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Natural ester is a good substitute for mineral oil. It has been successfully applied in power transformers, and its application scale will continue to increase. With the aging of oil-paper insulation in transformers, moisture and impurity particles will inevitably be generated. In this paper, dry and wet mineral oil and natural ester samples, as well as dry and wet cellulose particles, were prepared. The accumulation behavior of the cellulose particles and the conductivity current in mineral oil and natural ester under a non-uniform dc electrical field were compared by using eight combinations of dry or wet cellulose particles and dry or wet insulation oils. The results show that the speed of the cellulose particles and the growth of impurity bridges are faster in wet mineral oil and natural ester, especially when the cellulose particles are also wet. The increase of electrical field strength accelerates the appearance of this phenomenon. The cellulose particle bridging phenomenon in the mineral oil is more significant than that in the natural ester. Under the same dc voltage, the saturated conductivity current in every mineral oil sample decreases as the electrode distance increases; however, the opposite phenomenon is observed in the natural ester samples. Moreover, the oil moisture content increase has a more significant influence on the saturated current and the dc breakdown voltage than the particles' moisture content increase. For the same particle concentration level, the natural ester has a lower conductivity current and a larger dc breakdown voltage than the mineral oil. The natural ester presents better resistance to particle pollution and better insulation properties than the mineral oil.

Researches of the theories and application of polymer composites with nonlinear conductivity are useful for dealing with the nonuniform electrical fields widely existing in the cable accessory insulation. In the present work, we fabricated CCTO (CaCu3Ti4O12)/EPDM (Ethylene Propylene Diene Monomer) composites and investigated their breakdown strength, dielectric and nonlinear conductivity properties in detail, the microstructures of fillers and composites were characterized by scanning electron microscopy (SEM) and X-ray diffraction. CCTO particles are uniformly dispersed in CCTO/EPDM composites, and the composites showed nonlinear conductivity with electric field changes. When the CCTO particle content is low, the conductivity of CCTO/EPDM composites does not present obvious nonlinearity. However, when CCTO content exceeds 2 vol %, the conductivity experiences a nonlinear change with increasing electric field strength and the threshold field (Eth) of nonlinear conductivity declines with the increase of CCTO contents. In addition, it can be found from experiment and simulation results that 8 vol % CCTO/EPDM exhibit significant nonlinear conductivity and dielectric properties as expected, and homogenizing the electrical field much more effectively. Therefore, this paper offers a preliminary discussion about the variation trend of nonlinear conductivity CCTO/EDPM composites, providing an effective reference to solve the application of nonlinear conductivity materials for cable accessories.

There are several reasons to test the breakdown voltage of insulating liquids: acceptance tests on new deliveries, testing of treated liquids prior to or during filling into electrical equipment, and monitoring and maintaining oil-filled apparatus in service. Among others, the test for the determination of breakdown voltage at 50 or 60 Hz is one of the most important. In fact, it is worldwide accepted that this test gives quickly very useful information even if mainly related with moisture and particle content than the intrinsic properties of the insulating liquids. This test is standardized by IEC and ASTM but has some limitations in terms of the statistical scatter of mean value of the breakdown voltage. The present paper presents an improvement on the scatter of the mean values of the breakdown voltage by increasing the number of requested shots and discarding the most disperse results. In addition, it is presented as a revised test cell having stirring device on the top and round edges at the bottom may further reduce the standard deviation of the same breakdown voltage. Others experimental parameters which may still influence the scatter of the same standardized test are also examined and discussed.

We have studied the thixotropic behaviour of clay particle-stabilised emulsions using a combination of rheology and confocal fluorescence microscopy. Oil drops were stabilised by laponite particles attached to their surfaces and incorporated into a three dimensional super-aggregate structure of laponite particles that formed in the water. The self-supporting, three-dimensional network of drops and laponite particles gives the emulsions pronounced gel-like properties. The transient flow behaviour of the emulsions depends on the mechanical properties of the laponite dispersions and the oil drop volume fraction. Refereed/Peer-reviewed