Study On Optimization Design Of Shed Configurations Of AC Composite Insulators Used In Ice Regions

Composite insulators have been widely used in high voltage(HV) transmission system due to their advantages of light weight, high mechanism strength, excellent resistance to pollution flashover and less maintenance. However, both the artificial test results and operation experience have indicated that the insulating performance of the existing composite insulators under icing conditions is not better than that of ceramic and glass insulator strings. At present, a lot of investigations have been conducted for the icing characteristics and relevant electrical performance of composite insulators around the world. However, there has no report about the shed optimization design schemes based on the icing flashover theory due to the lack of systematic study of the discharge process, discharge paths and flashover model of ice-covered composite insulators Therefore, a systematic research of composite insulators with different shed configurations from the perspective of the combinations of icing experiments and flashover theory has been carried out in this paper. Then, specific optimization design plans have been put forward. These studies can improve the insulating performance of composite insulators used in ice regions and reduce the icing disasters in power grid, which has important academic and engineering practical application values.In this paper, firstly, the icing and flashover performance of composite insulators with different shed configurations under different icing conditions are systematically studied. These conditions include non-energized and energized ice accumulation, different icing degrees and different grading ring arrangements. Then the potential and electric field distributions of ice-covered composite insulators before and after the appearance of the partial arc are simulated. Moreover, a “multi-arc, multi-ice-layer” icing flashover model for different composite insulators is established. On these bases, the variations of shed configuration for composite insulators which have excellent icing flashover performance are obtained. Finally, shed configurations of composite insulators suitable for different ice regions are optimized and designed. The main work and achievements are as follows:① Based on icing flashover tests for 5 kinds of composite insulators, the effects of installation position, dimension and structure of the grading ring on their icing and flashover performance are studied. The differences between the influences of grading ring and shed configuration on the ac icing flashover voltages of composite insulators are compared and analyzed. Results indicate that the grading ring installed at the HV end has an obvious influence on the icing flashover voltage. When different grading rings are installed at the HV end, the effects of freezing water conductivity on minimum icing flashover voltage will weaken with the increase of the diameter of grading ring and the decrease of the height of grading ring, while change slightly with the variation of the pipe diameter and structure of grading ring. Although both the grading ring and shed configuration can affect the icing insulation performance of composite insulators, their influnence rules are independent to each other, and the optimization design of shed configurations generally has more effects on improving the icing flashover performance.② The non-energized and energized icing and relevant flashover tests for 12 kinds of composite insulators under different icing degrees are systematically carried out in the multi-function artificial climate chamber. The influences of various shed configuration parameters on icing characteristics and ac icing flashover voltages are deeply analyzed. Results show that a larger shed diameter will cause larger length and diameter of icicles, larger thickness of surface ice layer and bigger ice weight. When the shed spacing is larger, the length of icicles and ice thickness will become larger while the diameter of icicles becomes relative smaller. In general, the greater k2(the ratio of shed spacing and shed spread) can delay the icicle bridging and promote the growth of ice layer on the shed surface. The characteristic exponent c describing the influence of ice thickness on icing flashover voltage gradually decreases with the reduction of k1(the ratio of shed spread) and the increase of k3(the ratio of creepage distance of one shed unit and the sum of shed spacings of this unit). The value of c increases first and then decreases with the raise of k2, and there is no obvious relationship between c and C.F. factor. Besides, the installation position of extra large shed also has certain effects on the value of c.③ The development process and propagation path of the flashover arc on icecovered composite insulators with different shed configurations are shot by the highspeed camera. The relationships between the dripping water conductivity, the total length of air gap arc at the icicle tips and the icing flashover voltage are qualitatively discussed. Results indicate that the ratio of air gap arc and residual ice layer resistance during the flashover exist at differences among different ice-covered composite insulators. These differences lead to different icing flashover voltages. Compared to the variations of the surface water film conductivity, the changes of the length of air gap have more effects on the icing flashover performance.④ According to the icing characteristics obtained from tests and using the finite element software COMSOL, the potential and electric field distributions of ice-covered composite insulators with different shed configurations before and after the appearance of the partial arc are simulated. Simulations include the effects of grading ring arrangements, icing degree, shed configuration, air gap, water film conductivity, ice layer conductivity, and the appearance of the partial arc. Combining the simulation results with the actual icing flashover phenomenon, the development paths of the flashover arc on the ice- covered insulators are comprehensively estimated.⑤ Based on the consideration of the distinctions of the icicle resistance and ice layer resistance and the differences of the air gap arc and ice surface arc characteristics, and combining with the flashover arc path, a “multi-arc, multi-ice-layer” flashover model for ice-covered composite insulators is then established. The relationships between shed parameters, icing degree and icing flashover voltage are analyzed. The critical flashover voltages for different ice-covered composite insulators are calculated. Research results indicate that a good agreement is achieved between the predicted results calculated from the model and the experimental values with the errors within the range of 10%, which validate the rationality of the model.⑥ According to preceding research results in this paper, three key characteristics of shed configuration which have obvious effects on the flashover voltages of ice-covered composite insulators, namely the largest shed diameter(D1), the second largest shed diameter(D2) and the largest shed spacing(d1), are extracted and their influence rules are obtained. On these bases, shed configurations of composite insulators suitable for the light, moderate and heavy ice regions are designed and optimized, and their electrical performance are also validated and analyzed.