Search

DC power transmission is gaining considerable importance and converter transformers form an important component in it. An insulation of these transformers is stressed by both ac and dc voltages. Particle contamination is one of the major problems in insulation structures as it generates partial discharges (PD). If a particle is moving between the ground electrode and barrier, the magnitude of partial discharges formed is less compared to that of its movement between two metal electrodes. On the other hand, when the particle is lying over the barrier insulation material, it does not lead to any partial discharge activities as observed under ac/dc voltages. Broadband measurement of UHF signals produced by partial discharges due to particle movement in transformer oil under dc voltages, at high applied electric fields, indicates that the frequency content of the signal lies in the entire UHF range of 300 MHz-3GHz. Narrow band measurements of UHF signal due to PD formed by particle movement, at 1GHz, indicates that, in the presence of barrier, an increase in applied voltage results in increase in number of partial discharge pulses. This phenomenon occurs in converse in oil gap without any barrier. In the presence of barrier, the sequential movement of particle occurs and the magnitude of discharges increases with increase in applied voltage, irrespective of polarity of the dc voltage. The number of discharges that occur under ac voltage is high compared to the dc voltages. Index Terms--Partial discharges, UHF sensors, oil insulation, power transformer insulation.

In recent times, liquid nitrogen ([LN.sub.2]) is used as an insulant as well as coolant in high temperature superconducting power equipments. One of the major problems in the liquid insulation is the particle contamination, which can generate incipient discharges that can lead to complete failure of the insulation system during operation. An attempt has been made in the present study to understand the feasibility of using Ultra High Frequency (UHF) sensors for identification of partial discharge formation due to particle movement in liquid nitrogen under dc voltages. It is observed that partial discharge formed in [LN.sub.2] radiates UHF signal. For the purpose of comparison, the studies were also carried out with air as an insulating medium. The UHF signal generated under dc voltage characterized by measurement of its amplitude, rise time, duration of signal and through frequency domain analysis. It is observed that intensity of partial discharge due to particle movement in nitrogen is high under negative dc voltage compared to positive dc voltage. Also, the frequency content of UHF signal generated due to particle movement in liquid nitrogen lies in the entire bandwidth of 1-3 GHz. Index Terms--Liquid nitrogen, insulation, partial discharges, UHF technique, cryogenic, HTSC.

In the present study, the influence of water ageing on the surface characteristics of epoxy nanocomposites was analyzed through Atomic Force Microscopy (AFM) studies. The hydrophobic properties of the epoxy nanocomposite material were analyzed through contact angle and diffusion coefficient measurements. Fractal dimension were calculated by adopting a Multi Resolution Signal Decomposition (MRSD) to the 1DAFM signal through power spectral density calculations at each level of decomposition. 2D-MRSD studies were applied to AFM pictures to understand the surface characteristics of the material. The result of the study shows the presence of high and low frequency signals indicating characteristic variation in surface roughness of the material. The surface roughness parameter measured through the MRSD technique indicates that the surface roughness is lower for epoxy nanocomposites (up to 5Wt%) compared to pure epoxy resin. It is observed that the fractal dimension and the root mean square roughness values vary proportionately. Also it is realized that diffusion coefficient and surface roughness of the material shows an inverse relationship. Index Terms--Insulation, nanocomposites, surface roughness, contact angle, hydrophobicity, atomic force microscopy, wavelets, Fractals.

In the present paper, the tracking phenomena of silicone rubber material has been studied under ac voltages, with ammonium chloride as a contaminant. The characteristic changes in the tracking time of the aged specimens are analyzed through leakage current measurement. The observed trends in the leakage current during the tracking study are analyzed using a moving average current technique. The contact angle of the specimen varies with the type of aging, multiresolution signal decomposition technique was adopted to identify the characteristic change in the hydrophobicity levels of the specimens using the standard deviation multiresolution analysis curve. It is concluded that the material with less contact angle where the leakage current magnitude is high, allows tracking to occur early. Index Terms--Contact angle, hydrophobicity, leakage current, moving average, multiresolution signal decomposition (MRSD), surface discharge, tracking, wavelet.

This work reports an experimental and theoretical analysis of the rheological properties of mineral oil-based SiO2 nanofluid for their potential applications in transformer insulation. The flow electrification mechanism on the nanofluids with different surfactants such as cetyl trimethyl ammonium bromide (CTAB), oleic acid, and Span 80 is studied using a spinning disk technique. The results show a higher streaming current for the nanofluids with CTAB as a surfactant compared to oleic acid and Span 80. The rheological behavior of nanofluids is explored with the double gap concentric cylinder geometry. The variation of shear stress with shear rate follows a power law relationship along with a yield stress observed for all the nanofluids. A transition is seen from storage modulus to dominant loss modulus for the nanofluids during the frequency sweep analysis, whereas no transition is observed in the case of mineral oil. In addition, regression analysis using artificial neural network (ANN) algorithms are performed on the experimentally measured viscosity of the nanofluids in order to estimate theoretical parameters and provide insights into the streaming current formation. The desirable rheological characteristics of nanofluids are identified for achieving enhanced insulation performance in transformers. [ABSTRACT FROM AUTHOR]

This article reports the static electrification of ester nanofluids for its use in transformers adopting the spinning disk model. The results show that the streaming current is higher for the nanofluid involving surfactant compared to its effect without surfactant. The streaming current observed is negative at the fluid–metal interface and positive at the fluid/pressboard interface. The streamline pattern along with surface velocity magnitude is simulated under different disk velocities using COMSOL and the convection current model is formulated for the identification of streaming current. The mass transfer rate with surfactant-based nanofluid shows a shift from transport regime to reaction limited regime at higher Reynolds number. Furthermore, the interfacial zone of nanofluids is discussed for the streaming behavior considering its influence with the addition of benzotriazole (BTA). [ABSTRACT FROM AUTHOR]