Influence of quench-induced thermal bubble disturbance on insulation performance of liquid helium for superconducting power apparatus

PII: S0011-2275(98)00062-9

Cryogenics 38 (1998) 931–936  1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0011-2275/98/$—see front matter

Influence of quench-induced thermal bubble disturbance on insulation performance of liquid helium for superconducting power apparatus S. Chigusa, Y. Taniguchi, N. Hayakawa and H. Okubo Department of Electrical Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Received 19 February 1998 Quench of superconductor is an unavoidable phenomenon in superconducting power apparatus. The insulation performance of liquid helium (LHe) may be drastically degraded in thermal bubble disturbance under quench conditions. In this paper, we investigated the influence of quench-induced thermal bubble disturbance on insulation performance of LHe for practical design of superconducting power apparatus. Experimental results revealed that the dynamic breakdown voltage of LHe under uniform and non-uniform electric fields could be quantitatively evaluated by injected thermal energy during the quench period of superconductors. Moreover, suppression of thermal bubbles in pressurized LHe was verified to be effective to improve the insulation performance of LHe under quench conditions.  1998 Elsevier Science Ltd. All rights reserved Keywords: quench-induced thermal bubble disturbance; dynamic breakdown; pressurized LHe

In order to develop superconducting power apparatus such as superconducting generators, fault current limiters, magnet energy storages, magnets for fusion reactors, etc, it has been requested to establish the electrical insulation technique in cryogenic liquids1,2. Many papers on breakdown characteristics in cryogenic liquids have been published. However, most of their investigations have so far focused only on fundamental or ‘static’ breakdown characteristics of cryogenic liquids3–5. Thus, it is important to evaluate the insulation performance of cryogenic liquids under practical operating conditions for electrical insulation design of the superconducting power apparatus6,7. It is well known that superconducting power apparatus have an inherent phenomenon called quench, i.e. transition from superconducting to normal state. Quench phenomena in superconducting apparatus cause the generation and propagation of voltage and ohmic heat along the superconducting wire, resulting in thermal bubble generation in LHe. Thus, under not only the ‘static’ condition but also the ‘dynamic’ bubble disturbance, the insulation characteristics should be investigated and clarified. We have investigated the ‘dynamic’ breakdown characteristics of LHe caused by quench of superconducting wire and coil8–10. As a result, it has been derived that dynamic breakdown voltage in the quench-induced thermal bubble condition fell to 10–20% compared with the static one. In this paper, we discuss the

dependence of thermal energy injected by quench on the dynamic breakdown characteristics of LHe under non-uniform and uniform electric fields. Moreover, we measured the pressure dependence of the insulation performance of LHe under quench-induced dynamic conditions.

Experimental Figure 1 shows the schematic view of the stainless cryostat used for the dynamic breakdown experiment with pressurized LHe (0.1–0.5 MPa). FRP capacitor bushing with 75 kVrms PD free ability in LHe was mounted on the stainless cryostat. Figures 2 and 3 show the experimental setup for the measurement of static and dynamic breakdown characteristics of LHe. This circuit consists of a large-current source including superconducting wire or coil, and a high-voltage source including a plane electrode. The plane electrode was placed in parallel above the superconductors as shown in Figure 3, and they were immersed in LHe. Figure 3a is a superconducting wire–plane electrode system for non-uniform electric field distribution, while Figure 3b and c are for the uniform field. The superconducting coil in Figure 3c has a flat configuration so as to make a uniform electric field in the gap space. We used five kinds of supercon-

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Quench of superconducter: S. Chigusa et al. ducting wires with different specifications as listed in Table 1, in order to change the injected energy to superconductors by quench. Thermal energy injected in superconducting wires or coils by quench can be controlled by using different superconducting wires as well as by changing current flowing cycles (12–45 ms) after quench and prospective current level (145–750 Apeak ) of the large-current source. Positive dc high voltage was applied to the plane electrode and kept at a constant value below a static breakdown level. Under this condition, a large ac current of frequency 60 Hz caused quench of the superconductors, resulting in the injection of thermal energy. Owing to the quench, the gap space was filled with generated thermal bubbles, which induced dynamic breakdown of LHe. We observed the generation and propagation of bubbles in the gap space using a high-speed video system (400 fields/s). The injected thermal energy into the superconductors by quench was calculated from the waveforms of current Isc flowing in the superconducting wire and the terminal voltage Vsc.

Dependence of injected thermal energy by quench on insulation performance of LHe

Figure 1 Schematic view of stainless cryostat

Dynamic breakdown characteristics under nonuniform field Figure 4 shows the typical waveforms of voltage and current at the quench-induced dynamic breakdown of LHe under non-uniform electric field with the electrode system of Figure 3a. As shown in Figure 4, the quench of superconducting wire occurred when Isc reached 365 A, at which point the terminal voltage Vsc suddenly emerged. Immediately after the quench-onset, the current Isc plunged owing to the generated resistance in the superconducting wire. Having been maintained at a constant value for a moment after the quench-onset, the applied positive dc high voltage Vdc to the plane electrode suddenly collapsed 14.2 ms after the quench-onset. This result means that the insulation performance of LHe was degraded by the quench of the superconducting wire. Figure 5 shows the gap length dependence of static and dynamic breakdown voltages of LHe for different thermal

Figure 2 Experimental setup for measuring dynamic breakdown characteristics of LHe

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Figure 3 Superconducting wire- and coil–plane electrode configuration. (a) Wire–plane; (b) plane–plane; (c) coil–plane Table 1 Specifications of superconducting wires

Matrix ratio (Cu:CuNi:NbTi) Number of filaments Number of strands Diameter (mm)

#1

#2

#3

#4

#5

0:1.84:1

0:2.1:1

0.16:2.17:1

0:2.3:1

0.6:2.3:1

23 749 7 0.42

51 000 6 0.50

21 336 7 0.46

15 367 7 0.128

1 269 400 6 1.0

Figure 4 Waveforms of Isc, Vsc and Vdc at quench-induced dynamic breakdown of LHe

energy injected. The static breakdown voltage Vs was measured by increasing the applied voltage at a rate of 1 kV/s without current flowing in the superconducting wire. On the other hand, the dynamic breakdown voltage Vd was defined as a minimum value of the applied voltage below which the breakdown is no longer induced by the quench at each gap length. As shown in Figure 5, Vd is much lower than Vs for a certain gap length depending on the thermal energy injected, and increases exponentially with the gap length. For the larger gap length, Vd tends to equal Vs, since the volume of the quench-induced thermal bubbles would decrease compared with the gap volume. In other words, the larger gap length would be necessary to prevent the dynamic breakdown of LHe under the quench conditions. Figure 6 shows thermal energy dependence of Vd at gap length g = 5,7 and 10 mm. As shown in Figure 6, it is obvious that Vd decreases as the thermal energy increases. This result is attributed to the expansion of thermal bubble disturbance in the gap space with increasing the injected thermal energy by quench. Note that for a given gap length, the dynamic breakdown voltage of LHe Vd can be expressed as a function of injected thermal energy irrespec-

Figure 5 Static and dynamic breakdown voltages Vs, Vd as a function of gap length for different thermal energy under nonuniform electric field

tive of the specifications of superconducting wires. This result means that dynamic breakdown voltage Vd under non-uniform electric field can be evaluated by injected thermal energy during the quench period of superconducting wires.

Dynamic breakdown characteristics under uniform field Figure 7 shows dynamic breakdown strength Ed for the superconducting wire- and coil–plane electrode system under uniform electric field as a function of injected thermal energy density Junit by quench. Junit was defined as normalized thermal energy density/cm of superconducting wire length, in order to systematically evaluate Ed for different lengths of superconducting wires and coils. As shown in

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Figure 6 Dynamic breakdown voltage Vd as a function of thermal energy under non-uniform electric field

Figure 8 Dynamic breakdown strength Ed as a function of effective thermal energy density Jeff under uniform electric field

effective thermal energy density Jeff could be defined as Junit ⫻ 5(mm)/a(mm), where a is the diameter of superconducting wire. Figure 8 thus shows obtained dynamic breakdown strength Ed as a function of Jeff. As shown in this figure, Ed for the coil–plane electrode system agrees roughly with Ed for the wire–plane electrode system under uniform electric field. These results suggest that it is possible to comprehensively evaluate Ed under uniform field by Jeff from the viewpoint of injected thermal energy density by quench. From the experimental data in Figure 8, the dynamic breakdown strength of LHe under quench condition may be degraded to about 10% of the static one at higher effective thermal energy density Jeff than 0.3 J/cm.

Pressure dependence of insulation performance of LHe under quench condition

Figure 7 Dynamic breakdown strength Ed as a function of thermal energy density Junit under uniform electric field

Figure 7, both Ed(wire) and Ed(coil) decrease with the increase in magnitude of Junit, as in the case of Ed under non-uniform field in Figure 6. Ed tends to be saturated for the higher Junit with plenty of quench-induced thermal bubbles, where the dynamic breakdown can be regarded as the breakdown in gaseous helium at the cryogenic temperature. Note that Ed(coil) is lower than Ed(wire). This result can be explained as follows. Quench of the coil propagates 2-dimensionally on the coil surface. Thus, quench-induced thermal bubble disturbance in the gap space for the coil–plane system is larger than that for the wire–plane system. In other words, it was clarified that the dynamic breakdown for the superconducting coil is affected by the thermal energy from adjacent superconducting wires. From the above deductions, in the case of the coil–plane system, it is necessary to take into account the thermal energy from adjacent superconducting wires. According to the observed results of thermal bubble behavior for the coil–plane electrode system, it was verified that a bubble cluster of width approx. 5 mm on the coil surface would induce the dynamic breakdown of LHe. Therefore, the

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In the previous section, it was established that the quenchinduced thermal bubble disturbance in the gap space would drastically deteriorate the insulation performance of LHe. Therefore, improvement of the insulation performance of LHe under quench condition is inevitable to develop superconducting power apparatus. As one of the measures to improve the dynamic breakdown strength, pressurized LHe is expected to be effective. Thus, we measured pressure dependence of dynamic breakdown strength of LHe for the superconducting coil–plane electrode system. Figure 9 shows the time-resolved propagation of thermal bubble disturbance in the gap space after quench-onset at (a) 0.1 MPa (atmospheric pressure) and (b) 0.2 MPa. In Figure 9a at 0.1 MPa, the gap space is clear without thermal bubbles at the instant of quench-onset (0 ms). At 50 ms after the quench-onset, the gap space is dark because a lot of generated thermal bubbles propagate into the gap space. Furthermore, even at 300 ms later, the gap space is still filled with residual thermal bubbles, though the current flowing into the superconducting coil was interrupted at 16 ms. On the other hand, in Figure 9b at 0.2 MPa, the generated thermal bubble density is found to be much lower at 50 ms after the quench-onset. The gap space is empty of bubble at 300 ms. These results verified that the thermal bubble disturbance in the gap spaces under quench condition greatly decreased under pressurized LHe. Figure 10 shows pressure dependence of dynamic break-

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Figure 9 Propagation of thermal bubble disturbance after quench-onset for different pressures of LHe. (a) 0.1 MPa; (b) 0.2 MPa

lation performance of LHe under quench condition is obviously brought about by the suppression of thermal bubble disturbance under pressurized LHe as shown in Figure 9b. Figure 11 shows pressure dependence of static and dynamic breakdown strength of LHe at g = 3 mm. In Figure 11 the dynamic breakdown strength Ed is lower than the static breakdown strength Es. Moreover, it was clarified that both static and dynamic breakdown strength increased with pressure. Figure 12 shows the reduction rate of insulation performance of LHe under quench condition Ed/Es as a function of pressure at g = 3 mm. Ed/Es is calculated from Figure 11. As shown in Figure 12, at both 0.1 and 0.15 MPa, insulation performance of LHe fell to 18% by quench of the superconducting coil. On the other hand, it was improved up to 31% by pressurization of LHe to 0.2 MPa. It is concluded that the suppression of the thermal bubble disturbance by quench is effective to improve the dynamic breakdown characteristics of LHe. Figure 10 Dynamic breakdown voltage Vd as a function of pressure of LHe for different gap lengths under uniform electric field

down voltage Vd of LHe with Jeff = 0.17 J/cm for different gap lengths. As shown in Figure 10, Vd increased with pressure at each gap length. In the case of gap length g = 5 mm at 0.2 MPa, Vd reached 37 kV, which corresponds to about 4 ⫻ of that at 0.1 MPa. This improvement of insu-

Conclusions We investigated that influence of quench-induced thermal bubble disturbance on insulation performance of LHe. Experimental results revealed that under both non-uniform and uniform electric field, dynamic breakdown voltage was reduced in the thermal bubble disturbance due to quench of superconductors. The insulation performance of LHe under quench condition was verified to be evaluated by injected

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Figure 12 Reduction rate of breakdown strength Ed/Es as a function of pressure of LHe at gap length g = 3 mm Figure 11 Static and dynamic breakdown strength Es, Ed as a function of pressure of LHe at gap length g = 3 mm

thermal energy during the quench period. For example, the dynamic breakdown strength of LHe under a uniform electric field degraded to about 10% of the static one at higher effective thermal energy density Jeff than 0.3 J/cm. Moreover, pressurized LHe exhibited higher dynamic insulation performance at 0.2 MPa, as high as 2–4 ⫻ of that at atmospheric pressure. The time-resolved thermal bubble behavior was observed using a high-speed video system, which clarified that the quench-induced thermal bubble disturbance was greatly suppressed in pressurized LHe, resulting in improvement of the insulation performance of LHe under quench conditions.

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3. Hara, M., Kaneko, T. and Honda, K., Thermal-bubble initiated breakdown characteristics of liquid helium and nitrogen at atmospheric pressure. IEEE Transactions on Electrical Insulation, 1988, 23(4), 769–778. 4. Belevtsev, A. A., Dielectric strength of condensed cryogenic helium. Proceedings of the 11th ICDL, 1993, pp. 224–228. 5. Gerhold, J., Gap size effect on liquid helium breakdown. Cryogenics, 1994, 34(7), 579–586. 6. Gerhold, J. and Hara, M., Procedure of electrical insulation design for superconducting coils. Proceedings of the 8th ISH, 1993, pp. 567–570. 7. Okubo, H., Hikita, M., Goshima, H., Sakakibara, H. and Hayakawa, N., High voltage insulation performance of cryogenic liquids for superconducting power apparatus. IEEE Transactions on Power Delivery, 1996, 11(3), 1400–1406. 8. Hayakawa, N., Hirose, M., Goshima, H., Hikita, M., Uchida, K. and Okubo, H., Quench-induced breakdown characteristics of liquid helium and optical observation of thermal bubbles. Cryogenics, 1995, 35(2), 135–142. 9. Hayakawa,N., Wakita, M., Hirose, M., Goshima, H., Hikita, M., Uchida, K. and Okubo, H., Quench-induced breakdown mechanism of liquid helium. Proceedings of the 8th ISH, Subject 7, No. 7047, 1995. 10. Okubo, H., Wakita, M., Chigusa, S., Hayakawa, N. and Hikita, M., Dynamic breakdown characteristics of liquid helium induced by a quench of superconducting wire and coil. IEEE Transactions on Dielectrics and Electrical Insulation, 1997, 4(1), 120–126.