Lichun Shu

Flashover Performance of Pre-contaminated and Ice-covered Composite Insulators to be Used in 1000 kV UHV AC Transmission Lines

A method is introduced for testing insulators under icing conditions. The method is based on and developed from those described in the standard of IEC60-1, IEC 60507 and IEEE Task Force on Insulator Icing Test Methods, which give instructions to test insulators under pollution. Then, the paper explores the effects of various factors, including ice thickness, pollution severity on the surface of insulators before ice accretion, atmospheric pressure, and shed profiles, on the flashover performance of short samples of two different types of silicone rubber (SIR) composite long rod insulators intended for ultra high voltage (UHV) ac transmission lines. The experiments were carried out in the multi-function artificial climate chamber in the High Voltage and Insulation Technological Laboratory of Chongqing University, China. The experimental results reveal the regularity of the effects of ice thickness, pollution severity and atmospheric pressure on the average flashover voltage of two types of composite insulators. Those can provide reference for the outdoor insulation design for UHV ac transmission lines.

Effects of shed configuration on AC flashover performance of ice-covered composite long-rod insulators

The flashover tests of artificially ice-covered composite long-rod insulators, including four types with a nominal voltage of 110 kV, four types of 220 kV and four types of 500 kV, were carried out in the multi-function artificial climate chamber. The result indicates that, firstly, icing flashover performance of the improved composite insulators for icing regions is not always superior to that of the standard composite insulators when icing is light. Meanwhile, Eh (flashover voltage gradient of dry arcing distance) first increases and then decreases with the increase of CF (creepage factor, namely the ratio of creepage to dry arcing distance). Secondly, when icing is moderate, the icing flashover performance is influenced by such main factors as shed diameter, shed spacing and the configuration of sheds with different diameters. In this situation, Eh of the improved composite insulators is higher than that of the standard composite insulators, and ice-covered composite insulator with three or four different shed diameters has better flashover performance than that with two different shed diameters. Thirdly, when icing is heavy, compared with the moderate icing condition, the difference of Eh between the improved composite insulators and the standard composite insulators diminishes. Finally, when icing is moderate or heavy, there is no obvious regular relation between CF and Eh.

DC flashover performance and effect of sheds configuration on polluted and ice–covered composite insulators at low atmospheric pressure

How to choose external insulation under high altitude, pollution and icing conditions for UHVDC transmission project is a complicated problem. To provide technical reference to this engineering roadblock, the dc icing flashover performance of seven types of pre-polluted short samples of composite insulators at low air pressure simulated by the multifunction artificial climate chamber was researched in this paper. The influences of ice thickness, pollution and air pressure on icing flashover performance were analyzed. The correct formula of dc icing flashover gradient of seven types of pre-polluted specimens at high altitude was presented. The influence of the sheds configuration parameters on the flashover gradient was discussed. And the selection of sheds configuration was suggested.

Effects of artificial polluting methods on AC flashover voltage of composite insulators

In artificial pollution tests, there are three polluting methods frequently used, including quantitative Brushing Method (BM), Dipping Method (DM) and Spraying Method (SM). Based on measurements of ac flashover performance of four types of silicone rubber composite insulators in the artificial climate chamber, this paper studied the differences on the flashover voltage of polluted composite insulators using these methods. The tests show that there are some differences on the pollution flashover characteristics, and the influences vary on flashover voltage of specimens using different polluting methods. Further, the differences are determined by the geometry of composite insulators as well as the polluting degree. However, the fitting formulas, which are simulated on ac pollution flashover voltages at the various pollution degrees for the same type of insulator, are consistent with three different methods. Moreover, the differences on exponents a characterizing the influence of Salt Deposit Density (SDD) on the flashover voltage are negligible for specimens whichever polluting method is used.

Simulation and Experimental Investigation of DC Ice-Melting Process on an Iced Conductor

Ice melting with dc is one of the key technologies to prevent the Chinese power grid from ice storms. Especially after the severe ice storm in the southern part of China in early 2008, dc ice-melting technology has drawn more attention than ever before. However, there are few satisfactory methods to select correct parameters so the ice-melting project rarely achieves desired effects in some cases when applied in Hunan, Guizhou, and other provinces in China. Therefore, it is of great significance to develop a method to estimate parameters which is applicable in the practical situation for dc ice melting. To handle this, the factors, such as wind velocity, ambient temperature, current density and ice-layer thickness were analyzed and then a dc ice-melting model is put forward in this paper. Both the results of simulations and experiments show that the ice-melting process can be divided into three stages composed of temperature rising, ice melting, and ice shedding, among which the ice melting is the key stage consuming most of the ice-melting time. In this stage, an elliptic airgap is formed and widened gradually with an influence on the ice-melting time and the temperature of the conductor. The experiments in the artificial climate chamber demonstrate that the results of the presented model are consistent with those of the experiments generally, so it can be employed to estimate the parameters of ice melting in practical engineering as reference.

Flashover Voltage Prediction of Composite Insulators Based on the Characteristics of Leakage Current

Effective prediction of flashover voltage (FOV) of insulators is an important approach to the prevention of pollution flashover accidents. In order to predict the FOV of insulators and prevent pollution flashover accidents, first, a large number of artificial pollution tests, which simulate the impact of contamination level and hydrophobicity classification (HC) on FOV and leakage current, have been investigated. Second, based on the experimental data, the relationship between the FOV and contamination level, HC, has been obtained; the four characteristics of leakage current, namely, the entropy of pulse amplitude (S), the maximum pulse amplitude (Ih), the energy ration (K) and the energy (E), have been extracted. They jointly reflect how severe the contamination level and the HC of composite insulators are from different perspectives. Third, the variation laws between the four characteristics and the contamination level, HC, have been obtained. Finally, the FOV prediction least squares-support vector machines (LS-SVM) model has been presented, in which the four characteristics are used as the inputs of model, and the FOV is used as the output of model. The prediction results are basically consistent with the test results. Therefore, the model is acceptable to predict the FOV of composite insulators and is of significance for the prevention of pollution flashover accidents.

Modeling of DC Flashover on Ice-Covered HV Insulators Based on Dynamic Electric Field Analysis

In this paper, a model based on dynamic electric field analysis has been developed to predict the flashover voltage of the ice-covered HV insulators, under dc voltage. The potential and electric field calculation models before and after air gap breakdown are built respectively based on finite element method (FEM). The arc initiation process is determined based on the model before air gap breakdown. The critical applied voltage and leakage current to maintain an arc with certain length are obtained based on the electric field calculation model after air gap breakdown and the U-I characteristic of the arc. Moreover, the improved Hampton criterion has been employed to determine the critical flashover of the ice-covered insulator. The results obtained from the dynamic electric field analysis model have been compared with other mathematical and experimental results of other researchers and got a great agreement.

Effects of Space Charge on the Discharge Process in an Icicle/Iced-Plate Electrode System under Positive DC Voltage

Using an icicle/iced-plate system, the corona discharge at the icicle tip was investigated under positive DC voltage. The discharge pulses were recorded and analyzed. The relationship between the discharge magnitude and the time interval before the pulse was examined. The mechanisms underlying the corona discharges at icicle tips were investigated. The influence of space charge on the discharge and the effects of the applied voltage on the time interval between two pulses were also investigated. Finally, a model for calculating the drift and diffusion of the space charge cloud was presented.

Study of DC flashover performance of ice-covered insulators at high altitude

This paper examines the direct current (DC) icing flashover performance of typical DC porcelain, glass, and composite insulators in a complex environment of high altitude, icing, and pollution. The effects of the insulator string length, ice thickness, pollution level, freezing water conductivity, air pressure, and arrangement of insulator strings on the DC icing flashover performance are analyzed, and the correction formula for the DC icing flashover voltage is obtained. The DC flashover process of ice-covered insulators at high altitude is classified into dynamic changes of surface arcs, air-gap arcs, and residual resistance of the ice layer, as observed through a high-speed camera, and a DC flashover circuit model explaining the flashover performance for ice-covered insulators at high altitude is established.

Effect of shed configuration on DC flashover performance of ice-covered 110 kV composite insulators

Composite insulators have been used in power system for more than 20 years in China. Compared to porcelain and glass insulators, the pollution flashover performance of composite insulators is better, whereas the icing flashover performance is worse. Shed configuration is a key factor that influences flashover voltage of composite insulators covered ice or snow in icing regions. The DC flashover tests of artificially ice-covered 110 kV composite insulators with different shed configuration were carried out in the multi-function artificial climate chamber. The test results revealed that there are huge differences between the icing flashover voltages of the composite insulators with different shed configuration. The test results indicate that icing flashover performance of the improved composite insulators for icing regions is not always superior to that of the standard composite insulators.