Arpan Kumar Pradhan

This paper primarily focuses on the transformer oil ageing sensitive diagnostic approach utilizing the inflection frequency derived from the differential capacitance spectrum. Continuous transformer operation results oxidation in oil, leading to the formation of low molecular weight (LMW) acids which accelerate cellulose hydrolysis and insulation degradation. Therefore, accurately monitoring of this acid-induced ageing is crucial. Considering this fact, for emulating different acid aged stages (0-30 years), formic, acetic and levulinic acids with different proportions are mixed with mineral oil and its different nanofluids (doped with 0.01 g/L of Alumina (Al₂O₃), Titania (TiO₂) and Hexagonal Boron Nitride (h-BN) separately). Then, Nuclear Magnetic Resonance (NMR) spectroscopy and Frequency Domain Spectroscopy (FDS) are performed to analyze the effect of acid ageing on the physicochemical changes and the dipolar orientation of liquid dielectric. From FDS spectra, the inflection points in differential capacitance spectrum are derived from which the defect densities of the prepared oil samples are estimated. From the results of inflection point and defect density, three ageing sensitive parameters are identified and their empirical correlations with ageing severity were established.

This article analyzes the effects of aging on the dielectric properties of mineral oil (MO), natural ester (NE), and synthetic ester (SE), with artificially acid-induced changes estimated through the analysis of the relaxation frequency distribution (RFD) spectrum. For this purpose, liquid insulation samples having equivalent acid-induced aging periods of 0, 5, 10, 15, and 20 years are emulated by adding formic, acetic, and levulinic acids to the mineral and ester oils. Thereafter, Fourier transform infrared (FTIR) spectroscopy and relaxation current measurement are conducted on these aged samples to investigate the impact of aging on physicochemical changes and different polarization characteristics of the insulation liquid, respectively. The RFD spectrum is derived from the relaxation current for different-aged samples. The analysis of the RFD spectrum reveals different dipolar relaxation-sensitive zones. Every zone of the RFD spectrum carries signature information about the dipolar characteristics relating the aging state with respect to the acid content of the liquid insulation. From the RFD spectrum, four aging-sensitive parameters (peaks and corresponding frequencies of the RFD spectrum) are identified, and based on these parameters, two empirical relations are derived. Experimental investigation shows that the four parameters can be good indicators of the aging status of liquid insulation.

This paper focuses on the impact of injected space charges on the hybrid nanoparticle (NP)-induced modulation of molecular orbital (MO) energy distribution in epoxy insulation. Quantum chemical calculation (QCC) reveals that impact of injected space charges on the MO distribution is mitigated by the stabilizing ability of the rutile-TiO2 and AlN molecule. Further, density of states (DOS) analysis reveals deeper electron traps (3.24 eV and 3.12 eV) under the presence of trapped electrons. On the other hand, the trapped holes cause a slight increment in the hole trap levels from 0.80 eV to 1.28 eV under the presence of an electric field. Furthermore, 2 stage isothermal relaxation current (IRC) measurement is employed, based on which the trap distribution shows that incorporation of hybrid NP prompts deep electron (0.93 eV) and deep hole (0.90 eV) traps to develop in nanocomposite (NC) insulation. In addition, a time dependent density functional theory (TD-DFT) based excitation characteristics is investigated to understand the influence of the charge recombination induced luminescence (RIL) on the complex mechanism of electronic transition, which is responsible for the modulation in the intermolecular charge transfer (ICT). As a whole, this study presents a new approach of studying hybrid TiO2 /AlN NP-induced inhibition of charge injection into the epoxy insulation.

This research investigated the influence of insulating (Al2O3), semiconducting (TiO2), and conducting (Fe3O4) Nanoparticles (NPs) on the breakdown strength of Nomex-based polymeric Nanocomposites (NCs). Seven samples were fabricated and thermally aged at 145 °C for 600 h to simulate accelerated degradation. Fourier Transform Infrared Spectroscopy (FTIR) was conducted to unaged, 300 h, and 600 h aging durations to examine the chemical alteration and understand the NP-induced resistance of meta-aramid polymeric chains against the thermal aging. The AC Breakdown Voltage (ACBDV) was measured every 100 h of aging, and the results were analyzed using the Weibull distribution to extract the scale parameters, shape parameters, and p-values. Cumulative breakdown probabilities at 1%, 10%, and 50% risk levels were evaluated to assess the impact of different NPs on the breakdown mechanism. After 600 h of aging, the incorporation of 1 wt% Al2O3 improved the BDV of pristine Nomex by 28.2%. At a 50% risk probability, BDV enhancements of 63.16% (Al2O3), 48.89% (TiO2), and 42.39% (Fe3O4) were achieved, demonstrating the superior performance of NP-reinforced Nomex insulation under thermal stress..

In this study, aging impact analysis on dielectric behaviour of mineral oil, natural and synthetic ester is performed and their aging status is estimated using the hump phenomenon obtained from dielectric modulus spectrum. For this purpose, equivalent aging of 5, 10, 15 and 20 years of each oil are emulated by mixing formic, acetic, and levulinic acid with mineral and ester oils. Thereafter, Fourier Transform Infrared Spectroscopy and Frequency Domain Spectroscopy (FDS) are conducted on the prepared aged samples to assess the impact of aging on physicochemical alteration and low-frequency interfacial polarization characteristics at different temperatures, respectively. Based on the FDS results, dielectric modulus spectrum is obtained from which hump phenomenon is identified. Besides, another important factor i.e. conduction dominance factor (CDF) is introduced to quantitatively investigate the dominance of conduction mechanism over relaxation polarization. Based on the experimental results, a correlation with the aging status of liquid insulation with the three aging sensitive parameters (hump frequency, hump peak and CDF) is established which can reliably be used for estimation of their aging status.

In this work, a novel method is adopted to sense the condition of the transformer oil based nanofluid insulation considering moisture content by dielectric relaxation current. For this investigation, mineral oil, vegetable oil and their nano-fluids (TiO₂ and Al₂O₃ based) impregnated paper insulation prototypes with different moisture content are prepared in the laboratory. It is to be mentioned here that, lifespan of a transformer predominantly relies on the state of its insulation which consists of oil and paper. From the experimentally measured dielectric relaxation current data, Relaxation Frequency Distribution (RFD) spectrum of the prepared samples is evaluated. From RFD spectrum, the condition of oil and paper can be sensed distinctly. In addition, from the RFD spectrum, various parameters are identified to be moisture sensitive. Using these parameters, empirical relations are proposed to sense the moisture content and activation energy which are further verified experimentally. The experimental results show that the suggested moisture sensing method is reliable for assessing the insulation condition which can pave a new way for maintenance plan of transformers.

This article proposes a methodology to estimate the aging state of the Nomex based dry-type nano-composite (NDNC) insulation. For this purpose, seven types of NDNC insulation samples are prepared in the laboratory for experimental investigation. The test samples are undergone through accelerated thermal aging at 145°C for 600 hours in steps of 100 hours. The dielectric spectroscopy measurements are conducted on the NDNC type insulation samples at each aging stages. Three aging sensitive parameters (i.e. transfer function pole, de-trapped charge and hopping conductivity) are extracted from the experimental results obtained by dielectric spectroscopy (i.e. polarization and depolarization current (PDC) and complex permittivity). From the experimental results, it is observed that these three parameters are altered with the duration of the thermal aging. This fact signifies that these parameters can be utilized as aging sensitive markers. Therefore, using these three aging sensitive parameters, three empirical relationships are derived to correlate the thermal aging status of the NDNC insulation. Finally, the derived relationships are validated through different set of test samples having known aging status. Hence, this experimental investigation reveals that the proposed technique can suitably assess the thermal aging state of the NDNC insulation.

In this paper, polarization and depolarization current (PDC) measurement and quantum chemical calculation (QCC) are performed to investigate the impact of nano-filler addition on the space charge characteristics of the epoxy based polymeric insulation. In this context, single type and hybrid type epoxy composite insulation samples are prepared by incorporating h-BN nanoparticle (NP) along with a combination of h-BN and Al ₂ O ₃ nanoparticle, respectively. After that, hole and electron de-trapping current are estimated from experimentally recorded PDC, which shows that incorporation of nano-fillers results in a suppression of de-trapping of charges (i.e. hole and electron). From the de-trapping current, the distribution of electron and hole traps is estimated, which stipulates the nanofiller induced modification on the charge mobility in the bulk insulation. Further, QCC analysis reveals that the electron trap depth is 2.18 eV and 2.26 eV for single type and hybrid type nanocomposite (NC) molecule, respectively. However, the trap depth levels of holes are estimated to be 3.34 eV and 3.02 eV for single type and hybrid type NC molecule, respectively. In addition, this paper has proposed an approach to study the impact of nanofiller addition on the intra-molecular space charge migration in polymer based insulation through the distribution of molecular orbital (MO) energy levels and electron cloud.

This article presents an advanced diagnostic technique to estimate the moisture content (m.c.) in the XLPE cable insulation by investigating dielectric relaxation current. For this purpose, the experimentation is carried out on the short-section of the moisture injected XLPE cables. The relaxation characteristics of the XLPE cable insulation are analysed using the relaxation frequency distribution (RFD) function. The RFD function reveals the relaxation phenomenon of the XLPE cable and correlates with the clustering model. Based on the relaxation frequency distribution, five moisture sensitive parameters (i.e. two relaxation peaks, their ratio and corresponding relaxation frequencies) are identified. These five moisture sensitive parameters are correlated with moisture content by empirical relationships. Finally, another set of laboratory prepared XLPE cable samples with a known moisture content are used for experimentally validating the proposed technique. This experimental investigation illustrates that, the relaxation frequency corresponding to the highest peak of the RFD function can reliably be used for estimating the moisture content of the XLPE cable insulation in power system network application.