High-voltage engineering

Contamination on the surface of high-voltage outdoor insulators can lead to the formation of quasistable gas discharges, which burn in series with the resistive contaminative film. Under certain conditions, these discharges may grow to cause a complete flashover. In this paper, a model for the case of a discharge burning on a rectangular polluted strip is introduced; solution of Laplace's equation in two dimensions enables accurate values for the series resistance of the pollution film to be calculated, and a numerical method is used to calculate a factor which accounts for the change in resistance due to heating. Application of the flashover criterion di/dx > 0 to this model yields results which are in good agreement with experiment. The model can also be applied to axisymmetric insulators with complex shapes by replacing the practical insulator by its `equivalent cylinder' Good agreement is obtained between the calculated flashover voltages and those measured by the VDE 0448 test method.

Flashover voltage of high-voltage insulators with uniform surface-pollution films

This paper presents a dynamic model allowing the prediction of discharge activity leading to flashover of polluted insulators. It takes into account the instantaneous changes of the discharge (arc) parameters. For a given voltage, geometry and pollution severity, the model enables one to determine the following parameters: temporal evolution of the current, arc resistance, propagation velocity, charge injection and flashover time lag. The flashover voltage characteristic as a function of the surface conductivity of insulators calculated by this model is quite satisfactory in contrast to the empirical and semi-empirical models reported in the literature. The leakage currents computed by our model are found to be in a good agreement with those measured in the laboratory.

Flashover dynamic model of polluted insulators under ac voltage

The importance of the research on insulator pollution has been increased considerably with the rise of the voltage of transmission lines. In order to determine the flashover behavior of polluted high voltage insulators and to identify to physical mechanisms that govern this phenomenon, the researchers have been brought to establish a modeling. In this paper, a dynamic model of AC flashover voltages of the polluted insulators is constructed using the least square support vector machine (LS-SVM) regression method. For this purpose, a training set is generated by using a numerical method based on Finite Element Method (FEM) for several of common insulators with different geometries. To improve the resulting model's generalization ability, an efficient optimization algorithm known as the grid search are adopted to tune parameters in LS-SVM design. In addition, two different testing set, which are not introduced to the LS-SVM during the training procedures, is used to evaluate the effectiveness and feasibility of the proposed method. Then, optimum LS-SVM model is firstly obtained and the performance of the proposed system with other intelligence method based on ANN is compared. It can be concluded that the performance of LS-SVM model outperforms those of ANN, for the data set available, which indicates that the LS-SVM model has better generalization ability.

Prediction of flashover voltage of insulators using least squares support vector machines

In this paper, we present results dealing with the non-uniform pollution carried out under 50 Hz applied voltage on a plane model simulating the 1512 L outdoor insulator largely used by the Algerian Company of Gas and Electric Power (SONELGAZ). Many configurations in non-uniform pollution are studied in the ENP's (Ecole Nationale Polytechnique d'Alger) High Voltage Laboratory in order to analyze the impact of polluted layer distribution on the insulator dielectric performances. The polluted solution has a conductivity of 1.2 mS/cm obtained with distilled water and NaCl. Our investigations are particularly focused on the on line monitoring of both position and width of the contaminated layer. The flashover voltage and the leakage current magnitude have been investigated in order to study the flashover process on this insulating surface. A video apparatus is used to reflect the parallel discharges behavior, appearing when the polluted layer reaches a critical width. Phase angle values between applied voltage and leakage current (LC) signals at the fundamental frequency (50 Hz) are calculated using the Fast Fourier Transform (FFT) spectral analysis. Phase angle measurements indicate that the equivalent impedance of the insulator behaves like RC circuit with a high capacitive effect engendered by the pre-established clean band. This effect decreases when electric discharges occur at a particular voltage level. The Discrete Wavelet Transform (DWT) is adopted for the leakage current decomposition in several time-frequency bands. The STD-MRA (Standard deviation-Multi Resolution Analysis) of these frequency bands is calculated and is employed to choose the most interesting details that detect both position and width increasing of the conducting layer. Reported results show that the pollution surface state and the severity of this conducting layer deposited on insulator surface could be identified from the STD-MRA representation of leakage current frequency bands. It was shown that the high frequency band of the leakage current increases before the final flashover when the polluted layer is located in the middle of the plane model. It was established that a good correlation has been found between the insulator state surface and details of the leakage current obtained through the DWT decomposition. In fact, these details provide relevant information on both position and width of the polluted layer nonuniformly distributed on the insulator surface.

Flashover process and frequency analysis of the leakage current on insulator model under non-uniform pollution conditions

When a leakage current flows over the wet polluted surface of an insulator, high-resistance dry bands form. Discharges across these dry bands usually become extinguished but, exceptionally, may develop into a flashover of the insulator. The formation of dry bands and the subsequent growth of discharges on the polluted surface of a flat-strip insulator have been studied by scanning the voltage distribution along the strip at high speed. The behaviour of an arc rooted on a water surface has been investigated and the voltage gradients in the columns of arcs burning both in air and steam have been measured. Experiments show that when a water surface flashes over the arc burns in an atmosphere of steam, and that the condition for an arc to propagate over a resistive surface is that the voltage gradient on the surface exceeds that in the arc column. On this basis the flashover voltage of a water column is predictable to within 5%.

B.F. Hampton

Papers

Flashover mechanism of polluted insulation

Flashover mechanism of polluted insulation