Characteristic Test of HTS Pancake Coil Modules for ... - IEEE Xplore

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

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Characteristic Test of HTS Pancake Coil Modules for Small-Sized SMES Ji Hoon Kim, Woo-Seok Kim, Song-Yop Hahn, Jae Moon Lee, Myung Hwan Rue, Bo Hyung Cho, Chang Hwan Im, and Hyun Kyo Jung

Abstract—In this paper, 16 HTS pancake coil modules made with 32 double pancake coils were designed, analyzed, built, assembled, and tested to show feasibility of small sized HTS SMES ( -SMES). Rated current is 200 A and operating temperature 30 K. Evolution strategy was used for coil optimization is 20 and FEM was used for magnetic field calculation. After building the modular toroid coils, a vacuum chamber was built to contain the coil. Three GM cryocoolers were used to reach the operating temperature. Bridge type converter was used to supply the current to the SMES coil modules. Operation showed a close agreement with the calculated result.

TABLE I AMSC WIRE SPECIFICATION (COURTESY AMSC)

Index Terms—Conduction cooling, HTS SMES, modular toroid.

I. INTRODUCTION S INDUSTRY develops, more and more complicated and sensitive loads are being attached to the power system. In most cases, those “sensitive” loads are the key components of the system and power shortage in relatively short time (0.1–1.5 s) can often lead to considerable loss. To minimize the loss, “Development of Customized Power Quality Service Systems for 21st Century” project was started, which is based on FACTS and small sized SMES was included for short-term power supply. SMES has fastest response time compared to other energy reserves and this feature is ideal to compensate short term (0.1–1.5 s) power shortages [1]. Superconductor can be classified into two types depending on the operation temperature: LTS (Low Temperature Superconductors) and HTS (High Temperature Superconductors). LTS are cooled with liquid helium materials such as NbTi and (LHe) and HTS materials such as YBCO and BSCCO-2223 can be cooled with liquid nitrogen. Both coolants have a drawback, which fix the operating temperature at 4.2 K and 77 K, respectively [2]. On the other hand, if cryocoolers such as GM coolers are used, operation temperature can be decided according to heat load requirement (20 K in this research) [3]. The system in this paper uses BSCCO-2223 wire, and GM cryocooler which works at optional operating temperature with increased superconducting critical transport current.

A

Manuscript received October 4, 2004. This work was supported by the Ministry of Industry and Energy through “Development of Customized Power Quality Service Systems for 21st Century.” J. H. Kim, S.-Y. Hahn, J. M. Lee, M. H. Rue, B. H. Cho, C. H. Im, and H. K. Jung are with the Seoul National University, Seoul, Korea 151-744 (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; ichism@ elecmech.snu.ac.kr; [email protected]). W.-S. Kim is with the Korea Polytechnic University, Si Heung Si, Korea 429-450 (e-mail: [email protected]). Digital Object Identifier 10.1109/TASC.2005.849334

To make a coil module, two double pancake coils were used and 16 coil modules were arranged to resemble the toroid shape. The width of the toroid coil is approximately 800 mm and height is 320 mm. Operation test using bridge type converter was carried out and the result matched with that of calculation in a close fashion. II. DESIGN AND MANUFACTURE OF THE COIL A. Design of the Coil There are three separate elements that have to be considered when designing a HTS SMES coil. First is the property of the wire, second is the total length of the wire and last is the stray field [4], [5]. The second and third elements are somewhat interrelated and are considered in the optimization process at the same time. The wire used in this research was made by American Superconductor (AMSC). It has critical transport current of 115 A at 77 K and is reinforced with stainless steel strips. Table I shows the wire specifications. From the shape and manufacturing process, HTS wire acquires severely anisotropic characteristics according to the direction of the external magnetic field. The configuration of the coil should be decided considering the place of installation in account. Three types of coils were considered [6]–[8]. Solenoid type coil is easy to build but the stray field is too high to be viable in densely populated area. Multiple solenoid type coil can be built with even number of solenoid coils and relatively easy to build but it requires significantly more conductor than simple solenoids. A toroid type coil is ideal from the stray field point of view but it is hard to wind with tape-shaped HTS wires. In this research, the HTS SMES coil made of several modules arranged in toroid shape is proposed. Fig. 1 shows the operation condition of the HTS SMES system. Several double pancake coils are stacked for a single coil module. It is desirable to fix the number of double pancake coils

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TABLE III COIL OPTIMIZATION RESULT

Fig. 1. Conditions for the small scale HTS SMES. The size of the machine should be small enough to be installed in the basement of the building or on the top of the building. The stray field should be small enough to be used in densely populated area. The cooling system should be simple.

Fig. 2. Final coil arrangement according to optimization result. Terminals of each coil were arranged to meet the connection requirement. TABLE IV COOLER SPECIFICATIONS

TABLE II DOUBLE PANCAKE AND MODULE OPTIMIZATION RESULT

in one module and change the number of turns to optimize the length of the conductor. The number of the modules can also influence the length of the conductor. Table II shows the optimization result of the double pancake coil and module number [9]. The optimization process was largely based on evolution strategy and FEM [10]. Once the object function was decided, 1 1 evolution strategy were applied to calculate the length of the wire and the number of double pancake and modules. If the number of double pancake per module increases, it becomes less and less like a real toroid coil and if the number of modules increases over certain number, connection problems arise. After deciding the number of double pancake coils per module and the number of modules, (4, 12) the evolution strategy was applied to fine tune the optimization. Table III shows optimization result and Fig. 2 shows the coil arrangement. Fig. 5 shows the completed SMES coil. B. Manufacture of the Coil When building the actual HTS SMES system, cooling system, container, and mechanical support system should be considered also. For the sake of operation temperature of 20 K

Fig. 3.

Equivalent heat circuit. Temperature values were measured.

and simplicity, conduction cooling is recommended and three GM coolers were used for the experiment. To operate the is required and a coolers, a vacuum environment of vacuum chamber must be used. Normally, if the coil emits large stray field, a metallic chamber would produce eddy current heating and the generated heat will significantly influence the cooling capacity of the coolers. But since the shape of the coil in this research is nearly toroidal, the stray field is small enough to ignore. Thus the chamber was built with stainless steel and the heat shield was built with aluminum. All three coolers were made by different vendors. The cooling power is shown in Table IV and the heat circuit is shown in Fig. 3. The sum of 2nd stage cold fingers has 50% more cooling power than required.

KIM et al.: CHARACTERISTIC TEST OF HTS PANCAKE COIL MODULES FOR SMALL-SIZED SMES

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TABLE V ELECTRICAL PROPERTIES OF THE HTS COIL

Fig. 4. Completed module. Cooling channels are attached to each side of the double pancake coils. Thermal compound was applied between the channels and the coils to enhance the contact. Steel bolts shown in the picture were replaced with stainless steel and PC bolts. To retain the tension, a clamp was attached to the top of the module. The right side of the bobbin wings were cut for assembly.

Fig. 6. Comparison of the magnetic flux density between calculated and experimental results.

the cooling power should be larger than that of coil heat load calculation. Current leads consist of HTS leads and copper leads. HTS lead was purchased from AMSC and has 250 A capacity at 77 K. The heat penetration in the specification was 90 mW from 64 K to 4.2 K and in the research, it marked 120 mW from 60 K to 20 K. For copper leads, heat conduction optimization was performed respect to the cross-section area and the length of the and the length was 722 mm with leads. The area was 38 11 W loss for the pair. C. Driver for SMES Fig. 5.

Completed SMES coil. Thick legs are needed for physical support.

The mechanical support system should be nonconducting material with high tensile stress to compensate for electromagnetic force toward axis of the toroid, which can be 17 000 N for a single module. Moreover, the material should be a poor thermal conductor. GFRP was selected for such purpose. All of the support system was made out of GFRP and each component was assembled using stainless steel or PC (polycarbonate) bolts to reduce the influence of the magnetic field. Cooling channels are the part of the cooling system and are made of copper mostly. Copper bars and sheets were used to build the channel and copper bulks were adopted to secure heat buffer. Fig. 4 shows a completed module. After completing 16 coil modules, the modules were arranged in a toroid shape and were built on the base of the chamber. This “build-up” scheme is good for secure physical support but has disadvantage in heat conduction and it is the reason why

A bridge type converter was used for the driver for the HTS SMES. Fig. 7 shows the driving schematics and the electrical properties are shown in Table V. III. EXPERIMENTAL RESULT After cooling, which took 132 hr, 350 V pulses were applied to the coil until it reached 150 A. Fig. 8 shows the voltage and current of the SMES coil. The experiment was carried out to determine the influence of dI/dt on temperature and dI/dt is only measured in charge/discharge period, there is no freewheeling period. The experiment showed that dI/dt of 6.8 A/sec, did not increase the temperature significantly. To confirm the design, four hall generators were attached to the side of a module and the magnetic flux density was measured. Fig. 6 shows the applied current and the measured magnetic flux density in Tesla. After the measurement, the values were compared to those of the calculated values and it is shown in Fig. 9., which showed good agreement.

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Fig. 9. Relationship between applied current and the magnetic flux density. Four hall generators were used.

the profiling is over, it is expected to have a complete driving schematics of small-sized HTS SMES. Although the price of the HTS wire is considerably high ( 200 $/kA) at the moment, this study shows that HTS SMES using conduction cooling is realizable. The SMES can be used as power conditioning device for a building, or emergency power source for planes and boats thanks to its low stray field emission. ACKNOWLEDGMENT Fig. 7. Charge-discharge cycle of the bridge type converter. When charging, the converter switches between “charge” and “steady state”. When discharging, the converter switches between “discharge” and “steady state”.

J. H. Kim would like to express his deepest gratitude toward the Korea Polytechnic University staff for their enormous hospitality with winding machines. REFERENCES

Fig. 8. Wave patterns for voltage and current applied on the coil. Ch. 1 is voltage, ch. 2 is current (50 A/div), ch 3. source current, ch. 4 DC link voltage.

Currently, temperature stability test is being carried out and characteristic profiling will be completed by the end of 2004. IV. CONCLUSION In this research, a modular toroid coil with modules made from a number of double pancakes is proposed and tested. Once

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