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AC losses of the 5 m BSCCO cables with shield
Physica C 470 (2010) 1606–1610
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Physica C journal homepage: www.elsevier.com/locate/physc
AC losses of the 5 m BSCCO cables with shield K. Ryu a,*, Y.H. Ma a, Z.Y. Li a, S.D. Hwang b, H.J. Song c a
Department of Electrical Engineering, Chonnam National University, Kwangju 500-757, Republic of Korea Korea Electric Power Research Institute, 103-16 Munji-dong, Yusung-ku, Daejeon 305-380, Republic of Korea c School of Nano Engineering, Inje University, Gimhae 621-749, Republic of Korea b
a r t i c l e
i n f o
Article history: Available online 16 May 2010 Keywords: AC loss Conductor Pulse transport Shield
a b s t r a c t In order to research the AC loss characteristics of a multi-layered conductor and a shield in a high temperature superconductor (HTS) cable, we prepared two short cable samples, which are the same as the 22.9 kV/50 MVA HTS-cable installed at Gochang test yard of Korea Electric Power Corporation, and attached voltage-leads to both the conductor and the shield. To investigate the effect of transport period on their AC losses, we also applied current with the same magnitude and opposite direction to the conductor and the shield from a few cycles to several minutes. The tests show that the AC loss measured from the lead attached to the shield (shield-lead) is constant regardless of transport period. But the measured loss from the lead attached to the conductor (conductorlead) is greatly dependent on transport period. It seems to be caused by difficulty in heat transfer to the surrounding coolant due to thick insulator around the conductor. As transport period becomes longer, the conductor’s temperature rises and thus the AC loss measured from the conductor-lead increases. In addition, the measured loss from the conductor-lead is 1.5 times larger than that from the shield-lead, particularly for the transport period of a few cycles. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction A high temperature superconductor (HTS) cable composed of numerous HTS wires is at the phase of field test in power networks according to advance in surrounding technologies, and therefore several related studies are being actively performed [1,2]. However, AC loss is an obstacle to the commercialization of the HTScable in an economic point of view. Then the reduction of AC loss is indispensable. In order to decrease AC loss, its exact evaluation and understanding of loss characteristics must be done first. But owing to the complexities of the HTS-cable with a multi-layered conductor and a shield, their AC loss evaluation is very difficult and thus loss characteristics are not cleared yet. We have been investigating the AC loss characteristics of the 100 m 22.9 kV/ 50 MVA HTS-cable system under field test at Gochang test yard of Korea Electric Power Corporation (KEPCO) since it was installed in 2005 [2]. In this work, two 5 m Bi-2223 (Bi2Sr2Ca2Cu3O10) cable samples, which are the same as the KEPCO HTS-cable, were prepared and current with the same amplitude and opposite direction was applied to the conductor and shield composing the samples in liquid nitrogen. To examine the influence of transport period on AC losses, we measured the losses from the voltage-leads attached * Corresponding author. Tel.: +82 62 530 1748; fax: +82 62 530 1749. E-mail address: [email protected] (K. Ryu). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.05.172
to the conductor and shield by changing the transport period from a few cycles to several minutes. Moreover, their inductances, which are one of important parameters in power network analysis, were measured and compared with the results of a numerical analysis. 2. Experiment In order to study the AC loss characteristic of a multi-layered conductor and a shield in an HTS-cable, we prepared two 5 mcable samples (samples A and B), which are the same as the KEPCO HTS-cable except length, and investigated the AC losses of these cable samples experimentally. Fig. 1 shows the cross-sectional view and experimental setup of the cable samples, and their main specifications are listed in Table 1. As shown in the cross-sectional view of the samples in Fig. 1a, a conductor insulated with a 4.5 mm thick polypropylene laminated paper (PPLP) is placed on a cylindrical copper (Cu) former and a shield on the PPLP in turn. A copper tape and a cover for protecting the shield and the cable are coaxially arranged, respectively. In addition, the conductor is doubly layered using 23 Bi-2223 wires summarized in Table 1. The shield is in a single layer consisting of the same wires of 19. The conductor and shield are twisted and their pitches also listed in Table 1. A thin insulator is inserted between the Cu-former and the conductor or between the shield and the Cu-tape.
K. Ryu et al. / Physica C 470 (2010) 1606–1610
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Fig. 1. Cross-sectional view and experimental setup of the 5 m-cable.
Table 1 Specifications of the 5 m-cable. Critical current of Bi-2223 tape n-Value of Bi-2223 tape Width and thickness of tape Width and thickness of core region
90 A at 77 K 23 at 77 K 4.3 mm 0.22 mm 3.8 mm 0.11 mm
Diameter of Cu-former Outer diameter of conductor Outer diameter of PPLP Outer diameter of shield Outer diameter of cover Number of tapes in conductor Twist pitch of conductor Number of tapes in shield Twist pitch of shield
16 mm 16.88 mm 25.88 mm 26.32 mm 35 mm 23 (double-layer) 142 mm (inner), 301 mm (outer) 19 (single-layer) 227 mm
Copper wires (short-ring) in a ring shape were first soldered on surfaces of the conductor and shield. Next the voltage-leads (conductor-lead and shield-lead) for the measurement of AC loss were attached to the short-rings of the conductor and shield, and arranged along the surface of the cover in Fig. 1b. The distances between voltage taps are 5.5 m and 5.34 m, respectively as shown in Fig. 1b. Considering that the current induced on the shield of the KEPCO HTS-cable has the same magnitude and opposite direction with the conductor current, the current of the samples is supplied to the conductor and then returns to a power supply through the shield in series connection as shown in Fig. 1b. The power supply (PS3000) has the capacity of 3000 A/20 V and the frequency range of DC 200 Hz. Current-leads made of copper braid were soldered on the Bi-2223 wires composing the conductor and shield. Especially in the case of the conductor as shown in Fig. 1a, heat transfer of the generated AC losses to a surrounding coolant becomes difficult because of the thick insulator of PPLP. Therefore as transport period increases, the conductor’s temperature gradu-
ally increases according to the accumulation of heat, and ultimately results in the increase of AC loss [3]. In this work, two kinds of transports with different transport periods were used to investigate the influence of transport period on the AC loss characteristic of the HTS-cable: one is the short pulse transport of a few cycles and the other is a relatively long conventional transport of several minutes, which is generally used in measuring AC loss. Fig. 2 shows the voltage measured from the conductor-lead of Fig. 1b when AC current is applied to sample A with pulse transport. The voltage here is a loss voltage remaining after canceling an inductive voltage. The AC loss energy was determined by integrating the product of the measured loss voltage and the current for one cycle [4]. All the experiments through this work were done at the liquid nitrogen temperature of 77 K.
Fig. 2. AC pulse current and loss voltage of the sample A.
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K. Ryu et al. / Physica C 470 (2010) 1606–1610
3. Results and discussion Fig. 3 illustrates the critical currents of the conductor and shield measured for the sample A and B, which are determined from the measured DC V–I curve and a criterion of 1 10 4 V/m. The critical currents of the shield with 19 Bi-2223 wires are the same, whereas those of the conductor with 23 wires differ about 10%. Fig. 4 shows the self-inductances measured for the conductor and shield along with a mutual inductance between them when a current of 1117 Arms and 60 Hz flows in the sample A. Particularly in the measurement of inductance, which is different from loss measurement, we made the loop size of the voltage-leads large enough to completely pick up magnetic flux which links with the sample A. In addition, the inductances calculated by numerical analysis are shown together for the comparison with the measured data [3]. As shown in Fig. 4, the measured inductances of the conductor and shield are in good agreement with the numerically calculated ones. Fig. 5 indicates the frequency dependence of the AC losses measured from the conductor-lead by the pulse transport of Fig. 2. As shown in Fig. 5, the measured losses are identical regardless of current frequency. This well corresponds with the experimental results by the conventional transport [4,5]. It is well known generally that the measured AC loss of a conductor composed of numerous HTS wires is seriously dependent on contact position of a voltage-lead [6,7]. To examine the non-
Fig. 5. Frequency dependence of the AC losses in the sample A.
uniformity of our cable along the length due to poor works during fabrication process, we investigated the losses of two samples, A and B, even if they originated from the same HTS-cable. Fig. 6 shows the AC losses measured from the conductor-lead and shield-lead when a current of 200 Hz is applied to the samples by the pulse transport. As shown in Fig. 6a, the losses of the sample A and B measured from the conductor-lead are relatively identical even though there is a little difference between them, particularly in the range of low
Fig. 3. Critical currents of the samples A and B.
Fig. 4. Inductances of the sample A.
Fig. 6. AC losses of the samples A and B.
K. Ryu et al. / Physica C 470 (2010) 1606–1610
current. This difference seems to be caused by their different critical currents in Fig. 3 or the different contact positions of the voltage-leads as mentioned above. On the other hand, the AC losses measured from the shield-lead in Fig. 6b are exactly identical in the range of all currents. We investigated the effect of transport period on the AC loss characteristics of the cable samples experimentally. The measured AC losses of the sample A by the pulse transport are shown in Fig. 7. The AC losses measured by the conventional transport are also plotted for comparison. Fig. 7a and b illustrates the test results measured from the conductor-lead and shield-lead, respectively. As shown in Fig. 7a, the AC loss measured with the conventional transport is about 1.7 times larger than that measured with the pulse transport. This is judged to be caused by difficulty in thermal diffusion to the surrounding coolant of liquid nitrogen due to the thick insulator around the conductor. On the other hand, the losses measured from the shield-lead in Fig. 7b are equal for both the pulse and conventional transports. This is because the shield is closely adjacent to the surrounding coolant: this is different from the conductor. These results imply that the transport period has a significant effect on the AC loss characteristic, particularly in power applications because a thick insulator is essential. Moreover for an exact analysis of the AC loss in the conductor insulated with the thick insulator, its actual temperature should be reflected instead of the surrounding coolant temperature. Because the number of HTS wires in the conductor and shield is different from each other as shown in Table 1, their critical currents are not the same. For example, the critical currents of the
1609
Fig. 8. AC losses of the sample A for conductor and shield voltage-leads.
conductor and shield in sample A are 1830 A and 1600 A, respectively as shown in Fig. 3. Even though their critical currents are different, we measured the AC losses of the sample A from the conductor-lead and shield-lead by connecting them in series as shown in Fig. 1b. The results are depicted in Fig. 8 along with their ratio. To exclude the secondary parameters influencing AC loss, such as temperature rise of the conductor mentioned in Fig. 7, we tested by the pulse transport of Fig. 2 proposed in this work. For the currents less than the shield’s critical current of 1600 A, the AC losses measured from the conductor-lead are about 1.5 times larger than those from the shield-lead. Of course the shield’s critical current is 90% of the conductor. As soon as the current becomes larger than the shield’s critical current, the ratio of the loss measured from the conductor-lead to the shield-lead decreases. This seems to be due to the fact that the conductor is still lower than its critical current, keeping in a superconducting state, whereas the shield exceeds its critical current, increasing the losses. 4. Conclusions
Fig. 7. AC losses of the sample A for pulse and continuous transports.
We experimentally investigated the AC losses of two short cable samples, which are the same as the KEPCO 100 m 50 MVA HTScable system, by the voltage-leads attached to both the conductor and shield for two different transport periods. Considering the actual transport circumstances of the KEPCO HTS-cable system, we conducted the test by applying current of the same amplitude and opposite direction with the conductor to the shield. It is found from our examination result of the influence of transport period on AC loss that the loss measured from the conductorlead is greatly dependent upon the transport period. It is considered that thick insulator around the conductor for high voltage insulation makes heat transfer to the surrounding coolant difficult: as transport period becomes longer, the conductor’s temperature rises, and finally the AC loss increases owing to the reduction of the conductor’s critical current. On the other hand, as the shield is close to the surrounding coolant and then its generated AC loss can be removed easily, the loss measured from the shield-lead is regardless of the transport period. The critical current of the shield is about 90% of the conductor. Nevertheless, the losses measured from the conductor-lead are 1.5 times the ones measured from the shield-lead even in the case of pulse transport test to exclude the conductor’s temperature rising effect. In addition, the measured AC losses for two cable samples are comparatively identical. Of course there is a little difference for the conductor-lead, particularly in the low current range. Finally the measured inductances
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K. Ryu et al. / Physica C 470 (2010) 1606–1610
of the cable sample, which are of the importance in power network analysis, well correspond with the numerical analysis. Acknowledgments This study was partially supported by the Electric Power Industry Technology Evaluation and Planning (ETEP), an agency of the Korean government’s Ministry of Knowledge Economy (MKE). References [1] T. Masuda, H. Yumura, M. Watanabe, Physica C 468 (2008) 2014.
[2] S.H. Sohn, H.S. Choi, H.R. Kim, O.B. Hyun, S.W. Yim, T. Masuda, K. Yatsuka, M. Wantanabe, H.S. Ryoo, H.S. Yang, D.L. Kim, S.D. Hwang, J. Phys.: Conf. Ser. 43 (2006) 885. [3] H.B. Jin, Z.Y. Li, Y.H. Ma, K. Ryu, K.B. Park, I.S. Oh, IEEE Trans. Appl. Supercond. 19 (2009) 1847. [4] K. Ryu, S.D. Hwang, S.W. Kim, S. Hahn, IEEE Trans. Appl. Supercond. 15 (2005) 1611. [5] Y.H. Ma, Z.Y. Li, K. Ryu, S. Choi, K.B. Park, I.S. Oh, H.J. Song, G. Cha, IEEE Trans. Appl. Supercond. 17 (2007) 3140. [6] J. Souc, F. Gomory, B. Klincok, L. Frolek, M. Vojenciak, A. Usoskin, A. Rutt, IEEE Trans. Appl. Supercond. 17 (2007) 1718. [7] Y.H. Ma, Z.Y. Li, K. Ryu, S.H. Sohn, S.D. Hwang, IEEE Trans. Appl. Supercond. 18 (2008) 725.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
AC losses of the 5 m BSCCO cables with shield K. Ryu a,*, Y.H. Ma a, Z.Y. Li a, S.D. Hwang b, H.J. Song c a
Department of Electrical Engineering, Chonnam National University, Kwangju 500-757, Republic of Korea Korea Electric Power Research Institute, 103-16 Munji-dong, Yusung-ku, Daejeon 305-380, Republic of Korea c School of Nano Engineering, Inje University, Gimhae 621-749, Republic of Korea b
a r t i c l e
i n f o
Article history: Available online 16 May 2010 Keywords: AC loss Conductor Pulse transport Shield
a b s t r a c t In order to research the AC loss characteristics of a multi-layered conductor and a shield in a high temperature superconductor (HTS) cable, we prepared two short cable samples, which are the same as the 22.9 kV/50 MVA HTS-cable installed at Gochang test yard of Korea Electric Power Corporation, and attached voltage-leads to both the conductor and the shield. To investigate the effect of transport period on their AC losses, we also applied current with the same magnitude and opposite direction to the conductor and the shield from a few cycles to several minutes. The tests show that the AC loss measured from the lead attached to the shield (shield-lead) is constant regardless of transport period. But the measured loss from the lead attached to the conductor (conductorlead) is greatly dependent on transport period. It seems to be caused by difficulty in heat transfer to the surrounding coolant due to thick insulator around the conductor. As transport period becomes longer, the conductor’s temperature rises and thus the AC loss measured from the conductor-lead increases. In addition, the measured loss from the conductor-lead is 1.5 times larger than that from the shield-lead, particularly for the transport period of a few cycles. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction A high temperature superconductor (HTS) cable composed of numerous HTS wires is at the phase of field test in power networks according to advance in surrounding technologies, and therefore several related studies are being actively performed [1,2]. However, AC loss is an obstacle to the commercialization of the HTScable in an economic point of view. Then the reduction of AC loss is indispensable. In order to decrease AC loss, its exact evaluation and understanding of loss characteristics must be done first. But owing to the complexities of the HTS-cable with a multi-layered conductor and a shield, their AC loss evaluation is very difficult and thus loss characteristics are not cleared yet. We have been investigating the AC loss characteristics of the 100 m 22.9 kV/ 50 MVA HTS-cable system under field test at Gochang test yard of Korea Electric Power Corporation (KEPCO) since it was installed in 2005 [2]. In this work, two 5 m Bi-2223 (Bi2Sr2Ca2Cu3O10) cable samples, which are the same as the KEPCO HTS-cable, were prepared and current with the same amplitude and opposite direction was applied to the conductor and shield composing the samples in liquid nitrogen. To examine the influence of transport period on AC losses, we measured the losses from the voltage-leads attached * Corresponding author. Tel.: +82 62 530 1748; fax: +82 62 530 1749. E-mail address: [email protected] (K. Ryu). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.05.172
to the conductor and shield by changing the transport period from a few cycles to several minutes. Moreover, their inductances, which are one of important parameters in power network analysis, were measured and compared with the results of a numerical analysis. 2. Experiment In order to study the AC loss characteristic of a multi-layered conductor and a shield in an HTS-cable, we prepared two 5 mcable samples (samples A and B), which are the same as the KEPCO HTS-cable except length, and investigated the AC losses of these cable samples experimentally. Fig. 1 shows the cross-sectional view and experimental setup of the cable samples, and their main specifications are listed in Table 1. As shown in the cross-sectional view of the samples in Fig. 1a, a conductor insulated with a 4.5 mm thick polypropylene laminated paper (PPLP) is placed on a cylindrical copper (Cu) former and a shield on the PPLP in turn. A copper tape and a cover for protecting the shield and the cable are coaxially arranged, respectively. In addition, the conductor is doubly layered using 23 Bi-2223 wires summarized in Table 1. The shield is in a single layer consisting of the same wires of 19. The conductor and shield are twisted and their pitches also listed in Table 1. A thin insulator is inserted between the Cu-former and the conductor or between the shield and the Cu-tape.
K. Ryu et al. / Physica C 470 (2010) 1606–1610
1607
Fig. 1. Cross-sectional view and experimental setup of the 5 m-cable.
Table 1 Specifications of the 5 m-cable. Critical current of Bi-2223 tape n-Value of Bi-2223 tape Width and thickness of tape Width and thickness of core region
90 A at 77 K 23 at 77 K 4.3 mm 0.22 mm 3.8 mm 0.11 mm
Diameter of Cu-former Outer diameter of conductor Outer diameter of PPLP Outer diameter of shield Outer diameter of cover Number of tapes in conductor Twist pitch of conductor Number of tapes in shield Twist pitch of shield
16 mm 16.88 mm 25.88 mm 26.32 mm 35 mm 23 (double-layer) 142 mm (inner), 301 mm (outer) 19 (single-layer) 227 mm
Copper wires (short-ring) in a ring shape were first soldered on surfaces of the conductor and shield. Next the voltage-leads (conductor-lead and shield-lead) for the measurement of AC loss were attached to the short-rings of the conductor and shield, and arranged along the surface of the cover in Fig. 1b. The distances between voltage taps are 5.5 m and 5.34 m, respectively as shown in Fig. 1b. Considering that the current induced on the shield of the KEPCO HTS-cable has the same magnitude and opposite direction with the conductor current, the current of the samples is supplied to the conductor and then returns to a power supply through the shield in series connection as shown in Fig. 1b. The power supply (PS3000) has the capacity of 3000 A/20 V and the frequency range of DC 200 Hz. Current-leads made of copper braid were soldered on the Bi-2223 wires composing the conductor and shield. Especially in the case of the conductor as shown in Fig. 1a, heat transfer of the generated AC losses to a surrounding coolant becomes difficult because of the thick insulator of PPLP. Therefore as transport period increases, the conductor’s temperature gradu-
ally increases according to the accumulation of heat, and ultimately results in the increase of AC loss [3]. In this work, two kinds of transports with different transport periods were used to investigate the influence of transport period on the AC loss characteristic of the HTS-cable: one is the short pulse transport of a few cycles and the other is a relatively long conventional transport of several minutes, which is generally used in measuring AC loss. Fig. 2 shows the voltage measured from the conductor-lead of Fig. 1b when AC current is applied to sample A with pulse transport. The voltage here is a loss voltage remaining after canceling an inductive voltage. The AC loss energy was determined by integrating the product of the measured loss voltage and the current for one cycle [4]. All the experiments through this work were done at the liquid nitrogen temperature of 77 K.
Fig. 2. AC pulse current and loss voltage of the sample A.
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K. Ryu et al. / Physica C 470 (2010) 1606–1610
3. Results and discussion Fig. 3 illustrates the critical currents of the conductor and shield measured for the sample A and B, which are determined from the measured DC V–I curve and a criterion of 1 10 4 V/m. The critical currents of the shield with 19 Bi-2223 wires are the same, whereas those of the conductor with 23 wires differ about 10%. Fig. 4 shows the self-inductances measured for the conductor and shield along with a mutual inductance between them when a current of 1117 Arms and 60 Hz flows in the sample A. Particularly in the measurement of inductance, which is different from loss measurement, we made the loop size of the voltage-leads large enough to completely pick up magnetic flux which links with the sample A. In addition, the inductances calculated by numerical analysis are shown together for the comparison with the measured data [3]. As shown in Fig. 4, the measured inductances of the conductor and shield are in good agreement with the numerically calculated ones. Fig. 5 indicates the frequency dependence of the AC losses measured from the conductor-lead by the pulse transport of Fig. 2. As shown in Fig. 5, the measured losses are identical regardless of current frequency. This well corresponds with the experimental results by the conventional transport [4,5]. It is well known generally that the measured AC loss of a conductor composed of numerous HTS wires is seriously dependent on contact position of a voltage-lead [6,7]. To examine the non-
Fig. 5. Frequency dependence of the AC losses in the sample A.
uniformity of our cable along the length due to poor works during fabrication process, we investigated the losses of two samples, A and B, even if they originated from the same HTS-cable. Fig. 6 shows the AC losses measured from the conductor-lead and shield-lead when a current of 200 Hz is applied to the samples by the pulse transport. As shown in Fig. 6a, the losses of the sample A and B measured from the conductor-lead are relatively identical even though there is a little difference between them, particularly in the range of low
Fig. 3. Critical currents of the samples A and B.
Fig. 4. Inductances of the sample A.
Fig. 6. AC losses of the samples A and B.
K. Ryu et al. / Physica C 470 (2010) 1606–1610
current. This difference seems to be caused by their different critical currents in Fig. 3 or the different contact positions of the voltage-leads as mentioned above. On the other hand, the AC losses measured from the shield-lead in Fig. 6b are exactly identical in the range of all currents. We investigated the effect of transport period on the AC loss characteristics of the cable samples experimentally. The measured AC losses of the sample A by the pulse transport are shown in Fig. 7. The AC losses measured by the conventional transport are also plotted for comparison. Fig. 7a and b illustrates the test results measured from the conductor-lead and shield-lead, respectively. As shown in Fig. 7a, the AC loss measured with the conventional transport is about 1.7 times larger than that measured with the pulse transport. This is judged to be caused by difficulty in thermal diffusion to the surrounding coolant of liquid nitrogen due to the thick insulator around the conductor. On the other hand, the losses measured from the shield-lead in Fig. 7b are equal for both the pulse and conventional transports. This is because the shield is closely adjacent to the surrounding coolant: this is different from the conductor. These results imply that the transport period has a significant effect on the AC loss characteristic, particularly in power applications because a thick insulator is essential. Moreover for an exact analysis of the AC loss in the conductor insulated with the thick insulator, its actual temperature should be reflected instead of the surrounding coolant temperature. Because the number of HTS wires in the conductor and shield is different from each other as shown in Table 1, their critical currents are not the same. For example, the critical currents of the
1609
Fig. 8. AC losses of the sample A for conductor and shield voltage-leads.
conductor and shield in sample A are 1830 A and 1600 A, respectively as shown in Fig. 3. Even though their critical currents are different, we measured the AC losses of the sample A from the conductor-lead and shield-lead by connecting them in series as shown in Fig. 1b. The results are depicted in Fig. 8 along with their ratio. To exclude the secondary parameters influencing AC loss, such as temperature rise of the conductor mentioned in Fig. 7, we tested by the pulse transport of Fig. 2 proposed in this work. For the currents less than the shield’s critical current of 1600 A, the AC losses measured from the conductor-lead are about 1.5 times larger than those from the shield-lead. Of course the shield’s critical current is 90% of the conductor. As soon as the current becomes larger than the shield’s critical current, the ratio of the loss measured from the conductor-lead to the shield-lead decreases. This seems to be due to the fact that the conductor is still lower than its critical current, keeping in a superconducting state, whereas the shield exceeds its critical current, increasing the losses. 4. Conclusions
Fig. 7. AC losses of the sample A for pulse and continuous transports.
We experimentally investigated the AC losses of two short cable samples, which are the same as the KEPCO 100 m 50 MVA HTScable system, by the voltage-leads attached to both the conductor and shield for two different transport periods. Considering the actual transport circumstances of the KEPCO HTS-cable system, we conducted the test by applying current of the same amplitude and opposite direction with the conductor to the shield. It is found from our examination result of the influence of transport period on AC loss that the loss measured from the conductorlead is greatly dependent upon the transport period. It is considered that thick insulator around the conductor for high voltage insulation makes heat transfer to the surrounding coolant difficult: as transport period becomes longer, the conductor’s temperature rises, and finally the AC loss increases owing to the reduction of the conductor’s critical current. On the other hand, as the shield is close to the surrounding coolant and then its generated AC loss can be removed easily, the loss measured from the shield-lead is regardless of the transport period. The critical current of the shield is about 90% of the conductor. Nevertheless, the losses measured from the conductor-lead are 1.5 times the ones measured from the shield-lead even in the case of pulse transport test to exclude the conductor’s temperature rising effect. In addition, the measured AC losses for two cable samples are comparatively identical. Of course there is a little difference for the conductor-lead, particularly in the low current range. Finally the measured inductances
1610
K. Ryu et al. / Physica C 470 (2010) 1606–1610
of the cable sample, which are of the importance in power network analysis, well correspond with the numerical analysis. Acknowledgments This study was partially supported by the Electric Power Industry Technology Evaluation and Planning (ETEP), an agency of the Korean government’s Ministry of Knowledge Economy (MKE). References [1] T. Masuda, H. Yumura, M. Watanabe, Physica C 468 (2008) 2014.
[2] S.H. Sohn, H.S. Choi, H.R. Kim, O.B. Hyun, S.W. Yim, T. Masuda, K. Yatsuka, M. Wantanabe, H.S. Ryoo, H.S. Yang, D.L. Kim, S.D. Hwang, J. Phys.: Conf. Ser. 43 (2006) 885. [3] H.B. Jin, Z.Y. Li, Y.H. Ma, K. Ryu, K.B. Park, I.S. Oh, IEEE Trans. Appl. Supercond. 19 (2009) 1847. [4] K. Ryu, S.D. Hwang, S.W. Kim, S. Hahn, IEEE Trans. Appl. Supercond. 15 (2005) 1611. [5] Y.H. Ma, Z.Y. Li, K. Ryu, S. Choi, K.B. Park, I.S. Oh, H.J. Song, G. Cha, IEEE Trans. Appl. Supercond. 17 (2007) 3140. [6] J. Souc, F. Gomory, B. Klincok, L. Frolek, M. Vojenciak, A. Usoskin, A. Rutt, IEEE Trans. Appl. Supercond. 17 (2007) 1718. [7] Y.H. Ma, Z.Y. Li, K. Ryu, S.H. Sohn, S.D. Hwang, IEEE Trans. Appl. Supercond. 18 (2008) 725.