Electrical breakdown characteristics of nitrogen and air at cryogenic temperature in a quasi‐uniform electric field

The breakdown voltages of the longer‐gap configurations in gaseous nitrogen and air that are necessary in designing superconducting electrical power apparatuses are measured at temperatures of 293 and 93 K. The quasi‐uniform electric field made by a sphere‐to‐sphere electrode with a diameter of 150 mm and a gap length of about 10 to 100 mm is used in the measurement of the breakdown voltages. When 50‐Hz ac and dc voltages are applied to the sphere‐to‐sphere gap, the breakdown voltages in nitrogen and air obey Paschen's law even at cryogenic temperatures (93 K). When a 1.4/50‐μs lightning impulse voltage is applied to the gap, the 50% breakdown voltage of nitrogen also obeys Paschen's law under UV irradiation of the cathode electrode. However, the breakdown voltage in air at 93 K is higher in the case of lightning impulse voltage applications, and the delay from impulse voltage application to breakdown occurrence is apparently longer at 93 K than at 293 K. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 28–33, 2000     

DC pre-breakdown phenomena and breakdown characteristics in thepresence of conducting particles in liquid nitrogen

DC pre-breakdown phenomena and breakdown characteristics in the presence of free conducting particles in liquid nitrogen are studied experimentally. The results show that a microdischarge occurs when a charged particle is approaching an oppositely charged electrode. An intense microdischarge can trigger a complete breakdown of the gap. The breakdown voltage of a uniform field gap with a free metallic particle of mm size might be reduced well below that of a point-to-plane gap without a particle in liquid nitrogen. Heavy contamination by a metallic powder produces a large reduction in the breakdown voltage with a horizontal spacer surface. However carbon powder is less hazardous compared to metallic powder     

Effect of forced‐flowing liquid nitrogen on electrical insulation characteristics of cryogenic insulation systems

The effects of forced‐flowing liquid nitrogen on (1) breakdown voltage of liquid nitrogen, (2) surface flashover voltage along polymeric rod immersed in liquid nitrogen, and (3) lifetime to breakdown of polymeric film by partial discharges in liquid nitrogen were examined. Breakdown voltage of forced‐flowing liquid nitrogen was a little higher than that of liquid nitrogen flowing without force. A slight increase of surface flashover voltage along acrylic rods was observed in forced‐flowing liquid nitrogen. Lifetime to breakdown of polyethylene film subjected to partial discharges is longer in forced‐flowing liquid nitrogen. Considering the results of partial discharge measurements, the phenomena may be attributed to restraint of the propagation of discharges by forced‐flowing liquid nitrogen. © 2000 Scripta Technica, Electr Eng Jpn, 132(3): 1–7, 2000     

Dielectric behavior of insulating materials under liquid nitrogen

Measurements were made to assess the AC breakdown voltages in liquid nitrogen (LN/sub 2/) with different electrode configurations such as sphere-sphere, needle-needle, hemisphere-hemisphere, plane-plane, sphere-needle, etc. Experimental results reveal that the breakdown voltage is a function of electrode geometry and gap length. This study also addresses the effect on the breakdown strength of solid insulating materials under LN/sub 2/ environment with a sphere-sphere electrode configuration. In this paper, special emphasis has been attributed to the effect on loss index of a variety of dielectrics dipped in LN/sub 2/. The measured values of breakdown strength and loss index have been compared with those obtained under atmospheric condition. The study reveals that the breakdown strength of cellulosic materials like paper or pressboard increases manifold while the loss index decreases significantly when dipped in LN/sub 2/ with variations of the order of 50% to 90%. However, for impervious noncellulosic materials like Perspex (acrylic glass) or presspahn, the increase in breakdown strength is not that pronounced and the decrease in loss index is of the order of 2 to 30%.     

Thermal bubble breakdown in liquid nitrogen under nonuniform fields

The electrical forces acting on thermally induced bubbles in nonuniform field gaps with/without fins and the correlation between bubble behavior and dielectric characteristics in liquid nitrogen are studied. The results show that the gradient force and Maxwell stress strongly affect the bubble dynamics and bubble shape in the gap. Especially the pronounced gradient force near the fin tip reduces the growth of bubble there. These electrical forces acting on the bubble result in a lesser effect of thermal bubble on the breakdown voltage if the fins are formed to guide a bubble trapped in the groove from a higher field region to a lower one     

Initial images of complete breakdown of liquid nitrogen/polymerfilm

This paper describes an experimental study,on the prebreakdown phenomena in a liquid-nitrogen/polymer-film composite subjected to dc ramp and step voltages under point-plane electrode system. In order to clarify the formative process of the complete breakdown of the composite, photographic observation was carried out using a long image guide scope and an image intensifier with high speed gate in the nanosecond region. When a negative-plane electrode was covered by a polymer film (N-cover arrangement), a luminous region on the initial image of the complete breakdown was located near the negative-plane under both the ramp and the step voltages. These results suggested that the complete breakdown of the composite in the N-cover arrangement was caused by the film breakdown due to positive charges deposition and subsequent electron emission from the film breakdown spot. When a positive-plane electrode was covered by the film (P-cover arrangement), film breakdown could not trigger the complete breakdown under the ramp voltage because the film breakdown occurred at a low applied voltage. However, intense light near the positive-plane was observed at the initial stage of the complete breakdown under the relatively high step voltage. This was because the film breakdown under the high applied voltage induced the positive streamer propagation from the film breakdown spot     

Area and volume effects on breakdown strength in liquid nitrogen

We measured dc and ac breakdown voltages in liquid nitrogen (LN ₂) with a sphere-to-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN₂ did not increase monotonously but partially decreased as the sphere diameter increased at a constant gap length. Thus, the existence of the area and the volume effects on the breakdown voltage in LN₂ was verified quantitatively; the breakdown strength decreased when increasing the {SEA}₉₀ (90% stressed electrode area) and the {SLV}₉₀ (90% stressed liquid volume). By changing the experimental conditions, it was verified that both area and volume effects, having a mutual correlation, simultaneously lead to the degradation of the breakdown strength in LN₂. In order to examine the area and the volume effects for the larger {SEA}₉₀ and {SLV}₉₀, we also measured the breakdown voltage with a coaxial cylindrical electrode. It was concluded that the dc and ac breakdown strengths in LN₂ decreased as the {SEA}₉₀ and the {SLV}₉₀ increased varying widely from 10⁰ to 10⁵ mm² and from 10 ^-1 to 10⁵ mm³, respectively     

Gaseous electrical discharge characteristics in air and nitrogen at cryogenic temperature

In designing superconducting electrical power apparatus, the knowledge of cryogenic gas and liquid insulation characteristics is essential. The authors have studied the discharge characteristics of relatively long-gap configurations in air and nitrogen at a cryogenic temperature. A sphere-to-sphere electrode with a gap length of 20 to 150 mm is used for measurements in uniform electric field. The breakdown voltage characteristics basically obey Paschen's law at cryogenic temperature for 50 Hz, AC, DC and lightning impulse voltage applications. A rod-to-plane electrode with a gap length of 20 to 330 mm is used for measurements in nonuniform electric field. In air at cryogenic temperature and nitrogen gas at both room and cryogenic temperature, streamer-like corona discharge appears near the tip of the rod electrode before the breakdown, and the breakdown voltage increases linearly with gap length. In air at room temperature thin film-like corona discharge, however, appears near the tip of the rod electrode before breakdown, and the breakdown voltage becomes higher than the other case. In order to examine the variation of corona discharge characteristics, some additional experiments are conducted. As a result, it becomes clear that thin film-like glow corona discharge appears when electronegative gas is contained and sufficient electrons are supplied from the cathode.     

Dielectric loss behavior of liquid nitrogen in uniform fields:Millimeter gap

Liquid nitrogen is a simple molecular liquid having no internal dipole moment. Thus it is not expected to show any dielectric loss below microwave frequencies. However, measurements carried out on liquid nitrogen at uniform fields in millimeter gap show that it exhibits measurable loss even at power frequency. Interestingly, this loss shows a tendency to decrease with increasing electric stress over a certain range, the so-called Garton effect. Data on dielectric loss of liquid nitrogen in uniform electric field and millimeter gap, both as a function of stress as well as pressure, are presented and the origin of this loss discussed     

Dielectric breakdown of low-density polyethylene under simulatedinverter voltages

Breakdown characteristics have been investigated for low-density polyethylene immersed in silicone oil by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter. It was found that the breakdown voltage decreases as the high-frequency component increases. However, the number of zero-crossings of the applied voltage, which is very influential in water-tree growth, does not seem to play an important role in the dielectric breakdown process     

Prebreakdown and breakdown phenomena under uniform field in liquid nitrogen and comparison with mineral oil

This paper presents a study of breakdown and prebreakdown phenomena (streamers) in liquid nitrogen and mineral oil under quasi uniform electric field, under ac and impulse voltage. Streamers preceding breakdown are studied up to 0.5 MPa by high-speed visualization and recording of emitted light. In these conditions, breakdown in LN/sub 2/ is mainly due to negative streamers, initiated at lower voltage than the corresponding positive voltage. Hydrostatic pressure has a limited effect on breakdown voltage, such as in mineral oil. It is shown that the ratio of impulse to ac breakdown voltage in LN/sub 2/ is surprisingly low (close to 1), whereas in the same conditions ac breakdown voltage in mineral oil is lower than impulse breakdown voltage. Practical consequences for the design of HV insulation in superconducting systems are discussed.     

AC breakdown properties of bamboo pulp-ice composite system at cryogenic temperature

In recent years, biomaterials have attracted attention in various fields in response to environmental problems. We pay attention to bamboo because it naturally decomposes and is characterized by its excellent elasticity and water absorption properties. We had proposed a bamboo-ice composite system as a substitute for the glass fiber reinforced plastics (GFRP) in electrical insulation systems at cryogenic temperatures. In this paper we will report the AC breakdown properties of a bamboo pulp-ice composite system at liquid nitrogen temperature. The bamboo pulp-ice composite system had AC breakdown strength comparable with GFRP and can be easily molded in various shapes. We consider that the bamboo pulp-ice composite system is promising as a substitute for the GFRP in the electrical insulating systems at cryogenic temperatures     

DC breakdown voltage characteristics of saturated liquid helium in the presence of metallic particles

This paper deals with the breakdown voltage characteristics of saturated liquid helium in the presence of a needle-shaped or spherical metallic particle to obtain insulation design data for pool-cooled, low-temperature superconducting coils and to find the predominant factor affecting the breakdown voltage of liquid helium at a given state. The results show the following. (1) The generation of bubbles at the instant of collision of a particle with an electrode is caused mainly by the kinetic energy released from the moving particle to the liquid helium, while the electrostatic energy accompanying the microdischarge between the particle and the electrode has a lesser effect on it. (2) The lowest breakdown voltage at different particle conditions appears in the case of a free needle particle. The low breakdown voltage is caused by the bubble triggered by the particle collision, the high electric field at the tip of the needle particle, and the rich initial electrons supplied by the microdischarge.     

Dynamic breakdown characteristics of liquid helium induced by aquench of superconducting wire and coil

For practical insulation design of superconducting power apparatus, it is necessary to take into account an inherent phenomenon known as quench, the transition from the superconducting to the normal state. We investigated quench-induced dynamic breakdown and prebreakdown characteristics of liquid helium (LHe). Experimental results revealed that the quench of the superconductor drastically reduced the breakdown voltage of LHe in the thermal bubble disturbance. Moreover, dynamic breakdown characteristics in a quasi-uniform field using a superconducting coil were investigated. The results revealed that the breakdown was induced in the quench-initiated region where the largest thermal energy was generated