Supriyo Das

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.

Space charge mapping and conduction current measurements in epoxy and polyethylene

The study of the effect of space charge within dielectric materials is important with respect to understanding the mechanisms of aging and breakdown. Accumulation of space charge in insulation can accelerate degradation through local field enhancement, and consequent ionisation/excitation and electromechanical energy storage. Aging and breakdown of the polymers are related to the release of the electromechanical energy around trapped charges. In this work, space charge mapping and conduction current measurement are carried out on similar dielectric specimens. Quantitative information about the space charge within the dielectric volume are obtained from these experiments e.g. space charge distribution, charge transport mechanisms. These are expected to give us an insight into the aging and breakdown process in polymeric dielectrics. The space charge distribution is mapped using Pulsed Electroacoustic technique. Conduction current measurement is carried out using three electrode systems. In both these measurements, dielectric material is polarised for a given time and then depolarised for the same duration under dc field. Aluminium films are used as the electrode material. Two different types of polymeric materials are used in this work in order to understand the effect of material properties. These are epoxy resin which is an amorphous material, and polyethylene which is a semi crystalline material. The information obtained from these techniques about the charge trapping and detrapping are used to correlate the operative charge accumulation and transport processes. Further, the effect of electric field on these processes is studied.

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.

Study of space charge characteristics in epoxy resin and its nanocomposites

In recent times, there has been increased awareness of the fact that space charge within an insulating material considerably governs its dielectric behaviour. In this work, we study the nature of space charge trapping and detrapping in epoxy specimens, with and without nano sized fillers. After application of a poling field for a specific period of time, depolarisation characteristics are obtained by removing the voltage and placing the specimen under short circuit conditions. The release of space charge from deep traps provides a current in the external circuit which is measured using an electrometer. Recombination and transit time is considered to be negligible. Trap depth is calculated on the basis of depolarisation characteristics [1]. The trap depth distribution is studied with variation in specimen thickness, applied electric field and temperature. Further, the effect of ageing is studied. In order to estimate the effect of fillers, alumina and other nano fillers are incorporated into the bulk epoxy resin to investigate whether this affects the trapped charge distribution.

Space charge accumulation in epoxy-PET composites

Composite insulation systems are commonly used in high voltage apparatus. In the presence of multiple materials, interfaces between the materials assume considerable significance. Epoxy-PET composites are used as class B insulating materials in transformers but little is known about the interfacial charge behaviour in such a system. In this work, a comprehensive study is undertaken to understand the charge dynamics at the amorphous (epoxy)/semi-crystalline (PET) interface of Epoxy-PET composites. On application of electrical stress, mismatch in permittivities and conductivities at dielectric-dielectric interface leads to accumulation of charge at the interface. This charge accumulation is governed by charge injection at the electrodes and generation of charge within the material due to dissociation. In this work, the Pulsed Electroacoustic (PEA) technique is used to map the space charge distribution in epoxy-PET insulation system. The present study is aimed at understanding the phenomena of space charge distribution in epoxy-PET multi dielectric material. The two-layer multi-dielectric system is first oriented such that the PET layer lies proximal to the high voltage electrode. Subsequently, the epoxy layer is placed in contact with the high voltage electrode. Apart from investigating the space charge behaviour in the specific composite, the broader objective of this work is to understand the effect of a dielectric-dielectric interface on the space charge dynamics of the composite dielectric. Aluminium is used as the electrode material.

Optimal placement of protective devices in a distributed power system

Distribution networks are designed for reliability. In all physical systems, reliability is dependent on the system configuration that is on the arrangement and connection of various components that make up the system design. In a distributed generation based power system, island operation of micro-grid is made possible by the protective devices present in the system. Hence their optimal location will ensure that maximum number of customers are supplied with energy and thus improve reliability. Increase in the number of protective devices will ensure high reliability of power supply but this is limited by cost considerations. In this paper, the optimal number and location of protective devices is obtained by using simulated annealing algorithm to meet the reliability and economic requirements of a distribution test system. The system reliability indices SAIDI (System Average Interruption Duration Index), System ECOST and System EENS (Expected Energy Not Supplied) are strategically considered to obtain the solution. The simulations are carried out in MATLAB R2014a.

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.

Space chargediagnostics in aged polymeric insulation

Condition assessment of DC insulation systems is important from the point of view of operation and reliability. Under DC field, space charge accumulates within the bulk of the insulation in polymeric insulators. These accumulated charges cause local field intensification within the volume of the insulator, and subject the material to additional stresses, promoting aging. Therefore assessment of space charge in DC insulation is important. In this work, we study the effect of thermal aging on space charge accumulation in polyethylene, epoxy, and epoxy based nanocompositesunder DC applied field. From space charge measurements, trap depth and trap controlled mobility are estimated. Further, dielectric loss factor is estimated from depolarization current characteristics using Hamon approximation. Thermal aging causes the material to become increasingly lossy, and this is seen to be accompanied by a proportional change in space charge characteristics i.e. accumulated charge density, trap depth, and mobility.

Space charge accumulation in epoxy resin and polyethylene

Accumulation of space charge within a polymeric dielectric material, especially under dc voltage, is associated with its degradation. Space charge accumulation modifies the local field within the material and might contribute to its aging. Further, it would modify the conduction through the material. Formation of space charge within the dielectric material depends on the charge injection from metal-dielectric interface, and the nature of generation and transport of charge within the dielectric material. These are affected by various factors like material composition (including additives or cross linking), applied field, temperature and electrode material. In this work, Pulsed Electro Acoustic (PEA) technique is used to study the effect of field on charge accumulation and distribution in two commonly-used polymeric dielectric materials. The threshold field for space charge accumulation is identified. PE and epoxy resin are chosen since the former is semi-crystalline while the latter is amorphous. An estimate of trap depth and mobility at different fields are also obtained. The effect of two different electrode materials viz. gold and aluminum is also investigated. In this work, information about various parameters related to accumulation of space charge within the material is estimated from measured space charge distribution data obtained from PEA measurements. Maximum amount of charge stored in the material is obtained for different materials and as well as different electrode material under different applied fields.

Effect of lightning on soil characteristics with and without underground cable

Indirect lightning stroke i.e. discharge between cloud and ground has strong impact on the UG (Under Ground) cable insulation. In this paper, the impact of lightning stroke in UG cable is investigated. High current carried by lightning stroke to ground creates breakdown channel in soil if the resulting field exceeds soil breakdown strength. Through this channel current can reach cable and if the resulting field generated on cable surface exceeds breakdown strength of cable insulation then also the cable can damage. So proper assessment of Electric field distribution in soil and on cable surface due to lightning current is necessary to envisage proper planning of protection scheme and strategy for mitigating severity to UG cable. In this paper, a complete study is done to investigate electric field distribution in soil due to lightning stroke, minimum lightning current required for soil breakdown, current intercepted by buried cable and overvoltage generated on cable surface due to that current. Also the effects of various factors like soil resistivity, cable location, lightning strike position, lightning current etc. are studied.