Nandini Gupta

Interfacial charge behaviour at dielectric - dielectric interfaces

Multi-dielectric insulation systems are commonly used in high voltage apparatus. Interfacial charge accumulated at the dielectric - dielectric interface governs the dielectric behaviour of the composites considerably. In this work, we perform space charge measurements on polymeric composites of epoxy resin and polyethylene, to investigate the behaviour of interfacial charge at the interfaces, under various experimental conditions. Pulsed Electro-Acoustic (PEA) system is used for measurement of charge distribution. The interfacial charge at the discontinuities, in similar as well as dissimilar dielectrics, is studied. Interfacial charge is seen to develop at discontinuities within otherwise homogeneous materials. Accumulation of space charge above threshold fields, at interfaces of different materials, ultimately dominates the polarity and magnitude of accumulated charge.

Effect of thermal and humidity ageing on space charge accumulation in epoxy resin

Insulation systems are subjected to environmental stresses such as temperature and moisture. These atmospheric stresses cause ageing of polymeric dielectric materials through change in material properties. Under application of DC field, ageing might affect the accumulation of space charge within the dielectric material. As we are aware, space charge accumulation in a material has an important role to play in determining subsequent behaviour of the dielectric and its expected life. In this work, space charge measurement is carried out using the Pulsed Electro Acoustic (PEA) technique. Epoxy resin is chosen as the dielectric material on account of its prevalence in the high voltage industry. The specimens are subjected to thermal and and/or ambient (humidity) aging. For thermal aging, the samples are subjected to elevated temperatures for given time periods. Humidity ageing is carried out under environmental conditions with high moisture content. Space charge measurements are performed before and after aging. Also FTIR analysis is carried out to estimate the change in material properties due to heat/humidity treatment. A quantitative estimation of charge accumulation is done on epoxy samples aged under different conditions. Ageing markers like trap depth, trap controlled mobility are estimated and compared with unaged material. Charge trapping and transport mechanisms are investigated under different ageing conditions. Further, breakdown tests are also performed on the unaged/ aged specimens with an aim to correlate the charge accumulation and charge transport mechanisms to the breakdown strength of the material.

Tracking and surface degradation of barium titanate filled silicone rubber nanocomposites

Silicone rubber (SR) based composite insulators are being increasingly used in HV transmission lines because of their superior performance in polluted environments. The surface hydrophobicity of SR based insulators makes them ideal for use as outdoor insulation especially in polluted environments. Research in nanodielectrics in the last decade has shown that addition of nanosized fillers into bulk polymers often improve their electrical and non-electrical properties significantly. In this work, we study the effect of incorporating barium titanate nanofillers into room temperature vulcanized silicone rubber (RTVSR). Nano-sized barium titanate (BaTiO3) particles are used as fillers in the RTVSR matrix with volume fractions of 0.5%, 1%, and 2%. While preparing samples, mechanical mixing and ultrasonic mixing are used for reducing the agglomeration of nano fillers. Barium titanate fillers are expected to increase the permittivity of the composite, thus helping in electric field stress control. One of the most important issues in outdoor insulation is tracking. Tracking on the surface of the insulator leads to degradation, and even complete failure of the insulator. In this work, we study the effect of incorporation of nano-fillers on propensity of RTVSR to tracking and associated degradation. Inclined Plane Test (IPT) for erosion as per the IEC60587 is performed. Additionally, surface degradation tests are performed to estimate how the addition of nano fillers increases the resistance to degradation due to surface discharges. It is seen that SR filled with nanosized barium titanate fillers show better resistance to tracking as well as surface degradation.

Thermal aging assessment of epoxy-based nanocomposites by space charge and conduction current measurements

Modification in the physico-chemical properties under long term operating stresses affects charge injection, transport and trapping. Hence space charge measurements are expected to provide an indication of the extent of material degradation. In the present work, space charge measurements are carried out using the Pulsed Electro Acoustic (PEA) technique on thermally aged, epoxy resin and epoxy alumina nanocomposites. Time domain dielectric spectroscopy is carried out on thermally aged specimens in order to compute the change in real and imaginary permittivity. Epoxy resin (LY 556) is used as the polymer matrix, and aluminium oxide particles (size <;50nm) are used as a nanometric filler. Accumulated space charge density, threshold field, apparent trap controlled mobility, and trap depths are computed from the space charge measurements. Further, change in the material due to thermal aging is investigated through Fourier Transform Infra-Red (FTIR) spectroscopy. It is observed that the nanocomposites accumulate less space charge than the base resin. From the results, it is evident that space charge characteristics like charge density and threshold field may be used as effective aging markers. Through space charge measurements as well as time domain dielectric spectroscopy, nanocomposites are observed to be comparatively more resistant to thermal aging.

Estimation of interphase thickness of epoxy-based nanocomposites

Recent literature attributes the improvement often seen in the properties of nanocomposites to the interphase layer between the polymer matrix and the filler material. However, an understanding of the interphase in nanocomposites is still elusive. The present work aims at estimating the interphase thickness of epoxy based nanocomposites. For experimental studies, epoxy resin is used as the base polymer, and aluminium oxide (Al2O3) particles with an average size of 50 nm are used as fillers. The nanocomposites are characterized in terms of space charge density, conductivity, permittivity, and breakdown strength. All properties are measured for different nanofiller loadings (0.5 vol. % to 2 vol. %). The experimental findings reveal a correlation between interphase volume fraction and change in dielectric properties. It is assumed that the greatest change in property is achieved at that filler loading where the interphase volume fraction is maximized, provided adequate dispersion is achieved at all filler loadings. An approximate interphase thickness is estimated for epoxy alumina nanocomposites by using experimental results in conjunction with a simulation model. Accuracy of the estimated interphase thickness is seen to be dependent on the scatter in particle size and the uniformity of particle dispersion in the polymer matrix.

Effect of ageing on space charge distribution in homogeneous and composite dielectrics

Homogeneous and composite polymeric insulating systems under operation are subjected to various stresses, including thermal, causing degradation and deterioration to the material. Ageing increases charge accumulation within the volume of the material. In this paper, charge trapping and transport phenomena in thermally aged homogeneous epoxy resin and polyethylene are studied. Pulsed Electro-Acoustic (PEA) measurements are performed to obtain the spatial distribution of charge within the volume of a dielectric. Apart from the space charge behaviour at dielectric - electrode interfaces, the behaviour of interfacial charge at dielectric - dielectric interfaces is also investigated. To this end, composite dielectrics are created with epoxy and polyethylene as constituents. The effect of thermal ageing on the interfacial charge evolution and accumulation in dielectrics with interfaces is investigated. Due to thermal ageing of homogeneous materials, charge is seen to accumulate within the bulk of the material, and not only at the interfaces. In aged composites, interfacial charge evolves at the interfaces, as well as within the bulk of the material. However, space charge measurements in aged materials with interfaces require special care, as signals reflected from space charge sites within the volume of the aged dielectrics tend to distort the real signal.

Functionally graded material composites for effective stress control in insulators

Functionally graded materials (FGM) are materials in which one or several material properties vary continuously as a function of space. In this work, we are interested in a specific spatial distribution of dielectric permittivity with the objective of efficient electric stress control. This is of significance to solid insulators like spacers for GIS (Gas Insulated Switchgears), bushings, cable terminations, winding insulation, etc. In order to create an FGM, micro-sized fillers are incorporated into a base polymer matrix to obtain a spatial gradation of the filler percentage, resulting in a consequent gradation in the permittivity. Here, one dimensional (1D) FGM specimens are prepared by a sedimentation process. Bisphenol-A (LY556) epoxy resin is used as the base polymer and micro sized alumina (Al2O3) is used as a filler material. Relative permittivity is measured along the direction of the gradient of the volume fraction of microfillers, using an LCR meter. A linear variation of permittivity is observed. The experimental variation is matched against values obtained from a Finite Element Method (FEM) based numerical model developed to compute permittivity of the composite based on filler volume fraction. Further, the values are compared to those also predicted by several existing theoretical models. The consequent field distribution is also obtained.

Epoxy based high permittivity nanodielectrics for electric stress mitigation

In this work, we investigate the possibility of using nano-sized fillers in conventional polymeric insulating materials to obtain a high permittivity nanodielectric in order to mitigate field stress. In this work, Barium titanate (BT) fillers are used in an epoxy (Bisphenol A) polymer matrix. The average BaTiO3 nanoparticle diameter is 90 nm. Specimens with various filler contents up to 5% by volume are studied. The nanoparticles used are either unmodified or surface treated with 3-glycidoxypropyltrimethoxysilane (GPS). The preparation of nanocomposite has a marked effect on their eventual properties, so it is important to ensure that there is a proper dispersion of the nanophase material. The specimens are thoroughly examined under FESEM (Field Emission Scanning Electron Microscopy) in order to verify homogeneity of particle dispersion. Dielectric breakdown strength, permittivity and conductivity are measured. Dielectric permittivity measurements are performed using an LCR meter. An electrometer, with a built in voltage source, is used to measure dc conductivities of the specimens, from polarization and depolarisation current measurements. Analysis of the measured data indicates a small increase in breakdown strength of nanodielectrics with silane-coated nanoparticles compared to those with the as-received nanoparticles. Preliminary results indicate improvement in permittivity at the higher volume fractions. Conductivity is seen to increase with increasing field for all specimens.

Space charge estimation in epoxy-based nanodielectrics using complementary techniques

One of the major problems of polymeric insulation is their tendency to accumulate space charge at higher electric fields, especially in HVDC (High Voltage DC) applications. Nanodielectrics are an emerging area of research because of their potential as a novel insulating material in electrical apparatus and systems. Understanding space charge behavior in these new materials is crucial to their application in high field environment. In the present work, space charge behavior in epoxy alumina nanocomposites is investigated by using Pulse Electro Acoustics (PEA), Conduction Current (CC), and Electroluminescence techniques in conjunction. The PEA technique is used to measure accumulated charge density, and maps the space charge distribution in the volume of the dielectric. Polarization- Depolarization current measurements are carried out to extract conduction current density, dc conductivity, and permittivity. Electroluminescence measurements are related to injection and impact ionization of charge carriers. Space charge density, conduction current density, and electroluminescence counts, show a transition above a certain electric field. The electric field above which all three characteristics undergo a transition is associated with the threshold field for accumulation of space charge. Nano-dielectrics show lower space charge, and lower conductivity at higher fields than neat epoxy. The permittivity of the nanodielectrics is seen to be higher than neat epoxy at lower electric fields. At higher fields their permittivities are comparable. The complementary findings from all three techniques prove to be effective in characterizing the space charge in the nano dielectrics.

Space charge characteristics of thermally aged epoxy based nanocomposite

Space charge accumulation has been identified as one of the principal aging factors in polymeric materials, typically through electric field distortion and energy dissipative processes. Thus characterization of space charge behavior in aging insulating materials is important. In this paper, we study the space charge characteristics of epoxy and an epoxy-based nanocomposite. Bisphenol-A type epoxy resin (LY 556) is used as polymer matrix. Aluminum oxide (size 40 to 47nm, density 3.97 g/cm3 at 25°C) is used as nano filler. Pulsed Electro-Acoustic (PEA) technique and Electroluminescence (EL) measurement using a Photo Multiplier Tube (PMT) operated in photon counting mode are used in order to estimate space charge accumulation. Further, we investigate the effect of thermal aging on space charge in epoxy and the nanocomposite. Aging is characterised by estimating material changes through X-Ray Diffraction (XRD) analysis and Fourier Transform Infra Red (FTIR) spectroscopy. The nanocomposite is found to exhibit lower space charge accumulation and a higher threshold field for aging.