K. Sahitya Yadav

Bio: K. Sahitya Yadav is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Transformer oil & Pressboard. The author has an hindex of 3, co-authored 4 publications receiving 38 citations.

Understanding Corona discharge activity in titania nanoparticles dispersed in transformer oil under AC and DC voltages

Abstract: Titania (TiO2) nanoparticles dispersed transformer oil has high Corona Inception Voltage (CIV) and breakdown strength, and gets further enhanced on insertion of the barrier in the electrode gap. The volume of nanoparticles and the amount of surfactant have a strong influence on the dispersion of nanoparticles in transformer oil and were characterized through viscosity measurement, particle size analysis and by zeta potential measurements. It was also observed that with needle plane configuration, negative DC voltages have high breakdown voltage as compared to positive DC and AC voltages. Improvement in breakdown voltage with barrier can be obtained by placing the barrier at 20–80% of electrode gap from the tip of the high-voltage electrode. The breakdown voltage of needle plane electrode configuration was calculated based on normal distribution parameters. It was observed that the corona activity generates Ultra High Frequency (UHF) signal in the frequency range of 0.7–2 GHZ with its dominant frequency at 0.9 GHz and was characterized using a ternary plot. The discharge activity was studied by using a spectrum analyzer by operating it in zero span mode with 0.9 GHz as center frequency. It was observed that for a given voltage under AC and DC voltages, the magnitude of discharges is less with nanoparticles dispersed transformer oil. A pyrolysis study indicated no variation in the composition of the oil with the addition of titania. It was observed that the esters present in the oils were alkyl substituted oxalic acid and alkyl substituted sulfurous acid esters.

Understanding the surface discharge characteristics of thermally aged copper sulphide diffused oil impregnated pressboard material

Abstract: An attempt has been made to understand the impact of thermal aging with copper sulphide diffused oil impregnated pressboard (OIP) material for surface discharge inception voltage (SDIV) variation. The SDIV of Copper sulphide diffused OIP material reduces with increase in thermal aging. The UHF signals generated due to surface discharges with copper sulphide diffused OIP at the point of inception have frequency content in the range 0.7-1.5 GHz with its dominant frequency at 0.9GHz. HFCT measurement indicated that the rise time of injected current pulse due to surface discharge activity in the negative and positive half cycle of the AC voltage are about 0.8ns and 1ns respectively. It is also found that increase in thermal aging of copper sulphide diffused OIP, the amplitude and width of the current pulse increases. The magnitude of surface charge and its mean life is high with thermal aging temperature of copper sulphide diffused OIP material. Polarity of surface charge acquired by OIP due to AC corona charging depends on the surface condition of the pressboard material and its charge decay rate is lower than under DC voltage. Atomic Absorption Spectroscopy (AAS) and Energy Dispersive Spectrometry (EDS) study confirms the presence of copper sulphide on the surface of OIP and is found to be increased for copper sulfide diffused OIP material compare to unused OIP material. Scanning Electron Microscope (SEM) analysis indicates formation of cracks with thermally aged OIP material. Phase Resolved Partial Discharge (PRPD) study indicates that the surface discharge activity occurs near the peaks of the applied AC voltage at inception and at the rising portion of the applied AC voltage at higher voltages. It is found that the tensile and flexural strength reduces drastically for thermally aged copper sulphide diffused OIP material compare to unused specimen.

Influence of thermally aged barrier on corona discharge activity in transformer oil under AC voltages

Abstract: Corona Inception voltage (CIV) / breakdown voltage increases when the barrier is placed anywhere from needle tip to the 20% of the needle-plane electrode gap above which a marginal reduction in breakdown voltage is observed but its value is much higher than the breakdown strength of the electrode gap without barrier. Thermally aged oil impregnated pressboard insulation, as barrier, have high impact on CIV and breakdown voltage. The injected current due to corona activity, its rise time, measured using HFCT, in the negative and positive half cycle of the AC voltage measured to be 600 ps and 500 ps respectively. The shape of the current pulse is the same with the aged pressboard insulation barrier in the electrode gap. The corona inception voltage was measured by using UHF sensor and the signal has frequency contents between 0.5 -2.2 GHz. The surface charge measurements indicates that the amount of charge accumulated is high with thermally aged oil impregnated pressboard (OIP) than the unused material. Polarity of surface charge acquired by OIP due to corona charging depends on the surface condition of the material. To understand the mechanical behavior of the aged specimen, tensile test and flexural test were carried out and could conclude that ageing of material reduces the fundamental mechanical properties of the material. SEM analysis indicates that scission of fibers occurs due to thermal ageing at high temperatures. Partial Discharge studies on corona activity indicate, on corona inception, discharges occur near peak of the applied AC voltage, irrespective of position of barrier. The phase width of the PD generated due to corona discharge activity is slightly high with thermally aged specimen compared with unused specimen.

Understanding the dielectric properties of pressboard material thermally aged in Dibenzyl Disulphide (DBDS) included transformer oil

Abstract: Surface Discharge Inception Voltage (SDIV) of unaged and copper sulphide diffused Oil Impregnated Pressboard (OIP) material was studied by adopting Ultra High Frequency (UHF) technique. Copper sulphide diffusion into OIP material was carried out by thermal aging of pressboard material in Dibenzyl Disulphide (DBDS) added transformer oil. The SDIV reduces with increase of thermal aging temperature. The current pulse injected in positive and negative half cycle during the surface discharge of the AC shows the rise time of 0.8ns and 1ns respectively. The FFT analysis of UHF signal measured during surface discharge process has shown frequency content in the range 0.7–1.2 GHz with its dominant frequency at 0.9 GHz. The life of the charge presented on the pressboard increases with thermal aged specimen. Energy dispersive spectroscopy (EDS) analysis of OIP material indicates presence of copper and sulphur content. Frequency Domain Dielectric Spectroscopy (FDDS) studies indicate the permittivity(er), dissipation factor (tan(δ)), conductivity (σdc) increases and relaxation time (τ), coefficient (α) decrease with OIP material (in air / Nitrogen) aged at higher temperature in DBDS added transformer oil. Cole-Cole double relaxation model was used to understand the dielectric parameters of copper sulphide diffused OIP material. The DC conductivity of pressboard material aged in DBDS included transformer oil follows Arrhenius law and the activation energy decreases with increase in temperature of thermal aging.

A multiscale computational study of the effects of fluid flow and drug metabolism on drug delivery

Abstract: In this study, a heterogeneous multiscale method based mass transport model is developed for delivering drug into biological tissues. In the model, fluid flow and drug metabolism are modelled at the continuum scale, whereas the diffusion phenomenon is incorporated through macro-micro coupling. Darcy's law is used for fluid flow calculation, and the drug metabolism is determined using the Michaelis-Menten equation. Using the model, penetration and distribution of a drug in a tissue are investigated. Mainly, the effects of fluid flow and drug metabolism together with the particle size effects on drug delivery are explored. It is observed that the particles of sizes 10 and 100 nm can penetrate the tissue with advection; however, without advection, such particles accumulate around the capillary, the source of the drug to the interstitium. In fluid flow regions, drug metabolism may not affect penetration and distribution of drug; the results are similar to those without drug metabolism. However, metabolism may significantly reduce drug penetration in the absence of fluid flow. It is observed from the sensitivity analysis that the model is most sensitive to the parameters that model the transport processes of extracellular space while least sensitive to those that incorporate the intracellular processes.

Robust Classification of Partial Discharges in Transformer Insulation Based on Acoustic Emissions Detected Using Fiber Bragg Gratings

Abstract: Incipient discharges formed due to corona activity, surface discharge, and particle movement in transformer insulation are identified based on acoustic emission signals captured using fiber Bragg grating sensors and analyzed in the frequency domain. To improve the SNR of these signals, the use of an adaptive line enhancement-based technique is systematically explored through simulations, and the associated parameters are optimized. The noise-filtered spectra analyzed through ternary diagrams suggest the possibility of classifying the discharges which are further validated using appropriate classifiers. The experimental comparison of discharges generated in different oil media like mineral oil, nanoparticle-dispersed mineral oil, ester oil, and nanoparticle-dispersed ester oil reveals that the discharge characteristics are similar across multiple media, and the classification holds good.

Effect of TiO2 and ZnO Nanoparticles on the Performance of Dielectric Nanofluids Based on Vegetable Esters During Their Aging.

Abstract: Over the last few decades the insulating performance of transformer oils has been broadly studied under the point of view of nanotechnology, which tries to improve the insulating and heat dissipation performance of transformer oils by suspending nanoparticles. Many authors have analyzed the thermal and dielectric behavior of vegetable oil based-nanofluids, however, very few works have studied the evolution of these liquids during thermal aging and their stability. In this paper has been evaluated the performance of aged vegetable oil based-nanofluids, which have been subjected to accelerated thermal aging at 150 °C. Nanoparticles of TiO2 and ZnO have been dispersed in a commercial natural ester. Breakdown voltage, resistivity, dissipation factor and acidity of nanofluid samples have been measured according to standard methods, as well as stability. Moreover, it has been analyzed the degradation of Kraft paper through the degree of polymerization (DP). The results have showed that although nanoparticles improve breakdown voltage, they increase the ageing of insulation liquids and dielectric paper.

Comparison of breakdown voltage of vegetable olive with mineral oil, natural and synthetic ester liquids under DC voltage

Abstract: This paper deals with a comparative study of the negative and positive DC breakdown voltages of different insulating liquids namely two natural esters (an extra virgin olive oil (OO) without additive and rapeseed oil), two synthetic ester fluids (a methyleolate obtained by transesterification of olive oil and tetraester), and a naphthenic mineral oil (MO); an oils mixture namely 80% OO + 20% MO is also investigated. Statistical analysis of the experimental data is also performed. The obtained results are also compared with those obtained in our previous work under AC and lightning impulse voltages. It is observed that the negative and positive DC breakdown voltages of natural esters are higher than those of mineral oil. On the other hand, the DC breakdown voltage of (80% OO + 20 % MO) mixture shows comparable behavior to that of mineral oil. The DC negative breakdown voltages are higher than the positive ones in the tested liquids. It is also shown that the DC breakdown voltage values of the investigated liquids generally obey the normal distribution.

Failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion

Abstract: The life of transformer oil-paper insulation is determined by the insulation property of transformer cellulosic insulation. Existing studies have shown that sulfur corrosion poses a serious threat to the properties of transformer oil-immersed cellulosic insulation due to the reaction of corrosive sulfur dissolved in oil with copper winding. This paper investigated the effect of sulfur corrosion on the characteristics of transformer cellulosic insulation, cellulosic insulation winding samples with different degrees of sulfur corrosion were obtained through an accelerated thermal aging experiment. Then, short-/long-term withstand voltage tests for the cellulosic insulation winding were conducted. Experimental results showed that the contamination of sulfur corrosion had a minimal effect on the power–frequency breakdown voltage of the cellulosic insulation winding but considerably decreased its long-term electrical life. Combined with the analysis of the electrical/physical and chemical parameters of oil-immersed cellulosic insulation, the failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion was synthetically discussed and analyzed.

Failure Characteristics and Mechanism of Nano-Modified Oil-Impregnated Paper Subjected to Repeated Impulse Voltage

Abstract: Nano-modification is a prospective method for improving the electrical properties of transformer oil. In most situations, transformer oil combined with cellulose paper is used to construct an insulation system for power equipment, such as power transformers. However, the influence of nanoparticles on the electrical performance of oil-impregnated paper is still unclear. Therefore, in this paper, we identify the failure characteristics of both fresh and nano-modified oil/paper. Specifically, the accumulative failure characteristics of nano-oil-impregnated paper (NOIP) are experimentally determined. The space charge distribution and trap characteristics of fresh paper and NOIP were measured, and the effect of nanoparticles on the space charge behavior are then analyzed. Finally, we measure the microstructure of fresh paper and NOIP subjected to repeated impulses. The test results indicate that nano-titanium oxide (TiO₂) particles have a limited effect on the breakdown voltage of NOIP. However, the particles can dramatically improve the resistant ability of NOIP against repeated impulses. For the NOIP with a nano-concentration of 0.25 g/L, the improvement reaches 62.5% compared with fresh paper. Under repeated applications of impulse voltages, the space charge density of NOIP is much lower than that of fresh paper. The deep trap density of NOIP is much higher than that of fresh OIP, whereas shallow trap density is relatively lower. Micropores are generated in paper insulation subjected to repeated impulses. The amount of the generated micropores in NOIP is lower than that in fresh paper. Nano-TiO₂ particles suppress the accumulation of space charge in the oil paper insulation, which weakens the electric field distortion in the dielectric. However, nanoparticles reduce the accumulative damage caused by repeated impulses. The above two points are considered the main reasons to improve the resistant ability against repeated impulses.