Covering insulating sheath on transmission lines is one of the commonly measures to prevent bird damage in engineering, and the breakdown voltage of the air gap between bird droppings and the surface of sheath directly affects the configuration scheme of sheath. In this paper, the variation characteristics of breakdown voltage with the thickness of sheath and the influence of sheath defect on the breakdown voltage under different air gap distance were investigated by experiment, in which the bird droppings were simulated by copper rod. The results show that compared with bare wires, the breakdown voltage of the gap in the wires covered sheath with the thickness of 2.0, 3.0, 3.5 mm increases by 17.7%, 33.1%, 41.6%, respectively. Compared with new sheath of the same thickness, the breakdown voltage of gap in the defective sheath with the thicknesses of 2.0, 3.0, 3.5 mm decreases by 70.6%, 62.8%, 44.2%, respectively. It is concluded that the smaller the thickness of the defective sheath, the larger the decreasing amplitude of breakdown voltage.

It is one of the important methods to obtain the ZnO varistor ceramics with high-performance by improving the distribution uniformity of grain. In this paper, ZnO varistor ceramics samples doping with different contents of Sb₂O₃ were studied from several aspects such as the average grain size, distribution uniformity of grain size, and shape of grain. In addition, the non-uniform coefficient of grain size and the shape parameter were applied for quantification. The results show that with the increase of the doping content of Sb₂O₃, the average grain sizes, the non-uniform coefficients of grain size, and the shape parameters of the samples decrease generally. It suggests that the distribution uniformity of grain size is improved, and the grain shapes develop from slender to regular, which can also explain the enhance of the electrical properties of the samples. According to the analysis, the spinel phase formed after doping Sb₂O₃ inhibits the anisotropic growth of the crystal grains, which decreases the grain size and improves the uniform distribution and the shape of the grains. In addition, the inhibiting effect of spinel phase on the growth of grains in the large-grain region is weaker than that in the little-grain region.

During the operation of high-frequency transformer, oil-paper insulation bears severe electric and thermal stress, and the insulation system is easy to fail prematurely. In order to study the effect of high frequency pulse voltage on the discharge characteristics of oil-paper insulation, we built a partial discharge (PD) test platform under high frequency pulse voltage to conduct partial discharge tests. The characteristics of partial discharge of oil-paper insulation under highfrequency voltage was studied and compared with the characteristics under power frequency voltage. The results show that under high frequency voltage, partial discharge mainly occurs at the rising and falling edge of voltage, the PD amplitude is much higher than that under power frequency, and there is an frequency-induced inflection point near the frequency of 10 kHz. Moreover, oil-paper insulation is punctured under high frequency impulse. The reason is that the rapid voltage rise rate caused by high frequency changes the local field strength and high frequency electrothermal coupling effect at the defect position, resulting in the change of the partial discharge intensity.

Optical fiber sensing technology has the advantages of anti-electromagnetic interference, small volume, multiplexing, etc. Intelligent on-line monitoring for power equipment by optical fiber sensing technology has become a new trend of power grid development. In this paper, the research progress of several optical fiber sensing technologies in the field of power equipment monitoring were reviewed, including fiber grating sensing technology, scattering distributed optical fiber sensing technology, fluorescent optical fiber sensing technology, and interferometric optical fiber sensing technology. From the aspects of temperature monitoring, strain monitoring, partial discharge monitoring, fiber-optic current sensor, pollution monitoring, and hydrogen monitoring, the application as well as the advantages and disadvantages of different sensing technologies in different power equipment were explained. At last, the development direction of the engineering and networking application of optical fiber sensing technology was proposed.

The electric field strength and potential distribution of insulator strings with different locations, numbers and deterioration degrees of insulators were studied by finite element method. The results show that the deteriorated insulators at high and low voltage terminals has the greatest effect on the axial electric field of the insulator string, which significantly increases the peak amplitude of electric field at the deterioration location. Compared with the insulator string with a heavily deteriorated insulator, the electric field peak near the second insulator is greatly affected by the presence of two (first and second) deteriorated insulators. When the number and location of insulator degradation are the same, the effect of slightly deteriorated insulator on the axial electric field strength of insulator string is the same as that of zero value insulator, but the influence degree increases with the increase of degradation degree.

In order to guide the application of composite insulation cross-arm in distribution networks, we carried out 42 h boiling (0.1%NaCl) tests on the sealed and unsealed composite insulation cross-arm according to the requirements of standard GB/T 20142—2006, and studied the influence of high temperature water environment on the performance of the insulation cross-arms treated by the two methods. The water absorption, specified bending load, tensile strength, bending strength, dry power frequency voltage, dry lightning impulse voltage, and breakdown strength of the insulation cross-arms were tested before and after boiling. Meanwhile, the microscopic morphology of the fracture was analyzed by SEM. The result shows that under high temperature water environment, the performance of the sealed cross-arm is better than that of the unsealed one. The specified bending load of all the samples is above 6.5 kN, the lightning impulse withstand voltage is greater than 350 kV, and the tensile and bending strengths is greater than 680 MPa and 1 000 MPa, respectively. The samples show good mechanical and electrical properties.

In order to study the effect of stress cone on the accumulation of space charge, we designed a cable model with stress cone structure. A platform was built to perform the combined electric and thermal ageing on the model cable. The distribution of space charge before and after ageing was measured by pulse electro-acoustic method, and the accumulation of space charge at different axial and radial positions was compared. The results show that more space charge accumulates at the root of the stress cone, and the accumulated space charge mainly distributes on the outside of the insulation layer of the cable. To make a preliminary explanation of this phenomenon, it is believed that the electric field distortion caused by the stress cone makes the space charge easy to accumulate in this area.

On the basis of the impregnation mechanism analysis of oil-paper insulation, a test model of mixed insulating oil-paper impregnation was designed, and the main factors affecting the impregnation process of oil-paper insulation were discussed. The relationship diagram of the insulation impregnation length of mixed insulation oil-paper insulation with time was obtained through experiments, and it was compared with that of mineral oil-paper insulation. The results show that the impregnation effect of mixed insulating oil-paper insulation is weaker than that of mineral insulating oil, but the difference is slight. When the impregnation temperature and vacuum degree increase at the same time, the impregnation effect of the two kinds of insulating oil-paper insulation improves significantly in the impregnation stage of 12-36 h. The oil injection process of the mixed oil transformer was implemented on actual transformer, and the effectiveness of the process is verified by the high voltage test.

Alkylbenzene insulating oil, represented by dodecylbenzene, is a kind of liquid medium that widely used in oil-filled submarine cables. Due to the stagnation of the development of oil-filled cable technology in China, the application and research of this type of insulating oil is relatively scarce. This paper introduced the physical and chemical, electrical, ageing, and biological toxicity of dodecylbenzene insulating oil for submarine cables.

In order to study the effect of crepe paper on the curing characteristics of epoxy resin, we analyzed the curing process of epoxy resin and epoxy impregnated paper system by differential scanning calorimetry (DSC). The kinetic model parameters of curing reaction were calculated by Flynn-Wall-Ozawa and Malek methods. The results show that both the curing reaction of epoxy resin and epoxy impregnated paper system conform to the autocatalytic kinetic model. The hydroxyl group in the crepe paper has a catalytic effect on the curing reaction, which leads to the decrease of the activation energy of the curing reaction. The restraint of crepe paper leads to the decrease of the frequency factor and reaction order, so that the curing rate of epoxy impregnated paper system is slower than that of epoxy resin. At the same temperature, the curing process of epoxy impregnated paper requires more time.