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

Preliminary observations on the non-stationary aspects of the conduction process in n-hexane indicate that two distinct types of pulse activity occur. Lowamplitude pulses (of about 5 fC) are always present, but at stresses above 400 kV cm−1 they are accompanied by intermittent bursts of much larger pulses (of about 1 pC) which often culminate in breakdown. By means of an audio-visual technique a relationship between the low-level pulse activity and the particle motion in the electrode gap has been established. Typical non-stationarities in a sequence of pulses are illustrated by a plot of pulse rate and mean charge against time.

http://iopscience.iop.org/article/10.1088/0022-3727/4/11/104/pdf

Non-stationary pulse conduction in a dielectric liquid

It was found that SF6 dissolved in transformer oil resulted in an increase in the conductivity of the liquid. Special attention was given to the stress conditioning period and the results showed that the conduction currents increased with time. After such stress conditioning, deposits were found on the anode surface which chemical analyses showed were due to sulphur and/or one of its compounds. Luminescence was observed near the cathode when an oil sample containing SF6 was stress conditioned for long periods. Stress conditioning was also found to cause an increase in the number and magnitude of the conduction current pulses. It is suggested that these results can be explained in terms of a model which invokes both anode and cathode space charges with the possibility of a feedback of positive ions from the vicinity of the anode to the cathode.

https://iopscience.iop.org/article/10.1088/0508-3443/16/11/303/pdf

Effect of dissolved SF6 on high field conductivity in mineral oils

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.

https://iopscience.iop.org/article/10.1088/0022-3727/4/3/314/pdf

Conduction current pulses in transformer oil

The pressure dependence of the effects of the dissolved high‐pressure (1–12 atm) SF6 gas on the electrical conduction and prebreakdown phenomena of an insulating oil has been studied. The pulsed conduction currents in the oil dissolved SF6 gas under a high dc electric field of more than 100 kV/cm depend on both high equilibrium pressure (solubility) and high applied hydrostatic pressure, i.e., the pulsed currents are suppressed by both pressures. The ac partial discharge‐inception voltage in the oil containing dissolved SF6 gas of an equilibrium pressure of 10 atm (466 vol %) under an applied hydrostatic pressure of 12 atm has a value of 1.4 times larger than that for a degassed oil at 5×10−5 Torr. The results obtained suggest that electron affinity of SF6 gas enhances the dielectric strength of the oil saturated with SF6 gas. The degree of enhancement is apparently related to the amount of SF6 gas absorbed by the oil, which in turn is a function of the applied hydrostatic pressure.

Effects of high‐pressure SF6 gas dissolved in an insulating oil on dc conduction currents and ac partial discharges

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

A. Nosseir

Papers

Pre-breakdown Conduction Current Pulses in Liquid Dielectrics