S.H. Jayaram

Comparison between silicone rubber containing micro- and nano- size silica fillers [insulating material applications]

In this study, the influence of nano-size silica filler is compared with micro-size filler in silicone rubber using the ASTM 2303 inclined plane tracking and erosion test. Silicone rubber samples filled with 5 /spl mu/m mean size ground silica are compared with silicone rubber filled with 12 nm average size fumed silica. Both the low frequency components of leakage current and eroded volume are used to evaluate the performance of the two types of composites. While the fundamental component of leakage current does not correlate with the erosion, the third harmonic component of the leakage current shows good correlation to the erosion, in terms of volume, and for both composites. Based on the results of the eroded volume, it is observed that 10% by weight of nano-size filler in the silicone rubber gives a performance that is similar to that obtained with 50% by weight of micro-size filler. The paper discusses the merits of nano-size filler in silicone composites.

Thermal characteristics of silicone rubber filled with ATH and silica under laser heating

This work explores the thermal behavior of silicone rubber filled at different concentrations with alumina trihydrate (ATH) and silica. Three particle sizes, 1.5 /spl mu/m, 5 /spl mu/m and 10 /spl mu/m, were employed in the sample preparation. A near-infrared laser was used to heat the samples. The concentration of the filler plays a major role on improving thermal conductivity when compared to particle size. Nevertheless, better thermal conductivity could be distinguished at 5 /spl mu/m for ATH and 1.5 /spl mu/m for silica, at all concentrations.

Prediction of leakage current of composite insulators in salt fog test using neural network

This paper presents a new prediction method for the level of the fundamental component of leakage current in the early aging period. Several silicone rubber (SIR) insulators were tested in salt-fog chamber and the LC was continuously recorded, A neural network has been used to predict the level of LC in the early stage of aging of the SIR insulators. Initial value of LC and its increasing slope in the first hour are used as the input of the network and the value of LC after 10 hours is the output of the network. It was found that a 2-layer feedforward back propagation with a biased output is a suitable network to predict the LC hours based on its initial values with a maximum of 15 % error.

Tracking and erosion resistance of RTV silicone rubber: effect of filler particle size and loading

In this study the effect of filler particle size and concentration on the tracking and erosion resistance of silicone rubber loaded with silica and alumina tri-hydrate (ATH) as fillers has been investigated. Samples with different filler particle size and concentrations were tested in an inclined plane tracking and erosion resistance test apparatus. The loss of material is linked with the power dissipated by the fundamental and harmonic components, due to dry band arcing. The results show that smaller particle sizes and higher percentages of fillers, up to 50%, improve the tracking and erosion resistance of silicone rubber.

INFLUENCE OF SECONDARY WINDING TERMINAL CONDITIONS ON IMPULSE DISTRIBUTION IN TRANSFORMER WINDINGS

A simple computational method for the calculation of natural frequencies and transient voltage distribution in both primary and secondary transformer windings was developed. The equivalent circuits developed for different test conditions for transient analysis account for various inductive and capacitive couplings between various sections of the windings. The validity of the computational method was verified by comparison between the measurements and calculations on a model transformer. The influence of secondary winding termination on the measured natural frequencies in the case of a three phase winding model was studied. The natural frequencies measured for similar excitation of the primary and different terminal conditions of the secondary were significantly different, and the differences were as high as 300%.<<ETX>>

Partial discharge activities in transformer insulation under steep front voltages

Gassing problems and premature insulation failures of wind turbine transformers have been documented lately, indicating concerns with insulation reliability of power electronic converter-fed devices. This fact shows that the transformer insulation system degrades under high dV/dt voltages generated in wind energy plants at a faster rate than those operated under power frequency AC voltages of the same amplitude. To study such degradation processes, unlike the cases of AC or DC voltages, under high dV/dt voltages, interferences coming from the pulse generator and fast oscillatory input voltages pose substantial challenges. In this work, a method to evaluate the effect of voltage waveform characteristics such as rate of rise (ROR) on the extent of partial discharge activities, based on the changes in hydrogen content in the transformer oil/paper insulation system is investigated.

Statistical indicators of silicone rubber erosion in the DC inclined plane test

In this paper the leakage current signature in the DC inclined plane test is examined for statistical indicators of the incipient erosion of silicone rubber. The mean, skewness and kurtosis are examined as useful indicators of the dry-band arcing current causing erosion. The moving window method is employed during the test to obtain an enhanced resolution in time. Simultaneous thermogravimetric-differential scanning calorimetry analysis is used to predetermine the temperature at which erosion begins, and temperature measurements taken during the inclined plane test are correlated to the leakage current leading to erosion. During erosion, more stable discharges as well as a recognizable change in the statistical distribution of the dry-band arcing current is recognizable through the analysis employed.

Thermal characteristics of filled silicone rubber under laser heating

This work represents the continuation of the work presented by Meyer et al. (IEEE CEIDP, p.848-52, 2002), where thermal conductivity of room temperature vulcanized samples was tested. The thermal behavior of hot pressed silicone rubber filled at 30% and 50% concentrations with alumina trihydrate (ATH) and silica is now explored. A near-infrared laser and a thermovision camera were used to calculate thermal conductivities based on a thermal transient model. In addition to thermal conductivity results, the measurement of temperature in a modified inclined plane test highlights the importance of the thermal conductivity on tracking and erosion resistance.

Charge injection into enamelled wires studied by thermally stimulated depolarization current (TSDC)

The paper reports on measurements of thermally stimulated depolarization current (TSDC) that is applied to characterize changes in medium voltage rotating machine enamelled wires that have been exposed to ageing under a pulse width modulated (PWM) waveform. An elevated temperature polarization process is used which provides various information on the relaxation processes in multiple coated magnetic wires such as the disorientation of dipoles and the release of charges from trapping sites. The thermoelectrets were prepared at various polarizing fields and measurements of TSDC are carried out both as a function of time and temperature over the range of 23 to 200/spl deg/C. The various trapping levels, which have discreet energy differences having distinct peaks in the thermograms, are discussed on the basis of space charge polarization.

Estimating erosion on polymeric materials using a laser based model

A novel laser technique is used to study the degradation of silicone rubber composites, along with inclined plane test for 18 different composites, using ATH and silica as fillers, with various concentrations and mean particle sizes. The results from the constant temperature approach are compared with the results from inclined plane test by means of a statistical correlation tool. The two empirical relationships observed in the experiments have shown that it is possible not only to detect erosion on a silicone rubber surface by knowing the third harmonic component of the discharge energy, but also quantify such damages.