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The liquid motion normally observed in electrically stressed dielectric liquids is mainly caused by two mechanisms: (a) the Coulombic force due to the interaction of the space charge with the field, and (b) the electromechanical force created by the field resulting from the spatial variation of the dielectric constant due to the variation of temperature from domain to domain and from time to time in the liquid. The direction of such a liquid motion depends on which of these two mechanisms is dominant. The schlieren images sometimes observed in liquids under applied fields show mainly the change of the already existing temperature gradients in the liquids, and this change is due to the liquid motion caused by either mechanisms (a) or (b) or both. It is concluded that the schlieren images are directly governed by the temperature distribution in the liquid and have no direct bearing on the formation and distribution of space charges.

On the principle that the spatial distribution of the refractive index produced by a small applied temperature difference is changed due to an electromechanical force created by the field resulting from the spatial variation of the dielectric constant, the electric field distributions in a dielectric system can be determined using the schlieren method. Some experimental results for benzene and n-hexane between two spherical electrodes obtained with this method show the essential features predicted from the space charge effects. This method can be used to study electric field and space charge distributions under both steady and transient conditions.

The structure of ethylene-vinyl-acetate (EVA) copolymers depends strongly on the comp9sition of E (ethylene) and VA (vinyl-acetate). With various compositions in VA:E ratio in weight, the results show that both the density and the melt index increase, while the molecular weight, the melt viscosity and the degree of crystallinity decrease with increasing VA:E ratio in EVA copolymers. This reflects that both the ethylene sequence length and the activation energy of fluidity decrease with increasing VA content. The change of the structure of EVA copolymers is directly correlated with the change of the mechanical and dielectric properties of this material. The peak values of the dielectric constant and the dielectric loss in the dielectric relaxation temperature characteristics increase, while the activation energy of polarization and the dielectric breakdown strength decrease with increasing VA:E ratio. Other mechanical properties, such as the tensile strength and the environment stress cracking, also correspondingly decrease as the molecular weight and the ethylene sequence length decrease due to the increase of VA content in EVA copolymers.

The shadowgraphic image reflects only those regions of disturbance occurring in a dielectric liquid, which differ in refractive index from their surroundings. The time required for the onset of a prebreakdown disturbance after the application of a step voltage, and the rate of growth of the disturbance depend strongly on the viscosity (or the temperature) of the liquid. The refractive index is lower inside the disturbance region than outside. The variation of the external current is consistent with the temporal change of the disturbance. It is proposed that the formation of a disturbance region is due to the coulombic repulsion of local homocharges which in turn causes a decrease in local density and hence a decrease in refractive index. At highfields the anode injects holes to form positive ions and the cathode injects electrons, and theelectric conduction is filamentary. The electric breakdown in condensed materials (liquids and solids) involves double injection and the creation of low density regions to enable the charge-carrier multiplication processes to take place. Thus the breakdown strength is governed by the parameters which control the conditions for such prebreakdown phenomena to arise.

Taking into account the combined electrode and bulk effects, an expression for the high-field filamentary conduction as a function of applied field and temperature for dielectric liquids has been derived. This expression can explain recent experimental results on the field-dependent thermal activation energy for electric conduction in hydrocarbon liquids.

A new theoretical model for high-field electric conduction and breakdown in dielectric liquids based on the theory of filamentary single injection is presented. The model is based on the following postulates. 1) The high-field conduction is mainly due to electronemission from the unavoidable asperities on the metallic cathodesurface and is confined in one or more filamentary paths, in which thecurrent density is much larger than that in other regions. 2) There ispractically no impact ionization avalanche in the liquid prior to breakdown,and, at the field of breakdown strength, there is one filamentarypath in which the current density may reach such a critical value thatthe Joule heating produced in it is sufficient to initiate the thermalinstability in the vicinity of the filament and hence the onset of thebreakdown process. Expressions for the high-field conduction currentand the breakdown strength as functions of temperature, hydrostaticpressure, gap length, molecular structure, and applied voltage waveformhave been derived. The computed results are in good agreementwith currently available experimental results for hydrocarbon dielectricliquids. Previous theories based on a uniform distribution ofcurrent density over the electrode area and on an impact ionization processare also critically examined.

On the basis of the principle that an applied electric field will cause a change in spatial distribution of the dielectric constant produced by a small applied temperature difference due to the interaction of the gradient of the dielectric constant with the field, the electric field distributions along the interface between a dielectric fluid and a dielectric solid can be measured using a Schlieren technique. Some experimental results for the interfaces between n-hexane and a glass plate surface, and between n-hexane and a semiconducting layer surface obtained with this technique show that the space charge is the dominant factor controlling the field distributions. The other factors, such as the dielectric constants and resistivities of the two materials forming the interface, would play an important role in determining the field distributions if and only if the space charge effect is negligibly small or can be ignored. This technique can be used to study electric field and space charge distributions along fluid-solid dielectric interfaces for practical insulation systems under both steady-state and transient conditions.

The electric strengths of n hexane and transformer oil have been measured at various pressures using linearly rising voltages having rates of rise between 50 kV/cm/?s and 20 MV/ cm/?s. Each measurement was made by applying a single pulse of sufficient magnitude to ensure breakdown in order to avoid the possible influence of prestressing with the normal multipulse technique. For both liquids the electric strengths are strongly dependent on the rate of rise of the applied field and on hydrostatic pressure, but the pressure dependence diminishes with increasing rate of rise of the applied field, and at rates of rise higher than about 4 MV/cm/?s the pressure dependence practically disappears. The results indicate that a certain time may be required for bubbles to form and deform in an electrically stressed liquid and that the bubble mechanism may not be operative in the breakdown process for fast-rising voltages.

The tree initiation voltage, the initial tree penetration length and breakdown voltage in polyethylene, cross-linked polyethylene and acetophenone (1%) doped polyethylene have been measured under dc and ac voltage conditions using both the needle-plate and the needle-needle electrode configurations. The results show that all these high-field dielectric properties are strongly dependent on the polarity of the applied voltage at the needle, the defects created by cross-linking or impurity-doping in polyethylene, the geometric arrangement of the electrodes and the symmetry of the initial electric fields applied to the samples. The strong dependence of the dielectric properties on these parameters indicates that tree initiation is associated with electron and hole injection from electrodes and impact ionization by electrons in high-field regions with free volumes or mechanical cracking. The latter may be created during the manufacturing process of the materials (pre-existing defects) or by the high. field effects (post-introducing defects). In this paper we shall present our model for tree initiation, some new results and a brief discussion.