H. Yamashita

Prebreakdown phenomena in hydrocarbon liquids

Prebreakdown current, photo current and conduction current are measured in n-hexane, cyclo-hexane, 1-hexene and benzene. Similar phenomena are observed in the saturated hydrocarbon liquids, n-hexane and cyclohexane, whereas great difference is observed in the unsaturated hydrocarbons, 1-hexene and benzene. In the case of bush-type light emission, the mean velocity is about 100 m/s, and the waveforms of the photo and the prebreakdown currents are pulsive. But in the cases of tree-type and radial-type light emission, the mean velocities of them both are a few km/s, and a continuous light emission with a sudden increase of prebreakdown current are observed just before breakdown.

Observation of partial discharge in hexane under high magnification

Partial discharges are observed in hexane by shadow photography under the application of DC voltages. A nonuniform field geometry is used and the growth of cavities associated with partial discharges at a point cathode was photographed at 200* magnification. The use of an image-preserving optical delay allows a record of the conditions which exist in the liquid prior to the initiation of the low-density streamer to be obtained. The cavity growth at a point cathode is anisotropic, which suggests that electrostatic forces are of primary importance in driving its expansion. The onset of instabilities in the cavity wall is suggested. The initial partial discharge current pulse precedes or is concurrent with the growth of the cavity. An upper bound of 60 ns between first current pulse and the growth of the cavity is obtained. >

Pre-Breakdown Current and Light Emission in Transformer Oil

Prebreakdown current, photo current, and light emission are measured simultaneously in transformer oil under nonuniform fields. For a negative needle point, the light emission is divided into two patterns. They change from a bush-type to a tree-type as the applied voltage or the oil pressure increase. In the case of the tree-type, they are cau-sed by the streamer only at the initial stage. In the case of the bush-type, the light is emitted intermittently within a few hundred us. The initial one is caused by a tiny streamer but those that follow seem to occur within bubbles created by the initial one. For a positive needle point, positive streamers emanate from the point radially. When they reach the cathode, a few bright spots are formed on the cathode from which negative streamers start.

Streamer formation in perfluoropolyether under impulse conditions

A family of perfluorinated fluids that recently has become available, called perfluoropolyethers, contain oxygen in addition to fluorine and carbon and are potentially interesting as electrical insulating fluids. A study of their behavior under electrical pulse conditions is reported. Under the prevailing nonuniform field conditions, the breakdown voltage was lower when the needle was a cathode than when it was an anode. The streamer structures and their inception voltages are studied in some detail. The effect of pressure on the streamer structure is evaluated. >

Partial discharge measurements in dielectric liquids under impulse voltage

Current pulses caused by partial discharges (PDs) are measured using light-emitting diodes (LEDs) and fiber optics under impulse voltage conditions. This system has the advantage of having good isolation from the HV power supply and no effect from electromagnetic noise as compared with ordinary systems. It can detect a PD pulse having a half width of 3 ns, and has the capability of measuring PD pulse charges in the range of 1 to 2000 pC, when the optimum DC bias is applied. PD pulses and light emission or streamer formation in dielectric liquids are measured simultaneously by using an image converter camera. >

Observations of partial discharges in hexane under high magnification

Partial discharges are observed in hexane by shadow photography under the application of DC voltages. A nonuniform field geometry is used and the growth of cavities associated with partial discharges at a point cathode was photographed at 200* magnification. The use of an image-preserving optical delay allows a record of the conditions which exist in the liquid prior to the initiation of the low-density streamer to be obtained. The cavity growth at a point cathode is anisotropic, which suggests that electrostatic forces are of primary importance in driving its expansion. The onset of instabilities in the cavity wall is suggested. The initial partial discharge current pulse precedes or is concurrent with the growth of the cavity. An upper bound of 60 ns between first current pulse and the growth of the cavity is obtained.<<ETX>>

Pressure effects on partial discharges in hexane under DC voltage

The pressure dependence of partial discharges (PD) has been experimentally investigated at a needle electrode in hexane from subatmospheric pressure (near hexane vapor pressure) to several atmospheres. Each PD produces a phase transition in the liquid near the needle, which is photographed in synchronism with a characteristic pattern of current pulses. An image-preserving optical delay allows photography to commence just before or at inception of the discharge. Individual current pulses comprising a characteristic pattern are resolved. The cathode event consists of a short pressure-insensitive inception phase, a pressure-sensitive growth at a decreasing rate, and finally a detachment and dissipation, sometimes with noticeable contraction before detachment; increased pressure reduces the growth rate and lifetime. The accompanying characteristic current pulse pattern always ceases during the growth of the PD. For the anode event, less extensive data similarly show slowing of growth with increased pressure and a (different) characteristic current pulse pattern.<<ETX>>

The characteristics of streamers in perfluoro liquids

This paper describes the effects of molecular structure and physicochemical characteristics on the impulse pre-breakdown phenomena in perfluoro liquids under non-uniform field. The test liquids were two types of PFPE (perfluoro polyether) with different molecular weight, D40 and XAD, and one type of PFC (perfluoro carbon), FC40. An impulse voltage was applied to a needle electrode, and the streamer generated at the needle tip was photographed using a shadowgraph optical system combined with an image converter camera (ICC). The characteristics of streamers in D40 were different compared to those in XAD and FC40. These experimental results show that the effect of viscosity plays an important role in perfluoro liquids rather than molecular structure.

The effect of high pressure on prebreakdown phenomena in n-hexane

The effect of pressure on initiation of prebreakdown streamers in n-hexane in a nonuniform field is investigated. Using a high-magnification (100*) high-resolution (1 mu m) optical system and a high-speed camera, the initial growth of low-density streamers propagating from a needle cathode is examined at pressures ranging from 0.1 to 1.1 MPa. The initial streamer is a single filament which grows to approximately 4 mu m in diameter and 8 mu m in length before the appearance of a dense bushy structure at its tip. Oscillatory behavior of the initial streamer is observed in which the diameter of the initial streamer expands and contracts as the streamer grows. The pressure dependence of the streamer initiation voltage is shown.<<ETX>>

The inception and structure of prebreakdown streamers in perfluorinated polyethers

Electrical measurements of perfluorinated polyethers were performed to determine the suitability of this class of material as a liquid dielectric. Measures of the breakdown strength and streamer inception voltage are obtained under electrical impulse stress. The breakdown strength is nearly independent of polarity. Under high magnification photography the cathode-originated streamers are seen to have a subsonic mode of growth. The transition to fast growth occurs on time and spatial scales shorter than those found in liquid hydrocarbons. The gas-phase electrical properties and the high heat of vaporization of this material are considered as mechanisms for this behavior.<<ETX>>