IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013
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Effects of Impregnating Materials on Thermal and Electrical Stabilities of the HTS Racetrack Pancake Coils Without Turn-to-Turn Insulation Hyun-Jin Shin, Kwang Lok Kim, Yoon Hyuck Choi, Oh Jun Kwon, Seungyong Hahn, Yukikazu Iwasa, and Haigun G. Lee Abstract—A high temperature superconducting (HTS) coil without turn-to-turn insulation is proposed for the field coil of a wind turbine. In the case of the field coil, epoxy impregnation is generally necessary to protect the coil from mechanical disturbances by time-varying magnetic fields and rotational vibrations to provide high mechanical integrity. This study examined the thermal and electrical stabilities of non-insulated GdBCO racetrack pancake coils impregnated with CTD-521, Stycast 2850 FT, and paraffin through cool down, over-current, and repetitive cooling tests. Among the three epoxy impregnated coils, the Stycast 2850 FT-impregnated coil exhibited the best thermal and electrical stabilities during over-current testing. In repetitive cooling conditions, the CTD-521-impregnated coil exhibited less degradation of its superconducting property due to the well-matching of the thermal contraction between the GdBCO racetrack pancake coil and the epoxy. Index Terms—Coil impregnation, CTD-521, GdBCO racetrack coil, paraffin, Stycast 2850 FT.
I. I NTRODUCTION
W
IND is the world’s fastest-growing electric power resource because wind energy is one of the most costeffective renewable technologies in terms of the cost per kWh of electricity generated. In recent years, the development of a high-power wind turbine generator for use in off-shore wind energy systems has been widely examined due to its high generating efficiency and the considerable available space off shore. Since the high temperature superconducting (HTS) magnet creates relatively high critical current density and magnetic fields, with dramatically reduced size compared to its conventional counterparts, it may have potential for the development of compact mega-watt generators for off-shore wind turbines in the near future [1]–[5]. Meanwhile, the newly-introduced winding technique, the socalled no-insulation (NI), enables the HTS magnet to be considerably downsized by eliminating the turn-to-turn insulators.
Manuscript received October 5, 2012; accepted December 10, 2012. Date of publication December 13, 2012; date of current version January 22, 2013. This work was supported by the International Collaborative R&D Program of the KETEP grant funded by the Korean government MKE (20118520020020). H.-J. Shin, K. L. Kim, Y. H. Choi, O. J. Kwon, and H. G. Lee are with the Department of Material Science and Engineering, Korea University, Seoul 136713, Korea (e-mail:
[email protected]). S. Hahn and Y. Iwasa are with the Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2012.2234179
Fig. 1. Schematic drawing of the GdBCO composite (SuNAM Corp.).
It has also been reported that the NI HTS magnet exhibited superior thermal stability and enhanced mechanical integrity [6]–[13]. For these reasons, the use of an NI HTS coil is newly proposed for the field coil in wind turbine generators. However, the reliability of the NI racetrack-type coils impregnated with epoxy resin needs to be guaranteed because the field coil in the wind turbine is essentially impregnated to enhance the mechanical integrity against rotational vibrations as well as to protect the coil from mechanical disturbances caused by time-varying magnetic fields. Therefore, in this study, GdBCO coated conductor racetrack pancake (RP) coils without turn-toturn insulation impregnated with CTD-521, Stycast 2850 FT, and paraffin were evaluated through cool down, over-current, and repetitive cooling tests to investigate the effects of various impregnating materials on the superconducting properties of the HTS RP coils. II. E XPERIMENTAL S ETUP A. Coil Construction Fig. 1 shows a schematic drawing of the GdBCO-coated conductor tape used in this study (SuNAM Corp.). It was 4.12 mm wide and 0.21 mm thick. Buffer layers (0.1 μm) were deposited on the Hastelloy substrate layer (58 μm) via e-beam and sputtering systems. The GdBCO superconducting layer (1 μm) was deposited on the buffer layers by reactive co-evaporation (RCE), and a silver protecting layer (1 μm) was then deposited on the GdBCO superconducting layer using a DC sputtering system. The GdBCO-coated conductor was surrounded by a copper stabilizer (20 μm) and the lead layers (15 μm) were then laminated with brass (40 μm) on both sides.
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TABLE I S PECIFICATIONS OF THE G D BCO RP C OILS
Fig. 3. Test coils impregnated with (a) CTD-521 epoxy (coil 1), (b) Stycast 2850 FT epoxy (coil 2), and (c) paraffin (coil 3). TABLE II S PECIFICATIONS OF I MPREGNATING M ATERIALS
Fig. 2. Arrangement of thermocouples (TCs) and voltage taps (VTs).
Table I lists the specifications of the GdBCO RP test coil wound without turn-to-turn insulation. The inner and outer diameters at the curvature of the RP coil were 50 and 54 mm, respectively. The length of the straight sections was 50 mm. The Bz per amp at center and inductance were 0.16 mT/A and 15.88 μH, respectively. In order to investigate the temperature profile of test coils during the cooling test, 2 E-type thermocouples (TCs) were installed in the curved (TC1) and straight (TC2) sections (see Fig. 2). The terminal voltages and temperatures were monitored and recorded using a data acquisition (DAQ) system during testing. B. Coil Impregnation Fig. 3 shows photographs of RP coils impregnated with CTD-521 (coil 1), Stycast 2850 FT (coil 2), and paraffin (coil 3). Impregnating materials were pasted on the surface of each coil after winding. The pasting thickness of the impregnation was 1 mm. Table II lists the specifications of the impregnating materials [14]–[17]. The thermal conductivities (at room temperature) of CTD-521, Stycast 2850 FT, and paraffin were 0.35, 1.15, and 0.22 W/m-K, respectively. Thermal contraction coefficients (at room temperature) of CTD-521, Stycast 2850 FT, and paraffin were −60 × 10−6 , −111.5 × 10−6 , and −108 × 10−6 m/m/K, respectively. III. R ESULTS AND D ISCUSSION A. Cooling Tests The cooling test was performed in a bath of liquid nitrogen to investigate the cooling performance of epoxy-impregnated
Fig. 4. Photo showing impregnation failure of the paraffin-impregnated coil (coil 3) during cool down testing.
coils. In the case of coil 3, paraffin used for coil impregnation was chipped off repetitively during cool down (see Fig. 4). This is because of its low thermal conductivity and high thermal contraction coefficient, implying that paraffin would not be appropriate as an impregnating material for the GdBCO RP test coil due to its thermo-mechanical instability. Therefore, no further tests using coil 3 were performed in this study. Fig. 5 shows the temperature traces of coils 1 and 2 obtained during cool down from room temperature to 77 K (i.e., liquid
SHIN et al.: EFFECTS OF IMPREGNATING MATERIALS ON STABILITIES OF HTS RACETRACK PANCAKE COILS
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Fig. 5. Cooling test results of coils 1 and 2 during cool down from roomtemperature to 77 K.
nitrogen boiling temperature). The time required to reach 77 K of coils 1 and 2 was 70.9 and 101.2 s, respectively. As expected, the coil impregnated with Stycast 2850 FT showed a faster cooling time than that impregnated with CTD-521 due to its high thermal conductivity (see Table II). B. Critical Current Measurements Fig. 6 shows Ic values of coils 1 and 2 measured at 77 K before and after epoxy impregnation. Each current was normalized to its Ic value to facilitate the comparison. In the case of coil 1, the Ic values before and after impregnation were identical. However, a 2.2% lower Ic value was observed in coil 2 after impregnation, implying that the coil impregnated with Stycast 2850 FT might incur degradation of the coil’s superconducting property because of the larger thermal contraction coefficient of Stycast 2850 FT compared to that of CTD-521 [18]. C. Over-Current Tests Fig. 7 shows the over-current test results of coils 1 and 2 at various applied currents. During over-current testing at 1.074 Ic , the terminal voltage of coil 1 started to increase initially at ca. 120 s, reaching 0.76 mV abruptly, and remaining at 0.89 mV [see Fig. 7(a)]. Note that the constant voltage (0.89 mV) of coil 1 indicates thermal equilibrium between the cooling by liquid nitrogen and the Joule heating induced by the overcurrent. The test results at 1.081, 1.088, and 1.096 Ic also exhibited the thermal equilibrium state. However, when the supply current increased further up to 1.103 Ic , the voltage kept increasing, implying that the RP coil had eventually quenched. On the other hand, the voltage of coil 2 at 1.103 Ic , remained constant at 0.92 mV [see Fig. 7(b)], implying that the thermal stability of coil 2 is better than that of coil 1. While increasing the current up to 1.281 Ic , the voltage also remained constant at 22.63 mV. In the subsequent test at 1.291 Ic , coil 2 exhibited the thermal runaway phenomenon. The thermal runaway voltage of coil 2 (1.291 Ic ) was ca. 1.17 times higher than that of coil 1 (1.103 Ic ), resulting in the better thermal and electrical stabilities of coil 2 due to the higher thermal conductivity of
Fig. 6. Critical current (Ic ) values of the RP test coils before and after impregnation with epoxy: (a) coil 1 and (b) coil 2.
Stycast 2850 FT, which allowed hot spots to be dissipated more effectively when a quench occurred. D. Repetitive Cooling Tests Fig. 8 shows repetitive cooling test results of coils 1 and 2 to examine the change in the superconducting property of impregnated coils in the repetitive cooling condition. Repetitive cooling tests were carried out using the following steps: 1) cool down the epoxy-impregnated coil to 77 K, 2) measure the Ic value of the coil at 77 K, and 3) warm up the coil at room temperature. These steps were repeated 25 times in this study. For accurate comparison, the measured Ic value of each coil was normalized to its initial value. The Ic of both coils gradually decreased as more tests were carried out. The normalized Ic values of coils 1 and 2 after the 25th test were 0.937 and 0.903 Ic , respectively. This result clearly indicated that the coil impregnated with Stycast 2850 FT in the repetitive cooling condition exhibited a 6.7% greater decrease in the superconducting property due to the large discrepancy of thermal contraction between Stycast 2850 FT and the HTS RP coil compared to that of CTD-521.
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electrical stabilities in the over current condition due to the higher thermal conductivity of Stycast 2850 FT. • In the repetitive cooling tests, the CTD-521-impregnated coil exhibited less degradation of its superconducting property due to the well-matching of the thermal contraction between the GdBCO racetrack pancake coil and the epoxy compared to that of Stycast 2850 FT. Overall, the development of an epoxy resin with high thermal conductivity and low thermal contraction coefficient is recommended to impregnate the HTS coil for use in practical applications. R EFERENCES
Fig. 7. Over-current test results of (a) coil 1 and (b) coil 2.
Fig. 8. Repetitive cooling test results of coils 1 and 2.
IV. C ONCLUSION Cool down, over-current, and repetitive cooling tests were performed on non-insulated GdBCO-coated conductor racetrack pancake coils impregnated with CTD-521, Stycast 2850 FT, and paraffin to investigate the effects of various impregnating materials on the superconducting properties of the HTS coil without insulation. Based on the test results, we conclude that: • Part of the paraffin used for coil impregnation was chipped off during the cool down process due to its low thermal conductivity and high thermal contraction coefficient. • Compared to the CTD-521-impregnated coil, the Stycast 2850 FT-impregnated coil exhibited better thermal and
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