Electrical breakdown of vacuum insulation at cryogenic temperature

IEEE Transactions on Electrical Insulation Vol. 25 No. 3, June 1990

Electrical Breakdown of Vacuum Insulation at Cryogenic Temperature

Hao Fengnian and w u Weihan

Department of Electrical Engineering, Tsinghua University, Beijing, China

ABSTRACT A breakdown experiment of vacuum insulation for the develop- ment of cryocables and other cryogenic electrical apparatus was performed. The influence of conditioning, pressure, gap spacing, electrode material and voltage waveshape on the breakdown voltage of vacuum insulation was measured at room temperature and at cryogenic temperature respectively. The results indicated that cooling the HV sphere electrode enhances the breakdown strength of vacuum insulation, and in general, almost all effects of electrical breakdown of vacuum insulation at room temperature are reproduced at cryogenic temperature at somewhat higher voltage, the close correlation between the low-temperature and room-temperature breakdown voltages of large vacuum gaps can be explained from the point of view of microparticle-initiated breakdown theory.

INTRODUCTION

N order to meet the ever growing demand for transmis- I sion of electrical energy, during the past two decades there were more than two dozen of cryogenic (superconducting and cryoresistive) cable projects in the world. The breakthrough in the field of high T, superconducting materials has stimulated the interest in superconducting cable and other superconducting electrical products again. This in turn requires adequate low-temperature HV insulation systems.

Vacuum can be an ideal thermal and electrical insulation simultaneously, and it has been proposed to apply vacuum in several electrical insulation types for cryogenic cables, such as:

1. Vacuum insulation for terminal bushing [l, 21; 2. Vacuum with spacer [3-51; 3. Plastic tapes in vacuum [6];

4. Extruded polyethylene in vacuum [2].

55 7

Much work has been done in the field of vacuum breakdown a t room temperature [7-91, but insufficient experimental data exist for vacuum insulation a t cryogenic temperature. The breakdown of vacuum insulation is very complicated, and affected by many factors. Large differences among the experimental data from different researchers exist, and further extensive investigations to verify the electrical insulation performance of vacuum a t low temperature are still desired. This paper compares the characteristics of vacuum breakdown a t room and a t cryogenic temperatures by means of experiments, and dis- cusses their close correlation.

A P PA RAT U S

HE measurements were performed in a vacuum cham- T ber which consisted of a Plexiglasm cylinder of 200

0018-9367/90/0600-557$1.00 @ 1990 IEEE

. . - -. - _-

558 Hao: Electrical Breakdown of Vacuum Insulation

LN2 vessel

Plexiglas cylinder

. Sphere to plane electrodes

Oil diffusion pump

Figure 1. Vacuum chamber.

mm diameter and 700 mm high, fitted with stainless steel endplates and sealed with rubber O-rings (Figure 1). The vacuum chamber was pumped by a rotary pump and an oil diffusion pump with a water-cooled baffle valve. The pressure of the chamber was measured with an ionization gauge and was adjusted by the high vacuum valve of the oil diffusion pump.

" I - 40 1

10-3 10-2 lo - ' Pressure (pa) Figure 2.

Effect of pressure on ac breakdown voltage (peak value), 2 mm vacuum gap. o o 0: Aluminum, x x X: Stainless steel. - : room temperature. - - -: cooled by liquid N2.

Three pairs of sphere-teplane electrodes were made of aluminum, copper, and stainless steel, respectively, and were commercially polished. The diameter of the sphere electrode was 50 mm and the diameter of the plane electrode was 100 mm with an edge radius of 5 mm. The HV electrode could be screwed to the bottom of the liquid nitrogen vessel. When the vessel was filled with liquid nitrogen, the ultimate pressure decreased from 1 . 3 3 ~ 1 0 - ~ to 2 . 6 7 ~ 1 0 ~ ~ Pa, and the temperature of the HV sphere electrode dropped to about 88 K, which was measured by

a platinum resistor. Due to the thermal contraction, the electrode n a D increased by = 0.95 mm.

0- -- o\

100

-0 4 0 1 r - ; I - m o l I ' I l l I k I~

Pressure (pa)

10-2 10-1

Figure 3. Effect of pressure on dc breakdown voltage, 2 mm vacuum gap. o o 0: Aluminum, - dc; x x x : copper, + dc. - : room temperature. - - -: cooled by liquid Nz.

The experiments were conducted under ac (50 Hz) voltages obtained f r o q a 500/500 kV/kVA test transformer, and dc voltages obtained from a 1 to 800 kV stabilized dc supply. The voltage was applied to the vacuum insulation system through a 4 Mil resistor.

Zero gap spacing was given by the point of electrical contact. The gap spacing was adjusted by a screw mechanism at a rate of 1.5 mm per turn, and was measured again by a height gauge with an accuracy of 2% at 1 mm spacing. The voltage between the electrodes was measured by a 200 kV electrostatic voltmeter with an accuracy of 1.5%. The breakdown was indicated by a sudden drop in voltage and a sudden jump of the ionization gauge pressure.

This apparatus can also be used to investigate the poly- mer insulation of short model cables in vacuum at low temperature.

RESULTS AND DISCUSSION

EFORE measuring the ac and dc breakdown voltages, B the electrodes were conditioned with approximately 100 breakdowns. With successive breakdowns, the increase in the breakdown strength followed a general trend, and R 30% increase was obtained. No significant dif- ference between low-temperature and room-temperature conditioning characteristics was observed. But during the low temperature conditioning, a darker glow discharge was observed and the breakdown voltages were higher.

IEEE Transactions on Electrical Insulation Vol. 25 No. 3, June 1990 559

200

160 M 4 2 120-

: 4 8 0 -

6 4 0 -

Y

0

-

-

L I

0 1 2 3 4 5 Gap spacing (mm)

both temperatures. As can be seen from Figure 2, the breakdown voltage reaches a maximum value a t the pressure of 2.67~10-’ P a and the breakdown voltage remains constant for vacuum < 1 . 0 6 ~ lo-’ Pa.

200 1

0

& 40

Figure 4.

ac breakdown voltage (peak value) vs. gap spacing.

- : room temperature, - - -: cooled by liquid Nz.

0 1 2 3 4 5 o o 0: Aluminum, x x x : stainless steel. Gap spacing (mm)

Figure 6.

200 1

1 I 1 I 1

0 1 2 3 4 5 Gap spacing (mm)

Figure 5. Positive dc breakdown voltage vs. gap spacing. o o 0: Aluminum, x x X: stainless steel. - : room temperature, - - -: cooled by liquid NZ.

All data presented are average values taken from a min- imum of three trials. The spread in breakdown voltages is about 10%.

The effects of pressure on the breakdown strength of 2 mm vacuum gaps were measured under ac and dc voltages a t room-temperature and at low-temperature respectively. As shown in Figures 2 and 3, the effects of pressure a t both temperatures are similar, but the breakdown strength a t low temperature is higher. For the stainless steel electrodes, the ‘pressure effect’ [9] was observed a t

Negative dc breakdown voltage vs. gap spacing. o o 0: Aluminum, x x X : stainless steel. - : room temperature. - - -: cooled by liquid Nz .

The relationship between the breakdown voltage and the gap spacing was measured at the pressure of < 5 . 3 2 ~

Pa. As can be seen from Figure 4 to 6, the rela- tionships a t both temperatures are also similar, and it can be stated to a rough approximation, that breakdown tends to be field-dependent for small gaps corresponding to field emission initiated breakdown, whereas it becomes voltage dependent for large gaps, corresponding to microparticle initiated breakdown. Even though the critical gap spacings between the two gap regions are different among various electrodes, the critical gap spacing at low temperature is almost equal to that one of the same electrode at room temperature, e.g. the critical gap spacing of the aluminum electrode is x 2 mm a t both room and low temperature. For large gaps (d > 2 mm) , the ratio of low to room temperature breakdown voltages under ac voltages is nearly a constant,> 1, which is the case also for positive dc voltages.

It can be seen from the results that the electrode materials strongly affect the breakdown voltages at both temperatures, but the relative differences between the breakdown voltages of aluminum electrodes and stainless steel electrodes decrease with gap spacing. For different electrode materials, the effects of the electrode temperature are somewhat different. When the aluminum HV electrode is cooled by liquid nitrogen, the breakdown strength is enhanced more significantly. Probably one of the main


reasons for this is the low melting temperature of aluminum.

At small gaps, the field is close to a uniform and sym- metric field, and the differences among ac, positive dc, and negative dc breakdown voltages are not significant. At large gaps, cooling the HV sphere anode increases the breakdown voltages distinctly, and the increase in breakdown voltage for aluminum electrodes is about 15% (shown in Figure 5); cooling the HV sphere cathode, however, slightly increases the breakdown voltage. This may imply that the anode plays an important role in the breakdown process for large gaps.

After the experiments a t low or at room temperature under positive dc voltage or under negative dc voltage, many microcraters were observed on the anode surface, regardless of polarity, whereas the surface of the cathode did not change much. This may also imply that the anode melting and vaporization plays a dominant role in the breakdown process.

Despite the large scatter in the breakdown voltages of vacuum insulation, the experimental results still indicate some definite trends. The phenomena and characteristics of vacuum breakdown a t both temperatures, such as conditioning characteristics, effects of pressure, electrode material, and separation, etc. are all very similar, hence there is some evidence from the experiments confirming that the breakdown mechanisms of vacuum insulation a t both temperatures are similar or identical.

Breakdown mechanisms of vacuum insulation a t room temperature can be divided into four groups

1. Field emission plus cathode heating. In this theory, the breakdown initiation comes from the production of vapor or melting a t a microprotrusion on the cathode due to intense field emission from the protrusion;

2. Field emission plus anode heating. This theory states that field emission electrons from the cathode bombard the anode causing a local temperature rise a t a local site, and release gases and vapors in which gaseous discharge occurs;

This theory as- sumes that loosely bound microparticles on an electrode surface become detached under the action of the electric field, and are accelerated across the gap, the breakdown occurs in vapor produced when a suf- ficiently energetic, charged microparticle collides with an electrode;

3. Microparticle initiated breakdown.

4. Microdischarge initiated breakdown.

Vacuum breakdown at low temperature has been in- vestigated in some papers [lo-151, but it was difficult to explain the phenomena and characteristics of vacuum breakdown at low temperature quantitatively. Mainly based on the field emission initiated breakdown mechanism, the influence of low temperature on the breakdown voltage of small plane-to-plane vacuum gaps was inves- tigated in most of the previous papers [ll-131, and the following explanations were stated. First, the condensed gas layer on the surface of the cathode forms on cooling and causes a prebreakdown current reduction due to its effect on the field intensification factor 0. Second, cooling the electrodes reduces the local temperature rise on the electrode surface caused by field electron emission. Third, cooling the cathode reduces its electrical resistivity and the internal resistive heating resulting from a high current density flowing into the microprotrusion. Therefore the breakdown voltage increases on cooling.

The above factors related to field emission initiated breakdown are important for small gaps and their influence probably decreases with gap spacing. As can be seen from the breakdown characteristics under negative dc voltage in Figure 6, the effect of cathode cooling indeed decreases with gap spacing.

For most power engineering applications, the vacuum insulation behavior of large gaps under slightly non-uniform field is of concern. From the theory of microparticle initiated breakdown proposed by Cranberg, the breakdown voltage VI, can be written as [7]

where W is the critical kinetic energy of the microparticle to initiate breakdown at room temperature, R the radius of the microparticle, and d the gap spacing.

When the electrodes are cooled, the microparticle must reach a higher critical energy value to vaporize itself and the local hot spot of the opposite electrode on which it strikes, and the resultant vapor must be sufficient to initiate a gas discharge. This means that the critical kinetic energy of the microparticle a t low temperature, W’ must be higher than that at room temperature W . The ratio of the critical energies W’/W can simply represent the relationship between the low and room temperature breakdown characteristics of large vacuum gaps

Kt is the low-temperature coefficient or the low-temperature to room temperature breakdown voltage ratio. The

IEEE Transactions on Electrical Insulation Vol. 25 No. 3, June lQQ0 561

dependence of breakdown voltage on the gap spacing under ac voltage or positive dc voltage in Figures 4 and 5 conforms the relationship in Equation (2). For instance, the breakdown voltage ratio of the aluminum electrodes under ac voltage or positive dc voltage are about 1.20 and 1.15, respectively, and the coefficients for stainless steel electrodes are about 1.10 and 1.08, respectively.

When the low-temperature coefficient is determined by means of experiments or theoretic calculations for various electrodes, it will be possible to utilize the experimental breakdown data a t room temperature in the design of low-temperature HV vacuum insulation systems.

If the influence of low temperature on the thermal ca- pacity and the thermal conductivity of the electrodes is not considered, and it is also assumed that the temperature rise A T of the microparticle and the local hot spot on the anode is proportional to the microparticle energy W , at both temperatures, i.e. A T cx W, then the low- temperature coefficient Kt can be approximated by

where T, is the low-temperature, T,. the room-temperature, and T, the electrode melting temperature.

For aluminum electrode, T, = 933, T, = 300, T, = 88 K, and the low-temperature coefficient Kt = 1.155. This calculated value is in good agreement with the experimental value obtained under positive dc voltage. Utiliz- ing the similarity in the temperature increase, many in- definite factors in the microparticle initiated breakdown mechanism are eliminated in Equation (3).

CONCLUSIONS

1. Cooling the HV sphere electrodes by liquid nitrogen enhances the breakdown voltage of vacuum insulation. For aluminum electrodes with low melting temperature, the ac breakdown voltage increases by M 20%.

2. The phenomena, characteristics and experimental data of vacuum breakdown at low temperature and a t room temperature are related. The conditioning processes, pressure effects, effects of electrode material and separation etc. are very similar a t both temperatures.

3. The ratio of the low temperature to room temperature critical microparticle energies W’/W or the low- temperature coefficient Kt based on microparticle initiated breakdown theory, approximately represents the correlation between the low-temperature and room-temperature breakdown voltages of large vacuum gaps.

4. In practical designs of low-temperature HV vacuum insulation systems, it is suitable and reliable to utilize or directly use the experimental data of breakdown voltages obtained at room temperature.

ACKNOWLEDGMENTS

The authors wish to thank Prof. Yan Jinji for helpful discussions. The authors are also grateful to Mr. Gao Ronghua and Mr. Shao Shizhao for their participation in the experiments.

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This manuscript is based on a paper given at the 2nd Interna- tional Conference on Properties and Applications of Dielectric Materials, Beiing, China, 12-16 September 1988.

Manuscript was received on 6 Mar 1989, in final form 2 Feb 1990.

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Electrical breakdown of vacuum insulation at cryogenic temperature