Electrical breakdown of small gaps in vacuum

The comparison of the characteristics of an electric arc breakdown in vacuum between two silver or silver-nickel alloy convex electrodes, enabled us to show experimentally that the process initiating discharge in vacuum is the electronic emission at the micrometer scale. Increased content of nickel in the alloy improves the insulation and achieves high resistance to erosion for low power arcs. Heating in vacuum demonstrates the multiple layer structure of studied alloys. Secondary electron microscopy and energy dispersing spectroscopy of the electrode surfaces after breakdown, as well as the measurements of fall and delay times, enabled us to evidence the origin of the electric arc     

Investigation of dielectric breakdown characteristics of double‐break vacuum interrupter and dielectric breakdown probability distribution in vacuum interrupter

To investigate the reliability of equipment of vacuum insulation, a study was carried out to clarify breakdown probability distributions in a vacuum gap. Further, a double‐break vacuum circuit breaker was investigated for breakdown probability distribution. The test results show that the breakdown probability distribution of the vacuum gap can be represented by a Weibull distribution using a location parameter, which shows the voltage that permits a zero breakdown probability. The location parameter obtained from Weibull plot depends on electrode area. The shape parameter obtained from Weibull plot of vacuum gap was 10 to 14, and was constant irrespective of the nonuniform field factor. The breakdown probability distribution after no‐load switching can be represented by Weibull distribution using a location parameter. The shape parameter after no‐load switching was 6 to 8.5, and was constant irrespective of the gap length. This indicates that the scatter of the breakdown voltage was increased by no‐load switching. If the vacuum circuit breaker uses a double break, the breakdown probability at low voltage becomes lower than the single‐break probability. Although the potential distribution is a concern in the double‐break vacuum circuit breaker, its insulation reliability is better than that of the single‐break vacuum interrupter even if the bias of the vacuum interrupter's sharing voltage is taken into account. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 175(2): 13–20, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21060     

Visualization of late breakdown in vacuum by ICCD camera

Experimental observations of late breakdowns in a vacuum circuit breaker were made using an intensified charge coupled device camera (ICCD). These late restrikes can cause arc reignition and interruption failure. This type of failure is rare and can be initiated by a number of mechanisms. This paper illustrates the method used to observe this phenomenon. Some key image examples of the phenomenon occurring during breakdown initiation are shown.     

Impulse breakdown voltages of triggered multistage vacuum gap

Several sealed-off triggered vacuum gaps are connected in series to improve hold-off voltage. The characteristics of impulse breakdown voltage of these series-connected gaps are investigated experimentally. The sum hold-off voltage of series-connected gaps decreases to a unit hold-off voltage when the maximum value of voltage division ratio across the gaps increases to unity. Self-breakdown probability of the series-connected gaps is always higher than that of a single gap under the same conditions. Hence, stage efficiency of the multistage gap decreases with increasing number of stages. Its value is 90 percent with 2-stage gap and 75 percent with 5-stage gap, respectively, under the same voltage division ratio and the same gap length (2.0 mm) in each stage. Triggered breakdown voltage of 2- or 3- stage gap is several hundred volts when all gaps are triggered simultaneously at the peak of the main impulse wave and a working voltage range is nearly 100 percent in this case. The working voltage range decreases with number of stages. Its value is 45 percent with 3-stage gap and 15 percent with 5-stage gap, respectively, when one triggered gap is fired for switching.     

Parameters influencing breakdown characteristics of vacuum gapsduring spark conditioning

Experiments have been made to identify the key parameters in the increase of the breakdown strength of a vacuum gap through repetitive breakdowns. The current-voltage waveforms were observed, the residual stress in electrodes was measured, and the chemical composition of the electrode surfaces and the energy of the valence-band electrons were analyzed. These experiments revealed that prebreakdown currents (field emission currents) were sometimes observed and sometimes not. The residual stress before and after 500 breakdowns changed significantly. X-ray photoelectron spectroscopy analysis before and after 500 breakdowns showed that the spectral lines of oxygen and carbon due to contaminants on the electrode surface were completely removed and the copper ones appeared clearly after 500 breakdowns. The valence-band electron energy spectrum of the anode after 500 breakdowns shifted towards the lower binding energy to almost coincide with the theoretical spectrum, while the energy spectrum for the cathode shifted towards sometimes higher and sometimes lower binding energy     

Relation between breakdown voltage and prebreakdown current in vacuum gap

Two different surface treatments (mechanical polishing, thin film deposition) were performed on cathode surfaces, and the field emission currents from the cathodes were measured with a microchannel plate. In order to discuss the relationship between the breakdown voltage and prebreakdown current in the vacuum gap, the breakdown voltage was measured after field emission measurement. The V–I characteristics of the field emission and breakdown voltage were influenced by surface treatment, and the breakdown voltages of mechanical polished cathodes were lower than those of the thin film deposited cathodes. It was found that the probability of breakdown increased when the field emission current reached 10^–11 A. Atomic force microscope (AFM) measurements showed numerous protrusions on the cathode surface in the case of thin film deposition treatment, but we estimated by the finite element method that these protrusions make the field enhancement effect low. It was inferred that the breakdown voltage in vacuum gaps could be increased by the thin film deposition method. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 11–18, 2000     

On the variety of triggering mechanisms of high-voltage breakdown in vacuum

The paper is devoted to study of the mechanisms responsible for vacuum breakdown as a whole and the total voltage effect in particular. Experiments at DC and pulsed voltages were carried out. It has been shown that there is no manifestation of the total voltage effect at DC voltages up to 20 kV. The strong dependence of hold-off on anode temperature was recognized at DC voltages while pulsed hold-off turned out to be almost the same with heating the electrodes. This gives a basis to consider gas desorption as an insufficient factor in the initiation of pulsed breakdown. An attempt to enhance hold-off with electrostatic removing of loosely bound particles with assistance of electron flow from a thermionic cathode was undertaken in the work. The approach turned out to be ineffective.     

Estimation of dc breakdown mechanisms in air gaps containingfloating metallic particles

The characteristics of breakdown voltages against the position of floating metallic particles in atmospheric air are investigated experimentally. The breakdown mechanism is estimated on the basis of electric field calculation and visual observation of discharges at different voltages and a set of flow charts for calculation of breakdown voltage is proposed. The studied gaps are needle-to-plane and sphere-to-sphere electrodes of 120 mm in length forming Poissonian and Laplacian fields respectively and the shape of tested floating objects is a needle or sphere. The theoretical breakdown voltages obtained by the flow charts agree reasonably with the experimental ones     

Effect of electrode surface roughness on breakdown conditioning under non-uniform electric field in vacuum

Breakdown (BD) characteristics in vacuum are strongly dependent on the electrode surface conditions, such as surface roughness. However, there is little known concerning the details of the relationship between the surface roughness and BD conditioning effect. In practical application, it is important to clarify how the surface roughness affects the breakdown conditioning characteristics, especially for the non-uniform field configuration. This paper discusses the effect of surface roughness on breakdown conditioning characteristics under non-uniform electric field in vacuum under applications of negative standard lightning impulse voltage. For this purpose, we examined the BD conditioning of a rod-to-plane electrode made of SUS304 and Cu-Cr. The surface roughness of R_a is controlled from 0.3 to 2.5 mum. Experimental results revealed that the enhancement of surface roughness of electrodes increases the number of BD to complete the conditioning effect. We explained the results from the observed results of the electrode surface. Consequently, we could clarify the effect of surface roughness on the conditioning effect under non-uniform electric field in vacuum quantitatively.     

On priorities of cathode and anode contaminations in triggering the short-pulsed voltage breakdown in vacuum

Modern theoretical notations on electrical breakdown in vacuum consider cathode triggering mechanisms to be most responsible on short-pulsed (<1 /spl mu/s) breakdowns while anode mechanisms to be responsible in a part on DC and long-pulsed breakdowns. Following those notations, we tried to reveal conditions at which either mechanism steps aside to another one. The study involved several experimental techniques including the anode-probe surface scanning, pulsed electron-beam surface melting in vacuum for surface cleaning, and intentional dust particle contamination of electrode surfaces. Breakdown tests were performed using a pulser capable of producing 220 kV quasi-square pulses that were adjustable to /spl sim/30 to 80 ns pulse length. Our experiments showed that cathode emission sites are responsible for breakdowns at relatively low hold-off fields. At higher electric fields of up to 1 MV/cm, the anode share in the mechanism of triggering breakdown becomes probably more significant than the cathode mechanism.     

Recovery of breakdown strength of a vacuum interrupter after extinction of high currents

Relevant phenomena during the period of the recovery of the dielectric strength of vacuum interrupter (VI) are reviewed. Metal vapor, residual charge and the effects of molten contact surfaces reduce the breakdown strength after the interruption of high currents. Metal droplets seem to play only a secondary role during the recovery phase. Instability of liquid protrusions is suspected to initiate breakdown if the electric field strength exceeds 10 kV/mm at the molten contact surfaces (undisturbed field). Key parameters are surface temperature and vapor density. Breakdown during recovery of the dielectric strength depends not only on the value of the voltage applied but also on the duration of the high voltage stress. A spontaneous triggering event in the presence of metal vapor appears to initiate breakdown at conditions below the Paschen breakdown limit. This type of breakdown events limits the interruption performance of VI's.     

Partial vacuum breakdown characteristics of helium at 20 kHz for inhomogeneous field gap

In general, power devices and systems operating in vacuum or space environment are more susceptible to partial discharges, corona, or volume discharges due to the partial vacuum conditions. Additionally, high frequency operation of a power system is a contributing factor in lowering the breakdown voltage of insulation. In this paper we present our studies on the breakdown characteristics of helium operating in DC and 20 kHz AC field in partial vacuum, for a point-to-point and point-to-plane electrode configurations. Breakdown voltage as a function of pressure in the range of 27 to 400 Pa (0.2 to 3 torr) for both the DC and 20 kHz AC cases is presented. Voltage and current waveforms along with the optical emission waveform of the breakdown events are also presented. A variable DC power supply for DC and an in-house built variable DC-offset-AC power supply for the high frequency breakdown experiments are used. A high voltage probe and a Pearson current sensor are used for the voltage and current detection, and a photo-multiplier-tube with a digital pico-ammeter and a video camera are used for the optical signal detection of this set-up. The breakdown voltage as a function of pressure for both the AC and DC experiments, along with voltage breakdown waveforms for both electrodes are presented.     

Estimation of thermal breakdown voltage of HVDC cables - A theoretical framework

The insulation in a DC cable is subjected to both thermal and electric stress at the same time. While the electric stress is generic to the cable, the temperature rise in the insulation is, by and large, due to the ohmic losses in the conductor. The consequence of this synergic effect is to reduce the maximum operating voltage and causes a premature failure of the cable. The authors examine this subject in some detail and propose a comprehensive theoretical formulation relating the maximum thermal voltage (MTV) to the physical and geometrical parameters of the insulation. The heat flow patterns and boundary conditions considered by the authors here and those found in earlier literature are provided. The MTV of a DC cable is shown to be a function of the load current apart from the resistance of the insulation. The results obtained using the expressions, developed by the authors, are compared with relevant results published in the literature and found to be in close conformity     

Relationship between electrode surface roughness and impulse breakdown voltage in vacuum gap of Cu and Cu-Cr electrodes

The relationship between the impulse breakdown voltage of vacuum gaps and electrode surface roughness was investigated for the purpose of controlling the surface roughness on HV conductors. The roughness of mechanically polished Cu and Cu-Cr electrodes was measured with a roughness meter, and the relationship between the breakdown voltage and surface roughness was obtained for plate-to-plate gaps. The discharge-conditioning effect increased with reduction in the surface roughness. The breakdown voltage depended not only on the roughness of the cathode but also on the anode surfaces. Reducing the surface roughness was found not to be an effective way to increase the breakdown voltage for non-uniform field gaps after discharge conditioning.     

Estimation of breakdown voltages of surface flashovers initiated from triple junction in SF6 gas

In this paper, a method is proposed to estimate the minimum breakdown voltages of surface flashovers initiated from the triple junction at which a solid insulator, SF₆ gas, and a metal electrode come in contact. Using four kinds of test samples, the flashover voltages V_f are measured. The voltages V_f become lower than the theoretical voltages V_s of streamer inception in the case of the test sample with the large electrode area at the triple junction. To understand the phenomena quantitatively, we estimated the real partial discharge voltages V_c from the voltages V_f, and investigated the relationship between the voltages V_c and the theoretical values V_s. The values of V_s/V_c, which characterize the properties of the partial discharges, can be shown by the product λS_90%E_av P of the following parameters: (1) coefficient λ representing the surface roughness of electrode at the triple junction; (2) effective area S_90% of the electrode; (3) average field E_av at the area S_90%; and (4) gas pressure p. The minimum surface flashover voltages in the insulators for coaxial electrode systems can be estimated precisely, considering the properties of V_s/V_c.     

Discussion of electrode conditioning mechanism based on pre‐breakdown current under nonuniform electric field in vacuum

Electrode conditioning is a very important technique for improvement of the insulation performance of vacuum circuit breakers (VCBs). This paper discusses the spark conditioning mechanism under a nonuniform electric field, focusing on the pre‐breakdown current. We quantitatively evaluated the spark conditioning effect by analyzing the pre‐breakdown current based on the Fowler‐Nordheim equation. The field enhancement factor β was found to decrease with increasing breakdown voltage at the beginning of the conditioning process, and finally became saturated with the saturation of the breakdown voltage. In addition, in the case of a nonuniform field, we found that β on a high‐voltage rod electrode after conditioning varied according to the electric field strength on the rod electrode. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 175(2): 21‐28, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21059     

真空灭弧室真空度测量方法的探讨

通过磁控放电原理的阐述,指出在外激励电源、真空灭弧室的几何尺寸、所用材料一定时,真空灭弧室内的真空度与电离的电荷量有非常准确的对应关系。介绍了采用电离电荷量测量真空灭弧室真空度的方法及在南京地区一年来的现场实测的应用情况。     

Methods for estimation of the vacuum status in vacuum circuitbreakers

In this paper several methods are described for the estimation of the pressure inside vacuum interrupters which have been in operation for many years. The methods are applicable without demounting the interrupters and use only electrical measuring techniques. According to these methods the vacuum circuit breakers are subjected to synthetic circuits, switch-off limited currents, either 10 A dc or 300 A ac. With the 10 A dc method the height of arc voltage spikes are measured and represented in a histogram. The shape of the histogram is discriminative for pressures below or above 10^-2 Pa. Switching tests with currents of 200 to 400 A ac in inductive and capacitive circuits also showed different behavior for internal pressures below or above 10^-1 Pa. A three phase 200 A test device with a 10 kV recovery voltage, fed from a standard 380 V supply, showed similar results     

Gate oxide breakdown on nMOSFET cutoff frequency and breakdown resistance

Gate oxide breakdown effect on CMOS RF devices has been examined. The breakdown spot resistance and total gate capacitance of nMOS transistors decrease with stress. The analytical equation of cutoff frequency including the gate oxide breakdown effect is derived. The impact of oxide breakdown on the performance of an LC oscillator is evaluated. The oscillation frequency of the LC oscillator increases with oxide breakdown.