of a vacuum thermal insulation layer to prevent heat invasion. The maximum output voltage of high voltage power is DC 100 kV. The experiments for insulation elements of an immersed method investigated insulation properties by filling a cryostat with LN2 and installing a simulated electrode system for each element. The insulation elements of a conduction method were cooled in a conduction method up to 40 K by operating a cryocooler after exhausting the inside of a cryostat provided with a simulated electrode system into a high vacuum less than 2 10 6 Torr. High voltage for each insulation test increased at a speed of 2 kV/s.
4. Experiment result and consideration 4.1. Turn-to-turn insulation Kapton films (0.025 mm thickness, 10 mm width) were wound on a copper wire (0.3 mm thickness, 4 mm width), with 30% overlapping, in order to simulate a turn-to-turn insulation model. The thickness of Kapton films wound once on a wire is 0.05 mm. Two insulated wires were joined, and high voltage and grounding were connected to each opposite side. Therefore, the total thickness of Kapton wound once on a turn-to-turn electrode system is 0.1 mm.
Fig. 2. Puncture breakdown properties depending on the winding number of Kapton films.
Fig. 2 shows a DC puncture breakdown properties depending on the winding number of Kapton films for turn-to-turn insulation. As winding number increases, breakdown voltage increases, and electric strength decreases. However, considering that turn-to-turn voltage of magnets for SMES is less than a maximum of several tens to several hundreds volts, it can be seen that enough insulation can be secured by one winding of Kapton films. Nevertheless, the thickness of a film is very thin, so it is judged that care is required to avoid mechanical damage while winding an insulated superconducting wire. Besides, it is thought that 50% overlapping will be appropriate to complement defects, etc. considering the mechanical property of insulating film.
4.2. Layer-to-layer insulation The layer-to-layer insulation of SMES is dependent on the creeping discharge property of GFRP. GFRP is excellent in electric insulation and mechanical properties and low in contraction rate at a cryogenic temperature. Accordingly, this is widely used as a cryogenic insulating material and structure. Creeping discharge has diverse properties due to kinds and shapes of insulating materials, surface roughness of electrodes, types of applied voltage, surrounding medium and environments, etc. Fig. 3 shows the creeping discharge properties of GFRP in LN2 and in a vacuum. Experiments were performed by attaching a triangular-plane electrode to the surface of GFRP. The acute angle of the triangular electrode corner is 60°, its curvature radius is 25 lm, the length of the plane electrode is 30 mm, and the curvature radius of the corner is 10 mm. Breakdown voltage was measured
1705
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5
40
4
30
20
10 vacuum
Break down Voltage (kV)’’
Surface Flash over Voltage (kV)
J.H. Choi et al. / Physica C 470 (2010) 1703–1706
3
2
1
LN2 0 0
2
4
6
0
Surface Length (mm)
0
0.02
0.04
0.06
0.08
Thickness (mm)
Fig. 3. Creeping discharge properties of GFRP in LN2 and in a vacuum. Fig. 4. Puncture breakdown properties of Al2O3 in a vacuum.
4.3. Cryocooler-to-coil insulation The cryocooler-to-coil insulation is an element applied to the SMES of the conduction method. The cooling head of a cryocooler and the coil are combined for conduction cooling. Insulation is needed for a connection to protect the cryocooler from high voltage in the coil. The insulating materials used for a connection should have both high thermal conductivity and excellent insulation properties. However, general insulating materials are good in insulation properties but low in thermal conductivity. This research used Al2O3 that has electrically excellent insulation properties as well as maximizes cooling effects. If an aluminium bobbin is oxidized in oxygen, the surface of the bobbin becomes Al2O3. The bobbin insulated with Al2O3 enables both efficient conduction cooling and electrical insulation. This type of insulation is dependent on the puncture breakdown property of Al2O3 coated on the bobbin, so insulation properties were investigated by changing its thickness. Fig. 4 shows the puncture breakdown properties of Al2O3 in a vacuum. Puncture breakdown properties were investigated by changing the thickness of Al2O3 coated on a aluminium plate into 25–66 lm. As shown in the figure, as the thickness increases, puncture breakdown voltage also increases. Besides, it can be seen that errors in breakdown voltage are very large. It is thought that such a big error in experimental results is due to an error in the thickness of Al2O3. Therefore, it is considered that accurate and exact production technology should be backed in the production process of a bobbin. Besides, it is necessary to research mechanical properties such as cracks and breakage which are properties of a ceramic insulating material. 4.4. Coil-to-coil insulation The coil-to-coil insulation applies to insulation structure of an immersed method. This is dependent on the creeping discharge properties of GFRP, used as a structure between a coil and a coil, in LN2. A toroidal magnet in which pancake coils are connected in series is an insulation element to which maximum voltage of the equipment is applied.
Fig. 5 shows the creeping discharge properties of GFRP in LN2. A circular GFRP spacer was inserted between two pancake coils in which copper wires of 4 mm width are wound on a GFRP disc of 50 mm diameter and 4 mm thickness. Two pancake coils were connected to high voltage and grounding respectively. Creeping discharge distance was changed by applying the diameter of a circular GFRP spacer differently. As shown in the figure, as length increases, creeping discharge voltage increases. Besides, it can be seen that electric strength per unit length is higher than the creeping discharge properties of GFRP in layer-to-layer insulation. It is thought that this is because the electric potential gradient of a non-uniform electric field is slower than a triangular-plane simulated electrode system used in layer-to-layer insulation. Besides, the layer-to-layer insulation has a one-dimensional discharge route in the longitudinal direction, but the coil-to-coil insulation has a two-dimensional discharge route in the longitudinal direction and in the collar direction. For the creeping discharge of the same length, the twodimensional route shows a little higher voltage than the onedimensional route even though there is difference depending on the ratio of length and collars. 4.5. Ground insulation A cryostat is generally fabricated of metals, having ground potential. The insulation of a cryostat from the coil inside the SMES system and the high voltage in the current lead is defined ground 50
Surface Flash over Voltage (kV)
by changing the electrode gap in LN2 and in a vacuum by 1 – 6 mm respectively. Both tests show that as the electrode gap increases, breakdown voltage increases, but electric strength decreases. Besides, it can be seen that the insulation properties of LN2 is similar to the insulation properties of a vacuum.
40
30
20
10
0 0
2
4
6
8
Collor Length (mm) Fig. 5. Creeping discharge properties of GFRP in LN2.
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60
Break down Voltage (kV)
50
40
30
20
10
vacuum LN2 0
0
0.2
0.4
0.6
0.8
1
Gap Distance (mm) Fig. 6. Puncture breakdown properties of LN2 and a vacuum.
insulation. The inside of a cryostat in SMES was simulated to determine the insulation distance of ground insulation. Puncture breakdown properties of LN2 and a vacuum were investigated respectively by fabricating a spherical stainless steel electrode of 10 mm diameter and a plane electrode system of 40 mm diameter, 15 mm height and 10 mm end curvature radius. Fig. 6 shows puncture breakdown properties of LN2 and a vacuum. As shown in the figure, the puncture breakdown properties of ground insulation is very excellent in electric strength per unit length. Several ten kilovolt of breakdown voltage is provided by only several hundred micrometers of insulation distance. However, this is a quantitatively measured value by a simulation experiment. Therefore, an insulation distance considering a margin should be determined by considering the mechanical properties such as shocks, oscillations, etc. outside the cryostat during actual operation, the shape of a non-uniform electric field in the internal devices. 5. Conclusions This paper classified it into an immersed method and a conduction method depending on the cooling method of SMES. It
was divided into turn-to-turn insulation, layer-to-layer insulation, cryocooler-to-coil insulation, coil-to-coil insulation, ground insulation, etc. depending on the configuration of each method. For insulating materials for each element, the insulation properties were investigated in LN2 and in a vacuum. For turn-to-turn insulation, its puncture breakdown properties were investigated using a wire on which Kapton films of 25 lm thickness were wound with 30% overlapping. For layer-to-layer insulation, its creeping discharge properties were tested by attaching a triangular-plane electrode to the surface of GFRP plate. For cryocooler-to-coil insulation, its puncture breakdown properties of Al2O3 coated with oxidized aluminium were investigated. For coil-to-coil insulation, a creeping discharge tests for the GFRP disc inserted between pancake coils were performed. For ground insulation, puncture breakdown properties of LN2 and a vacuum were investigated respectively. It is expected that the experiment results in this paper can be used as basic data for the insulation design of SMES of an immersed method and a conduction method under planning in the future. Besides, the fabrication and insulation performance test of a mini model based on the insulation design of each SMES are under planning. Acknowledgement This work was supported by Electric Power Industry Technology Evaluation & Planning. References [1] K. Tasaki, Y. Sumiyoshi, M. Tezuka, H. Hayashi, K. Tsutsumi, K. Funaki, M. Iwakuma, Physica C 357–360 (2001) 1332. [2] P. Tixador, B. Bellin, M. Deleglise, J.C. Vallier, C.E. Bruzek, S. Pavard, J.M. Saugrain, IEEE Trans. Appl. Supercond. 15 (2005) 1907. [3] M. Ono, S. Hanai, K. Tasaki, M. Hiragishi, K. Koyanagi, C. Noma, T. Yazawa, Y. Otani, T. Kuriyama, Y. Sumiyoshi, S. Nomura, Y. Dozono, H. Maeda, T. Hikata, K. Hayashi, H. Takei, K. Sato, M. Kimura, T. Masui, IEEE Trans. Appl. Supercond. 10 (2000) 499. [4] K. Funaki, M. Iwakuma, K. Kajikawa, M. Takeo, J. Suehiro, M. Hara, K. Yamafuji, M. Konno, Y. Kasagawa, K. Okubo, Y. Yasukawa, S. Nose, M. Ueyyama, K. Hayashi, K. Sato, Cryogenics 38 (1998) 211. [5] H. Hayashi, K. Tsutsumi, K. Funaki, M. Iwakuma, K. Tasaki, Y. Sumiyoshi, M. Tezuka, Physica C 357–360 (2001) 1327.