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An investigation into electrical degradation mechanisms within air-filled cavities in solid dielectric materials
Abstract
Degradation and failure mechanisms in electrical insulation systems (EIS) in high voltage apparatus are not only caused by the thermal and mechanical stresses but also by the electrical stress during operational service. Imperfections in terms of cavities, craters, contaminants and electrical trees might seriously affect in insulation material of HV equipment due to the changes in microstructural levels at these defects under the enhancement of electric fields. Air-filled cavities embedded in solid dielectric material have been considered to be one of the most complicated issues that partial discharge (PD) activity plays an important role, which induces progressive deterioration processes in the cavity from localised erosion to complete failure. Phase-resolved partial discharge (PRPD) analysis is an effective condition monitoring tool for diagnosing the identifiable stages of degradation mechanisms in the relationship between PD characteristics and morphological changes in an airfilled cavity within the insulation.
The main aim of this thesis is to experimentally investigate the progressive degradation mechanisms within an air-filled cavity embedded in polymers by PD analysis. The samples used in the PD experiment are made from two different shaped cavities embedded in solid polymers regarding a cylindrical flat-shaped cavity and a spherical-shaped cavity. Based on the results, the recognisable
variation in PRPD patterns and PD statistical quantities can be directly linked to distinguish the progressive levels of localised erosion on the surface wall inside an enclosed cavity in terms of surface erosion, erosion depth, electrical tree growth and upcoming failure. Interestingly, the results also show that the distinct PD characteristics can be analysed to accurately validate the presence of
corrosive by-products regarding micro-craters and electrical tree initiation within the cavity wall before the treeing growth progressively propagates towards the whole dielectric between parallel electrodes, leading to total failure. The findings from these studies are further examined to explore the microscopic structure on the cavity surface due to accelerated ageing processes by various
analytical techniques of microanalysis instruments used for additional prognoses in terms of physical, thermal and chemical properties. In particular, the noticeable PRPD patterns and PD diagnostic quantities, i.e. the average apparent charge, the maximum apparent charge and the average number of PDs per cycle are reproduced through a PD model with good agreement between measurement
and simulation results.
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The aim of this paper is to investigate the characteristics of partial discharge (PD) within dielectric-bounded and electrode-bounded cavities which are embedded in Ethylene Propylene Rubber (EPR) by using plane to plane electrode. The theoretical analysis is carried on the electric field strength and PD inception voltage of cylindrical and ellipsoidal cavities in different dimensions. The samples with the different dimensions of artificial spherical cavities in EPR are made and the PD characteristics in the cavities, such as PD number, amplitude and phase, are measured by professional instrument in laboratory. It has been found by the experiments that the symmetric PD distribution changes from "rabbit-like" to "turtle-like" shape with the time of applied voltage. With the increase of applied voltage on the samples, the PD inception phase becomes prior to the zero crossing of positive and negative half-cycle. Meanwhile, PD pulse repetition rate has an increase trend with increasing the experimental voltage and the amplitude of PDs occurred in dielectric-bounded cavities changes from a higher value to normal during a short time. However, no obvious amplitude change of PDs occurred in electrode-bounded cavities has been observed in the experiment. As a result, internal PDs occurred in cavities is one of the prime reasons resulted in the degradation and corrosion of EPR.
The comparison between physical properties and chemical structures of several polysiloxanes was studied at initial state and upon ageings. Techniques such as densimetry, gel permeation chromatography, FTIR spectroscopy, thermogravim-etric analysis coupled with IR spectroscopy and the new tool photo-DSC were used to give assessments of average molecular weight, chemical evolution, cristallinity and thermal stability. Different types of ageing such as thermal ageing (60˚C and 100˚C), photo-ageing and acid vapour ageing were performed. After ageing we observed an evolution of chemical structures and physical properties for all samples. The main pathway of degradation is given for each sample. It results in oxidation, chain scission or cross-linking. Cross-linking levels are dependant on the type of ageing, the chemical structure and the initial rate of cristallinity. Cross-linking reactions are favoured after photo-ageing. Oxidation is higher for polysiloxane with aliphatic carbon chain.
In this study, the ageing process of cavities in solid insulation of power cables is divided into three stages, and for each stage a circuit model is proposed. Each of these circuit models preserves the basic characteristics of the cavity under the circumstances of the corresponding stage and provides the expected behaviour of generated partial discharge (PD) pulses. This set of models simulate cavity wall effects, the non-uniformity of internal electrical field and charge deployed on the wall surfaces with considering the existence of electronegative gases according to the requirements of the three defined ageing stages. Finally, the simulation results, in the forms of phase resolved PD patterns are presented showing good agreement with the published experimental measurements drawn from accelerated ageing process setups.
First comprehensive selection of FT-Raman and FTIR spectra of 500 polymers followed by structural information and CAS numbers
For high-voltage components, the measurement of partial discharge (PD) is a useful tool for performance assessment of electrical insulation. In this study, experimental measurements of PD activity for different spherical cavity sizes and material temperatures have been performed. A simulation model representing PD behaviour within spherical cavities in homogeneous dielectric materials has also been developed. The model has been used to study the influence of cavity size and material temperature on PD activity. Comparison of measurement and simulation results has been undertaken. The model uses a finite element analysis (FEA) method along with MATLAB code. It has been found that certain parameters in the model are both cavity size and temperature dependent. Thus, critical parameters influencing PD behaviour for different cavity sizes within the material and material temperatures can be identified; these are the charge decay time constant, cavity surface conductivity, electron generation rate (EGR), PD inception and extinction fields and the cavity temperature decay time constant. © 2012 The Institution of Engineering and Technology.
This paper presents a physico-chemical experimental study of treeing structures expanding in EPDM (Ethylene Propylene Diene Monomer) in a point - plane electrode geometry. Accelerated electrical degradation tests, under alternating 50 Hz electrical field, were realised on this polymer to initiate trees and to follow their propagation (i.e. the tree length) by measuring the partial discharges (PDs) occurring within this insulating material. A chemical analysis by Scanning Electron Microscopy (SEM) and by Infrared spectroscopy (IR) are undertaken to determine the damaged polymer microstructures and changes in its morphology. The chemical groups existing in this material and the changes due to its degradation under electrical field have been identified by Infrared spectroscopy. The microstructure modification of this material, by breaking chemical bonds and splitting molecular chains, explains the formation of tree channels, and accompanying off-gases that promote the appearance of partial discharges and develop the defect. This process is related to the amplitude and the duration of the electrical field which governs the tree forms and which depends on the diameter of the channels and their internal texture revealed by SEM analysis.
The new edition of this book incorporates the recent remarkable changes in electric power generation, transmission and distribution. The consequences of the latest development to High Voltage (HV) test and measuring techniques result in new chapters on Partial Discharge measurements, Measurements of Dielectric Properties, and some new thoughts on the Shannon Theorem and Impuls current measurements.
This standard reference of the international high-voltage community combines high voltage engineering with HV testing techniques and HV measuring methods. Based on long-term experience gained by the authors the book reflects the state of the art as well as the future trends in testing and diagnostics of HV equipment. It ensures a reliable generation, transmission and distribution of electrical energy. The book is intended not only for experts but also for students in electrical engineering and high-voltage engineering.
In China high temperature vulcanized (HTV) silicone rubber has been applied to manufacture the sheds and sheath of composite insulators for decades. Being exposed to environmental stressors including sunlight, UV radiation, oxidization, electrical stress and moisture throughout the whole operation period, various aging phenomena could be observed, among which color fading and hardening are two important indexes which may be directly related to the performance of composite insulators. According to polymer aging theory, thermo-oxidative could be the main aging reaction leading to the hardening of HTV silicone rubber and in this paper, Thermogravimetric Analysis (TGA) combined with Differential Scanning Calorimetry (DSC), Infrared Spectroscopy (IR) and Mass Spectroscopy (MS) are applied to study the degradation and decomposition process of HTV silicone rubber in air (oxidizing atmosphere) and nitrogen (inert atmosphere) respectively. 18 composite insulator specimens have been collected and their color fading degree, hardness, crosslink density as well as thermal performance have been measured and calculated. A correlation analysis showed that physical crosslink density had a strong positive relation with hardness of specimens whereas chemical crosslink density showed negative relations with hardness. It is concluded that TGA is an effective method to simulate and study the thermo-oxidative aging process of HTV silicone rubber and the measurement results of crosslink density shows that decomposition and over crosslinking reactions have taken place simultaneously in HTV silicone rubber and led to the aging of composite insulators together.
Surface charges generated by previous discharges at the solid-gas interface in a void can affect the characteristics of subsequent ones. In this paper, the effect of discharge area variation on the stochastic characters of partial discharge (PD) magnitude was investigated by changing the void size. It is found that the uniformity of PD magnitude increased when void diameter was reduced. In order to clarify this, the corresponding discharge area variation was obtained by Pockels effect of Bismuth Silicon Oxide (BSO) crystal. As void diameter became smaller, due to the limitation of cavity walls, the fluctuation of discharge area became weaker, and hence the uniformity of PD magnitude increased. Besides, the result that the fluctuation of PD magnitude decreased when PD void height became larger was obtained. Considering the safety of BSO crystal, surface charge distribution was not measured by experiment for this case. Instead, a simulation model was constructed to obtain it. As a result, discharge area became larger when void height increased. According to this, it is inferred that the expanding of discharge area may be affected by cavity walls in the experiment. Therefore, the changing tendency of the uniformity of PD magnitude with the void height can be explained when the void area remained unchanged. These results suggest that the variation of discharge area can affect the stochastic character of PD magnitude and further investigation is needed to clarify the PD mechanism.
Partial discharge (PD) is a localized electrical breakdown event inside solid or fluid insulating materials. PD does not link the two conductors directly. However, it can cause damage to the insulating material, and eventually build up a damaged channel leading to final breakdown. In general, PD events can be classified based on their discharge mechanisms, such as surface discharge, corona discharge, and free space cavity discharge which is the main focus of this work. Cavity discharge studied in this work is generated by an air-filled spherical cavity inside the insulating material, which is epoxy resin. Such small cavities exist in many high voltage equipment where polymeric insulating materials are involved. Detection of cavity PD is generally believed to be very concerned by industrial operators, and sometimes an immediate replacement of the plant is required. However, with the growing understanding of material degradation induced by cavity PD, it is possible to assess the degradation level by PD behaviour. By understanding the link between PD behaviour and degradation level, equipment’s’ service life may be extended to their maximum. Cavity PD in epoxy resin was found to develop itself through five stages in this work, defined by clear variations in terms of their phase resolved partial discharge (PRPD) patterns. Optical and scanning electron microscopic observations at degradation surfaces also confirmed the physics behind PD stage transition. Previous researchers usually focus on PRPD patterns against cavity air consumption for short life materials such as polyethylene, or singular pulse characters against degradation surface with long life materials such as epoxy resin. In this work, the feedforward (usually termed as feedback by other researchers, however, considering the fact that after each discharge event the cavity suffers deterioration and become a slightly different system, feedforward is a more accurate term from system control theory) or memory effect between adjacent pulses, short term surges, and long term behaviours were the main focus of discussion. The cavity losing its feedforward capability or memory capacity, as well as PD events getting localized onto specific damaged points, are experimentally shown to be the driving force from one degradation stages to another showed by PRPD pattern transitions. Results achieved in this work successfully established the link between various aspects of information which were usually studied in a less interactive manner. The results suggest that accumulative PD behaviour transition along degradation time is governed heavily by the cavity surface condition, the two of which form a closed loop. Pulse train analysis studies the capability of feedforward capability of degraded cavity surfaces. Both PRPD analysis and pulse train analysis take step changes in a synchronized manner with the discovery of new-born surface condition features. Some of the surface condition features can induce later stage discharge patterns such as swarming and treeing (“wing-like”) patterns, feedforward capability study of such patterns can directly be utilized to determine degradation level at such surface locations. Such information could be used to qualitatively, and quantitatively determine the level of degradation induced by cavity PD. At this stage, PRPD patterns can be used to predict degradation surface conditions, and a number of factors related to the PD feedforward capability of such degraded cavities are found. The feedforward capability was never investigated to such depth, and was never used as a condition monitoring tool in the past. This thesis contributes towards a new way of thinking for cavity PD degradation assessment, and if materialized, a longer and more reliable service life of polymeric insulating materials if cavities are involved.
A general physical model of partial discharge (PD) has been developed and used to simulate discharges within a void at the tip of a metallic spike defect within a three-phase 11 kV paper insulated lead covered (PILC) cable joint. Discharges are modeled by altering surface charge density at the void boundary using a logistic function distribution. The model was validated against experimental data, and a good agreement was observed with minimal free parameters. The model was then used to investigate the impact of single phase energization on PD activity in three-phase PILC cable joints. It was concluded that PD testing of three-phase PILC cable joints should be performed at raised temperatures with the cable fully energized as this results in a higher frequency of PD activity, and reduces the level of background PD from cable terminations. This research represents a further step towards developing PD models that can describe measurements taken from operational high voltage plant.
Previous papers have identified Raman microprobe analysis as a useful tool in the chemical analysis of electrical trees and have successfully applied the technique to trees grown in polyethylene. This paper comprises a detailed spectroscopic study of electrical trees grown in silicone rubber. A selection of trees of varying fractal dimensions were exposed using cryogenic microtomy and then the individual tree channels were subjected to surface specific Raman microprobe analysis. It was found that although some trees show that few chemical changes have occurred, some trees (including some where complete breakdown has occurred) show evidence of the presence of silica and carbon, the latter possibly originating from the side groups of the polymer chain. After this, some of the samples were then analyzed using scanning electron microscopy (SEM) to provide an alternative method of analysis and to reinforce the conclusions made. Results are discussed in comparison to previously published results from a similar analysis on trees in polyethylene.
The behavioural acceleration in electrical ageing tests for low-density polyethylene (LDPE) samples embedded a gas filled cavity is experimentally undertaken to examine the time evolution of partial discharge (PD) events so that the degradation mechanisms on the cavity surface by internal discharges until the growth of trees can be studied. The experimental data of localized erosion by treeing growth is closely analyzed to detail the corresponding PD characteristics with microscopic techniques in terms of chemical by-products to identify the distinct phenomenon of electrical tree growth from a cavity.
A prolonged period of ageing behavior on polyethylene samples containing a gas-filled cavity has been investigated using diagnostic assessment methods to develop understanding of the causes of dielectric failure. In this experiment, low density polyethylene (LDPE) samples are made using a multiple layer technique. The experimental results detail the investigation for correlation between the partial discharge (PD) characteristics and chemical reactions on the cavity surface. The evolution of discharge mechanisms can be distinguished into three stages with an elapsed time of ageing. Some visible evidence of eroded by-products on the inner surface of a cavity are examined by measuring further at the level of microanalysis as a result of oxidative ageing in terms of chemical by-products, microscopic structures and thermal degradation.
Recently many electronic parts are widely used for the power electric field, such as power transistor module, diode and solid state relay. These are encapsulated with solid insulating materials such as epoxy resin or silicon rubber. In these category of parts, majority of electrical failures has been caused by inadequate material selection or workmanship error for encapsulation systems. It is suspected that the cause of the electrical failures is due to partial discharge (PD) in voids of insulating materials or in cracks in the vicinity of conductors. Accordingly, the test on PD has been performed for these products. Although PD quantity (e.g. PD current, Qmax), decreases gradually during voltage application with ordinary electrical PD measurements, while solid insulating materials degrade along with time and finally result in electrical breakdown [1,2].
This paper considers the behavior of accelerated ageing on polyethylene materials containing cavities involving its
dielectric performance in terms of the electrical, chemical and morphological properties. As a result of internal discharges in an air-filled cavity within polymeric materials, this ageing mechanism might gradually deteriorate and progress further to complete failure. In this experiment, low density polyethylene (LDPE) test samples are made using a multiple layer technique. The results detail the effect of electrical ageing corresponding with analysis of partial discharge measurements. In addition, the combination of electrical properties and chemical compositions is examined experimentally to diagnose the electrochemical consequence of ageing using infrared spectroscopy and dielectric spectroscopy techniques.
Models of partial discharge (PD) activity have been widely used to investigate PD activity in cylindrical and spherical voids within insulation material. PD activity is known to alter void geometry due to the erosion of insulation material from the deployed charge. In this work, a PD model is produced to investigate the impact of void erosion on PD activity. The model is validated by reproducing measured phase resolved PD (PRPD) patterns for a spherical void in a homogeneous dielectric material. The impact of void erosion is then investigated by running the model with different void geometries. The results from the model show that the PRPD patterns depend heavily on the level of erosion due to the impact of the erosion on the electric field at the void surface.
As the use VLF diagnostic testing is a promising trend, it is important to investigate PD characteristics during aging at very low frequency range. Test samples of solid insulation with voids inside fabricated by a 3D printer are subjected to VLF excitation for PD measurements. PD characteristics at different aging states of test samples are investigated under power frequency (50 Hz) and VLF excitation (0.1 Hz). PD patterns show that behaviors of discharges at 0.1 Hz and 50 Hz can be similarly categorized into 2 stages. In the first stage, electrical discharges tend to happen at high instantaneous voltage value and have large discharge magnitudes due to lack of initial free electrons at the beginning of the test. Along the testing period in the first stage, discharge magnitudes gradually decrease and PD activities happen at lower voltage value as more free electrons are available. In the second stage, PD characteristics are hardly changed. However, there is a substantial difference in time duration in the first stage between the frequency of 0.1 Hz and 50 Hz. The first stage at 50 Hz excitation is much shorter than that under VLF excitation. This could be explained by significant dependence of physical conditions such as surface conductivity of the void and charge decay mechanisms on applied frequency. At VLF, charge decay rate is higher which may cause the discharge process taking longer to finish the first stage.
The physical properties of polyester film, such as thermal stability, combined with its mechanical properties, such as tensile break strength, and its electrical strength make it the preferred material for a wide variety of electrical insulation applications. Therefore, polyesters are widely used as insulating material for electric power system equipment such as cables, motors, transformers, and capacitors, since its high dielectric constant and resistance to degradation in high voltage and thermal stresses makes it ideal for mechanical barriers in wire and cable applications. For instance, in transformers and other high-voltage distribution equipment, polyester film is used to wrap the conductors [1]-[3].
Partial discharge (PD) diagnostic is a well proven method for detecting weak spots in medium and high voltage cables. Beside the key parameters, like partial discharge inception / extinction voltage and partial discharge level in Pico-coulomb, one of the most important parameter is the point of localization along the cable. Without exact localization even the interpretation of the key parameters could lead to wrong conclusions. However even when the PD is localized additional information about the type of defect would be very helpful in addressing the severity of the detected defect. Does the PD weak spot need to be replaced directly, or can the cable be safely energized and the replacement of the defective component can be scheduled on a later time period. This paper goes into detail to PD localization, PRPD patterns and finally localized PRPD patterns. With help from “localized” PRPD patterns new ways of identifying the type of defect are possible and more reliable conclusions can, with regards to the severity of the defect, be drawn.
In this work, emission optical spectroscopy is used as a technique to study partial discharges (PDs) in four unfilled epoxy samples encompassing an artificial air-filled cavity. It is shown that emission spectroscopy can be used to estimate the density and the chemical composition of a gas from the spectrally resolved emission and time-resolved pulse shape of the PDs at any time during the aging process. Two scenarios are observed: either PD continues until the sample breaks down (observed in 1 out of 4 samples) or PD stops at a certain point without sample breakdown (3 out of 4 samples). For both the scenarios, a stable initial phase with a gradual decrease of emission intensity from the discharge is typical for a few hundreds of hours of continuous discharge operation. At this stage, the spectrum of the second positive system of molecular nitrogen dominates in the entire spectral range of 350-500 nm studied in this work. Furthermore, time-resolved measurements indicate two types of discharges of very different frequencies and magnitudes as well as a decrease of the pressure in the voids as a function of aging time. Then, a sharp 500% increase of the N2 emission is observed 2 days before the breakdown; during the last day a spectrum of CO and some other C-N-O-H containing molecules is observed instead of the spectrum of molecular nitrogen. This allows predicting a breakdown at least a few hours before it happens by analyzing the broad emission spectra behavior. Additionally, the possible role of surface conductivity increase during aging on PD inhibition is discussed.
This paper addresses some shortcomings of existing IEC standards related to analyses of cable systems with polymer insulation (IEC 60840 and IEC 62067). The focus is on partial discharges measurement, completely neglected in present version of these standards, and justification of the needs for their inclusion in the standards through appropriate procedures. If included in the standards the procedures would provide an effective way to identify and detect the defects that might appear during the cable system installation and to forestall their appearance during exploitation. The simulation evidences, to suggest and justify the changes in the standards, through a modelling of partial discharge phenomena using contemporary software tool (COMSOL) are provided in this paper.
This Thesis is concerned with an experimental study into the degradation processes that occur when void s in solid dielectric materials experience high applied electric fields. A method has been developed for manufacturing 2 mm thick samples of silicone rubber that contain a single gas void of around 1mm diameter. Samples are simultaneously electrically stressed under an applied ac sinusoidal voltage of 11-12kV for 6 hours that is then increased to 12-17kV. During the stressing period, partial discharge (PD) data is regularly acquired. The samples are then inspected for signs of degradation. Degraded samples that have not suffered catastrophic failure and contain pits or evidence of damage were then cut open using an RMC MT-7 ultra- microtome equipped with a C R-21 cryo-system set at -110。C in order to provide a surface containing open segments that can be assessed using spectroscopic techniques.
The experiment is repeatable and the obtained degraded samples and the degradation areas of microtomed samples have been analysed using Raman spectroscopy to identify the chemical content of the degraded areas and obtained improved images using a scanning electron microscope (S EM) at the void /silicone rubber interface. In addition, the obtained PD patterns have been used to make a comparison between measurement and simulation results obtained using a physical model implemented in Matlab.
Due to its applications in high voltage insulation, a thorough understanding of the chemical reactions that occur during electrical ageing in polymers is needed. A confocal Raman microscope has a potential lateral resolution of ~1μm along both the lateral and optic axes and is able to characterise the localised chemical composition of a material; for this reason, it has been applied in the study of electrical ageing in solid dielectrics. Due to inaccurate assumptions about the optical processes involved in confocal Raman microprobe spectroscopy (CRMS), however, the exact characterisation of the processes and chemicals involved has previously proven to be difficult.
The objective of this study is to apply the technique of Raman microprobe spectroscopy in the analysis of the chemical structures of electrically aged polymers. It was found that, with the application of immersion oil and by using a refined version of a model of CRMS which is based on a photon scattering approach; CRMS is a valuable tool in the study of polymers. More accurate results can be obtained, however, by revealing the feature in question to the surface and applying non confocal Raman microprobe spectroscopy (RMS).
CRMS was applied to a variety of polymeric samples containing electrically aged voids and electrical trees. Results showed that within the electrically aged voids, chemical signatures similar to those previously found in electrical trees in PE can be found. Finally, a variety of polymeric insulators was subjected to spark ageing and corona discharge. The by-products of these ageing mechanisms were then characterized using RMS in an attempt to reproduce in bulk the chemical compounds formed in electrical treeing. The resulting Raman spectra indicated that the same by-products as those formed in voids and trees are indeed formed. Where possible all results were compared to comparative data obtained using Fourier transform infra red (FTIR) spectroscopy and scanning electron microscopy (SEM) and discussed in relation to previously published work.
For high voltage components, the measurement of partial discharge (PD) is used in the performance assessment of an insulation system. Through modelling the PD process, a
better understanding of the phenomenon may be attained. In this work, a model for a spherical cavity within a homogeneous dielectric material has been developed using
Finite Element Analysis (FEA) software in parallel with MATLAB programming code. The model has been used to study the effect of various applied stresses and cavity
conditions on PD activity and also the electric field and temperature distributions within the cavity.
The experimental measurement of PD activity within a spherical cavity has also been undertaken. The measurements were performed for different amplitudes and frequencies
of the applied voltage, a range of spherical cavity sizes and temperature variation of the material. The obtained results show that PD is strongly influenced by various conditions of the cavity and applied stress. The cycle to cycle behaviour of PD events, discharge phase and magnitude distributions, numbers of PDs per cycle, total charge magnitude per cycle, mean charge magnitude and maximum charge magnitude for each experiment have been obtained and analysed.
The simulation results from the PD model have been compared with the measurement results. It is found that certain model parameters are dependent on the applied stress and
cavity conditions. Parameters that clearly affect PD activity can be readily identifed. These parameters include; the effective charge decay time constant, the cavity surface conductivity, the initial electron generation rate, the inception field, the extinction field
and the temperature decay time constant in the cavity. The infuences of surface charge decay through conduction along the cavity wall and temperature and pressure change in the cavity on PD activity have also been studied
This paper presents the time varying characteristics of partial discharge (PD) activity in a cylindrical cavity within polymeric insulation. The samples are produced using a multiple layer technique with polyethylene terephthalate (PET) selected as the solid dielectric. The results highlight the degradation process of the cavity surface through the exposure to a HV AC electric field and the impact of this on PD activity. The variation in sample conditions are linked to PD analysis in each case with one sample having exhibited electrical tree growth. The morphological surface and chemical composites of the test samples are analysed using Raman spectroscopy and an optical microscope.
Following an examination of the basic gas-discharge concepts, the Townsend breakdown criterion is derived for nonuniform fields and a reformulation of the streamer criterion is undertaken. A direct application of the Townsend criterion to a practical situation is hardly possible, whereas the streamer criterion leads to applicable criteria for the onset of breakdown in air as well as in strongly electronegative gases. In this approach, a knowledge of the relevant ionization coefficients is not required, because these are replaced by parameters obtained from uniform-field Paschen curve data. In addition, the criteria developed do not contain any arbitrary constants. By introducing the electrode surface mean curvature, it is shown that the calculation of breakdown data for widely different electrode shapes can be addressed through a single formula. It is emphasized that the inherent roughness of practical electrode surface demands that caution be exercised in all design criteria. >
New developments in scanning electron microscopy (SEM) have resulted in a wealth of new applications for cell and molecular biology as well as related biological disciplines. New instrument developments coupled with new sample preparation techniques have been key factors in the increasing popularity of this versatile research tool. The desire to view biological material in native states at high resolution has stimulated new approaches to cryo-SEM applications and environmental SEM (ESEM). In addition, new staining techniques and novel processing applications that combine embedding and resin-extraction for imaging with high-resolution SEM has allowed new insights into structure–function relationships. Advances in immuno-labeling have further enabled the identification of specific molecules and their location within the cellular microenvironment. It is now possible to analyze macromolecular complexes within their three-dimensional cellular environment in near native states, and this in many cases has provided advantages over two-dimensional imaging with transmission electron microscopy (TEM). New instrument developments include helium ion microscopy that allows imaging greater details of cellular components; new technology approaches include automated block-face imaging combined with serial sectioning inside an SEM chamber that in recent years has increasingly been utilized for a variety of biological applications. Focused ion beam milling (FIB) combined with block-face SEM is among the newer approaches to analyze cellular structure in three dimensions. The present chapter introduces the basic features of SEM and sample processing for SEM, and highlights several advances in cell and molecular biology that have greatly benefited from using conventional, cryo-, immuno-, and high-resolution SEM.
With the aim of developing defect-based life models (i.e. life models in which breakdown is explicitly associated with a partial discharge-induced damage growth from a defect beyond a critical level), ageing tests were carried out in the lab on XLPE sandwich specimens containing artificial cavities. PD activity taking place in the cavities was continuously monitored during ageing. The behavior of discharge repetition rate and amplitude were extracted from the monitoring records. The modification of the cavity surface at various ageing times was investigated using microscope observation. Brownish solid byproducts, having oxygen containing moieties, were found covering uniformly the entire area of the cavity surface. After the byproduct was wiped off, degradation pits were observed. The variation of PD repetition rate and amplitude, as well as the relevant PD patterns, could be related to the modification of the cavity surface.
Because partial discharge (PD) in air-filled void embedded in the insulation system can cause the degradation and eventual insulation failure, it's necessary to study the PD mechanism. Our paper employed the equipment based on Pockels effect to investigate the change of surface charges distribution in the process of PD degradation. During PD sequences, the effect of previous discharge on subsequent discharge was also examined.
This chapter describes the design of the instruments used to acquire hyperspectral data. The analysis of localized regions of samples by vibrational microspectroscopy can be accomplished in two ways, mapping or imaging. Mapping involves the sequential measurement of the spectrum of each adjacent region of a sample by moving each region of the sample into the beam after recording the spectrum. In hyperspectral imaging, the images at more than 10 wavelength regions are recorded simultaneously with a two-dimensional array detector. Vibrational hyperspectral imaging can be accomplished through the measurement of either the mid-infrared, near-infrared (NIR), or Raman spectrum. The measurement of each type of spectrum is accomplished in different ways, although the instruments that have been developed for the measurement of NIR and Raman spectra are more closely related than that of mid-infrared hyperspectral imaging spectrometers.
This unified approach to polymer material science covers the wide range of underlying principles: from molecular structure to material properties; from the relationships between part design, processing and performance to mechanical properties and failure mechanisms. The book stands out with many full-color graphs and figures that visually convey what polymers can--and cannot--accomplish in a world that relies on this class of materials in basically all aspects of modern life. Real-world examples and a variety of problems help the reader apply their knowledge and sharpen their problem-solving skills. Materials Science of Polymers for Engineers 3E covers the 6Ps: polymers, process, product, performance, profit, and post-consumer life (sustainability). There are three major sections in the book.
This chapter outlines a variety of sample preparation procedures for performing scanning electron microscopy and x-ray microanalysis of polymer materials. Typical specimen preparation methods will be described and shown in a range of applications that are encountered every day in academic and industrial laboratories. The reader new to the study of polymer materials is directed to Chapter 12 of the Enhancements section on the accompanying CD, which provides background material for this chapter. The topics found on the CD are an introduction to polymer materials, E12.1, and polymer morphology, E12.2, and a description of some typical processes used to produce materials from polymers, E12.3. The book by Sawyer and Grubb (1996) is comprehensive, providing details of specimen preparation methods, dozens of practical examples, and references that describe both the structure and the microscopy of polymers.
Widely used as insulators, capacitors, and transducers in daily life, soft dielectrics based on polymers and polymeric gels play important roles in modern electrified society. Owning to their mechanical compliance, soft dielectrics subject to voltages frequently undergo large deformation and mechanical instabilities. The deformation and instabilities can lead to detrimental failures in some applications of soft dielectrics such as polymer capacitors and insulating gels but can also be rationally harnessed to enable novel functions such as artificial muscle, dynamic surface patterning, and energy harvesting. According to mechanical constraints on soft dielectrics, we classify their deformation and instabilities into three generic modes: (i) thinning and pull-in, (ii) electro-creasing to cratering, and (iii) electro-cavitation. We then provide a systematic understanding of different modes of deformation and instabilities of soft dielectrics by integrating state-of-the-art experimental methods and observations, theoretical models, and applications. Based on the understanding, a systematic set of strategies to prevent or harness the deformation and instabilities of soft dielectrics for diverse applications are discussed. The review is concluded with perspectives on future directions of research in this rapidly evolving field.
Despite the large amount of activities undertaken in both the area of partial discharge (PD) in cavities and polymeric insulation degradation, degradation of polymeric insulation induced by cavity PD remains a subject where there are more questions than answers. In this paper, an experimental approach is discussed which has the aim of allowing observation of degradation induced by cavity PD. A model based on electron scattering mechanisms in both the cavity gas gap and polymeric insulation is proposed. From observing electron avalanche behavior and its damage mechanisms to the insulation, it may be possible to develop approaches that improve our ability to estimate remaining life of polymeric insulation systems in high voltage plant.
This paper is concerned with an experimental study into the degradation processes that occur when voids in solid dielectric materials experience high applied electric fields. A method has been developed for manufacturing 2mm thick samples of silicone resin that contain a single void of around 1mm diameter. Five samples are simultaneously electrically stressed under an applied ac sinusoidal voltage of 12kV for 6 hours that is then increased to 15kV until a sample fails. During the stressing period, PD data is regularly acquired. The remaining 4 samples are then inspected for signs of degradation. Degraded samples that have not suffered catastrophic failure and contain pits or evidence of electrical trees were cut open using an RMC MT-7 ultra-microtome equipped with a CR-21 cryo-system set at -110°C in order to provide a surface containing open segments of pits or trees. The experiment is repeatable and the obtained degraded samples and the degradation areas of microtomed samples have been analysed using Raman spectroscopy to identify the chemical content of the degraded areas at the voids /silicone rubber interface. Initial results indicate that the degradation areas of microtomed samples are probably pits generated from the voids, which may be evidence of the initial development of a bow-tie electrical tree. This paper will detail the experiment, contain images of the obtained degraded samples and detail the chemical changes that occur in silicone rubber due to a electrical degradation process.
Describes the theory and practice of infrared and Raman spectroscopy as
applied to the study of the physical and chemical characteristics of
polymers. Its purpose is to give the beginning researcher in the field a
firm foundation and a starting point for the study of more advanced
literature. To this end the book concentrates on the fundamentals of
the theory and nomenclature, and on the discussion of well-documented
illustrations of these fundamental principles, including many
now-classic studies in the subject. No previous knowledge of either
polymers or vibrational spectroscopy is assumed.
Partial-discharge (PD) measurements in compliance with the standard IEC 60270 are based on the main quantity apparent charge. This term is deduced from a simple PD model comprising a capacitive network. However, from a physical point of view, a PD defect cannot be represented by a capacitance, as argued by Pedersen and his coworkers. As an alternative, Pedersen proposed a field theoretical approach that is based on a dipole model. This more sophisticated concept has been ignored in the past, whereas the capacitive PD model is accepted also nowadays. Apparently, the reason for that is that the charge transfer can easily be calculated for the capacitive equivalent circuit, which is not the case if the field theoretical approach based on the dipole concept is adopted. As will be reported in this article, the analysis can substantially be simplified if instead of gaseous discharges in spherical, elliptical, or cylindrical cavities, as commonly regarded in the relevant literature, the establishment of a dipole moment in a uniform field between solid dielectric layers is considered.
This article starts from the same points that generated the John Tanaka review, as an homage to his unforgettable contribution to the science of dielectrics and electrical insulation.
Results of recent research on physical and chemical processes in partial discharge (PD) phenomena are reviewed. The terminology used to specify different types or modes of PD are discussed in light of a general theory of electrical discharges. The limitations and assumptions inherent to present theoretical models are examined. The influence of memory propagation effects in controlling the stochastic behavior of PD is shown. Examples of experimental results are presented that demonstrate the nonstationary characteristics of PD which can be related to permanent or quasi-permanent discharge-induced modifications (aging) of the site where the PD occur. Recommendations for future research are proposed
A surface charge measurement system based on Pockels effect was constructed to detect the variation of residual surface charge distribution in partial discharge (PD) sequences. The current form (ns level) of the PD sequence was measured simultaneously. By using this system, it is found that surface charge distribution caused by PDs appeared as several isolated charges spots similar to that in dielectric barrier discharges (DBD). The result also agrees with the previous models, suggesting that charge distribution is not uniform and that it plays an important role in the stochastic behavior of PDs. As the applied voltage increased, the number of charge spots increased and the hetero-discharge occurred. Besides, a charge ring along the void edge was observed after aging due to PD. An approximate linear relationship existed between PD magnitude and the corresponding accumulated surface charges. In the PD sequence, the discharges tended to take place at the sites having heterocharges at dielectric surface after voltage polarity reversed. Moreover, there were no homodischarges observed at the decreasing part of applied voltage even at the area without surface charges existing. This could not be explained by the accumulation of surface charges. It was also observed that more than one charge spot in one PD could occur. This was named simultaneous discharge in our paper, and may be attributed to the effects of photoionization and time lag.
Gas insulated switchgear (GIS) has been widely applied in power equipment since the late 1960s due to its reliability and compactness. Because epoxy insulators in GIS are important insulation components that affect the dielectric withstand level and the equipment lifetime, high voltage tests and sensitive partial discharge (PD) tests have been carried out on them in the factory. If a latent defect occurs inside an actual epoxy insulator, its size is predicted to be small, such as a micro-defect, because it will have passed rigorous tests and multiple inspections at the factory. Although many studies have clarified the fundamental phenomena of deterioration caused by PD occurring due to defects in inner epoxy, little is known about the actual size of such defects and their effect on insulation properties. Therefore, to assess the risks associated with aging GIS equipment, it is essential to understand the actual size of latent defects in inner epoxy insulators of GIS and the impact the defects have on the insulation performance. The puncture properties and PD characteristics of artificial micro-defects of cracks, voids, and delaminations produced using an accurate estimation technique and precise production control were investigated and are described. The results suggest that an estimation method using electric field analysis is useful for designing the defect shape and that the breakdown voltage depends on the three-dimensional shape of the defect. Techniques for accurate estimation of small defects in epoxy resin and stable production of micro-defects have thus been achieved.