Latest Status of High Temperature Superconducting Cable Projects

Transcription

1 Latest Status of High Temperature Superconducting Cable Projects Y.Ashibe, H.Yumura, M.Watanabe, H.Takigawa, H.Ito, M.Ohya, T.Masuda and M.Hirose Sumitomo Electric Industries, Ltd.Osaka, Japan Abstract: High temperature superconducting (HTS) cables achieve large power capacity and low-loss power transmission in a compact size, and have economical and environmental advantages such as energy saving, resource conservation, carbon-dioxide reducing and electromagnetic interference (EMI)-free performance. Due to these advantages, HTS cable demonstration projects are being promoted around the world for commercialization. Sumitomo Electric has been developing HTS BSCCO wires as well as 3-in-One (three cores-in-one cryostat) HTS cables since the discovery of HTS materials, and the world s first in-grid operation of HTS cable system has been started since July 2006 in the Albany N.Y.. In the Albany project, the 350-meter-long HTS cable system with capacity of 34.5kV, 800A, are connected between two substations in National Grid Power Company s grid. A 320-meter and a 30-meter DI-BSCCO cables were installed into underground conduits successfully and jointed each other at a vault. The in-grid operation has been progressed satisfactorily at unattended condition. At the next step, the 30-meter section is planed to be replaced to the YBCO cable. Then, the operation will be resumed in this winter. On the other hand, in Japan, a national project which focuses on 66- to 77-kV class HTS cable with large capacity will be also started for the practical application. This paper describes the latest status of the HTS cable projects in the USA and Japan. Keywords: High-temperature superconductor, Superconducting power cable, In-grid operation 1. INTRODUCTION HTS cables achieve large-capacity, low-loss power transmission in a compact size and are expected to offer not only economical advantages but also environmental advantages like energy conservation resource conservation, carbon-dioxide reducing and electromagnetic interference (EMI)-free performance. Due to these advantages, HTS cable demonstration projects and practical application studies are being promoted around the world. In the United States, three HTS cable demonstration projects have been conducted on the actual power grids, funded by the U. S. Department of Energy (DOE). The Albany cable project, one of them, is to install the 350 meters long HTS cable system with capacity of 34.5kV, 800A, connecting between two substations in National Grid Power Company's network. A 320-meter and a 30-meter cable are installed into 152mm (6 ) underground conduit and jointed each other at a vault [1]. In the 1 st phase of this project, the HTS cable system with DI-BSCCO cables was installed and operated a long term in-grid operation [2]. In the 2 nd phase, the 30-meter cable section is to be replaced to new cable with YBCO coated conductors. 2. ALBANY PROJECT BSCCO HTS Cable Structure (1 st phase) The structure of the "3-in-One"HTS cable is shown in Figure 1. The three cores are housed in a cable cryostat. The "3-in-One" cable design can prevent the thermal contraction of the core by adopting "loosely stranded 3-core structure". In the 1 st phase, a 320-meter and 30-meter HTS cable with "3-in-One" type were manufactured with DI-BSCCO wires which were produced with an innovative CT-OP sintering method [3].Total amount of wires is approximately 70 km and their average critical current is 86 A for 500 m piece length at 77 K. The cable core is composed of 2-layer HTS conductor and 1-layer HTS shield. Polypropylene Laminated Paper (PPLP) is used as electrical insulation. Figure 1. "3-in-One"HTS cable structure Corresponding author: yumura-hiroyasu@sei.co.jp 548

2 To accommodate the fault of 23kA and 38 cycles, stranded copper wire is used as the former and the copper shield layer is adopted on the outside of HTS shield. The three cores are stranded loosely to create slack which absorbs the thermal contraction forces during the cool-down. The cable cryostat is made of co-axial stainless steel corrugated pipe with thermal insulation. The cryostat was evacuated and sealed at the factory with being maintained at an appropriate vacuum state. As this cable had a loosely stranded 3-cores structure, instead of pulling the cores, a tension member made of stainless steel tapes was attached to outside of the cable cryostat to share the pulling tension. The outer diameter is 135mm and it allows laying into the 152mm (6 ) underground conduit. The various shipping tests, shown in Table 1, were conducted with some short samples, successfully INSTALLATION OF CABLE SYSTEM HTS Cable Installation The route profile for the HTS cable installation is shown Figure 2. The 320-meter cable section has a 90 degree bend with a radius of 12 meters and maximum difference in elevation of approximately 5 meters. The cable drum was placed on a drum roller and the cable was pulled into 152mm (6 ) conduit by a winch. The HTS cable was installed into the conduit extremely smoothly and the maximum tension was approximately 2 tons, which was almost same as the tension value calculated in advance with a friction coefficient of Furthermore, after the cable installation was completed, it was confirmed that there was no abnormal elongation or external damage to the HTS cable, and there was no degradation in the degree of vacuum level of the cable cryostats. This successful cable installation demonstrated that the HTS cable could be installed using the same method and same equipment as those for installing conventional cables HTS Joint assembling Schematic view and photo of the cable-to-cable joint are shown in Figure 3. In order to assemble the joint inside the vault, a compact "3-in-One" normal joint (NJ) structure that could house the joint of the three cores in one case was applied. This joint structure is comprised of the connection of copper stranded former, the connection of HTS conductor layer, formation of a supplementary insulation layer, the connection of HTS shield layer and copper shield layer. The core joint was not fixed to the case (ground), and an allowance was provide inside the case have the "loosely stranded 3-core structure" which dose not produce large tension during cooling, and it contributes to be a compact design. The vacuum chamber of the thermal insulation vessel assembled on the outside of the cable core joint is separated from the cable cryostat. This structure enabled the vacuum maintenance of the cable during the joint assembly operation. Also this structure was designed to allow easy dismantling and re-assembling at the time of the scheduled cable replacement for a 30-meter section. After vacuum treatment of the thermal insulation vessel of the joint, waterproof and corrosion-proof treatment were applied HTS Termination assembling The basic structure and photo of the HTS termination are shown in Figure 4. The termination had a "3-in-One" structure which means that the termination of the 3-core HTS cable and the 3-phase bushings are housed in one vessel. At the inlet of the termination vessel, the three cores were connected with the central conductors of the three epoxy units fixed to the vessel and a supplementary insulation was formed, and the shield layers of 3 cores were short-circuited to generate the induction current. The outgoing line from the high voltage part was led out from the central conductors of the epoxy units to the three epoxy units fixed to the vessel and a supplementary insulation was formed, and the shield layers of 3 cores were Table 1. Results of Shipping Tests for BSCCO Cable Items Critical current (Ic) measurement AC loss measurement Withstand voltage tests Cable bending tests with diameter of 2.4m (18 D) Results 1800 A at 77K, defined by 1 µv/cm 0.7 W/m/phase at 0.8kArms, 60Hz AC: 69 kv for 10 minutes Imp: ± 200 kv, 10 shots / each DC: 100 kv for 5 minutes No Ic degradation No defect in HTS wires and electrical insulations North Termination 5m 90 Bend (12mR) 2m Vault South Termination 320m HTS Cable 30m HTS Cable Figure 2. The route profile for the HTS cable installation 549

3 short-circuited to generate the induction current. The outgoing line from the high voltage part was led out from the central conductors of the epoxy units to the outside of the vessel via the current leads inside the bushings. After being assembled, the termination vessel was fixed to the ground. As in the case of the cable, the assembling vacuum treatment of the main part of termination vessel was completed in Japan before shipment. Vacuum treatment was conducted only for the site assembling parts Initial Cooling Initial cooling for the HTS cable system was conducted with controlling the temperature in the lengthwise direction of the cable. The cable was cooled down gradually for the entire length using nitrogen gases at a temperature of -100 degrees Celsius, and then the temperature of the nitrogen gas flowing into the cable was gradually lowered. When the temperature at the inlet of the cable was -150 degrees Celsius and the temperature gradient for the entire length of the cable had turned sufficiently small, liquid nitrogen was begun to be injected into the cable. And then the entire length of the cable was cooled to the liquid nitrogen temperature. In the initial cooling process, the maximum tension 800 kg for the three cores. This is much lower than the tension produced when there is no allowance for absorption of heat-shrinkage (approximately 5 tons), and it reconfirmed the effectiveness of loosely stranded 3-core structure. In addition, the vacuum level in each section including the cable and the joint was kept at a good level, and the core behavior inside the joint was also confirmed to be within the normal range. Total length : approx. 5 m Cable cryostat Outer diameter of outer vessel : 360 mm Outer diameter of inner vessel : 200 mm Cable cryostat Figure 3. Schematic view and photo of the "3-in-One" HTS joint Porcelain Bushing Current lead 3-in-One HTS cable Cable core Epoxy unit connection Figure 4. Schematic view and photo of 3-in-One HTS termination 550

4 2.3. Long Term In-grid Operation The good results obtained in the commissioning tests confirmed that the HTS cable system had been constructed properly and demonstrated good performance, and that it met the required specifications. In addition, the back-up mode operation of the CRS in the event of refrigerator failure, LN2 pump failure or electric power failure was confirmed to maintain temperature and pressure stable [5]. In response to these favorable results, the cable system was connected to the actual power grid of National Grid on July 20, The operation of this cable system at the site can be monitored and controlled remotely. From that time, a long term in-grid operation had been progressed satisfactorily at unattended condition and completed on May 1st, The cable temperatures and transmitted electricity during the in-grid operation is shown in Figure 5. The in-grid operation was shut down just once, because the fault current flowed into the HTS cable line. The condition of the fault current is shown in Figure 6. The maximum current was 7 ka and it was cleared at 8 cycles by operating of the first protection system. There were no major changes in cable temperatures by this fault current. After that, the soundness of the HTS cables was confirmed and then in-grid operation was carried on immediately. At last, the long term in-grid operation was completed successfully on May 1st, 2007 and the HTS cable system has accumulated nearly 7000 hours of electricity transmission Temperature [K] Cable Outlet Temperature Cable Inlet Temperature Shut Down by Fault Current Transmitted Electricity 66 7/20 8/17 9/14 10/12 11/9 12/7 1/4 2/1 3/1 3/29 4/26 Date ( ) Transmitted Electricity [MVA] Figure 5. Stats of the long term in-grid operation of Albany HTS cable system (July 20, 06 - May 1, 07) 7kA 8 cycle Figure 6. The fault current condition flowed into HTS cable system 551

5 2.4. Warm-up HTS Cable System After the completion of a long term in-grid operation, the Megger test and Ic measurement were conducted in order to check the soundness of the HTS cable cores. The result of the Megger tests showed the cable maintained good electrical insulation properties. The Ic measurements were conducted for each phase conductor at an average cable temperature of 73 K as same as commissioning test. The Ic was 2.3 ka (defined by 1 µv/cm) for each of all 3 phases and they were same values as commissioning test. After the confirmation of the soundness of the cable, HTS cable system was warmed up in order to replace the 30-meter section. At the warm-up process, liquid nitrogen was pushed out into the bulk storage tank of the CRS initially, and then the cable system was warmed up naturally. The entire cable system was warmed to ambient temperature in about 3 weeks. In the warm-up process, the vacuum level in each section didn t indicate any leakage and the tension at the both termination returned to the original value of about 200 kgf compressive force YBCO 30-meter Cable (2 nd phase) YBCO Cable Structure For the 2nd phase of the Albany project, a new 30-meter cable with YBCO coated conductors was fabricated [4]. The YBCO cable has 3-core structure as similar as BSCCO cable. The cable core is made of Super Power s YBCO tapes which have surround copper stabilization with 4 mm width and 0.1 mm thickness. The total amount of wires is 9.7 km and their average critical current is approximately 70 A. The cable core is composed of the same Cu former as BSCCO cable, 3-layer conductor, PPLP dielectric of 4.5 mm thickness, 2-layer shield and Cu tape layer. The outer diameter is approximately 35 mm which is almost same as BSCCO core YBCO Cable Fabrication and sipping tests The 30-meter YBCO cable was manufactured with almost same condition as BSCCO cable at Osaka works, Japan. The results of the various shipping tests after the completion of cable manufacture were conducted successfully. As the results of these tests, it was confirmed that the cable had good properties as designed, and satisfied the required specifications. After the success of shipping tests, the cable was shipped to Albany from the Port of Kobe and arrived at Albany test site on June 14, YBCO Cable Installation The 30-meter section had been replaced to new YBCO cable. The YBCO cable-to-bscco cable joint and The YBCO cable termination are now under construction CONCLUSION In the 1st phase, the installation of HTS cable system with DI-BSCCO cables was completed and a long term in-grid operation was begun on July 20th, During a long term in-gird operation, it was shut down once because the fault current flowed into the HTS cable line. After that, the soundness of the HTS cables was confirmed and then in-grid operation was carried on immediately. At last, the long term operation was completed successfully on May 1st, In the 2nd phase, a new 30-meter cable with YBCO coated conductors has been fabricated and arrived at the Albany test site. The replacement of 30-meter cable is now under construction and the in-grid operation is planed to begin this winter. 3. HTS PROJECT IN JAPAN A national project which focuses on 66- to 77-kV class HTS cable with large capacity will be also started for the practical application. REFERENCES 1. Weber, C. et.al., IEEE Transaction on Applied Superconductivity Vol.15, No.2, pp (2005) 2. Yumura, H. et.al., World's first in-grid demonstration of long-length "3-in-One" HTS cable (Albany project),.sei Technical Review, No.64 (April 2007) 3. Kato, T. et.al., Development of Drastically Innovative BSCCO (DI-BSCCO) Wire, SEI Technical Review, No.62 (June 2006) 4. Ohya, M. et.al., "Design and Evaluation of YBCO Cable for Albany HTS Cable Project", presented at the 2007 Cryogenic Engineering Conference 5. Lee, R. et.al., Advances in Cryogenic Engineering Trans. of the Cryogenic Engineering Conference, Vol.51, pp (2006) 552