IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
5202905
Dynamic Response of No-Insulation and Partial-Insulation Coils for HTS Wind Power Generator Jung-Bin Song, Seungyong Hahn, Youngjae Kim, Daisuke Miyagi, John Voccio, Juan Bascuñán, Haigun Lee, and Yukikazu Iwasa
Abstract—In this paper, we present results, experimental and numerical, of the electromagnetic interaction forces between pairs of racetrack coils under time-varying conditions. Three turn-toturn insulation designs were applied to wind three racetrack coils with GdBCO coated conductor: 1) no insulation (NI); 2) partial insulation (PI) of a polyimide layer every eight turns; and 3) insulation (INS) of a polyimide layer between each, i.e., NI, PI, and INS racetracks. Two racetrack pairs, namely, NI-INS and PI-INS, were tested for their interaction forces, measured with load cell under current-ramping conditions in a bath of liquid nitrogen at 77 K. Good experimental and simulation results validate our equivalent circuit model to compute interaction forces of PI-INS racetrack pair. Overcurrent test of NI and PI coils, where each racetrack coil was charged above critical current (Ic ), was also performed to compare coil stability. This result implies that, although the PI winding technique improves the dynamic response, stability will be somewhat compromised. Index Terms—Electromagnetic force, equivalent circuit model, no-insulation, partial-insulation, wind power generator.
I. I NTRODUCTION
A
S, THE POWER generation capacity of a unit wind turbine continues to escalate, there is an urgent need to reduce the generator size and weight. The smaller and lighter generator reduces its overall system cost. The high-temperature superconductor (HTS) technology enables a “large” wind generator to be compact and lightweight, leading to innovative turbine designs that are not feasible with conventional technology. However, protection of HTS coils in the event of a quench still remains a major technical challenge to the HTS wind generators. Therefore, for practical HTS turbines, these protection and reliability issues need to be fully resolved [1]–[6]. The main feature of the no-insulation (NI) winding technique is complete elimination of turn-to-turn insulation in an HTS winding [7]. In the event of a quench, the NI coil current can Manuscript received August 12, 2014; accepted December 1, 2014. Date of publication December 19, 2014; date of current version February 12, 2015. This work was supported by the International Collaborative R&D Program of the KETEP grant funded by the Korean government MKE (20118520020020). (Corresponding author: Seungyong Hahn.) J.-B. Song, S. Hahn, Y. Kim, D. Miyagi, J. Voccio, J. Bascuñán, and Y. Iwasa are with the Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail:
[email protected]). H. Lee is with the Department of Material Science and Engineering, Korea University, Seoul 136-701, Korea (e-mail:
[email protected]). 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.2014.2384739
“automatically” bypass through the turn-to-turn contacts from its original spiral path and the coil becomes “self-protecting” without any additional protection circuitry. To date, we have demonstrated, with experiment and analysis, the built-in selfprotecting feature of the NI coils, absent in their conventional insulated counterparts [7]. Due to this self-protecting feature, NI coils require only a minimal thickness of stabilizer, typically < 10 um, electroplated chiefly for ease of handling and soldering, and thus becoming highly compact. Therefore, many studies for investigating thermal and electrical stabilities of NI magnet have been conducted [7]–[26]. However, for applying the NI technique to large-scale HTS rotating machine such as HTS wind power generator, it is necessary to investigate dynamic responses of the electromagnetic interaction force (Fz ) between the windings of an NI coil and its insulated (INS) armature. In 2013, we reported simulation and experimental results of Fz between NI and INS coils under time-varying condition for application of the NI technique to the HTS wind power generator. This earlier study showed that Fz between NI and INS coils lags during current ramping, and increasingly more so at faster ramp rate [27]. Recently, we proposed the partialinsulation (PI) winding technique that can significantly reduce charging delay [28], the source of lagging in Fz . As a follow-up study, we have investigated Fz between PI and INS coils under time-varying conditions. This paper presents results, empirical and numerical, of the Fz between PI and INS pairs of racetrack coils under timevarying conditions. Three turn-to-turn insulation designs were applied to wind three racetrack coils with Cu-electroplatedGdBCO coated conductor: 1) NI; 2) PI of a polyimide layer in every eight turns; and 3) INS of a polyimide layer in turnto-turn, i.e., NI, PI, and INS racetracks. Two racetrack pairs, NITOP -INSBOT and PITOP -INSBOT were tested for their interaction forces, measured with a load cell under current-ramping conditions in a bath of liquid nitrogen at 77 K. The subscripts TOP and BOT refer to the axial positions of the racetracks. Good agreement between experiment and simulation validates our equivalent circuit model applied to compute interaction forces of the PI-INS racetrack pair. Over-current test of NI and PI coils, where each racetrack coil was charged above critical current (Ic ), were also performed to compare the stability of the coils. This result implies that, although the PI winding technique improves the dynamic response, stability will be somewhat compromised.
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
Fig. 1. Pictures of three racetrack SP coils. (a) NITOP . (b) PITOP . (c) INSBOT .
Fig. 2. Schematic of a force measurement experimental setup for two racetrack coils, one on top of the other.
TABLE I S PECIFICATION OF T HREE R ACETRACK SP T EST C OILS
Fig. 3. Equivalent circuit model of NITOP -PITOP and PITOP -INSBOT pair [21].
II. E XPERIMENTAL S ETUP Fig. 1 shows pictures of the NI, PI and INS single-pancake (SP) racetrack coils. Each test coil was wound onto a phenolic bobbin with GdBCO coated conductor (CC) manufactured by SuNAM, with a winding tension of 20 N. Although the 25-mm inner diameter end sections and the 80-mm long straight section were the same for the three racetrack test coils, the outer diameters were 32.2, 29.3, and 29.8 mm, respectively, indicating a larger outer diameter for the INS than those for the NI and PI due to 0.06-mm thick polyimide insulation layers. The specifications of three racetrack SP coils are listed in Table I. Fig. 2 shows a schematic drawing of the experimental setup for force measurement between two racetrack coils. Two coil pairs, NI-INS and PI-INS, were prepared, each pair connected to a DC power supply. Hall sensor was located at the axial center of the SP racetrack pair. The dynamic response of two racetrack pairs, in a bath of LN2 at 77 K, were measured by a load cell (FUTEK, LBB200) connected with top racetrack coil for combinations of current amplitude and ramping rate.
The load cell constant was 0.5 mV/N, and its measurement uncertainty, due primarily to temperature change during measurement, was about 10%. To prevent damaging the load cell by thermal shock, it was always placed above the LN2. Over-current test was also performed to investigate stability of NI and PI racetrack coils. For the test, the NI and PI racetrack coils were tested by following three sequential steps: 1) charge to 1.25 Ic at a current-ramping rate of 10 A/min, 2) maintain at 1.25 Ic for 300 s, and 3) discharge at the rate numerically equal to the ramping up rate. III. E QUIVALENT C IRCUIT M ODEL Fig. 3 shows an equivalent circuit model for the NI-INS and PI-INS pairs. The current flows toward the radial direction through the turn-to-turn contacts as well as through the superconducting spiral direction. This circuit model can be formulated as following equation by applying Kirchhoff’s laws: Iθ + IR = IT OP LN I
dIθ dIBOT +M + Iθ Rθ = I R Rc dt dt
(1) (2)
where IT OP , IBOT , Iθ , IR , Rθ , LNI , and M are, respectively, power supply currents to the top and bottom coils, the currents through the spiral and radial directions of the NI or PI coil,
SONG et al.: DYNAMIC RESPONSE OF NI AND PI COILS FOR HTS WIND POWER GENERATOR
Fig. 4. Iop and Bz traces, experimental and analytical, of each top coil at ramp rate of 1 A/min. (a) NI racetrack coil. (b) PI racetrack coil.
azimuthal resistances, self and mutual inductances. Rc is the characteristic resistance of a NI pancake coil, NI or PI, representing the total sum of resistances. Here, Rθ , generated by the index and AC losses, may be neglected under nominal operating conditions, and Rc is mostly from the turn-to-turn contacts. In addition, dIBOT /dt = 0, because IBOT was maintained constant at 40 A in this test. IV. R ESULT AND D ISCUSSION A. Characteristic Resistance Rc of NI and PI Racetrack Coils To determine Rc of NI and PI racetrack coils, experiment and calculation were conducted on each coil in the following four steps: 1) charge a racetrack coil to 10 A at a currentramping rate of 1 A/min; 2) maintain at 10 A for 600 s; 3) discharge it to zero at a current-ramping rate of −1 A/min; 4) calculate Rc that minimizes disagreement between measuring and calculating magnetic fields [27]. Fig. 4(a) and (b) present magnetic fields, measured and calculated, respectively,
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Fig. 5. Fz as a function of IT op with ramping rates of 1, 5, 10, and 20 A/s during the tests at IBOT = 40 A. (a) NITOP -INSBOT pair. (b) PITOP INSBOT pair.
of the NI and PI racetracks. As seen from the figures, the calculated and measured magnetic fields agree well for both coils, and the measured Rc ’s are 69.4 and 658.9 μΩ, respectively. These results show that in a PI coil Rc increases by an order of magnitude from that of its NI counterpart. B. Dynamic Responses of NI and PI Racetrack Coils Two pairs, NITOP -INSBOT and PITOP -INSBOT , were each tested in a bath of LN2 at 77 K. In each pair, the coils were axially separated by 17.2 mm, and the calculated interaction force constant (Fc ) was −0.25 N/A, repulsive. For measuring dynamic responses of each pair, while the INSBOT racetrack coil carried a constant current of 40 A, a current was ramped at rates of 1, 5, 10, and 20 A/s to the top coil, from −40 to 40 A without pause. This test was repeated 4 times with the power supply controlled manually. Fig. 5 shows the electromagnetic interaction force (Fz ) as a function of IT op with ramp rates of 1, 5, 10 and 20 A/s and at IBOT = 40 A. For the NITOP -INSBOT pair [Fig 6(a)], as the ramp rate was increased from 1 A/s, the major axis slope of
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
Fig. 7. Overcurrent test of NI and PI racetrack coils at 1.25Ic .
(33.5 A) was ∼ 22 A higher than that of the NITOP -INSBOT pair (11.2 A). The PI racetrack coil has a significantly faster dynamic response Iθ than the NI coil, because IR decreases with Rc , which is increased by partial insulation, which in turn results in reduced Fz lag [see (2)]. Furthermore, it may be clearly observed that the experimental and calculated results are in good agreement, demonstrating that our equivalent circuit model is valid for computing the interaction force of PITOP INSBOT racetrack pair. C. Over-Current Test of NI and PI Racetrack Coils
Fig. 6. Fz and current versus time plots, (solid) experimental and (open) analytical, of NITOP -INSBOT and PITOP -INSBOT pairs at a ramping rate of 20 A/s, IBOT = 40 A. (a) NITOP -INSBOT pair. (b) PITOP -INSBOT pair.
the Fz loop abruptly decreased at 5 A/s, decreased further with ramp rate, reaching almost flat at 20A/s. In addition, a measured Fz value of 2.8 N of this pair at a ramp rate of 20A/s greatly differs from a calculated figure of 10 N for IT OP = 40 A. This disparity indicates that the dynamic response of the NITOP INSBOT pair under time-varying conditions is very slow. For the PITOP -INSBOT pair [Fig. 6(b)], the major axis slope of the Fz loop decreased only slightly with increasing ramp rate. At IT OP = 40 A, a measured Fz of 7.6 N at a ramp rate of 20A/s demonstrates that the PI technique can eliminate an abrupt change in the major axis slop of the Fz loop. We performed simulation work by using an equivalent circuit model to compute Iθ of NI and PI racetrack coils under the same conditions in Fig. 5. From this result, Fz of the NITOP INSBOT and PITOP -INSBOT pairs may be estimated by the following equation [27]: Fz = Fc + I θ .
(3)
Fig. 6 shows measured and numerically calculated values of Fz and current as a function of time at ramp rate of 20 A/s, corresponding to Fig. 5 (square data). As shown in Figs. 6(a) and (b), the maximum Iθ value of the PITOP -INSBOT pair
To estimate the thermal stability of NI and PI racetrack coils, the coils were charged up to 1.25Ic and discharged to zero at a current-ramp rates, respectively, of ±10 A/min (Fig. 7). As shown in Fig. 7, the voltage (13.3 mV) of the PI racetrack coil (closed star) was much higher than that (3.5 mV) of the NI racetrack (closed circle). In addition, when an over-current was maintained, the voltage of the PI racetrack coil dropped a little and reached a plateau, which indicates that the coil shows automatic turn-to-turn bypassing characteristics of an NI coil. This result implies that, although the PI winding technique improves the dynamic response, stability will be somewhat compromised. Therefore, optimization of dynamic response vs. stability is required when the PI technique is applied to the superconducting coils in the wind generator. V. C ONCLUSION Charging-discharging tests and numerical analyses were performed on two racetrack pairs, NI-INS and PI-INS, in order to investigate the dynamic response of each pair. Over-current test was also performed to determine the degree of stability in NI and PI racetrack coils. Based on the test results, we may conclude that: • The PITOP -INSBOT pair shows significantly faster dynamic response force than that of NITOP -INSBOT pair, because current (IR ) flowing through radial directions decreases as the characteristic resistance (Rc ) is increased by partial insulation.
SONG et al.: DYNAMIC RESPONSE OF NI AND PI COILS FOR HTS WIND POWER GENERATOR
• The numerical results based on an equivalent circuit model agreed well with experimental results. The agreement validates our equivalent circuit model to compute the interaction force of PI-INS racetrack pair. • For fast dynamic response, the PI technique is superior to the NI technique; however, the coil stability is somewhat compromised. Therefore, it is necessary to optimize dynamic response vs. stability when applying the PI technique to the superconducting coils in the wind generator. In sum, the PNI technique is suitable to the field coil of a wind generator if it’s optimized in terms of dynamic response vs. stability. R EFERENCES [1] S. Fukui et al., “Study of 10 mw-class wind turbine synchronous generators with HTS field winding,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 1151–1154, Jun. 2011. [2] C. Lewis and J. Muller, “A direct drive wind turbine HTS generator,” in Proc. IEEE Power Eng. Soc. Gen. Meet., Jun. 2007, pp. 1–8. [3] G. Snitchler, “Progress on high temperature superconductor propulsion motors and direct drive wind generators,” in Proc. IPEC, Jun. 2010, pp. 5–10. [4] G. Snitchler, B. Gamble, C. King, and P. Winn, “10 mw class superconductor wind turbine generators,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 1089–1092, Jun. 2011. [5] H. Jo et al., “Numerical analysis and design of damper layer for MW-class HTS synchronous wind turbine generator,” IEEE Trans. Appl. Supercond., vol. 24, no. 3, Jun. 2014, Art. ID. 5200905. [6] Y. Hwang et al., “Electromagnetic design of a 15 MW-class HTS flux switching synchronous generator considering mechanical stress of the rotor core,” IEEE Trans. Appl. Supercond., vol. 24, no. 3, Jun. 2014, Art. ID. 5202305. [7] S. Hahn, D. K. Park, J. Bascuñán, and Y. Iwasa, “HTS pancake coils without turn-to-turn insulation,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 1592–1595, Jun. 2011. [8] Y. H. Choi, S. Hahn, J. B. Song, D. G. Yang, and H. G. Lee, “Partial insulation of GdBCO single pancake coils for protection-free HTS power applications,” Supercond. Sci. Technol., vol. 24, no. 12, Nov. 2011, Art. ID. 125013. [9] S. Hahn, D. K. Park, J. Voccio, J. Bascuñán, and Y. Iwasa, “No-insulation (NI) HTS inserts for 1 GHz LTS/HTS NMR magnets,” IEEE Trans. Appl. Supercond., vol. 22, no. 3, Jun. 2012, Art. ID. 4302405. [10] Y. G. Kim, S. Hahn, K. L. Kim, O. J. Kwon, and H. G. Lee, “Investigation of HTS racetrack coil without turn-to-turn insulation for superconducting rotating machines,” IEEE Trans. Appl. Supercond., vol. 22, no. 3, Jun. 2012, Art. ID. 5200604. [11] X. Wang et al., “Turn-to-turn contact characteristics for an equivalent circuit model of no-insulation ReBCO pancake coil,” Supercond. Sci. Technol., vol. 26, no. 3, Mar. 2013, Art. ID. 035012.
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