# Design of 10 kA DC terminal connector for HTS cable

## Transcription

Design of 10 kA DC terminal connector for HTS cable

DESIGN OF 10 kA DC TERMINAL CONNECTOR FOR HTS CABLE Abhay S. Gour, Pankaj S., Sudharshan H., R. Karunanithi, Centre for Cryogenic Technology, Indian Institute of Science, Bangalore, India V. V. Rao, Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur, India Abstract HTS cables are designed to carry bulk power with high ampacity in superconducting state. These cables require terminal connectors of same ampacity to transfer power from power converter module at room temperature to the HTS cable maintained at cryogenic temperature. This paper deals with thermal analysis of 10 kA DC terminal connector for HTS cable. The ohmic losses were computed which are used as input for steady state thermal analysis of HTS terminal connector. Based on the thermal gradient obtained from thermal analysis length of conductor for terminal connector is calculated for thermal stability of HTS cable. conductor. An FEM based thermal analysis was carried out for different heat loads and feedthough positions. HTS DC CABLE DESCRIPTION Fig. 1 shows the detailed view of HTS cable. It comprises of HTS wire is wounded on supporting tube and surrounded by electrical insulation material enclosed inside a liquid nitrogen tube. The liquid nitrogen tube is thermal insulated by vacuum tube and non metallic jacket as shown in Fig. 1. INTRODUCTION HTS cables are used for transmission bulk power. These cables are capable of carrying high current in superconducting state. HTS cable specification varies with the operating conditions like number of phases, ampacity, operating voltage [1,2]. The designed cable should be able to comply with electrical, mechanical, structural, vacuum and cryogenic system. One of the important components of HTS cable is its end terminal. These terminals a intermediate links which connects the power converters at room temperature to the HTS cable maintained at cryogenic temperature. These links maintains temperature gradient across its length [3,4]. The terminal connectors have the same ampacity as that of connected HTS cable. HTS cable terminal design is complicated because of large temperature variation (77 K to 300 K), operating voltage, high ampacity and its integration and isolation with vacuum and cryogenic system. In this paper, design of HTS DC cable terminal is discussed for operating voltage of 300 V and 10 kA. This involves dimension optimization of cable terminal conductor, location of LN2 and vacuum feedthroughs to maintain the temperature gradient along the length of Figure 1: The detailed view of HTS cable[] DESIGN CONSIDERATION OF HTS CABLE There are two important design considerations for HTS cable terminal namely, electrical and thermal. The electrical specification includes ampacity (which decides size of conductor) and operating voltage (size and type of dielectrics and insulators). Thermal specification includes ambient temperature (it helps in calculating conduction heat flow) and maximum cable temperature (it defines the maximum joule losses permissible for the operating current) [5]. Fig. 2 shows schematic of the conductor along with the heat loads. π π ππ¨π©π = π β«π π π€ π π βπ β«π π π€πππ π ππ (7) When the transport current I is Iopt , the heat leakage from the conductor to the cryostat is minimal. Figure 2: Schematic of conduction cooled conductor Eq. 1 defines heat flow in a conduction cooled conductor. π ππ π(π)π₯π + [π€(π)π ] ππ± ππ± π =π (1) The first term of Eq. 1 corresponds to conduction heat load and second term corresponds to joule heat produced by the current flowing through the conductor. Heat conduction per unit length is given by Fourierβs law of thermal conduction (Eq. 2). ππ± = π€π ππ (2) π Combining Eq. 1 and Eq. 2, the net heat load at a given temperature (T) is given by π π(π) = βπππ β ππ₯π β«π π€πππ (3) π The ratio of length to cross-section is given by Eq. 4. π π π π€ π π βπππ βππ₯π β«π π€πππ π = β«π π ππ The analytical solution of the above equation is not straight forward to calculate as the thermal conductivity of material is a function of temperature as well as material itself. This involves the use of FEM based solution to determine the temperature rise for the applied heat. DESIGN SPECIFICATION AND SIMULATION An FEM simulation for the HTS cable terminal was carried using Ansys thermal steady state simulation. The terminal is designed with an ampacity of 10 kA and operating voltage of 300 V. This specification was selected because of its use in superconducting magnets at J-PARC [6]. Fig. 3 shows the quarter sectional 3-D drawing of HTS cable terminal. Two type of heat transfer simulation were carried out. In first, the thermal simulations were carried out for fixed conductor length in LN2 with varied feedthrough position to obtain the temperature gradient profile for different heat loads. In second type of analysis, feedthrough position were fixed with varied conductor length in LN2 to obtain its effect on the temperature gradient along the length of whole conductor at different heat loads. The boundary conditions and parameters used for the simulation are listed in the table 1. (4) When the conductor is conducting a current I, the minimum heat leakage QL to cryostat can be achieved by taking the minimum of Eq. 3given by Eq. 5. π (ππ )π¦π’π§ = πβπ β«π π π€πππ π (5) By substituting Eq. 5 in Eq. 4, an optimized operating current Iopt can be obtained. ππ (π) π¨π©π π π€ π π βπ β«π π π€πππ π = β«π π ππ (6) Figure 3: The sectional 3-D drawing of HTS terminal Table 1: The boundary conditions and parameters Boundary Conditions & Parameters Values Uniform Initial Temperature Liquid Nitrogen chamber temperature 300 K Vacuum Chamber, Feedthrough Perfectly Insulated 77 K RESULTS AND DISCUSSIONS Fig 4. shows the thermal steady state FEM simulation for HTS cable terminal. The input heat load was 600 W. The distance between the LN2 and vacuum feedthrough was 250 mm. The maximum temperature obtained at the end of conductor was 357 K. The end- joint temperature was found to be 263 K. Figure 5: Maximum temperature with change in FT distance for constant conductor length in LN2 300mm. Figure 6: Maximum temperature with change conductor length in LN2 for a fixed feedthorugh distance of 250 mm. Figure 4: The FEM simulation results Fig. 5 shows the variation of maximum temperature along the conductor for different heat loads. The distance between vacuum and LN2 feedthrough are varied. Conductor length inside LN2 is fixed to 300 mm. Fig. 6 shows the variation of maximum temperature along the conductor. The distance between vacuum and LN2 feedthrough is kept constant at 250mm. The conductor length in LN2 is varied for different heat loads. Fig. 7 shows the variation of end - joint temperature along the conductor for different heat loads. The distance between vacuum and LN2 feedthrough are varied. Conductor length inside LN2 is fixed to 300 mm. Fig. 8 shows the variation of end - joint temperature along the conductor. The distance between vacuum and LN2 feedthrough is kept constant of 250mm. The conductor length in LN2 is varied for different heat loads. Figure 7: End-joint temperature with change in FT distance with LN2 chamber length 300 mm Figure 8: End-joint temperature change in LN2 chamber conductor length with FT distance 250 mm CONCLUSIONS The thermal analysis 10 kA DC terminal connector for HTS cable was carried out. The ohmic losses were computed and were used as heat load input for the FEM simulation. The position of the LN 2 and vacuum feedthrough were optimized to have maximum temperature and end joint temperature within the limits of 360 K and 280 K respectively. REFERENCES [1] Sun J, Watanabe H, Hamabe M, Kawahara T, Yamaguchi S. Effects of HTS tape arrangements to increase critical current for the DC power cable. IEEE Transactions on Applied Superconductivity. 2013 Jun;23(3):5401104-. [2] Kim WJ, Kim HJ, Cho JW, Choi YS, Kim SH. Electrical and mechanical characteristics of insulating materials for HTS DC cable and cable joint. IEEE Transactions on Applied Superconductivity. 2015 Jun;25(3):1-4. [3] Kim JG, Salmani MA, Graber L, Kim CH, Pamidi SV. Electrical characteristics and transient analysis of HTS DC power cables for shipboard application. InElectric Ship Technologies Symposium (ESTS), 2015 IEEE 2015 Jun 21 (pp. 376-381). IEEE. [4] Zhang H, Wang Y, Xue J. Electromagnetic Field Analysis of a High Current Capacity DC HTS Cable With Self-shielding Characteristic by 2-D Simulation. IEEE Transactions on Applied Superconductivity. 2016 Oct;26(7):1-5. [5] Chen ZH, Jin JX. HTS DC cable and transmission development. InApplied Superconductivity and Electromagnetic Devices (ASEMD), 2015 IEEE International Conference on 2015 Nov 20 (pp. 23-24). IEEE. [6] https://www.bnl.gov/magnets/JPARC_Correctors/construction.php