- Email: [email protected]
Electric field analysis on the insulation design of the stop joint box for DC HTS power cable
Cryogenics xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Electric field analysis on the insulation design of the stop joint box for DC HTS power cable Jae-Sang Hwang a, Hee-Suk Ryoo b, Ja-Yoon Koo a, Jeon-Wook Cho b, Bang-Wook Lee a,⇑ a b
Hanyang University, 408-2, 4th Engineering Bldg, Hanyang University, Sa 3-dong, Sangrok-gu, Ansan 426-791, Republic of Korea1 Korea Electrotechnology Research Institute, Changwon, Gyungnam 641-120, Republic of Korea
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: DC HTS cable DC electric field distribution Joint box Insulation design
a b s t r a c t DC HTS power cable is a promising electric power transmission line for the future of smart grid, and it has been competitively investigated at many research institutes all over the world. For the commercialization of DC HTS cable, higher power transmission capacity and longer length for long distance transmission line should be prepared. In order to meet the needs of long distance DC HTS cable, a joint box should be developed for the connection of cable components. As for AC HTS cable, a number of patents of nominal joint box have been already reported. However, any conceptual designs of the joint box for DC HTS cable have not been suggested yet. One of the reasons is that the cryogenic high voltage insulation design, especially in DC environment is not fully investigated yet. Conventional normal joint box for AC HTS cable could not be directly applied to DC HTS cable because different electric field distributions compared to AC electric field which requires totally different electrical insulation design concepts. In this paper, in order to establish the basic insulation design of the stop joint box (SJB) for DC HTS cable, three kinds of SJB models were designed and electric field analyses have been conducted both considering AC and DC environment. And the critical factors affecting the DC insulation design of the stop joint box were analyzed. From the simulation results, it was observed that the electric field distribution was totally different both in AC and DC operating conditions. And it was possible to find the weakest regions in the insulation design of the SJB. Consequently, based on the DC electric field analysis, the insulation design criteria and the desirable configurations were suggested for the insulation design of the stop joint box for DC HTS cable. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Around the world, electricity production and consumption have been drastically increased for several decades. The demand of reliable and efficient electric energy supply is also continuously increasing. Thus, in order to meet these electric power requirements, the concept of super grid, which is a wide area transmission network that makes it possible to trade high volumes of electricity, has been proposed [1,2]. Recently, many large scale projects related to super grid for renewable energy in Europe have been planned and tried to realize for the future electric power networks [3,4]. In order to implement super grid successfully, high voltage direct current (HVDC) system could be one of feasible solutions. HVDC system has technical advantages of synchronization for two power systems having different frequencies. In addition HVDC ⇑ Corresponding author. Tel.: +82 31 400 5665; fax: +82 31 400 4752. 1
E-mail address: [email protected] (B.-W. Lee). [email protected].
system shows economical merits for long transmission distance compared to high voltage alternating current (HVAC) transmission line [5]. As a transmission line for HVDC system, HVDC power cable has been played important role to supply electric power, and recently high temperature superconducting (HTS) DC power cable has been considered as an ideal solution for future HVDC power cable. In order to develop DC HTS power cable, higher voltage, larger capacity and longer cable length should be implemented. And one of the key components of DC HTS cable for realizing longer cable length is the joint box of DC HTS cable. But few research works in this field have been reported yet [6]. One of the reasons is that the cryogenic high voltage insulation design, especially in DC environment is not fully investigated yet. For the insulation design of the joint box of DC HTS cable, DC electric field analysis should be performed considering the insulation structure of the joint box. In this work, in order to determine the difference between AC and DC electric field analysis, a simple model having a coaxial cylindrical structure has been modeled and simulated. Comparing
http://dx.doi.org/10.1016/j.cryogenics.2014.05.006 0011-2275/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hwang J-S et al. Electric field analysis on the insulation design of the stop joint box for DC HTS power cable. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.05.006
2
J.-S. Hwang et al. / Cryogenics xxx (2014) xxx–xxx
the simulation results to theoretical electric field calculation, both AC and DC electric field analysis techniques were confirmed. Then, as a fundamental step to develop the stop joint box (SJB) of DC HTS cable, an initial SJB model was designed based on a joint box for conventional AC oil-filled (OF) power cable. And electric field analysis has been conducted both considering AC and DC environment to compare the electric field distributions. Two kinds of improved SJB model were considered and DC electric field analysis was conducted in order to reduce severe electric field concentration in the initial SJB model of DC HTS cable. Finally, the critical factors affecting the DC insulation design of the stop joint box was analyzed. 2. Estimation of both AC and DC electric field intensity for a coaxial cylindrical structure In order to compare between electric field calculation and simulation result, a coaxial cylindrical geometry composed of double dielectrics with an insulation thickness of 3 mm was considered and their dimensions were indicated in Fig. 1(a). In a single dielectric, both AC and DC electric field intensity values are equal regardless of the types of applying voltage. In the double dielectrics, however, AC electric field distribution depends on the distance and relative permittivity ratio between dielectric materials, but in case of DC, the distance and electrical conductivity ratio between dielectric materials is a dominant factor. Equations for the electric field calculation of each dielectric media and their calculated results were shown in Table 1. For the electric field analysis, COMSOL Multiphysics 4.3b simulation tool was used and its simulation results were compared to the theoretical field calculation. Fig. 1(b) and (c) has shown AC and DC electric field distribution and their maximum electric field intensity values in each dielectric media. Comparing the maximum electric field intensity, both AC and DC simulation result values has shown exactly same results compared to the calculation values. Therefore, the reliability of electric field analysis techniques could be confirmed. From the simulation results, it was shown that the maximum electric field intensity in DC was higher than that of AC. It was already known that the field distribution of DC was governed by the relative ratio of electric conductivity of insulating materials. Typically, the relative permittivity values of different materials do not differ so much but the ratio between the electrical conductivities of different materials amounts to many decades [7]. For this reason, it is relatively more difficult to design the insulation structure of HVDC electric power apparatus compared to AC one. 3. Simulation set-up of the stop joint box for DC HTS power cable Among the various types of joint boxes for power cables, a stop joint box has been utilized especially for HVAC OF cable which uses liquid dielectric media. Similarly, it would be possible to utilize the stop joint box for DC HTS power cables using liquid nitrogen as cooling and dielectric media. By use of SJB, it is possible to connect cable sections for long distance and to provide proper electrical insulation. But up to now, satisfactory solution of SJB for DC HTS cable has not been proposed due to several unsolved problems. DC insulation design is critically different compared to that of AC insulation. Besides, cryogenic environment could cause different problems compared to normal operating condition at room temperature. Initial stop joint box model for DC HTS cable is designed considering the configuration of OF power cable as shown in Fig. 2. From Fig. 2, Epoxy spacer is located on the center of SJB in order to separate cooling sections and to provide proper electrical insulation.
Fig. 1. Simulation results of AC and DC electric field distribution for coaxial cylindrical geometry, (a) geometry, (b) AC electric field distribution and (c) DC electric field distribution.
Polypropylene laminated paper (PPLP) was selected for the main insulation material due to outstanding breakdown strength in
Please cite this article in press as: Hwang J-S et al. Electric field analysis on the insulation design of the stop joint box for DC HTS power cable. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.05.006
3
J.-S. Hwang et al. / Cryogenics xxx (2014) xxx–xxx Table 1 AC and DC electric field calculation of coaxial cylindrical structure with double dielectrics. Estimation
Field
The maximum electric field intensity (kV/mm)
Theoretical calculation results
AC
E1 ¼ h V r ln
r2 r1
E2 ¼ h
e r e1 ln 2
DC
e
þe1 ln 2
V r2 r1
þln
E1 ¼ h V r ln
E2 ¼ h
r
r2 r1
r r1 ln 2
r
þr1 ln 2
V r2 r1
þln
1 1 i ¼ ¼ 1:67 ffi 0:598 1½ln ð41Þþ24 ln ð74Þ r3 r2
i ¼ r3 r2
1 4½42 ln ð41Þþln ð74Þ
i ¼ h r3 r2
15
i¼
1 ln ð41Þþ1014 ln ð74Þ
1 1:44
ffi 0:694
10
i ¼ h r3 r2
1
1 ¼ 13:33 ffi 0:075
4
1015 1014
1
1 i ¼ 57:69 ffi 0:017
ln ð41Þþln ð74Þ
Fig. 2. The initial SJB model for electric field distribution simulation.
Table 2 Dielectric parameters of materials for electric field distribution simulation. Material
Relative permittivity
Electrical conductivity (S/m)
Epoxy PPLP Kraft
3.7 2.29 1.03
1.54e14 1.69e16 2.44e10
liquid nitrogen [8]. To assure the firm binding of PPLP layers, Kraft was used. And electric field distribution of the initial SJB model was simulated. In Fig. 2, applying voltage was set to 360 kV for both AC and DC simulations, and the outermost layer of Kraft was grounded. The vertical dotted arrow line represents a radial direction from epoxy to Kraft. The straight arrow line represents a path along the interface between PPLP and epoxy. As the SJB model has bilateral symmetry, electric field simulations were carried out only considering left half side of the SJB model. The dielectric parameters of materials for electric field distribution simulation are shown in Table 2 [9,10].
4. AC and DC electric field distribution analysis for initial SJB model Fig. 3(a) and (b) shows AC and DC electric field distribution of the initial SJB model. Due to lower relative permittivity ratio between dielectrics, there is no severe electric field concentration in AC electric field. But when DC electric field was exerted, electric field concentration was found inside the thinnest PPLP section located on the edge of epoxy spacer. This concentration of electric field is seen at the right side in Fig. 3(b). DC electric field concentration was caused inside PPLP layer because PPLP has the lowest electrical conductivity value compared to the other materials. DC electric field intensity was decreased with increasing PPLP thickness and this PPLP thickness control could be one of possible solutions for electric field grading in SJB. Fig. 3(c) and (d) shows the plot in radial and surface direction of electric field for the initial SJB model. Radial direction of electric field represents the path across different dielectrics (dotted arrow line in Fig. 2) and surface direction represents the path along the interface between PPLP and epoxy (straight arrow line in Fig. 2). Looking at electric field across dielectrics, the maximum electric field intensity in DC was 54 kV/mm, which was 3.45 times higher
Fig. 3. Electric field distribution simulation results of the initial SJB model; (a) AC electric field distribution, (b) DC electric field distribution, (c) radial direction and (d) surface direction.
Please cite this article in press as: Hwang J-S et al. Electric field analysis on the insulation design of the stop joint box for DC HTS power cable. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.05.006
4
J.-S. Hwang et al. / Cryogenics xxx (2014) xxx–xxx
Fig. 4. Electric field distribution simulation results of improved SJB model; (a) three steps SJB model, (b) one step SJB model, (c) radial direction and (d) surface direction.
value than 15.62 kV/mm in AC. The maximum DC electric field intensity along the interface of PPLP and epoxy was 59.1 kV/mm, which was 2.34 times higher than 25.56 kV/mm in AC. DC electric field intensity was higher than AC due to large electrical conductivity ratio between different dielectrics. Therefore, DC insulation design is totally different from AC and it is necessary to simulate the DC electric field distribution for DC insulation design. Consequently, this seven steps structure of AC SJB model led to higher DC electric stresses and could not be directly applicable to DC SJB model. This design should be improved. 5. DC electric field distribution analysis for improved SJB models DC electric field concentrates inside the thinnest material having the lowest electrical conductivity value. In order to reduce the electric field concentration at the thinnest part of PPLP in the initial SJB model, PPLP thickness should be controlled. Two kinds of improved SJB model were considered and DC electric field analysis was conducted. Simulation results of improved SJB models are shown in Fig. 4(a) and (b). Regarding the simulation results, the electric field concentration inside PPLP was reduced when the thickness of PPLP located on the edge of epoxy spacer was increased. This could be clearly seen at the right side in Fig. 4(b) and (c). Fig. 4(c) and (d) shows the plot in radial and surface direction of electric field for improved SJB models. In radial direction, the SJB model having only one step showed the lowest DC electric field intensity of 21.67 kV/mm. One step and three steps SJB models showed 60% and 39.5% reduction of electric field intensity respectively compared to the initial SJB model. In surface direction, the one step SJB model shows the lowest DC electric field intensity of 30.92 kV/mm. Comparing to the initial SJB model, the one step and three steps SJB models showed 47.7% and 21.1% reduction of electric field intensity. Therefore, step structure for AC electric field grading was not suitable for DC operation environment. Consequently, it was shown that the one step structure of epoxy spacer is more appropriate than multi-steps structure for DC SJB.
6. Conclusion As a fundamental step to determine the insulation design of the SJB for DC HTS power cable, three kinds of SJB models were considered and electric field analysis have been conducted considering DC steady state. An initial SJB model was designed based on the SJB for AC oil-filled cable and DC electric field analysis has been performed. In order to reduce the electric field concentration of the initial SJB model, two kinds of improved SJB models such as one step and three steps SJB models were considered. Regarding the simulation results of improved SJB models, it was observed that the one step structure of epoxy spacer is more appropriate than multi-steps structure for DC SJB. Acknowledgements This work was supported by a grant from Basic Science Research Program of Korea Electrotechnology Research Institute funded by the Korea Research Council for Industrial Science and Technology (KOCI). References [1] Kalcon G, Adam G, Anaya-Lara O, Burt G, Lo K. Int Conf Environ Electr Eng Rome 2011:1–4. [2] Feltes JW, Gemmell BD, Retzmann D. IEEE Power Energy Soc Gen Meet 2011. [3] Purvins A, Wilkening H, Fulli G, Tzimas E, Celli G, Mocci S, et al. J Cleaner Prod 2011;19:1909–16. [4] Elliott D. Energy Strategy Rev 2013. [5] Van Hertem D, Ghandhari M. Renew Sustain Energy Rev 2010:14. [6] Xiao L, Dai S, Lin L, Teng Y, Zhang H, Liang X, et al. IEEE Trans Appl Supercond 2011:22. [7] Kreuger FH. Industrial high voltage. Delft Univ Press; 1998. p. 15–22. [8] Nagao M, Kurimoto M, Takahashi R, Kawashima T, Murakami Y, Nishimura T, Ashibe Y, Masuda T. Annual report on electrical insulation and dielectric phenomena (CEIDP), 2011. [9] Hwang J, Seong J, Shin W, Lee J, Cho J, Ryoo H, et al. Phys C: Supercond 2013. [10] Masood A, Zuberi M, Husain E. IEEE Trans Dielectr Electr Insul 2008:15.
Please cite this article in press as: Hwang J-S et al. Electric field analysis on the insulation design of the stop joint box for DC HTS power cable. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.05.006
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Electric field analysis on the insulation design of the stop joint box for DC HTS power cable Jae-Sang Hwang a, Hee-Suk Ryoo b, Ja-Yoon Koo a, Jeon-Wook Cho b, Bang-Wook Lee a,⇑ a b
Hanyang University, 408-2, 4th Engineering Bldg, Hanyang University, Sa 3-dong, Sangrok-gu, Ansan 426-791, Republic of Korea1 Korea Electrotechnology Research Institute, Changwon, Gyungnam 641-120, Republic of Korea
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: DC HTS cable DC electric field distribution Joint box Insulation design
a b s t r a c t DC HTS power cable is a promising electric power transmission line for the future of smart grid, and it has been competitively investigated at many research institutes all over the world. For the commercialization of DC HTS cable, higher power transmission capacity and longer length for long distance transmission line should be prepared. In order to meet the needs of long distance DC HTS cable, a joint box should be developed for the connection of cable components. As for AC HTS cable, a number of patents of nominal joint box have been already reported. However, any conceptual designs of the joint box for DC HTS cable have not been suggested yet. One of the reasons is that the cryogenic high voltage insulation design, especially in DC environment is not fully investigated yet. Conventional normal joint box for AC HTS cable could not be directly applied to DC HTS cable because different electric field distributions compared to AC electric field which requires totally different electrical insulation design concepts. In this paper, in order to establish the basic insulation design of the stop joint box (SJB) for DC HTS cable, three kinds of SJB models were designed and electric field analyses have been conducted both considering AC and DC environment. And the critical factors affecting the DC insulation design of the stop joint box were analyzed. From the simulation results, it was observed that the electric field distribution was totally different both in AC and DC operating conditions. And it was possible to find the weakest regions in the insulation design of the SJB. Consequently, based on the DC electric field analysis, the insulation design criteria and the desirable configurations were suggested for the insulation design of the stop joint box for DC HTS cable. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Around the world, electricity production and consumption have been drastically increased for several decades. The demand of reliable and efficient electric energy supply is also continuously increasing. Thus, in order to meet these electric power requirements, the concept of super grid, which is a wide area transmission network that makes it possible to trade high volumes of electricity, has been proposed [1,2]. Recently, many large scale projects related to super grid for renewable energy in Europe have been planned and tried to realize for the future electric power networks [3,4]. In order to implement super grid successfully, high voltage direct current (HVDC) system could be one of feasible solutions. HVDC system has technical advantages of synchronization for two power systems having different frequencies. In addition HVDC ⇑ Corresponding author. Tel.: +82 31 400 5665; fax: +82 31 400 4752. 1
E-mail address: [email protected] (B.-W. Lee). [email protected].
system shows economical merits for long transmission distance compared to high voltage alternating current (HVAC) transmission line [5]. As a transmission line for HVDC system, HVDC power cable has been played important role to supply electric power, and recently high temperature superconducting (HTS) DC power cable has been considered as an ideal solution for future HVDC power cable. In order to develop DC HTS power cable, higher voltage, larger capacity and longer cable length should be implemented. And one of the key components of DC HTS cable for realizing longer cable length is the joint box of DC HTS cable. But few research works in this field have been reported yet [6]. One of the reasons is that the cryogenic high voltage insulation design, especially in DC environment is not fully investigated yet. For the insulation design of the joint box of DC HTS cable, DC electric field analysis should be performed considering the insulation structure of the joint box. In this work, in order to determine the difference between AC and DC electric field analysis, a simple model having a coaxial cylindrical structure has been modeled and simulated. Comparing
http://dx.doi.org/10.1016/j.cryogenics.2014.05.006 0011-2275/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hwang J-S et al. Electric field analysis on the insulation design of the stop joint box for DC HTS power cable. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.05.006
2
J.-S. Hwang et al. / Cryogenics xxx (2014) xxx–xxx
the simulation results to theoretical electric field calculation, both AC and DC electric field analysis techniques were confirmed. Then, as a fundamental step to develop the stop joint box (SJB) of DC HTS cable, an initial SJB model was designed based on a joint box for conventional AC oil-filled (OF) power cable. And electric field analysis has been conducted both considering AC and DC environment to compare the electric field distributions. Two kinds of improved SJB model were considered and DC electric field analysis was conducted in order to reduce severe electric field concentration in the initial SJB model of DC HTS cable. Finally, the critical factors affecting the DC insulation design of the stop joint box was analyzed. 2. Estimation of both AC and DC electric field intensity for a coaxial cylindrical structure In order to compare between electric field calculation and simulation result, a coaxial cylindrical geometry composed of double dielectrics with an insulation thickness of 3 mm was considered and their dimensions were indicated in Fig. 1(a). In a single dielectric, both AC and DC electric field intensity values are equal regardless of the types of applying voltage. In the double dielectrics, however, AC electric field distribution depends on the distance and relative permittivity ratio between dielectric materials, but in case of DC, the distance and electrical conductivity ratio between dielectric materials is a dominant factor. Equations for the electric field calculation of each dielectric media and their calculated results were shown in Table 1. For the electric field analysis, COMSOL Multiphysics 4.3b simulation tool was used and its simulation results were compared to the theoretical field calculation. Fig. 1(b) and (c) has shown AC and DC electric field distribution and their maximum electric field intensity values in each dielectric media. Comparing the maximum electric field intensity, both AC and DC simulation result values has shown exactly same results compared to the calculation values. Therefore, the reliability of electric field analysis techniques could be confirmed. From the simulation results, it was shown that the maximum electric field intensity in DC was higher than that of AC. It was already known that the field distribution of DC was governed by the relative ratio of electric conductivity of insulating materials. Typically, the relative permittivity values of different materials do not differ so much but the ratio between the electrical conductivities of different materials amounts to many decades [7]. For this reason, it is relatively more difficult to design the insulation structure of HVDC electric power apparatus compared to AC one. 3. Simulation set-up of the stop joint box for DC HTS power cable Among the various types of joint boxes for power cables, a stop joint box has been utilized especially for HVAC OF cable which uses liquid dielectric media. Similarly, it would be possible to utilize the stop joint box for DC HTS power cables using liquid nitrogen as cooling and dielectric media. By use of SJB, it is possible to connect cable sections for long distance and to provide proper electrical insulation. But up to now, satisfactory solution of SJB for DC HTS cable has not been proposed due to several unsolved problems. DC insulation design is critically different compared to that of AC insulation. Besides, cryogenic environment could cause different problems compared to normal operating condition at room temperature. Initial stop joint box model for DC HTS cable is designed considering the configuration of OF power cable as shown in Fig. 2. From Fig. 2, Epoxy spacer is located on the center of SJB in order to separate cooling sections and to provide proper electrical insulation.
Fig. 1. Simulation results of AC and DC electric field distribution for coaxial cylindrical geometry, (a) geometry, (b) AC electric field distribution and (c) DC electric field distribution.
Polypropylene laminated paper (PPLP) was selected for the main insulation material due to outstanding breakdown strength in
Please cite this article in press as: Hwang J-S et al. Electric field analysis on the insulation design of the stop joint box for DC HTS power cable. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.05.006
3
J.-S. Hwang et al. / Cryogenics xxx (2014) xxx–xxx Table 1 AC and DC electric field calculation of coaxial cylindrical structure with double dielectrics. Estimation
Field
The maximum electric field intensity (kV/mm)
Theoretical calculation results
AC
E1 ¼ h V r ln
r2 r1
E2 ¼ h
e r e1 ln 2
DC
e
þe1 ln 2
V r2 r1
þln
E1 ¼ h V r ln
E2 ¼ h
r
r2 r1
r r1 ln 2
r
þr1 ln 2
V r2 r1
þln
1 1 i ¼ ¼ 1:67 ffi 0:598 1½ln ð41Þþ24 ln ð74Þ r3 r2
i ¼ r3 r2
1 4½42 ln ð41Þþln ð74Þ
i ¼ h r3 r2
15
i¼
1 ln ð41Þþ1014 ln ð74Þ
1 1:44
ffi 0:694
10
i ¼ h r3 r2
1
1 ¼ 13:33 ffi 0:075
4
1015 1014
1
1 i ¼ 57:69 ffi 0:017
ln ð41Þþln ð74Þ
Fig. 2. The initial SJB model for electric field distribution simulation.
Table 2 Dielectric parameters of materials for electric field distribution simulation. Material
Relative permittivity
Electrical conductivity (S/m)
Epoxy PPLP Kraft
3.7 2.29 1.03
1.54e14 1.69e16 2.44e10
liquid nitrogen [8]. To assure the firm binding of PPLP layers, Kraft was used. And electric field distribution of the initial SJB model was simulated. In Fig. 2, applying voltage was set to 360 kV for both AC and DC simulations, and the outermost layer of Kraft was grounded. The vertical dotted arrow line represents a radial direction from epoxy to Kraft. The straight arrow line represents a path along the interface between PPLP and epoxy. As the SJB model has bilateral symmetry, electric field simulations were carried out only considering left half side of the SJB model. The dielectric parameters of materials for electric field distribution simulation are shown in Table 2 [9,10].
4. AC and DC electric field distribution analysis for initial SJB model Fig. 3(a) and (b) shows AC and DC electric field distribution of the initial SJB model. Due to lower relative permittivity ratio between dielectrics, there is no severe electric field concentration in AC electric field. But when DC electric field was exerted, electric field concentration was found inside the thinnest PPLP section located on the edge of epoxy spacer. This concentration of electric field is seen at the right side in Fig. 3(b). DC electric field concentration was caused inside PPLP layer because PPLP has the lowest electrical conductivity value compared to the other materials. DC electric field intensity was decreased with increasing PPLP thickness and this PPLP thickness control could be one of possible solutions for electric field grading in SJB. Fig. 3(c) and (d) shows the plot in radial and surface direction of electric field for the initial SJB model. Radial direction of electric field represents the path across different dielectrics (dotted arrow line in Fig. 2) and surface direction represents the path along the interface between PPLP and epoxy (straight arrow line in Fig. 2). Looking at electric field across dielectrics, the maximum electric field intensity in DC was 54 kV/mm, which was 3.45 times higher
Fig. 3. Electric field distribution simulation results of the initial SJB model; (a) AC electric field distribution, (b) DC electric field distribution, (c) radial direction and (d) surface direction.
Please cite this article in press as: Hwang J-S et al. Electric field analysis on the insulation design of the stop joint box for DC HTS power cable. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.05.006
4
J.-S. Hwang et al. / Cryogenics xxx (2014) xxx–xxx
Fig. 4. Electric field distribution simulation results of improved SJB model; (a) three steps SJB model, (b) one step SJB model, (c) radial direction and (d) surface direction.
value than 15.62 kV/mm in AC. The maximum DC electric field intensity along the interface of PPLP and epoxy was 59.1 kV/mm, which was 2.34 times higher than 25.56 kV/mm in AC. DC electric field intensity was higher than AC due to large electrical conductivity ratio between different dielectrics. Therefore, DC insulation design is totally different from AC and it is necessary to simulate the DC electric field distribution for DC insulation design. Consequently, this seven steps structure of AC SJB model led to higher DC electric stresses and could not be directly applicable to DC SJB model. This design should be improved. 5. DC electric field distribution analysis for improved SJB models DC electric field concentrates inside the thinnest material having the lowest electrical conductivity value. In order to reduce the electric field concentration at the thinnest part of PPLP in the initial SJB model, PPLP thickness should be controlled. Two kinds of improved SJB model were considered and DC electric field analysis was conducted. Simulation results of improved SJB models are shown in Fig. 4(a) and (b). Regarding the simulation results, the electric field concentration inside PPLP was reduced when the thickness of PPLP located on the edge of epoxy spacer was increased. This could be clearly seen at the right side in Fig. 4(b) and (c). Fig. 4(c) and (d) shows the plot in radial and surface direction of electric field for improved SJB models. In radial direction, the SJB model having only one step showed the lowest DC electric field intensity of 21.67 kV/mm. One step and three steps SJB models showed 60% and 39.5% reduction of electric field intensity respectively compared to the initial SJB model. In surface direction, the one step SJB model shows the lowest DC electric field intensity of 30.92 kV/mm. Comparing to the initial SJB model, the one step and three steps SJB models showed 47.7% and 21.1% reduction of electric field intensity. Therefore, step structure for AC electric field grading was not suitable for DC operation environment. Consequently, it was shown that the one step structure of epoxy spacer is more appropriate than multi-steps structure for DC SJB.
6. Conclusion As a fundamental step to determine the insulation design of the SJB for DC HTS power cable, three kinds of SJB models were considered and electric field analysis have been conducted considering DC steady state. An initial SJB model was designed based on the SJB for AC oil-filled cable and DC electric field analysis has been performed. In order to reduce the electric field concentration of the initial SJB model, two kinds of improved SJB models such as one step and three steps SJB models were considered. Regarding the simulation results of improved SJB models, it was observed that the one step structure of epoxy spacer is more appropriate than multi-steps structure for DC SJB. Acknowledgements This work was supported by a grant from Basic Science Research Program of Korea Electrotechnology Research Institute funded by the Korea Research Council for Industrial Science and Technology (KOCI). References [1] Kalcon G, Adam G, Anaya-Lara O, Burt G, Lo K. Int Conf Environ Electr Eng Rome 2011:1–4. [2] Feltes JW, Gemmell BD, Retzmann D. IEEE Power Energy Soc Gen Meet 2011. [3] Purvins A, Wilkening H, Fulli G, Tzimas E, Celli G, Mocci S, et al. J Cleaner Prod 2011;19:1909–16. [4] Elliott D. Energy Strategy Rev 2013. [5] Van Hertem D, Ghandhari M. Renew Sustain Energy Rev 2010:14. [6] Xiao L, Dai S, Lin L, Teng Y, Zhang H, Liang X, et al. IEEE Trans Appl Supercond 2011:22. [7] Kreuger FH. Industrial high voltage. Delft Univ Press; 1998. p. 15–22. [8] Nagao M, Kurimoto M, Takahashi R, Kawashima T, Murakami Y, Nishimura T, Ashibe Y, Masuda T. Annual report on electrical insulation and dielectric phenomena (CEIDP), 2011. [9] Hwang J, Seong J, Shin W, Lee J, Cho J, Ryoo H, et al. Phys C: Supercond 2013. [10] Masood A, Zuberi M, Husain E. IEEE Trans Dielectr Electr Insul 2008:15.
Please cite this article in press as: Hwang J-S et al. Electric field analysis on the insulation design of the stop joint box for DC HTS power cable. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.05.006