Ramanujam Sarathi

One of the major problems in an electrical insulation structure is its failure while operating at normal voltage stress which is due to electrical treeing. It is well-known that this electrical tree grows progressively and is damaging locally. To analyze the electrical treeing in the laboratory, not being cost effective, computer simulations are used. We have employed a known cellular automata method with new parallel processing techniques to reduce the computation time considering the available parallel processing systems. The major advantage is that the cellular automata algorithm inherently exhibits parallelism paving the way for ample exploitation. Moreover the parallelisation has helped to understand various aspects of electrical treeing also in a simple way.

This paper examines the impact of various nanoparticles on ester fluids with a special focus on their usage towards power transformers. The precautionary measures to be considered on the nanofluids such as preparation methodologies with an appropriate surfactant and its stability is well elucidated. The electrical double layer (EDL) formation around the nanoparticles on its diffusion in the insulating fluid is explained by its different layers away from the particle surface. The partial discharge of ester nanofluids with different detection methods is elaborated on its comparison with conventional IEC 60270 measurements. The field configurations on ester-nanofluids govern the breakdown mechanism with variations in the streamer patterns. The equation of relaxation time towards breakdown is valid only when it is lower than the initiation time for streamers. The flow charges induced at the solid/liquid interface inside transformers depends on the structure of the nanofluid and the condition of pressboard/paper insulation. The impact of different concentrations of nanoparticles on ester nanofluids observes a change in its flow behaviour affecting the streaming current. The permittivity of nanofluid depends on the polarization of nanoparticles where the Clausius-Mossotti equation governing this mechanism is explained towards ester-nanofluids. The viscosity of nanofluids observed no significant variation whereas the other physio-chemical properties such as flash point, interfacial tension, and oxidation stability improved depending on the type of nanoparticle. The addition of metal-oxide nanoparticles on ester fluids increases thermal conductivity with different models proposed based on the structure and shape of a nanoparticle. The impact of ageing on nanofluids observes an instability over a longer ageing duration with specific nanoparticles which should be better understood before implementing them in real-time power transformers.

This paper reports the critical reviews on the gassing tendency of different insulating fluids along with the precautionary measures to be considered during their fault diagnosis in transformer insulation. The experimental techniques and procedures for identifying the gassing due to electrical and thermal stress along with the stray gassing phenomenon has been elucidated. The different interpretation schemes used for determining the faults in transformers results in unexpected errors when the historical data relating to mineral oil is used for the other alternative fluids. Mineral oil and natural ester show a positive gassing tendency compared to synthetic ester which exhibit a negative gassing tendency. The stray gases are mostly due to breakage of C-C bonds under normal operating temperature of transformer. Among the different hydrocarbons, hydrogen and ethylene are more predominantly formed under lower temperatures. The silicone oil and ester fluids are more stable even under localised hot spots simulated observing a lesser gassing compared to the mineral oil. The impact of additives along with the oxygen and water content in the insulating fluids can lead to the stray gas’s causing confusion towards the identification of actual faults occurring in transformers. Furthermore, the regeneration of insulating fluids using different adsorbents reduces the gassing tendency depending on the number of cycles used for its reclamation.

Ultra-high-frequency signals are generated due to particle movement, floating conductors, corona, and surface discharges in gas-insulated switchgear. The ultra-high-frequency signal generated due to particle movement is independent of operating pressure and applied voltage. The bandwidth of the ultra-high-frequency signal formed due to corona and surface discharges vary with applied voltage. In a ternary plot, each type of discharge has a unique location. If a variety of discharge occurs simultaneously, it is difficult to classify the type of discharges through the ternary plot. It is demonstrated that the type of discharge can be classified efficiently by adopting a multi-class support vector machine. © 2011 Copyright Taylor and Francis Group, LLC.

This paper deals with surface discharges and flashover voltages on epoxy resin nanocomposite surfaces. Factors, such as, water droplet number and volume, droplet conductivity as well as the distance of the droplets from the electrodes affect both surface discharges and flashover voltages. Moreover, the degradation of the insulting material is indicated in this paper.

Converter transformers play an important role in the high-voltage DC power transmission system. The insulation of these transformers is stressed by both AC and DC voltages. Particle contamination is one of the major problems in insulation structures, as they generate partial discharges. A methodical experimental study was carried out to understand the partial discharge formation because of single macroscopic conducting particle movement in transformer oil at different temperatures under DC voltages using ultra-high-frequency (UHF) technique. It is observed that the single particle moves in the vertical electrode gap under the DC voltage, and generates UHF signal in the range of 1-3GHz. At high electric fields, the levitated particle sometime floats in the oil medium and as and when the applied voltage is removed, the particle slowly drops down. When it comes in contact with ground electrode, it generates UHF signals with the frequency content lying in the range of 1-3GHz. Oil breakdown tests were carried out (without any particle in the oil electrode gap) under DC voltages and fast Fourier transform (FFT) analysis of the signal generated by the UHF sensor because of breakdown indicates that the frequency content of the UHF sensor signal lies up to 1GHz. © 2009 The Institution of Engineering and Technology.

Thermal aging of natural ester oil shows drastic reduction in partial discharge inception voltage (PDIV) and a significant variation is observed only above a certain aging time, under AC, DC, high frequency AC voltages and with harmonic voltages with different total harmonic distortion. Weibull distribution studies on PDIV measurements indicate a reduction in scale parameter (α) with increase in thermal aging temperature. A characteristic reduction in breakdown voltage was observed with the thermally aged ester oil, under AC, DC and standard lightning impulse voltage. The breakdown voltage variation with aged ester oil follows normal distribution. Ultraviolet (UV) analysis of ester oil thermally aged at 160°C has revealed a regular shift of the derived absorbance parameter to longer wavelengths. The interfacial tension and turbidity exhibits an inverse relationship with the thermally aged ester oil. Gas chromatography/mass spectrometric analysis of the thermally aged ester oil predicted the formation of more carboxylic acids and ketones with aging duration. The steady-state fluorescence on thermally aged ester oil exhibits a shift in its emission profile, which is in tandem with the UV absorption spectroscopic analysis. Fluorescence analysis can be adopted as a real-time monitoring tool in transformers, to understand the condition of liquid insulation. The viscosity dependence on the wavelength of derivative absorption maxima follows a direct relationship with the thermally aged natural ester oil.

Titania (TiO2) nanoparticles with cetyl trimethyl ammonium bromide (CTAB) surfactant dispersed in transformer oil has higher corona inception voltage than the normal transformer oil, under AC, DC, and composite AC and DC voltages. It is observed that corona inception voltage is lower under composite voltages compared to AC and DC voltages. The corona activity radiates ultra high frequency (UHF) signals with its dominant frequency near 1 GHz. Operating a spectrum analyzer in zero span mode, the UHF signals generated from the corona activity, the number of discharges is observed to be more under composite voltages than under AC voltages, both at inception and at higher voltages. A reduction in interfacial tension and an improvement in flash point is observed in nano-titania with surfactant dispersed transformer oil. The addition of surfactant reduces the turbidity of the nanofluid. Phase Resolved Partial Discharge (PRPD) analysis with UHF signals measured, indicates that corona discharge activity occurs around the peak and the pre-peak rising portions of the supply voltage. It is observed that more discharges occur in the positive half cycle for AC superimposed with positive DC voltage and in the negative half cycle for AC superimposed with negative DC voltage. The magnitude of UHF signals formed due to corona activity is less with nanofluid, irrespective of voltage profiles.

Water droplet residing on surface of roughened silicone rubber generates corona activity at much lower voltage compared with the virgin specimen. An increase in conductivity of water droplet, under AC voltage, shows a reduction in corona inception. Further, a combination of time series analysis and fractal feature extraction techniques are used to classify the different phases that occurs on application of AC voltage to the water droplet sitting on silicone rubber specimen, in the electrode gap. The fractal dimension and the lacunarity were calculated using the 2D Hilbert energy spectrum obtained with leakage current measured during water droplet initiated discharges, shows inverse relationship.

This study reports the effect of adding silver sulfide to mineral oil and mixed fluid on their static electrification and visco-elastic properties. The flow electrification studies indicate an increase in streaming current for higher silver sulfide concentration with the hysteresis pattern indicating a higher current during the decrement of disc velocity. The transition from loss modulus to storage modulus was visible in the mixed fluid compared to mineral oil. Further, the effect of silver sulfide has reduced the storage modulus with frequency sweep and temperature sweep testing on both mineral oil and mixed fluid.