Prebreakdown Current Pulses Research Articles

Secondary Ionization Coefficient of Dielectric Electrode -I. Observation and Stabilization of Discharge Paths for Measurement of .GAMMA. of MgO Electrode-

Experiments for observations and stabilization of discharge paths in several electrode systems are carried out aiming at precise measurement of the secondary ionization coefficient γ of MgO film electrode. The discharge chamber is filled with Ar gas. The waveforms of the applied voltage between the electrodes and the discharge current are measured with visual observation of the discharge light. Two MgO coated electrodes are placed so that they are facing each other. For these MgO electrodes, the discharge paths take a detour, not the shortest distance. Smaller prebreakdown current pulses are observed before the breakdown. After breakdown, discontinuous discharge current is observed. Therefore, it is prepared a glass tube surrounding the discharge area. As the result, the discharge paths take a straight perpendicular for the electrode surface, and the discharge is stabilized.

Conduction and breakdown mechanismsin transformer oil

With a fast coaxial test setup using high speed electrical and optical diagnostics, prebreakdown current pulses and shadowgraphy images are measured for direct current (dc) breakdown in Univolt 61 transformer oil. Also, dc currents across the gap are measured using a high sensitivity electrometer. The conduction and breakdown mechanisms in transformer oil as function of applied hydrostatic pressures are quantified. Together, this information provides data on the development of current flow in the system. We have identified three stages in the conduction process prior to breakdown for highly nonuniform fields. Stage 1 is characterized by a resistive current at low fields. Increasing the applied electric field lowers the effective barrier at the metal/dielectric interface allowing a "tunneling" mechanism to begin, leading to the rapid rise in the injection current observed in stage 2. In stage 3, at high fields, the current reaches space charge saturation with an apparent mobility of 3/spl middot/10/sup -3/ cm/sup 2//V/spl middot/s prior to breakdown. The processes of final breakdown show a distinct polarity dependence. A strong pressure dependence of the breakdown voltage is recorded for negative needle/plane breakdown; a 50% reduction in breakdown voltage is observed when the hydrostatic pressure is lowered from atmospheric pressure to hundreds of mtorr. Positive needle discharges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. Weak pressure dependence indicates the breakdown mechanism does not have a strong gaseous component. We will discuss possible links between conduction current and dc breakdown.

Prebreakdown Light Emission and Current Pulses in Liquid Nitrogen

Prebreakdown light emission phenomena and current pulses in liquid nitrogen under positive and negative impulse voltages of both polarities were simultaneously observed. Two structures of light emission, bush-like and filamentary, were observed in both polarities. The light pulses could be classified into a few patterns. A close relation between these patterns and the structures of the light emission was found. From the streak photographs using the image converter camera, it was found that light emission appears intermittently and propagates in a stepwise manner. The prebreakdown current pulses occur prior to the corresponding light pulses, and the time difference between them is 40 ns for both polarities. From the results of these observations, the mechanisms that cause the difference of the light emission structures is considered. For the negative polarity, the structure is determined by the pulse activity after the initiation of the phenomena. In contrast, the structure is already determined at the instance of the initiation for the positive polarity.

Low-noise wide-band amplification system for acquiring prebreakdown current pulses in liquid dielectrics

A low-noise wideband amplification system consisting of a preamplifier with input protection circuit and an amplifier were developed for measuring the prebreakdown currents in liquid dielectrics. The system design considerations and experimental results are presented. The preamplifier was voltage-sensitive with a voltage gain of 154 V/V, and had a bandwidth of 56 MHz and an equivalent noise voltage spectral density of 0.93 nV/Hz/sup 1/2/. The amplifier had a voltage gain of about 12 and a bandwidth of 58 MHz and was capable of driving mod 2 mod V across two paralleled 50 Omega loads. The overall system had a gain of 92400 V/A when loaded with 50 Omega , and it had a minimum detectable current of 1.9 mu A for an overall bandwidth of about 29 MHz. >

Investigations of the occurrence of erratic low breakdown voltages in compressed air

Previous investigations of the influence of electrode surface irregularities on the breakdown voltage in compressed gases were continued in order to get a better understanding of the reasons which cause the erratic low breakdown voltages in pressurized gas insulations. This paper contains the results of breakdown voltage investigations of smooth or roughened electrodes in air with pressures up to 30 bar. Measurements were carried out with + d.c. as well as 50 Hz a.c. using uniform and coaxial fields with gap lengths from 2.0 to 5.0 mm. The a.c. breakdown voltage measurements displayed that the lowest spark voltages of polished or sandblasted electrodes are in the same order of magnitude. With- d.c. an irreversible prebreakdown current behaviour of sand-blasted electrodes was found in air at pressures higher than about 5 bar, respectively at field gradients greater than about 80 kV/cm. Scanning electron microscope pictures of a single electrode surface protrusion showed the removal of the top of the irregularity by irreversible prebreakdown current pulses at -d.c.. It is assumed that field electron emission accompanied by space charges and ionization causes a heating as well as a removal of sharp electrode irregularities and without doubt this heating process facilitates the occurence of erratic low breakdown voltages.

Conduction current pulses in transformer oil

A systematic study of the pre-breakdown conduction current pulses in transformer oil using static electric stresses showed that these pulses are very sensitive to test conditions. The pulses were found to depend on gap length, applied stress, temperature, electrode material and hydrostatic pressure. The distribution of the pulse heights was exponential, indicating the presence of a multiplication process in the liquid. It is suggested that the pulses are due to the ionization of microscopic gas bubbles by electrons emitted at the cathode.

Pre-breakdown conduction current pulses in insulating oils under non-uniform field conditions

The pre-breakdown conduction current pulses in degassed transformer oil were studied using needle-sphere electrodes. The current pulses were recorded on a magnetic tape and then analysed using a pulse height analyser. The results showed that these current pulses were sensitive to the applied hydrostatic pressure as well as to the liquid temperature. With the needle positive the number of larger pulses reached a minimum at a pressure between 250 and 500 torr, while with the needle negative the pulse rate was greatest in this range of pressure. Increasing the temperature of the liquid or the applied voltage increased the pulse activity. When a needle-needle electrode system was used, pulse activity also showed a pronounced response to the applied hydrostatic pressure. It is suggested that a gaseous phase is involved in the breakdown process of liquids, which is most likely to be due to dissociation of the liquid molecules by energetic electrons.

Pre-breakdown current pulses in high vacuum

Pre-breakdown current pulses in high vacuum

Measurement of autocorrelation function and power spectral density of pre-breakdown conduction current pulses in liquid dielectrics

A technique for the study of the pre-breakdown conduction current pulses in insulating liquids has been developed. It consisted of recording these pulses during the experiments on a magnetic tape and subsequently subjecting them to a number of statistical analyses. The analyses included the determination of the autocorrelation function and computation of the power spectrum. Typical examples showing the effects of applied electric stress, temperature and additives to the liquid on the autocorrelation function and power spectrum of the current pulses are given. The autocorrelation function showed that such fluctuations of the current were random and did not contain hidden periodic components. From the corresponding power spectra it was found that the fluctuations lay in the low audio frequency range.

Effect of Temperature on the Pre-breakdown Pulses of Conduction Current in Insulating Oils

IT has been previously1,2 reported that the pre-breakdown conduction current pulses in liquid dielectrics are very sensitive to test conditions and the composition of the test liquid. It has been found that a convenient way of investigating these pulses is to record them as a double trace on a high-speed magnetic tape (speed 75 in./sec). By using two play-back heads which can be displaced relative to each other it is possible to obtain the autocorrelation function of the random pulses. To reduce distortion and interference, frequency modulation before recording and demodulation during play-back were used.

Discussion on “Effect of discharge products upon corona discharge and spark breakdown voltage” and “Analysis of pre-breakdown current pulses in gas discharge gaps”

Discussion on “Effect of discharge products upon corona discharge and spark breakdown voltage” and “Analysis of pre-breakdown current pulses in gas discharge gaps”

Pre-breakdown Conduction Current Pulses in Liquid Dielectrics

THE erratic nature of the conduction current which precedes the electrical breakdown of a liquid dielectric has been observed by several investigators1,2.

Analysis of pre-breakdown current pulses in gas discharge gaps

The paper presents the theory relating the shape of the observed current pulses of individual electron avalanches to the physical processes involved, and indicates how measurements from the observed pulses can yield quantitative data on these processes. The theory is developed separately for gases which do not form negative ions and for those which do. The initial part of the paper is similar to previous work by K. J. Schmidt, with the differences that, in the present work, the case of avalanches initiated part-way across the gap is discussed and that it is not assumed here that the positive and negative ions have equal drift velocities. The final Section of the paper considers the effect of inductance in the circuit containing the discharge gap.

The Rise-time of Pre-breakdown Current Pulses in Nitrogen and Air

Observations on the growth of the current pulses of electron avalanches in nitrogen, dry air and humid air, for pd-values in the range 100 to 2300 mmHg cm, are reported. A possible explanation for the delayed rise-time of the pulses in nitrogen and dry air is suggested.