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Development and testing of a 1 G based high temperature superconducting (HTS) double pancake coil for HTS synchronous machines
Physica C: Superconductivity and its applications 562 (2019) 36–41
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Development and testing of a 1 G based high temperature superconducting (HTS) double pancake coil for HTS synchronous machines
T
V A S Muralidhar Bathulaa,b, , U K Choudhuryb, V V Raoa ⁎
a b
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur, West Bengal 721302, India Corporate R&D Division, Bharat Heavy Electricals Limited, Hyderabad, Telangana 500093, India
ARTICLE INFO
ABSTRACT
Keywords: HTS synchronous machine HTS coil Racetrack Double pancake 1 G HTS tape
High Temperature Superconducting (HTS) synchronous machines are highly efficient, compact, lighter, low noise, high output power to weight ratio and better dynamic response machines compared to conventional copper conductor based machines. HTS synchronous machines are used worldwide for various strategic and critical industrial applications, because of the above mentioned advantages. Successful development of HTS synchronous machine requires HTS, electrical, mechanical, cryogenic and vacuum technologies. The major components of a HTS rotor and copper winding stator topology synchronous machine are air gap copper winding stator, HTS rotor, excitation system, cryogenic cooling and vacuum systems. HTS rotor consists of HTS tape wound pole coils, which generate DC magnetic field when DC current passes through them. There will be no joule heat in the HTS pole coils due to superconductivity. Each pole coil consists of few numbers of field coils (in the form of double pancake coils) which are stacked to form a pole. One of the main technical challenges for manufacturing HTS rotor is development of HTS field coil. The development of a HTS field coil involves the selection of HTS tape, winding technique, handling of the HTS tapes while winding and testing of the HTS coil at cryogenic temperatures of 77 K and 35 K. In the present work, a HTS racetrack double pancake coil using 1 G HTS tapes was fabricated and tested. The details of design, development and cryogenic testing of 1 G HTS tapes wound double pancake coil are presented in this paper.
1. Introduction
2. Design of HTS field coil
One of the promising topologies available for High Temperature Superconducting (HTS) synchronous machine is HTS pole coils in rotor and air gap copper coils in stator. HTS coils are used in a rotor of the HTS synchronous machines, in place of the conventional copper coils. Compact, less weight, highly efficient, greater overloading capacity and less noise machines can be developed using this technology. These machines can be used for strategic applications as well as industrial applications, i.e. ship propulsion, compact wind generators etc. [1–5]. The schematic of a HTS synchronous machine (generated in CAD) consisting of copper windings stator and HTS rotor is shown in Fig. 1. The HTS field coil is one of the critical components in the development of a HTS synchronous machine rotor [6–9]. In the present paper, the development details of a HTS field coil for HTS synchronous machine application and its testing at cryogenic temperatures like 77 K and 35 K are presented.
The double pancake HTS field coil is designed for 200 kVA, 415 V, 250 RPM, 6 pole synchronous machine. Each pole consists of five number of stacked field coils (i.e. double pancake coils). An electromagnetic analysis was carried out on HTS synchronous machine with six pole configuration to finalize the specifications of required field coil. The technical details of the HTS field coil are shown in Table 1. A sketch of the double pancake HTS field coil is shown in Fig. 2.
⁎
3. Selection of HTS tape The important selection criteria of HTS tapes for the development of HTS field coil are operating current and applied magnetic fields on HTS tapes at the given operating temperature. By considering operating current (150 A), perpendicular magnetic field (2 T), parallel magnetic field (3 T) and operating temperature (35 K) of HTS coil, the HTS tape was selected for development of HTS field coil. A suitable factor of safety was also taken into consideration during the selection of HTS
Corresponding author. E-mail address: [email protected] (V.A.S.M. Bathula).
https://doi.org/10.1016/j.physc.2019.03.017 Received 11 February 2019; Received in revised form 17 March 2019; Accepted 25 March 2019 Available online 28 March 2019 0921-4534/ © 2019 Elsevier B.V. All rights reserved.
Physica C: Superconductivity and its applications 562 (2019) 36–41
V.A.S.M. Bathula, et al.
Fig. 1. Schematic of HTS synchronous machine.
tapes. The HTS tape selected for development of field coil was a first generation (1 G) BSCCO-2223 (Trade name: HTS - High strength plus) manufactured by American Superconductor Corporation, USA [10]. The technical specifications of the HTS tapes are listed in Table 2. The critical current (Ic) characteristics of the above mentioned HTS tape as a function of perpendicular (B⊥) and parallel (B||) magnetic fields at the operating temperature of 35 K are shown in Fig. 3. From Fig. 3, it can be observed that the Ic of HTS tape at 2 T perpendicular magnetic field is 170 A (approx.) and at 3 T parallel magnetic field is 260 A (approx.) at an operating temperature of 35 K, which are above the required operating current of 150 A. Based on the above, it was found that the selected BSCCO HTS tape (HTS - High strength plus) is quite suitable for the development of specified HTS field coil.
Table 1 Specifications of double pancake HTS field coil. Parameters
Specifications
Current carrying capacity Nominal operating temperature Dimensions of the HTS field coil Perpendicular DC flux density Parallel DC flux density Type of coil Geometry of the coil Number of turns in each layer Number of turns in a double pancake Number of terminals
150 A (DC) 35 K 700 × 200 × 14.8 mm 2.0 T 3.0 T Double pancake Racetrack type 150 numbers 2 × 150 numbers 2 numbers
4. Winding procedure of HTS field coil The HTS field coil was fabricated using 1 G BSCCO-2223 HTS tapes and by means of a specially developed winding machine (Fig. 4). The HTS field coil has been wound in a double pancake configuration. Based on the dimensions of HTS field coil and size of selected HTS tape, the copper former was fabricated. The former was made out of a single
Fig. 2. A sketch of the double pancake HTS field coil (all dimensions in mm).
Table 2 Specifications of the HTS tapes [10]. Parameters
Details
Type of HTS tape Trade name Critical Current (Ic) Average Width of tape Average thickness of tape Insulation / wrapping Critical bending diameter Critical tensile stress
BSCCO-2223 High strength plus 135 A @ 77 K, self-field 4.4 mm 0.285 mm Kapton 38 mm (room temperature) 200 MPa (room temperature)
Fig. 3. HTS tape characteristics in (a) parallel magnetic field, (b) perpendicular magnetic field (Courtesy: American Superconductor Corporation, USA) [10]. 37
Physica C: Superconductivity and its applications 562 (2019) 36–41
V.A.S.M. Bathula, et al.
Fig. 7 shows the Ic - B⊥ characteristics of HTS tape at 35 K, 77 K and the load line of HTS field coil. The critical current of 1 G BSCCO HTS tape (HTS - High strength plus) at 77 K and 35 K under perpendicular magnetic fields are computed based on the data provided by AMSC, USA [10]. Based on the intersections of Ic - B⊥ curves of HTS tape and load line of HTS coil, the critical operating current (Ic-op) of HTS field coil at self-field is estimated. From the graph, the theoretical value of the Ic-op for the developed HTS field coil are estimated as 53 A at 77 K and 250 A at 35 K. 6. Testing of HTS field coil The manufactured 1 G BSCCO based HTS field coil was tested in two stages. The strategy of two stages was conceived to ensure that there was no damage to the developed coil during manufacturing and to measure the critical current values at 77 K and 35 K. In the first stage of testing, Liquid Nitrogen (LN2) was used as coolant while helium gas was used as coolant for the second stage testing. The test setup and procedure adopted for cooling and energising the HTS coil at 77 K and 35 K are discussed in the subsequent sections.
Fig. 4. Photograph of the HTS field coil winding on specially developed winding machine.
copper block with two grooves and one centre cut for joint-less winding. The reason behind choosing copper as the former material is that, it provides thermal stability during the operation of HTS field coil at cryogenic temperatures. The copper former is fixed at the centre of winding machine using necessary fixtures. Prior to winding, half of the required HTS tape of length (220 m.) is transferred to a secondary spool (empty initially) from the primary spool. The primary spool is loaded on one arm, while the other spool is kept at the centre of winding machine. Now, the primary HTS spool is made to rotate around the bottom groove of former with desired number of turns, programmed in control panel of the machine. After the completion of desired number of turns from primary spool in the bottom groove, the secondary spool is loaded on other arm of the winding machine, and made to revolve in reverse direction to realize the required number of turns in the top groove of copper former. A photograph of the developed double pancake HTS field coil using the above mentioned winding method is shown in Fig. 5.
6.1. Testing of the HTS field coil at 77 K The development of HTS field coil involves various mechanical operations that induce stresses over the HTS tape. These stresses in the HTS tape need to be maintained well below the stress levels provided by the manufacturer. The developed HTS coil was tested for critical current measurement at LN2 temperature (77 K). The photograph of the experimental setup for testing the HTS coil at 77 K is shown in Fig. 8. The HTS field coil was placed in an insulated LN2 bath container made of styrofoam along with extended copper connectors and electrically insulated bakelite holders which were specially manufactured for the testing purpose. Two pre-calibrated platinum based (Pt-102) resistance temperature detectors (RTDs) were provided on the top and bottom surface of HTS field coil to measure the temperature of HTS coil and these RTDs were integrated with a cryogenic temperature monitor. For measuring various related electrical parameters, other essential instruments like milli voltmeter, milli ohm meter and micro ohm meter were also provided. To energize the HTS coil at 77 K, a low-voltage high-current DC power source (0–1000 A) was selected. An HTS coil operating at high current densities stores the electrical energy in the form of electromagnetic field surrounding it. In the event of DC power source failure, the current carrying HTS coil poses a serious high voltage risk. To mitigate such risk, a dump resistor (0.1 Ω, 500 V) is connected across the HTS coil. The end terminals of HTS coil were soldered to two copper connectors for feeding the current. The LN2 at standard atmospheric pressure is used to cool the developed HTS coil to 77 K. The styrofoam container was filled with LN2 and the HTS coil was fully immersed in the bath. A sufficiently large time gap was given for cool down of HTS coil to 77 K and the same was confirmed using cryogenic temperature monitor. Four-probe measurement was employed to measure the resistance of developed HTS coil. After the HTS coil attained thermal equilibrium at 77 K showing zero resistance, the current through the coil is increased in steps using a stable DC power source (Make: PowerTen, USA) to measure the I-V characteristic. The obtained electric field (µV/cm) as a function of current is shown in Fig. 10. Under 1.0 μV/cm criterion of superconductivity, the measured critical current of HTS field coil was ≈ 55 A, which is closer to the theoretical estimation of 53 A (explained in Section 5).
5. Estimation of the critical operating current (Ic-op) of HTS field coil at self field The theoretical estimation of critical operating current (Ic-op) of the developed HTS field coil at 77 K and 35 K is required for establishment of cryogenic test setups and validation of obtained test results. The Ic-op of HTS field coil is mainly decided by the perpendicular magnetic field values on the HTS tape of field coil and operating temperature. The Ic-op of HTS field coil at 77 K and 35 K can be estimated using the Ic characteristics of HTS tape at 77 K and 35 K for different perpendicular magnetic fields and load line of HTS field coil. The load line of HTS field coil is estimated using electromagnetic simulation. Fig. 6 shows the 2D magnetic field plot of the HTS double pancake coil obtained using FEM based electromagnetic software to obtain maximum perpendicular magnetic field values on HTS tape for different operating currents.
6.2. Testing of the HTS field coil at 35 K The main purpose of testing the HTS field coil at 35 K is to measure the critical operating current at this temperature. Hence, the developed HTS coil was subsequently tested at 35 K using helium gas as cryogen. A
Fig. 5. Photograph of developed HTS field coil and its zoomed-in side view showing double pancake layers. 38
Physica C: Superconductivity and its applications 562 (2019) 36–41
V.A.S.M. Bathula, et al.
Fig. 6. Magnetic field plot (2D) of HTS field coil.
Stirling cryocooler (Model no.: SPC 4T, Make: Stirling Cryogenics) with rated cooling capacity of 300 W at 20 K was used for this purpose. A separate test facility was developed for the testing of HTS coil at 35 K. The schematic and photograph of the experimental test setup of HTS field coil at 35 K are shown in Fig. 9. In this regard, a test chamber with low heat-in-leak was manufactured and was integrated with cryocooler. The test chamber contains two sub-chambers viz. inner cold helium gas chamber which contains HTS coil at 35 K and an outer vacuum chamber which enables the reduction of convective heat-in-leak by means of a vacuum envelope over helium gas chamber. Two precalibrated Pt-102 RTDs were provided on the top and bottom surface of coil to measure the temperature of coil. The end terminals of HTS coil were soldered to two copper connectors for feeding the current. The electrical and instrumentation connections were routed with the help of a cryogenic feed through to minimize the heat-in-leak into the system. For measuring various related parameters, other essential instruments like milli voltmeter, milli ohm meter, micro ohm meter and vacuum gauge were also provided. To energize the HTS coil at 35 K, a low-
Fig. 7. Theoretical estimation of the critical current of HTS field coil.
Fig. 8. Photograph of the HTS field coil testing at 77 K. 39
Physica C: Superconductivity and its applications 562 (2019) 36–41
V.A.S.M. Bathula, et al.
Fig. 9. (a) Schematic of HTS field coil test setup at 35 K, (b) Photograph of HTS field coil test setup at 35 K.
voltage high-current DC power source (0–1000 A) was used. The HTS field coil was kept inside helium gas chamber using Polytetrafluoroethylene (PTFE) spacers. During the assembly of chambers, the top plate of helium gas chamber was welded to bottom container after ensuring electrical and cryogenic healthiness of power and instrumentation leads. Then, the hermetically sealed helium gas chamber was suitably placed inside the vacuum chamber with thermally insulating supports to reduce the heat-in-leak from ambient (300 K) to HTS coil (35 K). All the mechanical and weld joints were checked for vacuum leaks using a helium leak detector, to ensure a leak-free system. Finally the top plate of vacuum chamber was integrated to bottom container with O-rings and bolted joints. The vacuum space was evacuated down to 10−3 mbar. The test chamber with HTS field coil was then connected to the vacuum jacketed transfer line of cryocooler. A dump resistor (0.1 Ω, 500 V) is connected across the HTS coil for protection of HTS coil from high voltages in the event of input power failure. After complete integration of the test setup, the HTS field coil was cooled down to 35 K and was monitored with the help of instrumentation. Upon attaining the thermal equilibrium at 35 K, the current was supplied from DC power source in different steps with suitable time-interval between each step. The measured electric field (µV/cm) as a function of current is shown in Fig. 10. Under 1.0 μV/cm criterion of superconductivity, the measured critical current of HTS field coil was ≈ 257 A, which is relatively close to the theoretical estimation of 250 A. The ‘n’ value of HTS tape of developed HTS field coil at self field and 35 K temperature can be estimated using the power law, given by Eq. (1)
E (I ) = Ec (I /Ic
)n
Fig. 10. E-I curve of the developed HTS field coil.
where E(I) is the voltage drop (µV/cm) of HTS coil at operating current (I), Ec is the electric field criterion (1.0 µV/cm), I is the operating current and Ic is the critical current. Using Eq. (1), the n value of the HTS tape of field coil at self field is estimated as 7.27 at 35 K and 6.53 at 77 K for an operating current (I) of 150 A and 30 A respectively. The higher ‘n’ value confirms a satisfactory design of HTS coil. 7. Conclusions
(1)
In this paper, the selection of 1 G HTS tapes for field coil 40
Physica C: Superconductivity and its applications 562 (2019) 36–41
V.A.S.M. Bathula, et al.
development along with joint -less winding procedure for double pancake race track shape HTS coil was presented. Also, the successful development and testing of a racetrack shape double pancake HTS field coil for the rotor of a HTS synchronous machine was described. The developed HTS field coil has shown zero resistance at LN2 temperature (77 K) and its critical currents were measured at 77 K and 35 K. The obtained critical currents are comparable with theoretically estimated values. The ‘n’ value of HTS tape of the field coil is also estimated at self field. The successful demonstration of HTS field coil has significant implications towards the development of compact and high power density HTS synchronous machines in India.
the online version, at doi:10.1016/j.physc.2019.03.017. References [1] Kiruba S Haran, et al., High power density superconducting rotating machines – development status and technology roadmap, Supercond. Sci. Technol. 30 (12) (2017) 1–44 art no.123002. [2] Michael Frank, et al., High Temperature superconducting rotating machines for ship applications, IEEE Trans. Appl. Supercond. 16 (2) (2006) 1465–1468. [3] D. Sekiguchi, et al., Trial test of fully HTS Induction/Synchronous machine for next generation electric vehicle, IEEE Trans. Appl. Supercond. 22 (3) (2012) 5200904. [4] Yunying Pan, Danhzen Gu, Superconducting wind turbine generators, Trans. Environ. Electr. Eng. 1 (3) (2016) 9–15. [5] C D Manolopoulos, et al., Stator design and performance of superconducting motors for aerospace electric propulsion systems, IEEE Trans. Appl. Supercond. 28 (4) (2018) 1–5 art no. 5207005. [6] J.P. Voccio, B.B. Gamble, C.B. Prum, H.J. Picard, 125 HP HTS motor field winding development, IEEE Trans. Appl. Supercond. 7 (2) (1997) 519–522. [7] Marijn Oomen, et al., Manufacturing and test of 2G HTS coils for rotating machines: challenges, conductor requirements, realization, Physica C 482 (2012) 111–118. [8] E. Ueno, T. Kato, K. Hayashi, Race-track coils for a 3 MW HTS ship motor, Physica C 504 (2014) 111–114. [9] J. Leclerc, P.J. Masson, Testing of a subscale HTS coil for wind turbine generator, IEEE Trans. Appl. Supercond. 26 (3) (2016) 1–4 art no. 5206304. [10] BSCCO-2223 High strength plus HTS tape specification sheet, AMSC (2005) [Online]. Available https://www.amsc.com/gridtec/amperium-hts-wire/.
Acknowledgements The authors would like to acknowledge with thanks Mr. Sudheer Thadela, Cryogenic Engineering centre, IIT Kharagpur and Mr. Divya Kumar Sharma, Corporate R & D, Bharat Heavy Electricals Limited, Hyderabad for the support extended to this work. Supplementary materials Supplementary material associated with this article can be found, in
41
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Development and testing of a 1 G based high temperature superconducting (HTS) double pancake coil for HTS synchronous machines
T
V A S Muralidhar Bathulaa,b, , U K Choudhuryb, V V Raoa ⁎
a b
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur, West Bengal 721302, India Corporate R&D Division, Bharat Heavy Electricals Limited, Hyderabad, Telangana 500093, India
ARTICLE INFO
ABSTRACT
Keywords: HTS synchronous machine HTS coil Racetrack Double pancake 1 G HTS tape
High Temperature Superconducting (HTS) synchronous machines are highly efficient, compact, lighter, low noise, high output power to weight ratio and better dynamic response machines compared to conventional copper conductor based machines. HTS synchronous machines are used worldwide for various strategic and critical industrial applications, because of the above mentioned advantages. Successful development of HTS synchronous machine requires HTS, electrical, mechanical, cryogenic and vacuum technologies. The major components of a HTS rotor and copper winding stator topology synchronous machine are air gap copper winding stator, HTS rotor, excitation system, cryogenic cooling and vacuum systems. HTS rotor consists of HTS tape wound pole coils, which generate DC magnetic field when DC current passes through them. There will be no joule heat in the HTS pole coils due to superconductivity. Each pole coil consists of few numbers of field coils (in the form of double pancake coils) which are stacked to form a pole. One of the main technical challenges for manufacturing HTS rotor is development of HTS field coil. The development of a HTS field coil involves the selection of HTS tape, winding technique, handling of the HTS tapes while winding and testing of the HTS coil at cryogenic temperatures of 77 K and 35 K. In the present work, a HTS racetrack double pancake coil using 1 G HTS tapes was fabricated and tested. The details of design, development and cryogenic testing of 1 G HTS tapes wound double pancake coil are presented in this paper.
1. Introduction
2. Design of HTS field coil
One of the promising topologies available for High Temperature Superconducting (HTS) synchronous machine is HTS pole coils in rotor and air gap copper coils in stator. HTS coils are used in a rotor of the HTS synchronous machines, in place of the conventional copper coils. Compact, less weight, highly efficient, greater overloading capacity and less noise machines can be developed using this technology. These machines can be used for strategic applications as well as industrial applications, i.e. ship propulsion, compact wind generators etc. [1–5]. The schematic of a HTS synchronous machine (generated in CAD) consisting of copper windings stator and HTS rotor is shown in Fig. 1. The HTS field coil is one of the critical components in the development of a HTS synchronous machine rotor [6–9]. In the present paper, the development details of a HTS field coil for HTS synchronous machine application and its testing at cryogenic temperatures like 77 K and 35 K are presented.
The double pancake HTS field coil is designed for 200 kVA, 415 V, 250 RPM, 6 pole synchronous machine. Each pole consists of five number of stacked field coils (i.e. double pancake coils). An electromagnetic analysis was carried out on HTS synchronous machine with six pole configuration to finalize the specifications of required field coil. The technical details of the HTS field coil are shown in Table 1. A sketch of the double pancake HTS field coil is shown in Fig. 2.
⁎
3. Selection of HTS tape The important selection criteria of HTS tapes for the development of HTS field coil are operating current and applied magnetic fields on HTS tapes at the given operating temperature. By considering operating current (150 A), perpendicular magnetic field (2 T), parallel magnetic field (3 T) and operating temperature (35 K) of HTS coil, the HTS tape was selected for development of HTS field coil. A suitable factor of safety was also taken into consideration during the selection of HTS
Corresponding author. E-mail address: [email protected] (V.A.S.M. Bathula).
https://doi.org/10.1016/j.physc.2019.03.017 Received 11 February 2019; Received in revised form 17 March 2019; Accepted 25 March 2019 Available online 28 March 2019 0921-4534/ © 2019 Elsevier B.V. All rights reserved.
Physica C: Superconductivity and its applications 562 (2019) 36–41
V.A.S.M. Bathula, et al.
Fig. 1. Schematic of HTS synchronous machine.
tapes. The HTS tape selected for development of field coil was a first generation (1 G) BSCCO-2223 (Trade name: HTS - High strength plus) manufactured by American Superconductor Corporation, USA [10]. The technical specifications of the HTS tapes are listed in Table 2. The critical current (Ic) characteristics of the above mentioned HTS tape as a function of perpendicular (B⊥) and parallel (B||) magnetic fields at the operating temperature of 35 K are shown in Fig. 3. From Fig. 3, it can be observed that the Ic of HTS tape at 2 T perpendicular magnetic field is 170 A (approx.) and at 3 T parallel magnetic field is 260 A (approx.) at an operating temperature of 35 K, which are above the required operating current of 150 A. Based on the above, it was found that the selected BSCCO HTS tape (HTS - High strength plus) is quite suitable for the development of specified HTS field coil.
Table 1 Specifications of double pancake HTS field coil. Parameters
Specifications
Current carrying capacity Nominal operating temperature Dimensions of the HTS field coil Perpendicular DC flux density Parallel DC flux density Type of coil Geometry of the coil Number of turns in each layer Number of turns in a double pancake Number of terminals
150 A (DC) 35 K 700 × 200 × 14.8 mm 2.0 T 3.0 T Double pancake Racetrack type 150 numbers 2 × 150 numbers 2 numbers
4. Winding procedure of HTS field coil The HTS field coil was fabricated using 1 G BSCCO-2223 HTS tapes and by means of a specially developed winding machine (Fig. 4). The HTS field coil has been wound in a double pancake configuration. Based on the dimensions of HTS field coil and size of selected HTS tape, the copper former was fabricated. The former was made out of a single
Fig. 2. A sketch of the double pancake HTS field coil (all dimensions in mm).
Table 2 Specifications of the HTS tapes [10]. Parameters
Details
Type of HTS tape Trade name Critical Current (Ic) Average Width of tape Average thickness of tape Insulation / wrapping Critical bending diameter Critical tensile stress
BSCCO-2223 High strength plus 135 A @ 77 K, self-field 4.4 mm 0.285 mm Kapton 38 mm (room temperature) 200 MPa (room temperature)
Fig. 3. HTS tape characteristics in (a) parallel magnetic field, (b) perpendicular magnetic field (Courtesy: American Superconductor Corporation, USA) [10]. 37
Physica C: Superconductivity and its applications 562 (2019) 36–41
V.A.S.M. Bathula, et al.
Fig. 7 shows the Ic - B⊥ characteristics of HTS tape at 35 K, 77 K and the load line of HTS field coil. The critical current of 1 G BSCCO HTS tape (HTS - High strength plus) at 77 K and 35 K under perpendicular magnetic fields are computed based on the data provided by AMSC, USA [10]. Based on the intersections of Ic - B⊥ curves of HTS tape and load line of HTS coil, the critical operating current (Ic-op) of HTS field coil at self-field is estimated. From the graph, the theoretical value of the Ic-op for the developed HTS field coil are estimated as 53 A at 77 K and 250 A at 35 K. 6. Testing of HTS field coil The manufactured 1 G BSCCO based HTS field coil was tested in two stages. The strategy of two stages was conceived to ensure that there was no damage to the developed coil during manufacturing and to measure the critical current values at 77 K and 35 K. In the first stage of testing, Liquid Nitrogen (LN2) was used as coolant while helium gas was used as coolant for the second stage testing. The test setup and procedure adopted for cooling and energising the HTS coil at 77 K and 35 K are discussed in the subsequent sections.
Fig. 4. Photograph of the HTS field coil winding on specially developed winding machine.
copper block with two grooves and one centre cut for joint-less winding. The reason behind choosing copper as the former material is that, it provides thermal stability during the operation of HTS field coil at cryogenic temperatures. The copper former is fixed at the centre of winding machine using necessary fixtures. Prior to winding, half of the required HTS tape of length (220 m.) is transferred to a secondary spool (empty initially) from the primary spool. The primary spool is loaded on one arm, while the other spool is kept at the centre of winding machine. Now, the primary HTS spool is made to rotate around the bottom groove of former with desired number of turns, programmed in control panel of the machine. After the completion of desired number of turns from primary spool in the bottom groove, the secondary spool is loaded on other arm of the winding machine, and made to revolve in reverse direction to realize the required number of turns in the top groove of copper former. A photograph of the developed double pancake HTS field coil using the above mentioned winding method is shown in Fig. 5.
6.1. Testing of the HTS field coil at 77 K The development of HTS field coil involves various mechanical operations that induce stresses over the HTS tape. These stresses in the HTS tape need to be maintained well below the stress levels provided by the manufacturer. The developed HTS coil was tested for critical current measurement at LN2 temperature (77 K). The photograph of the experimental setup for testing the HTS coil at 77 K is shown in Fig. 8. The HTS field coil was placed in an insulated LN2 bath container made of styrofoam along with extended copper connectors and electrically insulated bakelite holders which were specially manufactured for the testing purpose. Two pre-calibrated platinum based (Pt-102) resistance temperature detectors (RTDs) were provided on the top and bottom surface of HTS field coil to measure the temperature of HTS coil and these RTDs were integrated with a cryogenic temperature monitor. For measuring various related electrical parameters, other essential instruments like milli voltmeter, milli ohm meter and micro ohm meter were also provided. To energize the HTS coil at 77 K, a low-voltage high-current DC power source (0–1000 A) was selected. An HTS coil operating at high current densities stores the electrical energy in the form of electromagnetic field surrounding it. In the event of DC power source failure, the current carrying HTS coil poses a serious high voltage risk. To mitigate such risk, a dump resistor (0.1 Ω, 500 V) is connected across the HTS coil. The end terminals of HTS coil were soldered to two copper connectors for feeding the current. The LN2 at standard atmospheric pressure is used to cool the developed HTS coil to 77 K. The styrofoam container was filled with LN2 and the HTS coil was fully immersed in the bath. A sufficiently large time gap was given for cool down of HTS coil to 77 K and the same was confirmed using cryogenic temperature monitor. Four-probe measurement was employed to measure the resistance of developed HTS coil. After the HTS coil attained thermal equilibrium at 77 K showing zero resistance, the current through the coil is increased in steps using a stable DC power source (Make: PowerTen, USA) to measure the I-V characteristic. The obtained electric field (µV/cm) as a function of current is shown in Fig. 10. Under 1.0 μV/cm criterion of superconductivity, the measured critical current of HTS field coil was ≈ 55 A, which is closer to the theoretical estimation of 53 A (explained in Section 5).
5. Estimation of the critical operating current (Ic-op) of HTS field coil at self field The theoretical estimation of critical operating current (Ic-op) of the developed HTS field coil at 77 K and 35 K is required for establishment of cryogenic test setups and validation of obtained test results. The Ic-op of HTS field coil is mainly decided by the perpendicular magnetic field values on the HTS tape of field coil and operating temperature. The Ic-op of HTS field coil at 77 K and 35 K can be estimated using the Ic characteristics of HTS tape at 77 K and 35 K for different perpendicular magnetic fields and load line of HTS field coil. The load line of HTS field coil is estimated using electromagnetic simulation. Fig. 6 shows the 2D magnetic field plot of the HTS double pancake coil obtained using FEM based electromagnetic software to obtain maximum perpendicular magnetic field values on HTS tape for different operating currents.
6.2. Testing of the HTS field coil at 35 K The main purpose of testing the HTS field coil at 35 K is to measure the critical operating current at this temperature. Hence, the developed HTS coil was subsequently tested at 35 K using helium gas as cryogen. A
Fig. 5. Photograph of developed HTS field coil and its zoomed-in side view showing double pancake layers. 38
Physica C: Superconductivity and its applications 562 (2019) 36–41
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Fig. 6. Magnetic field plot (2D) of HTS field coil.
Stirling cryocooler (Model no.: SPC 4T, Make: Stirling Cryogenics) with rated cooling capacity of 300 W at 20 K was used for this purpose. A separate test facility was developed for the testing of HTS coil at 35 K. The schematic and photograph of the experimental test setup of HTS field coil at 35 K are shown in Fig. 9. In this regard, a test chamber with low heat-in-leak was manufactured and was integrated with cryocooler. The test chamber contains two sub-chambers viz. inner cold helium gas chamber which contains HTS coil at 35 K and an outer vacuum chamber which enables the reduction of convective heat-in-leak by means of a vacuum envelope over helium gas chamber. Two precalibrated Pt-102 RTDs were provided on the top and bottom surface of coil to measure the temperature of coil. The end terminals of HTS coil were soldered to two copper connectors for feeding the current. The electrical and instrumentation connections were routed with the help of a cryogenic feed through to minimize the heat-in-leak into the system. For measuring various related parameters, other essential instruments like milli voltmeter, milli ohm meter, micro ohm meter and vacuum gauge were also provided. To energize the HTS coil at 35 K, a low-
Fig. 7. Theoretical estimation of the critical current of HTS field coil.
Fig. 8. Photograph of the HTS field coil testing at 77 K. 39
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Fig. 9. (a) Schematic of HTS field coil test setup at 35 K, (b) Photograph of HTS field coil test setup at 35 K.
voltage high-current DC power source (0–1000 A) was used. The HTS field coil was kept inside helium gas chamber using Polytetrafluoroethylene (PTFE) spacers. During the assembly of chambers, the top plate of helium gas chamber was welded to bottom container after ensuring electrical and cryogenic healthiness of power and instrumentation leads. Then, the hermetically sealed helium gas chamber was suitably placed inside the vacuum chamber with thermally insulating supports to reduce the heat-in-leak from ambient (300 K) to HTS coil (35 K). All the mechanical and weld joints were checked for vacuum leaks using a helium leak detector, to ensure a leak-free system. Finally the top plate of vacuum chamber was integrated to bottom container with O-rings and bolted joints. The vacuum space was evacuated down to 10−3 mbar. The test chamber with HTS field coil was then connected to the vacuum jacketed transfer line of cryocooler. A dump resistor (0.1 Ω, 500 V) is connected across the HTS coil for protection of HTS coil from high voltages in the event of input power failure. After complete integration of the test setup, the HTS field coil was cooled down to 35 K and was monitored with the help of instrumentation. Upon attaining the thermal equilibrium at 35 K, the current was supplied from DC power source in different steps with suitable time-interval between each step. The measured electric field (µV/cm) as a function of current is shown in Fig. 10. Under 1.0 μV/cm criterion of superconductivity, the measured critical current of HTS field coil was ≈ 257 A, which is relatively close to the theoretical estimation of 250 A. The ‘n’ value of HTS tape of developed HTS field coil at self field and 35 K temperature can be estimated using the power law, given by Eq. (1)
E (I ) = Ec (I /Ic
)n
Fig. 10. E-I curve of the developed HTS field coil.
where E(I) is the voltage drop (µV/cm) of HTS coil at operating current (I), Ec is the electric field criterion (1.0 µV/cm), I is the operating current and Ic is the critical current. Using Eq. (1), the n value of the HTS tape of field coil at self field is estimated as 7.27 at 35 K and 6.53 at 77 K for an operating current (I) of 150 A and 30 A respectively. The higher ‘n’ value confirms a satisfactory design of HTS coil. 7. Conclusions
(1)
In this paper, the selection of 1 G HTS tapes for field coil 40
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development along with joint -less winding procedure for double pancake race track shape HTS coil was presented. Also, the successful development and testing of a racetrack shape double pancake HTS field coil for the rotor of a HTS synchronous machine was described. The developed HTS field coil has shown zero resistance at LN2 temperature (77 K) and its critical currents were measured at 77 K and 35 K. The obtained critical currents are comparable with theoretically estimated values. The ‘n’ value of HTS tape of the field coil is also estimated at self field. The successful demonstration of HTS field coil has significant implications towards the development of compact and high power density HTS synchronous machines in India.
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Acknowledgements The authors would like to acknowledge with thanks Mr. Sudheer Thadela, Cryogenic Engineering centre, IIT Kharagpur and Mr. Divya Kumar Sharma, Corporate R & D, Bharat Heavy Electricals Limited, Hyderabad for the support extended to this work. Supplementary materials Supplementary material associated with this article can be found, in
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