M. A. R. M. Fernando

Leakage current patterns on contaminated polymeric surfaces

This paper presents measurements of low-level leakage current (LC) patterns on naturally aged insulators and artificially contaminated material samples as well as insulaton. A nonlinear be- haviorof the LC has been observed. Possible causes for this behavior are discussed and its rela- tions to surface hydrophobicity and electric stress are described. In addition, neural networks are trained to recognize the LC patterns as well as to estimate their harmonic contents.

Ageing of silicone rubber insulators in coastal and inland tropical environment

This paper presents investigations on the performance of 33 kV silicone rubber insulators characterized by different creepage lengths, which aimed to find out whether the insulators could permanently work when electrically stressed beyond the recommended limits in polluted and clean tropical environments. The study was performed under natural field and laboratory conditions. The insulators tested included eight types of silicone rubber composite insulators, one type of hybrid silicone-ceramic insulator and one semi-conducting glazed porcelain insulator, while ordinary porcelain and glass insulators were used as reference. During the field investigation, two sets of the insulators were separately installed and energized in coastal and inland parts of Sri Lanka, being by that exposed to marine and clean tropical environments. Their performances were periodically evaluated by visual inspections and measurements of hydrophobicity class. After five years of field exposure, the insulator performances were evaluated in laboratory by measurements of leakage currents under clean fog conditions and of wet flashover voltage. A third set of the insulators was aged in laboratory for 1000 hours inside a salt fog chamber where the insulators were continuously energized and daily sprayed with salt solution for eight hours and left to rest for remaining 16 hours. This treatment represented conditions similar as those in the field i.e. insulators exposed to salt sprays during monsoons. The insulator performances were investigated by measurements of leakage currents and classifying their patterns into different categories, i.e. capacitive, resistive, non-linear, discharge and strong discharge types, by means of fast Fourier transform and short time Fourier transform analyses. It was found that the long-term field exposure yielded weaker insulator deterioration than the salt fog chamber ageing, which indicated for a possibility to increase the electric stress on silicone rubber insulators to levels higher than the ones used today on glass and porcelain counterparts.

Leakage currents on non-ceramic insulators and materials

This paper presents a review of leakage current (Lc) measurements on nonceramic insulators and material samples. It is divided into two parts: First, the field investigations of LC, include a development of LC measuring systems and how the on-line LC measurements correlate to the insulator performance as well as to the atmospheric and pollution parameters. Then, the laho. ratory investigations, describe how the LC are used to evaluate materials, to develop new testingprocedures and to predict insulator performance. An extensive bibliography and additional references is provided.

Performance of nonceramic insulators under tropical field conditions

This paper presents the results of investigations on the performance of nonceramic insulators in tropical field conditions. Six different types of 33 kV insulators (silicone rubber, EPDM composite, porcelain, and RTV coated porcelain) were installed at six different test sites exposed to marine, industrial and clean environments in Sri Lanka and Tanzania. The visual scrutiny, hydrophobicity class (HC) and surface conductivity were used for the evaluation of the field performance. The silicone rubber and RTV coated insulators preserved the hydrophobic properties although some biological growth was found on them. The EPDM insulators showed distinct surface changes. At the same time, damage due to dry-band arcing appeared on the porcelain insulators exposed to marine pollution.

Performance of coconut oil as an alternative transformer liquid insulation

Transformer mineral oil has been replaced by alternative oils such as synthetic oil and natural esters due to their biodegradability and environmentally friendly nature. This paper presents performance of coconut oil as such an alternative. Generally, as the other alternative oils, coconut oil has high conductivity due to the presence of free fatty acids. In this study, three different types of coconut oil samples consisting of virgin, copra and RBD (refined, bleached and deodorized) were initially tested by frequency dielectric spectroscopy (FDS) measurements to see how the conductivity was improved by dehydration and neutralization. The FDS results were fitted by inverse power dependence and Cole-Cole models to estimate the conductivity and response functions. Afterwards, a set of new coconut oil samples extracted from copra were thermally aged at 120°C under sealed conditions and compared with that of mineral oil. The performances of oil samples were evaluated using the test results of breakdown voltage, acidity, interfacial tension and FDS measurements under different aging periods such as 2, 5 and 7 weeks. Another 4 sets of new coconut and mineral oil samples were subjected to simulated thermal faults and electrical faults which include aging for 12 hours at 160°C, one hour at 200°C, exposing to partial discharges for four hours and subjecting to 20 low energy breakdowns. The performance comparisons were done by FDS measurements and dissolved gas analysis. In parallel, field-aged coconut oil samples collected from a sealed distribution transformer with 11 years of service were also tested by FDS measurements. In general, coconut oil shows its suitability as an alternative to the mineral oils for transformers, despite limitations found in some of their physical properties. It was found that the FDS results were in good agreement with chemical test results and with the estimated conductivity values.

Effects of biological contamination on insulator performance

Results of investigations on biological growths on different insulators, especially porcelain and silicone rubber ones, are presented. The insulators have been exposed to tropical weather under polluted and clean conditions in Sri Lanka and Tanzania since 1995. Algae contamination was found on the insulators at the clean locations. The algae were identified as green algae. After performing a series of laboratory tests, it was concluded that the effect of algae was not too strong on the contaminated silicone rubber insulators. Suggestions how to control the algae growth are presented.

Leakage current patterns on artificially polluted composite insulators

This paper presents results and analyses of different leakage current (LC) waveforms on porcelain, RTV coated and polymeric composite insulators during the clean fog pollution testing under laboratory conditions. The resulting LC waveforms recorded with different time lapses after contamination, are compared both in, time and frequency domains. Flashover voltage measurements are also presented. The waveforms become deformed from sinusoidal shape as voltage level increases. A neural network is trained to recognize the patterns and to evaluate their third harmonic content. The results are compared with fast Fourier transform (FFT) analyses of the patterns.

Analysis of Insulation Characteristics of Coconut Oil as an Alternative to the Liquid Insulation of Power Transformers

This paper presents the experimental results of a study, which has been carried out to analyze the insulation characteristics of coconut oil in order to check the suitability as the liquid insulation in power transformers. In this study dielectric response measurements were carried out in frequency domain at temperatures of 25degC, 50degC, 80degC. In addition, results of mineral oil samples were taken for comparison. Further, breakdown voltage was also measured on samples. Based on the results, relative permittivity and conductivity values were obtained at room temperature (25degC) and elevated temperatures. Finally, the activation energy values were calculated on each sample. The dielectric breakdown voltages of the five coconut oil samples were in the range of 9.3 kV to 18.4 kV, which should be above 50 kV according to the IEC60296 standard. Conductivity values of coconut oil at room temperature were in a range from 10-11 to 10-12 S, which is considerably higher than that of new mineral oil. These results imply that further purification is required to bring coconut oil to the required standard to use as an insulating medium in transformers. Effort was done to identify the effect of temperature on dielectric response measurements of oil insulation system and correlation of it with the conductivity. The activation energy of five coconut oil samples was around 0.1, which is less than that of mineral oil, i.e. 0.4. This implies that the increase of conductivity with temperature is considerably low in coconut oil, when comparing with mineral oil.

Analysis of leakage current wave forms for field-aged and new composite insulators

Results of measurements and analyses of leakage current (LC) patterns on field-aged and non-aged insulators are presented. The field-aged insulators were exposed to marine and industrial pollutions as well as to tropical weather conditions for one year. The LC measurements were performed on them under natural and artificial contaminated conditions. The non-aged insulators were previously used for pollution testing. In most cases, the LC patterns on the field-aged insulators were sinusoidal in shape whereas they were highly deformed on the non-aged insulators. These different LC patterns were quantitatively analysed by means of a circuit model representing a polluted and wet insulator surface. A modification of the previously developed neural network for evaluation of the third and fifth harmonic contents of the LC patterns is also presented.

Condition assessment of stator insulation during drying, wetting and electrical ageing

Stator insulation failure due to moisture ingress is one of the common reasons for generator failures. Non-destructive tests such as DC ramp, insulation resistance (IR) and frequency domain spectroscopy (FDS) are usually conducted to assess the conditions of such insulations. This paper presents a laboratory investigation of generator spare windings under drying, wetting and cyclic electrical ageing conditions. First Dc ramp, IR and FDS tests were conducted on asphalt-mica and epoxy-mica insulated spare windings to obtain initial finger print values. Then the drying effects was investigated by DC ramp and FDS measurements conducted on asphalt-mica insulated winding samples by drying them up to three days at 80°C. The wetting effect was studied by wetting the polyester-mica insulated winding samples in a water bath up to one week at 25°C. Further a correlation has been established between the moisture content and FDS results by drying and wetting of two asphalt-mica insulated samples at 70°C and the room temperature respectively. The ageing effects was obtained by electrically stressing two asphaltmica and two epoxy-mica insulated winding samples at 300% of the rated voltage up to 9 weeks in a cyclic manner. For all cases, conductivity (σ) and parameters representing the response function were estimated by the Davidson-Cole and inverse power dependence functions. The estimated parameters under DC ramp, IR and FDS tests were in similar range for new samples. During ageing and drying, a clear variation was noted only on the σ values and FDS results. It was found that FDS results and estimated parameters provide useful information about the condition of generator insulation.