Refat Atef Ghunem

Erosion of silicone rubber composites in the AC and DC inclined plane tests

In this paper, the performance of silicone rubber composites containing 0, 10 and 30 wt % loading of silica filler is investigated in the initial-tracking voltage test method of the inclined plane test under AC and DC showing the most inferior erosion class under +DC followed by -DC and then AC. DC as compared to AC dry-band arcing characteristics are analyzed by conducting leakage current measurements prior to and after the inception of dry-band arcing. At high test voltages, instability of the contaminant rivulet is found to be an important factor in the development of dry band arcing and, consequently, biasing the test outcomes when equal voltages of rms AC and DC are selected. The initial tracking voltage in ASTM D2303 is, therefore, proposed as a reliable approach by which +DC and -DC test voltages can be reliably selected. A coefficient of variation parameter is also proposed as a useful mean of selecting the equivalent DC voltages, and the validity of the parameter is verified experimentally using the initialtracking voltage test method. Erosion in the constant voltage method is investigated under DC by applying the equivalent voltages and compared to AC. Based on this methodology, it is suggested that an improvement in silicone rubber compositions is needed for DC. Moreover, the need to select the creepage distance of polymer insulators designed for AC, yet to be used under DC, with reference to tracking/erosion performance is highlighted.

Artificial neural networks with stepwise regression for predicting transformer oil furan content

In this paper a prediction model is proposed for estimation of furan content in transformer oil using oil quality parameters and dissolved gases as inputs. Multi-layer perceptron feed forward neural networks were used to model the relationships between various transformer oil parameters and furan content. Seven transformer oil parameters, which are breakdown voltage, water content, acidity, total combustible hydrocarbon gases and hydrogen, total combustible gases, carbon monoxide and carbon dioxide concentrations, are proposed to be predictors of furan content in transformer oil. The predictors were chosen based on the physical nature of oil/paper insulation degradation under transformer operating conditions. Moreover, stepwise regression was used to further tune the prediction model by selecting the most significant predictors. The proposed model has been tested on in-service power transformers and prediction accuracy of 90% for furan content in transformer oil has been achieved.

Investigation into the eroding dry-band arcing of filled silicone rubber under DC using wavelet-based multiresolution analysis

This paper describes an investigation into the dry-band arcing mechanism leading to erosion of filled silicone rubber in the DC inclined plane test. Good correlation is obtained between the formation of surface residue, hotspots and the eroding dry-band arcing. The hotspots detected by infrared camera correlates well with the temperature of degradation determined by thermogravimetric analysis and differential scanning calorimetry. To characterize the physical mechanism of dry-band arcing, wavelet-based multiresolution analysis up to seven levels of resolution is applied to the leakage current waveforms. The analysis shows that both stable and intense discharges characterize the eroding dry-band arcing. A time-to-eroding parameter is proposed to determine the relative resistance to the formation of residue. Faster accumulation of residue is evident under -DC than +DC. Earlier inception of erosion is obtained for the silica filled- as compared to the alumina tri-hydrate filled- silicone, in which the release of the water of hydration is postulated to suppress the residue formation.

Suppression of silicone rubber erosion by alumina trihydrate and silica fillers from dry-band arcing under DC

This paper describes the suppression of dry-band arcing erosion of silicone rubber by alumina tri-hydrate and silica fillers in the DC inclined plane test, employing the wavelet based multiresolution analysis of leakage current. The third detail component of the leakage current as decomposed by the wavelet-based multiresolution analysis is shown to be an indicator of the effectiveness of the filler type in suppressing erosion by dry-band arcing. The addition of alumina tri-hydrate or silica filler to silicone rubber increases the thermal conductivity of the composites, retarding the development of the eroding temperature, and thus the evolution of the third detail. Additional effect is also obtained for the dehydration enthalpy, of alumina tri-hydrate in silicone rubber at a filler level of 30 wt%, in impeding the development of hot spots on the tested surface. A reduction in the magnitude of the third detail is evident with filler level, indicating that the increasing volume of silica or alumina tri-hydrate reduces the temperature of the dry-band arcing plasma. Comparable levels of the leakage current third detail is found between 30 wt% alumina tri-hydrate and silica filled composites which suggests the water of hydration plays a minor role in diluting the SiR but at 58 wt% an internal oxidation mechanism that produces gases diluting the arcing phase appears to suppress erosion.

RF-based monitoring and classification of partial discharge on wet silicone rubber surface

The main objective of this study is to investigate the feasibility of using an RF antenna to detect and classify partial discharges on polymer insulation surfaces. The detected discharges are identified based on their originating sources, employing a cascade of feature extraction, feature selection and the standard artificial neural network classifier. Both statistical and spectral analyses have been used for feature extraction. Feature extraction is followed by a feature selection stage with a novel implementation of stepwise regression method in order to derive representative feature vectors, yet with minimum dimensionality. Suppression of redundant features is therefore achieved, thereby enhancing the classification reliability. Finally, classification is performed using a standard feed forward neural network with back propagation training algorithm. The proposed method is found to be successful in classifying different types of partial discharge with recognition accuracy exceeding 96%. The proposed method can be an essential stage towards overhead line inspection to assess the status of outdoor polymer insulators, where partial discharges could be initiated from surface discharges due to pollution and/or from corona discharges from energized ends.

A Novel Selection Algorithm of a Wavelet-Based Transformer Differential Current Features

In this paper, a novel selection algorithm of wavelet- based transformer differential current features is proposed. The minimum description length with entropy criteria are employed for an initial selection of the mother wavelet and the resolution level, respectively; whereas stepwise regression is applied for obtaining the most statistically significant features. Dimensionality reduction is accordingly achieved, with an acceptable accuracy maintained for classification. The validity of the proposed algorithm is tested through a neuro-wavelet- based classifier of transformer inrush and internal fault differential currents. The proposed algorithm highlights the potential of utilizing synergism of integrating multiple feature selection techniques as opposed to an individual technique, which ensures optimal selection of the features.

Partial discharge on-line monitoring of outdoor insulators

The main objective of this study is to investigate the feasibility of using an RF antenna to detect partial discharges in outdoor insulators. Moreover, the study focuses on identifying the sources of partial discharge using artificial neural networks. In outdoor insulators, partial discharges could be initiated either from surface discharge due to pollution and/or from corona discharges from energized ends. Both statistical and spectral features have been used as an input vector to a feed forward neural network with back propagation training algorithm. The proposed method is found to be successful in classifying different types of partial discharge with a recognition accuracy of 93%. The proposed method can be used during overhead line inspection to assess the status of outdoor non-ceramic insulators.

The DC inclined-plane tracking and erosion test and the role of inorganic fillers in silicone rubber for DC insulation

The first polymer insulator for AC transmission lines was developed in the late 1950s. This insulator consisted of a flexibilized epoxy resin weathershed cast onto a fiberglass rod, and field trials on a 138-kV line took place in 1959. Insulators failed almost immediately on energization due to severe tracking and erosion of the bisphenol-A epoxy material, which was later, through inclined-plane tests, shown to be inherently unsuitable for use in outdoor insulation [1]. This article describes a study of the DC dry-band arcing mechanism as a foundation for the development of a standard DC inclined-plane test and its employment in the evaluation of silicone-rubber housing composites for outdoor DC insulation.

Comparative inclined plane tests on silicone and EPDM elastomers under DC

The paper presents comparative inclined plane test results of high temperature vulcanized silicone rubber and EPDM elastomers filled with alumina trihydrate to 25 wt% under DC. Both the initial tracking voltage and the constant voltage test methods are employed to evaluate the relative tracking/erosion resistance of both elastomers. While insignificant differences between the tracking/erosion resistance of the two elastomers is observed, earlier failure occurs under -DC. A recognizable distinction is also found between the failure pattern and the corresponding dry-band arcing current. Shorter non-conduction periods in the leakage current are observed on the silicone rubber elastomer prior to failure, a result that indicates stable discharge activity, which is more evident under +DC than under -DC. Imbedded in the stable dry band arcing current are transient leakage current pulses due to probable ionization activity, which can be seen to enhance the intensity of the discharge. In contrast, intermittent dry band arcing current is found to be dominant for the EPDM elastomer.

Erosion of ATH and silica filled silicone rubber in the DC inclined plane test

In this paper erosion of silicone rubber loaded with 30 wt% ATH and silica is investigated in the DC inclined plane tracking/erosion test as per ASTM D 2303. Reduced equivalent DC voltages have been applied in order to compare with the erosion under AC. Hot spot temperatures are used for comparative analysis of dry-band arcing severity on the tested materials. Also, depending on the type of filler, initiation of stable and intense dry-band arcing is discussed. Based on the results it is proposed that the existing material formulation used for AC insulators are not suitable for DC insulators as the erosion is more severe and different.