Asao Ohashi

Conduction currents in silicone liquid films

Abstract The current response to the application of a stepped field on silicone liquid films has been measured over a wide range of time duration (10 −5 – 10 4 s), field strength (5 – 500 kV/cm), temperature (20 – 160 °C) and sample thickness (4 – 25 μm). Both the transient and the quasi-steady-state responses are considered. The results indicate that the activation energy for the transient current increases rapidly with increasing time corresponding to the rapid decrease of the current, and the Activation energy for the quasi-steady-state current does not vary with time. It is also found that the transient current depends on the sample thickness (i.e. bulk limited) and the quasi-steady-state current is electrode limited. These time-dependent behaviors of the conduction are discussed in terms of the thermally activated hopping of injected excess electrons from the metal-liquid contact.

Thermal breakdown of silicone liquids under pulsed high frequency fields

The effects of the pulse duration of applied field and electrode geometry on the thermal-breakdown phenomenon have been studied by applying pulsed high-frequency field (1-10 MHz) with variable duration (10-1000, μs) to the silicone liquids immersed in the rod-plane electrodes (approximate uniform field) and the needle-plane electrodes (extreme nonuniform field), considering the temperature dependence of the electrical conductivity of silicone. It has been found that the thermal breakdown due to dielectric heating occurs when the field is applied for less than a millisecond. Thermal breakdown explains the experimental results in which the breakdown strength decreases in the rod-plane electrodes with increasing pulse duration and electrode separation, but is little influenced in the needle-plane electrodes by these same variables.

High field conduction in dielectric liquid films in time range of 10−5 − 10−3 s

Abstract In order to develop more enough the investigation of electrical conduction in dielectric liquid films, partly reported previously by the authors, the transient conductivity at higher electric field has been studied. The measurement has been carried out under the conditions as follows; time duration (10 −5 − 10 −3 s), field strength (100 kV/cm - 1 MV/cm), temperature (room temperature - 150°C), a coefficient of kinematic viscosity (1 – 100 cSt) and sample thickness (3 – 5 μm). The results indicate that 1) the transient current i has been observed to decay according to i ∝ t -n where the exponent n is independent of the field strength, 2) the transient conductivity σ varies with field strength E as σ ∝ exp (bE) where b is a constant, and 3) the hopping distance of excess electrons obtained from conductivity-field curves depends on time and temperature, and these dependences correspond to time and temperature dependences of the activation energy respectively. The transient processes of excess electrons in dielectric liquids from these results are discussed.

New Method of Decreasing the Oil Surface Potential in a Tank with an Electrostatic Screener

The discharging effectiveness of 100 and 200 mesh stainless screens fixed on top of a diffuser at the oil inlet to a 40 and 500 liter test tank is evaluated. To study the discharging mechanism in detail, the potential distribution along the axis of the test tanks, the ground current at each section of the oil - flow system, and the potential on the oil surface of the test tanks were measured. It was found that the peek potential along the axis of the tanks and the surface potential of the oil in the tanks was considerably reduced when either the 100 or 200 mesh screens were fitted. The screens were found to have some discharging effectiveness and lowered the potential not only on the surface but in the oil by reducing the oil velocity and changing the distribution of the charge density.