Shijing Wang

Effects of electric field distribution on icing and flashover performance of 220 kV composite insulators

The energized icing of composite insulators has been studied by the researchers around the world, but not much about the effects of electric field distribution on their icing and flashover performance. For this purpose, the ice accretion and flashover performance of 220 kV composite insulators with two shed configurations on occasions of different electric field distributions considering the effect of grading rings are analyzed in this paper. The test results indicate that the intensity and homogeneous distribution of electric field have positive effects on the appearance, density and weight of ice; the improvement of electric field distribution after installing grading ring has more effects than the icing characteristics, such as quality, density as well as appearance of ice, on flashover voltages of ice-covered composite insulators; the icing flashover voltages are increased by the improvement of electric field distribution and the increasing ranges of icing flashover voltage were relied on shed configuration. Icing accumulation patterns have less effect on flashover performance of composite insulators with alternatediameter or widely spaced shed configurations.

Modeling of AC flashover on ice-covered composite insulators with different shed configurations

The shed configuration has an obvious effect on flashover performance of ice-covered composite insulators. Based on the flashover arc path, a “multi-arc, multi-ice-layer” mathematical model is proposed to predict AC flashover voltages of ice-covered composite insulators with different shed configurations during the melting period. The model takes into account two different arc paths across the tip of each un-bridge icicle. Moreover, the arc across the air gap is distinguished from the arc on the ice surface, and the residual ice layer resistance is divided into icicle resistance, arc root resistance and shed surface ice resistance. In order to verify this model, four kinds of 220 kV composite insulators with different shed configurations are tested under different icing conditions. The theoretical results from the model are found to be in good agreement with the experimental results.

Influences of grading ring arrangement on AC flashover performance of 220 KV ice-covered composite insulators

The grading ring (GR) can adjust the potential distribution along the composite insulator and reduce the electric field (E-field) strengths near its end fittings. However, little research has been conducted about the effects of GR on the insulation performance of composite insulators under icing condition. Therefore, the ac icing flashover performance of two kinds of 220 kV composite insulators under different GR arrangement patterns are tested in the artificial climate chamber. Combining with the numerical simulation, the effects are analyzed. The results indicate that when GR is installed at the high voltage (HV) end, the E-field strength close to the end fitting and the icing uniformity on the whole insulators are both improved. These cause the effect of icing degree on the icing flashover voltage to be larger than that without GR at the HV end. When GR is installed at the grounded end, the perpendicular icicles on the GR will impact the flashover arc path, which causes the icing flashover voltages to be lower than that without GR at the grounded end. When different GRs are only installed at the HV end, the characteristic exponents b (characterizing the influence of freezing water conductivity on icing flashover voltage) are all larger than that without GR. Moreover, the value of b increases with the decrease of GRs diameter and with the raise of GRs height. But b changes slightly when there are variations in GRs pipe diameter and structure. The research can provide reference for the application of GR for 220 kV composite insulators in icing regions.

Comparison of AC icing flashover performances of 220 kV composite insulators with different shed configurations

The shed configuration has obvious effects on the flow field characteristics and electric field distributions around composite insulators, and then impacts their icing flashover performance. In this study, the AC flashover comparative tests for 12 types of 220 kV composite insulators with different shed configurations under non-energized and energized ice accumulation conditions are conducted in the artificial climate chamber. The influences of shed configuration on icing flashover voltages and flashover arc paths are analyzed. Results indicate that with the increase of the icing stress, both the flashover arc path and the ratio of the air gap arc gradually decrease. These variations lead the icing flashover voltage to gradually decrease and then approach to saturation. For ice-covered composite insulators with different shed configurations, the ratios of the air gap arc to the total flashover arc are different due to the differences of their shed spacings, icicle lengths, differences of shed diameters, (extra) large shed diameters. These result in the differences of icing flashover voltages and their characteristic exponent c describing the influence of ice thickness. Besides, for insulators iced under energized condition, their icing flashover voltages and flashover arc paths are different from that under non-energized condition. Hence, the energized icing test, which is also the only normative reference of icing test proposed by the IEEE standard, should be carried out to optimize the shed configuration of composite insulators used in ice areas.

Effect of grading ring on ice characteristics and flashover performance of 220 kVcomposite insulators with different shed configurations

The energized icing tests for five kinds of 220 kV composite insulators with and without the grading ring (GR) at the high voltage (HV) end are performed in the artificial climate chamber. Combining with the numerical simulations of electric field (E-field), the differences between the influence of GR and shed configuration on the ice characteristics and flashover performance of composite insulators are compared and analyzed. Results indicate that GR can improve both the ice growths and E-field distributions near the HV end. The improvements of GR on the icing flashover voltages under light icing degree is relatively obvious, while with the increase of icing degree, these effects gradually become less apparent. For different insulators, the effects of icing degree on icing flashover voltages under the presence of GRs are all higher than that under the absence of GRs. Moreover, the influence law of icing degree among different insulators remains unchanged under the presence and absence of GRs. In general, the improvement of optimized shed configuration for the icing flashover performance is more obvious than that of GR, which can be considered as one main measure to improve insulation performance of composite insulators in ice regions.

Studying corona onset characteristics after rime ice accumulation on energized stranded conductors

Rime makes conductor surface extremely rough and it will distort the electric field seriously. Numerous studies on icing conductors has been done but the fact that conductors operating under voltage are always ignored and Aluminum pipes are applied to study the actual conductors. Moreover, no clear information had been obtained into the decrease of regularities of corona onset voltage (COV) after energized icing. Therefore, a series of AC corona tests of charged rime icing had been done in the Multi-functional Climate Chamber. UV imaging technology and I-U curve fitting were applied to measure the COV and the finite element model was established to research field strength after icing. Results show that rime can bring COV down significantly. Ice-tree of rime in various shapes lead to different COV values. Increasing in icing time will raise COV values while the rising speed slowed down gradually. Conductivity has no impact on rime form or COV. The sharper ice-trees are, the more seriously electric field distorted and the lower COV values. This paper can provide information about design of transmission lines in regard to corona losses after icing.

Study on the effects of glaze icing on the corona onset characteristics of stranded conductors

Glaze affects corona onset voltage (COV) after icing on the conductor surface. Energized icing on operating wires is always ignored in many tests, and the COV downtrend has not been elucidated. A series of AC corona tests on stranded conductors after energized icing are implemented in the multifunction climate chamber, and the UV imaging camera and I-U curve fitting method are applied to measure and analyze the COV. Finite element models are established to research the effect of glaze icicles on the field strength of conductors. Results show that glaze significantly reduces COV, with different glaze shapes leading to distinct COV values. Wires with thick radius receive high COV even when such wires are iced under the same duration. An Increase in icing time slows the decrease rate of COV values. High conductivity decreases COV and slightly changes the glaze morphology. Sharper icicles correspond to more serious field distortions and lower COV values. This paper provides a reference for the selection and design of transmission lines in glaze icing areas.

Influence of shed configuration on icing characteristics and flashover performance of 220 kV composite insulators

Shed configuration is one of the important factors affecting the icing characteristics and icing flashover performance of composite insulators. In this paper, the non-energized and energized glaze icing tests are conducted on 12 types of typical 220 kV composite insulators with different shed configurations in the artificial climate chamber. Combining with numerical calculations of electrical field distributions, the monitoring of leakage current, the measurement of the ice-melting water conductivity and the shooting of flashover arc paths, the influences of shed configuration on the ice growth and icing flashover voltages of composite insulators are comprehensively analyzed. Results indicate that a larger shed diameter brings about the faster ice growth and the lower average electric field strength across the icicle air gap, which leads to the reduction of the icing flashover voltage. When the shed spacing is larger, both the icicle length and ice thickness become larger, and the ice-melting water conductivity during the flashover is higher. But the ratio of icicle air gap arc to the flashover arc is higher and its effects on the icing flashover performance are more obvious, which leads to the improvement of the icing flashover voltage. When the ice accretes on the energized insulator, the ice-melting water conductivity is smaller while the ratio of icicle air gap arc to the flashover arc is larger than that under non-energized icing condition. These variations lead to the higher icing flashover voltages for energized insulators. It is thus suggested that the icing and flashover tests under different icing degrees should be conducted on energized composite insulators to deeply study the influences of shed configuration.

Influence of size parameters of grading ring on electric field distribution and icing characteristics of 220kV composite insulator

In this paper, numerical simulations using the finite element method (FEM), were performed to study the electric field (E-field) distributions along ice-covered 220 kV composite insulator with different sizes of grading rings at the high voltage (HV) end. Combining with energized ice accretion tests in artificial climate chamber, the influence of size parameters of grading ring on icing characteristics of composite insulator was analyzed. The results show that the ice accretion of composite insulator with grading ring was more uniform than that without grading ring. With the increase of diameter of grading ring, the E-field distribution along the arcing distance becomes more uniform. The change of pipe diameter has no obvious effect on the E-field distribution at the HV end. The increase of height of grading ring could accordingly lift the main position of E-field distortion and thus enhance the E-field intensity along the string. The ice accretion uniformity, situation of shed space bridged by icicle and the variation of ice weight of composite insulator obtain by icing tests were well accordant with the E-field calculation results.