Showing papers in "IEEE Transactions on Dielectrics and Electrical Insulation in 2011"
TL;DR: The goal is to show how much space charge can contribute directly or indirectly, through local electric field magnification or alteration, to increase ageing rate or even trigger more energetic, thus faster, degradation processes in insulation design and electrical apparatus long-term performance/reliability.
Abstract: Considering the title of this paper, one might think that everything has been said, shown, proven and discussed already on ageing and failure of electrical insulating materials, based on at least half century of research work and field feedbacks. However, perhaps there is still room to collect ideas and speculate on apparently proven or brand new concepts, such as the interrelation between ageing mechanisms and physically measurable quantities such as space charge, partial discharges and conduction current. The latter is, indeed, the scope of this paper, which is taken from the Whitehead Memorial Lecture given at the 2010 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP). On one hand we will revisit the concepts of space charge accumulation in insulating (polymeric) materials, considering also the latest achievements which deal with ultra-fast charge pulses (solitons), on the other hand we will discuss how space charge can contribute to ageing rate under dc, ac or impulsive voltage, directly through electric field alteration, or indirectly modifying a fundamental ageing factor such as partial discharges.The goal is, at the very end, to show how much such basic investigations can have a fundamental impact on insulation design and electrical apparatus long-term performance/ reliability.
234 citations
TL;DR: In this article, experimental studies on streamer and breakdown in synthetic and natural esters were conducted, and it was found that fast streamer led breakdown becomes statistically dominant in a large gap under lightning impulse and ester liquids have lower breakdown voltage than mineral oil due to their low tolerance to fast streamers.
Abstract: Interests on ester liquids as replacement of mineral oil in large power transformers are growing in recent years. This paper presents experimental studies on streamer and breakdown in synthetic and natural esters. Lightning impulse voltage was chosen when considering the practicality of standard tests of transformer industry. First, streamer length, velocity, shape and mode were analyzed based on optical images obtained by a multi-channel ultra high speed camera. Under the same voltage level, streamer in both natural and synthetic esters propagates faster and further, with more branches, than in mineral oil. Breakdown tests at various gaps from 15 mm to 100 mm were conducted, and it was found that fast streamer led breakdown becomes statistically dominant in a large gap under lightning impulse and ester liquids have lower breakdown voltage than mineral oil due to their low tolerance to fast streamer. Therefore a relationship between the results under lightning impulse and previously published results under step voltage was built up and an empirical formula was obtained to predict the breakdown voltage of esters at very large gaps.
204 citations
TL;DR: In this article, a comparative study of physicochemical, ac breakdown strength and thermal stability behavior of BIOTEMP natural ester/pressboard insulation and Karamay 25# naphthenic mineral oil/press board insulation after long term thermal ageing was conducted.
Abstract: Natural ester is considered to be a substitute of mineral oil in the future To apply natural ester in large transformers safely, natural ester impregnated solid insulation should be proved to have comparable dielectric strength and thermal stability to mineral oil impregnated solid insulation This paper mainly focuses on a comparative study of physicochemical, ac breakdown strength and thermal stability behavior of BIOTEMP natural ester/pressboard insulation and Karamay 25# naphthenic mineral oil/pressboard insulation after long term thermal ageing The physicochemical and dielectric parameters including moisture, acids and the ac breakdown strength of these two oil/pressboard insulation systems at different ageing status were compared The permittivity and ac breakdown strength of these two oil/pressboard insulation systems at different temperatures were also investigated And a comparative result of the thermal stability behavior of these two oil/pressboard insulation systems with different ageing status was provided at last Results show that though natural ester has higher absolute humidity and acidity during the long ageing period, the lower relative humidity of natural ester helps to keep its ac breakdown strength higher than mineral oil The pressboard aged in natural ester also has higher ac breakdown strength than that aged in mineral oil The lower relative permittivity ratio of natural ester impregnated paper to natural ester is beneficial to its dielectric strength Using natural ester in transformer, the resistance to thermal decomposition of the oil/pressboard insulation system could be also effectively improved
173 citations
TL;DR: In this article, a new potential barrier model is proposed to restrain the particles in the interaction zone when nanoparticles are in an isolated dispersion, leading to a decrease in both mobility and density of carriers.
Abstract: The dielectric constant and the conductivity of epoxy resin nanocomposites exhibit a lower value at slight filler loading compared with the host epoxy resin. The electrical strength has an increase and presents an optimal value at filler loading of 1 wt%. The interaction zone between the nanoparticles and the polymeric matrix is considered as an independent region. Accordingly, a new potential barrier model is proposed. Based on the model, carriers are restrained in the interaction zone when nanoparticle is in an isolated dispersion, leading to a decrease in both mobility and density of carriers. As a result, the conductivity decrease and the electrical strength increase. The restriction of dipole movement in the interaction zone and the increase of free volume are collectively contributed to the reduction of the dielectric constant. With increasing filler loading, the thickness of the interaction zone extends due to the overlap of the interaction zone, even a conductive path occurs when filler loading exceeds the percolation threshold, leading to a great increase in both mobility and density of carriers. Consequently, the conductivity increase and the electrical strength decrease. The increase of the dielectric constant is chiefly ascribed to the particles.
170 citations
TL;DR: In this article, the incorporation of graphene sheets (GSs) increased the permittivity, thermal conductivity and thermal stability of polyvinylidene fluoride (PVDF)/graphene nanocomposites.
Abstract: Poly(vinylidene fluoride) (PVDF)/graphene nanocomposites were prepared by solution blending. The incorporation of graphene sheets (GSs) increased the permittivity, thermal conductivity and thermal stability of PVDF, resulting in a transition from electrical insulator to semiconductor with a percolation threshold of 4.5 wt%. The composite containing 7.5% GSs had a permittivity higher than 300 at 1000 Hz, which is about 45 times that of pure PVDF. The thermal conductivity of the composite with 0.5% GSs was increased by approximately a factor of 2 when compared with the pure PVDF. The addition of 0.05% GSs produced an increase in the maximum decomposition temperature of PVDF of over 20°C.
165 citations
TL;DR: In this article, a simulation model for spherical cavities within a homogeneous dielectric material has been developed using finite element analysis (FEA) software in parallel with a mathematical package, which provides many advantages over previous PD models because discharge events can be simulated dynamically and the electric field in the cavity can be calculated numerically.
Abstract: Modeling of the partial discharge (PD) process allows a better understanding of the phenomena. In this paper, a simulation model for spherical cavities within a homogeneous dielectric material has been developed. The model is implemented using Finite Element Analysis (FEA) software in parallel with a mathematical package. This method provides many advantages over previous PD models because discharge events can be simulated dynamically and the electric field in the cavity can be calculated numerically. The model has been used to study the effect of different amplitudes and frequencies of the applied voltage and simulation results have been compared with experimental measurement results. It is found that certain model parameters are dependent on the applied stress and parameters that clearly affect PD activity can be readily identified, these parameters include; the electron detrapping time constant, the cavity surface conductivity, the initial electron generation rate and the extinction voltage. The influence of surface charge decay through conduction along the cavity wall on PD activity has also been studied.
149 citations
TL;DR: In this article, a review of the experimental literature regarding nanodielectrics indicates numerous inconsistencies in the results obtained and the likely cause is due to a lack of quality control during nanocomposite processing.
Abstract: Study of the experimental literature regarding nanodielectrics indicates numerous inconsistencies in the results obtained. In many cases, the likely cause is due to a lack of quality control during nanocomposite processing. By examining examples from the literature along with an alumina/polyamideimide nanocomposite and silica/crosslinked polyethylene nanocomposite, this contribution seeks to shed light on some of the likely causes for these inconsistencies. Measurements of the dielectric breakdown strength and voltage endurance confirm that poor dispersion can lead to poor material performance and the use of quantitative techniques is highlighted. Good dispersion alone is not sufficient to achieve improved properties. The addition of nanoparticles can alter the resulting structure of the nanocomposites, can introduce water into the system resulting in cavity formation and can also result in degradation of the polymer if the processing parameters are not carefully selected. In this review, understanding the effects of nanoparticle addition requires not only characterization of relevant dielectric properties, but also careful control of the processing parameters and characterization of changes in polymer structure, particle dispersion, and water content.
148 citations
TL;DR: A comprehensive experimental investigation of XLPE and its nanocomposite with fumed silica (SiO2) has been performed by CIGRE Working Group D1.24, in cooperative tests conducted by a number of members; covering materials characterization, real and imaginary permittivity, dc conductivity, space charge formation, dielectric breakdown strength, and partial discharge resistance as discussed by the authors.
Abstract: A comprehensive experimental investigation of XLPE and its nanocomposite with fumed silica (SiO2) has been performed by CIGRE Working Group D1.24, in cooperative tests conducted by a number of members; covering materials characterization, real and imaginary permittivity, dc conductivity, space charge formation, dielectric breakdown strength, and partial discharge resistance. The research is unique, since all test samples were prepared by one source, and then evaluated by several expert members and their research organizations. The XLPE used for preparation of the nanocomposites was a standard commercial material used for extruded power cables. The improved XLPE samples, based on nanocomposite formulations with fumed silica, were prepared specifically for this study. Results of the different investigations are summarized in each section; conclusions are given. Overall, several important improvements over unfilled XLPE are confirmed, which augur well for future potential application in the field of extruded HV and EHV cables. Some differences/discrepancies in the data of participants are thought to be the result of instrumental and individual experimental technique differences.
140 citations
TL;DR: In this paper, the authors used BN (boron nitride) with high thermal conductivity, low permittivity and low thermal expansion coefficient of filler to obtain composite with higher thermal conductivities and lower thermal expansion coefficients.
Abstract: The aim of this research is to find a way to achieve the epoxy composites with both high thermal conductivity and acceptable dielectric breakdown (BD) strength. As high thermal conductivity, low permittivity and low thermal expansion coefficient of filler can endow composite with higher thermal conductivity, higher BD strength and lower thermal expansion coefficient respectively, BN (boron nitride) with high thermal conductivity, low permittivity and low thermal expansion coefficient was adopted as main filler in the research. Thermal conductivity was investigated in this part. The BD strength of samples will be discussed in Part II. Neat epoxy and other 25 kinds of epoxy/BN composites were prepared by a hot press method. Most of BN fillers were surface modified with silane coupling agent through ethanol/water reflux method to improve thermal conductivity. The values of 2.91 W/m·K, 3.95 W/m·K and 10.1 W/m·K as thermal conductivity were obtained for the composites that was single-loaded with h-BN(hexagonal boron nitride), c-BN (cubic boron nitride) or conglomerated h-BN, respectively. They were further improved to 5.26 W/m·K, 5.94 W/m·K and 12.3 W/m·K, respectively, by adding extra smaller A1N (aluminum nitride) to fill the voids in sample. Thermal conductivity of samples changes with the ratio of c-BN and h-BN when c-BN and h-BN were co-loaded. A value of 5.74 W/m·K as maximum was obtained at their ratio of 1 to 1 when total filler content is 80 wt%. A much higher value of 7.69 W/m·K was obtained by adding extra AIN. From the experiment data, it is concluded that the filler orientation in vertical direction of sample surface and the decrease of voids in sample are very important to obtain high thermal conductivity, and that the filler surface modification is also necessary to improve thermal conductivity especially for epoxy/c-BN composites, and addition of nano silica in small amount can also increase thermal conductivity if sample is prepared appropriately.
135 citations
TL;DR: In this paper, the role of space charge and interfaces in epoxy polymer with micro, nano and micro + nano silica fillers has been evaluated for their electrical performance in high voltage insulation applications.
Abstract: Epoxy polymer with micro, nano and micro + nano silica fillers have been evaluated for their electrical performance in high voltage insulation applications. The dielectric strength of these samples was measured in accordance with the ASTM D-149 standard. Dielectric spectroscopy was used to understand the role of space charge and interfaces in these materials. The results of dielectric spectroscopy suggest that significant improvement in the electrical performance can be expected by using samples containing nanofillers and micro + nanofillers when compared to materials containing only microfillers. However, the dielectric strength measurement showed no statistically significant improvement for the nanofilled samples. Techniques other than dielectric breakdown may be required to adequately characterize differences in the electrical performance of the dielectrics. For example, a partial discharge test using a highly non-uniform field may be more useful as it would correspond to simulate actual service conditions.
130 citations
TL;DR: A procedure for the identification of PD signals generated by multiple sources based on the comparison of signal energies associated with particular wavelet-decomposition levels and the density-based spatial clustering of applications with noise (DBSCAN) clustering algorithm is presented.
Abstract: Partial discharge (PD) signals generated within electrical power equipment can be used to assess the condition of the insulation. In practice, testing often results in multiple PD sources. In order to assess the impact of individual PD sources, signals must first be discriminated from one another. This paper presents a procedure for the identification of PD signals generated by multiple sources. Starting with the assumption that different PD sources will display unique signal profiles this will be manifested in the distribution of energies with respect to frequency and time. Therefore the technique presented is based on the comparison of signal energies associated with particular wavelet-decomposition levels. Principal component analysis is adopted to reduce the dimensionality of the data, whilst minimizing lost information in the data concentration step. Physical parameters are extracted from individual PD pulses and projected into 3-dimensional space to allow clustering of data from specific PD sources. The density-based spatial clustering of applications with noise (DBSCAN) clustering algorithm is chosen for its ability to discover clusters of arbitrary shape in n-dimension space. PD data from individual clusters can then be further analyzed by projecting the clustered data with respect to the original phase relationship. Results and analysis of the technique are compared for experimentally measured PD data from a range of sources commonly found in three different types of high voltage (HV) equipment; ac synchronous generators, induction motors and distribution cables. These experiments collect data using varied test arrangements including sensors with different bandwidths to demonstrate the robustness and indicate the potential for wide applicability of the technique to PD analysis for a range of insulation systems.
TL;DR: In this paper, the results of an investigation into electrical tree growth in XLPE cable insulation using an embedded needle electrode are reported for a range of voltages from 9 kV rms to 27 kv rms.
Abstract: The results of an investigation into electrical tree growth in XLPE cable insulation using an embedded needle electrode are reported for a range of voltages from 9 kV rms to 27 kV rms. The partial discharge (PD) activity and tree structures were measured simultaneously throughout the tree growth and the trees were recorded from initiation up to and including the final runaway stage. A multifractal analysis was also performed on the tree structures as they propagated, and it was found that their fractal dimension increased and the distribution of embedded structures changed as small side channels were added to the tree as it grew. At 11 kV rms only branch trees were found and only bush (bush-branch) trees at higher voltages, but at 9 kV rms trees of three different shapes were formed. Observation of the tree shapes at 9 kV rms under reflected light followed by a detailed analysis using con-focal Raman spectroscopy, showed that the stagnated and branch-pine (monkey puzzle) tree shapes were due to the formation of a conducting graphitic deposit upon the walls on tree branches in the region of the needle electrode. This was not present in the branch trees produced at 9 kV rms. A simple scheme is presented for the formation of branch-pine trees and their corresponding PD activity based on the concept of conducting branch generation. The trees produced at 13 kV rms and above have a bush shape, which converts into a bush branch shape when a runaway branch grows from their periphery. This is shown to happen when the field at the bush tree periphery exceeded a voltage independent critical value, which was estimated to be 100 MV/m. The consequence of this result for the initiation of the runaway stage in branch trees is commented upon.
TL;DR: In this article, the authors investigated the influence of vegetable oil on the thermal aging rate of transformer paper, and found that vegetable oil retarded paper degradation rate and extended its useful lifetime.
Abstract: With the development of new insulation materials, vegetable oil-the best substitute for mineral oil-has gradually been widely used in the liquid insulation of transformers. To investigate the influence of vegetable oil on the thermal aging rate of oil paper, transformer paper impregnated with mineral oil and vegetable oil underwent thermally accelerated aging at three different temperatures. Degree Polymerization (DP) of paper was measured to indicate the aging degree of transformer paper. The aging rate of paper in mineral oil and vegetable oil was compared quantitatively, and results showed that vegetable oil retarded paper's degradation rate and extended its useful lifetime. The reasons contributing to such phenomenon were analyzed using X-ray Photoelectron Spectroscopy (XPS) and molecular simulation software. Transformer paper in vegetable oil had larger activation energy. Due to the larger interaction force between water and natural ester molecules, water molecule was easily bonded with natural ester, weakening the hydrolysis process of cellulose. Cellulose was chemically modified by natural ester during thermal aging process, and the reactive -OH (hydroxyl) groups on the cellulose became esterified with fatty acid esters. The water molecule was firmly bounded to the ester groups on glucose produced by esterification. The long-chain fatty acid esterified to cellulose was parallel with cellulose chains and acted as a "water barrier" to further weaken the hydrolysis process.
TL;DR: In this article, the relationship between the IEC 60270 pC and Radio Frequency (RF) signal energy is analyzed for a given test configuration, and the correlation between the two techniques produces characteristic patterns specific to each defect type.
Abstract: Partial discharge (PD) measurement is an established condition monitoring technique used to facilitate the detection of incipient faults in high voltage electrical insulation systems such as gas-insulated switchgear and power transformers. By simultaneously recording partial discharge using both the IEC60270 'apparent charge' measurement technique and the more recent Radio Frequency method, which measures the energy radiated from the discharge, it is thought that more information can be gained about the nature of the PD source. Combined measurement has been carried out on various common PD source topologies recreated under laboratory conditions using PD test cells in a range of insulating media including SF6, oil and epoxy resin. Test cell geometries include floating electrodes, free particles and protrusions. By plotting apparent charge against the energy of the RF signal for a large number of PD pulses, the relationship between the two techniques can be quantified for a given test configuration. It has been found that the correlation between the two techniques produces characteristic patterns specific to each defect type. It is envisioned that combined RF/IEC measurements will contribute to a more widespread acceptance of the RF technique in terms of its ability to quantify PD severity by relating the RF energy to the more widely accepted IEC60270 pC level.
TL;DR: In this paper, the effect of thickness on the ac breakdown strength of epoxy nanocomposites was studied and it was observed that the decrease in ac electric breakdown strength with an increase in sample thickness follows an inverse power-law dependence.
Abstract: Experiments were conducted to measure the ac breakdown strength of epoxy alumina nanocomposites with different filler loadings of 0.1, 1 and 5 wt%. The experiments were performed as per the ASTM D 149 standard on samples of thickness 0.5 mm, 1 mm and 3 mm in order to study the effect of thickness on the ac breakdown strength of epoxy nanocomposites. In the case of epoxy alumina nanocomposites it was observed that the ac breakdown strength was marginally lower for 0.1 wt% and 1 wt% filler loadings and then increased at 5 wt% filler loading as compared to the unfilled epoxy. The Weibull shape parameter (β) increased with the addition of nanoparticles to epoxy as well as with the increasing sample thickness for all the filler loadings considered. DSC analysis was done to study the material properties at the filler resin interface in order to understand the effect of the filler loading and thereby the influence of the interface on the ac breakdown strength of epoxy nanocomposites. It was also observed that the decrease in ac electric breakdown strength with an increase in sample thickness follows an inverse power-law dependence. In addition, the ac breakdown strength of epoxy silica nanocomposites have also been studied in order to understand the influence of the filler type on the breakdown strength.
TL;DR: In this paper, the time characteristics of the surface charge accumulation process on cylindrical polymeric model insulators under dc stress were investigated in a partial discharge free, dry air environment.
Abstract: The time characteristics of the surface charge accumulation process on cylindrical polymeric model insulators under dc stress were investigated in a partial discharge free, dry air environment. In order to simulate the influence of different dielectric and electric material properties on surface charge accumulation, a tool was developed which considers also the nonlinear behavior of the electric conduction mechanism within the gas volume. The results of simulation have been verified by measurements of the surface potential distribution along the samples at different ambient temperatures.
TL;DR: In this article, the effect of the size and quantity of silica particles on the electrical properties of the analyzed materials was examined by transmission electron microscopy, dielectric spectroscopy, conduction current and space charge measurements.
Abstract: This work deals with the study of micro and nanosilica filled epoxy resin samples carried out in the framework of CIGRE WG D1.24 cooperative test program. This program focused on chemical, electrical and electrostatic properties of epoxy based nanodielectrics for electrical engineering applications. Epoxy based samples filled with micro and/or nanoparticles of silica were characterized by transmission electron microscopy, dielectric spectroscopy, conduction current and space charge measurements. These mutually complementary techniques were used to examine the effect of the size and quantity of silica particles on the electrical properties of the analyzed materials. The analysis of charge injection, polarization, trapping and conduction phenomena has allowed the modeling of dielectric behavior of the studied materials under multiple stresses. The Schottky Injection and Space Charge Limited Current models were studied to explain conduction phenomena. A composition of micro and nano-sized silica particles accumulating the smallest amount of space charge is also proposed.
TL;DR: In this paper, the authors proposed an epoxy/ conglomerated h-BN composite with co-loaded nano SiO2 and micro AIN filler, which has 12.3 W/m·K in thermal conductivity, 75.1 kVpeak/mm in BD strength and 260 % of BD time for neat epoxy.
Abstract: The aim of this research is to find a way to achieve the epoxy composites with high thermal conductivity and acceptable dielectric breakdown (BD) strength. A value 12.3 W/m·K is the highest thermal conductivity obtained for epoxy composite in Part I. Dielectric breakdown performances such as short-time dielectric breakdown strength (BD strength), partial discharge (PD) resistance and BD time for composites were investigated in the Part II. In general, micro filler inclusion will increase thermal conductivity and decrease dielectric breakdown performance. Influencing factors are considered to be the orientation of filler, the content of void space, the content ratio in the case of co-mixing, the addition of nano filler, and filler surface modification. Twenty six kinds of composites were prepared in consideration of the above influencing factors. There are two options for most appropriate ones among the composites evaluated in the research. One is an epoxy/ conglomerated h-BN composite with co-loaded nano SiO2 and micro AIN filler. It has 12.3 W/m·K in thermal conductivity, 75.1 kVpeak/mm in BD strength and 260 % of BD time for neat epoxy. It is most suitable when low BD strength and high thermal conductivity is needed. The other one is an epoxy/ h-BN composite with co-loaded nano silica and AIN filler for requirement of very high BD strength but lower thermal conductivity. Optimum thermal conductivity is obtained if flaky h-BN filler is oriented in parallel to heat flow. Since it is difficult to realize full orientation, the use of conglomerated h-BN filler is a suitable option. Optimum BD performance is obtained if void space is reduced by certain methods such as co-dispersion of different size fillers and addition of nano filler.
TL;DR: In this article, the authors present results of calculation of the electric field distribution for composite insulators up to 1200 kV, using a 3D software package based on the Boundary Element Method.
Abstract: Satisfactory operation of composite insulators is intimately related to the surface electric field (stress) distribution. This paper presents results of calculation of the electric field distribution for composite insulators up to 1200 kV, using a 3D software package based on the Boundary Element Method. The impact of corona and grading rings, single and bundled conductors, insulator orientation (dead-end and suspension), single and double units, and surface condition (dry and wet) on the electric field distribution has been analyzed. For UHV systems (higher than 750 kV ac) the use of dual insulators with individual corona rings at the line and ground end and a common grading ring at the line end is beneficial. The existence of optimal dimensions of corona and grading arrangement has been illustrated on a 1000 kV insulator string. It was found that the dead end insulators experience higher electric stress when compared to their suspension counterparts and this difference gets more prominent as the system voltage increases.
TL;DR: In this paper, the dielectric response of crosslinked polyethylene (XLPE) insulated, miniature power cables, extruded with inner and outer semicons, was measured over the frequency range 10-4 to 104 Hz at temperatures from 20 to 100 °C.
Abstract: The dielectric response of crosslinked polyethylene (XLPE) insulated, miniature power cables, extruded with inner and outer semicons, was measured over the frequency range 10-4 to 104 Hz at temperatures from 20 to 100 °C. A dielectric spectrometer was used for the frequency range 10-4 to 10-2 Hz. A bespoke noise-free power supply was constructed and used to measure the dc conductivity and, using a Fourier transform technique, it was also used to measure the very low dielectric tanδ losses encountered at frequencies of 1 to 100 Hz. Tanδ measurements of <;10-5 were found in this frequency range and attributed to a β-mode dielectric relaxation lying above 100 Hz due to motion of chain segments in the amorphous region and an β-mode relaxation lying below 1 Hz window due to twists of chains in the crystal lamellae. The dc conductivity measurements were consistent with those of the dielectric spectrometer and indicate lower dc conductivities in vacuum degassed cables than have been previously reported for XLPE (less than 10-17 S.m-1). The conduction process is thermally activated with an activation energy of approximately 1.1 eV. Higher conductivities were found for non-degassed cables. A transformer ratio bridge was used for measurements in the range 1 to 10 kHz; loss in this region was shown to be due to the series resistance of the semicon layers. Thermal ageing of the cables at 135 °C for 60 days caused significant increases in the conductivity and tanδ and it is considered that such measurements may be a sensitive way of measuring electrical degradation due to thermal aging.
TL;DR: In this article, surface charge decay on HTV silicone rubbers charged by impulse corona is studied, where surfaces of the materials exposed to corona were kept open to ambient air whereas the opposite surfaces were in contact with a grounded copper plate and surface potential distributions on the samples were measured using Kelvin type electrostatic probe.
Abstract: Surface charge decay on thick flat samples of HTV silicone rubbers charged by impulse corona is studied. In the experiments, surfaces of the materials exposed to corona were kept open to ambient air whereas the opposite surfaces were in contact with a grounded copper plate and surface potential distributions on the samples were measured using Kelvin type electrostatic probe. The developed procedure allowed for implementation of three study cases when (i) neutralization of pre-deposited charges by free ions present in air was prevented and surface potential decay occurred mainly due to bulk neutralization; (ii) gas neutralization took place under natural conditions and (iii) gas neutralization was enhanced due to increased amount of free ions in ambient air provided by nearby corona. Potential decay observed only due to bulk neutralization was used to evaluate voltage dependent conductivity of the materials and allowed for comparing them with those measured by the standard method. Comparison of decay characteristics observed for different test conditions were used to evaluate the relative importance of each mechanism on the total process of charge decay.
TL;DR: In this article, the degradation of the sample surfaces were analyzed using SEM techniques, surface profile studies, FTIR spectroscopy as well as partial discharge (PD) experiments were conducted at a voltage of 10 kV for different durations using IEC (b) type electrodes.
Abstract: Epoxy nanocomposite samples with a good dispersion of alumina nanoparticles in epoxy matrix were prepared and experiments were performed to measure their partial discharge resistant characteristics. Epoxy alumina nanocomposites with 0.1, 1, 5, 10 and 15 wt% nanofillers were prepared in the laboratory and partial discharge (PD) experiments were conducted at a voltage of 10 kV for different durations using IEC (b) type electrodes. The degradation of the sample surfaces were analyzed using SEM techniques, surface profile studies, FTIR spectroscopy as well as PD studies. An attempt was made to understand the interaction dynamics between the nanoparticle and the epoxy chain by measuring the glass transition temperature of the nanocomposites. The partial discharge resistance obtained for the nanocomposites are compared with those of unfilled epoxy and epoxy microcomposites. It was observed that even with 0.1 wt% of nanofiller added to the epoxy matrix, the partial discharge resistance to degradation gets improved considerably. It was also observed that the inter particle distance has a significant effect on the discharge resistance to degradation. The improvement in the degradation resistance is attributed to the interactions between the nanoparticle and the epoxy chain. A possible mechanism for the surface degradation of nanocomposites has been proposed.
TL;DR: In this paper, a method of determining the electric field distribution in dc power cables was proposed by considering the influence of space charge on the conductivity of the insulating material under different temperatures.
Abstract: When power cables are loaded under high voltage direct current (HVDC), an accumulation of space charge and a radial distribution of temperature gradient are developed across the insulation material. Such existence and accumulation of space charge within the insulating material poses a threat to the reliability of the operation of dc power cables. The electric field of a practical dc power cable is affected by the conductivity of the material, which is a function of both temperature and electric field. This causes difficulties in identifying the electric field distribution. In this paper, a method of determining the electric field distribution in dc power cables was proposed by considering the influence of space charge on the conductivity of the insulating material under different temperatures. Commercial 11 kV ac cross-linked polyethylene (XLPE) power cables were used and the space charge in these cables under dc conditions was measured using a modified pulsed electroacoustic (PEA) system with an attached current transformer. Therefore, a replica of a power cable under load conditions is obtained, which allows an investigation of the formation, migration and accumulation of space charge in a power cable with and without temperature gradients across the insulating material. COMSOL Multiphysics software package was used to accurately determine the electric field distribution in the dc power cable with consideration of the influence of electric field on the conductivity of the insulating material. The numerical modelling is based on the hopping conduction mechanism and its parameters were obtained from experiments carried out on the XLPE insulation material.
TL;DR: In this article, an analytical GC-MS method coupled with headspace injection was developed, followed by a validation, and it seemed that while no 2FAL was present some considerable amount of MeOH was detected which revealed some paper degradation.
Abstract: This study was designed to examine methanol as a new aging marker of insulating paper. An analytical GC-MS method coupled to headspace injection was developed, followed by a validation. To further validate the possible applicability of methanol for different transformer conditions some laboratory studies were performed such as a stability and an aging test. Oil samples of power transformers of several Belgian nuclear and coal-fired power plants have been analyzed since mid 2009 and are followed-up periodically to correlate the results with the analysis of the furanic compounds which have been used since mid 1980s as oil soluble degradation products of the insulating winding paper. It seemed that while no 2FAL was present some considerable amount of MeOH was detected which revealed some paper degradation. A second real case study was started to further correlate MeOH and 2FAL results. From these laboratory and real case studies it seemed that MeOH is a very promising marker for paper degradation in power transformers.
TL;DR: In this paper, an accelerated aging condition assessment of oil-paper transformer insulation based on partial discharge analysis was presented to realize statistical parameters reduction, and the extracted feature factors of this proposed model were used to identify oil paper samples with different aging degrees.
Abstract: This paper presents aging condition assessment of oil-paper transformer insulation based on partial discharge analysis in order to realize statistical parameters reduction. The extracted feature factors of this proposed model were used to identify oil-paper samples with different aging degrees. An accelerated aging test was implemented using artificial oil-paper samples with an internal flat air gap. During the aging test, partial discharge signal acquisition was conducted periodically. In the new model, conventional statistical parameters of phase resolved partial discharge (PRPD) patterns were analyzed using principal component and factor analysis (PCFA), and a group of new features constituted by the extracted factors was obtained. These factors were not only independent of one another, they had their own specific properties. To a great extent, these factors represent information on PRPD patterns through a limited number of variables. Through the use of the new features extracted from PCFA method, the clustering and discriminating results of the samples with different aging stages provided significantly referenced information on the condition assessment of oil-paper insulation.
TL;DR: In this article, the influence of accelerated water treeing test (AWTT) on the properties of 10 kV cross-linked polyethylene (XLPE) cable insulating materials was investigated.
Abstract: The influence of accelerated water treeing test (AWTT) on the properties of 10 kV cross-linked polyethylene (XLPE) cable insulating materials was investigated in this paper. The dielectric and physicochemical properties of both aged samples and unaged samples were tested. Dielectric property investigation found a new peak of the dielectric loss tangent with an activation of 0.14 eV in the low-frequency domain for aged samples. With increased ageing time, the conductivity in low frequency region of less than 50 Hz increases obviously, while it nearly keeps unchanged in the frequency above 50 Hz. Physicochemical investigation of FTIR, DSC, XRD and density measurement results indicated that AWTT ageing of XLPE insulated cables will lead to decreased crystallinity and density, and methyl group at the outer insulation layer cannot be found in aged samples. The melting temperature (Tm) was also decreased with ageing time. It was suggested that water was introduced from the outer layer of insulation and the degradation was developed by a combination of mechanical forces and electric force. During the ageing process, water will choose a path between surface of lamella and amorphous region, which consequently lead to the increase of low frequency conductivity and decrease of lamella thickness. Furthermore, a model was proposed to explain how the microstructure of XLPE cable insulation will change during AWTT ageing process. It is shown that XLPE cable insulation will be changed both in dielectric and physicochemical properties before any water tree was found, and a combination of dielectric and physicochemical methods is effective to detect the degradation of the XLPE cable insulation materials.
TL;DR: In this paper, the authors investigated the effect of nano-Al2O3 fillers on the breakdown strength of gate bipolar transistors (IGBTs) and found that the nano-fillers can improve the PD resistance of composites.
Abstract: Epoxy composite is expected to become the substrate insulation of gate bipolar transistors (IGBTs) that can replace ceramics if it has high thermal conductivity. Fillers with high thermal conductivity were added to the epoxy resin, while this composite often suffers from lowering in breakdown (BD) strength. In order to reveal the above phenomena and to clarify the breakdown (BD) mechanism, we carried out experiments using several electrode structures, i.e. an MB-PWB (metal-base printed wiring board) insulation simulated structure, a rod-to-plane electrode for PD erosion resistance, and a sphere-to-sphere electrode for BD strength. As a result, it was clarified that the IGBT insulation breaks down after successive partial discharges (PDs), and the BD strength of microcomposite decreases with increasing the content of micro-fillers, while PD resistance of microcomposite is improved with increasing the content of micro-fillers. A trial was made to raise the once-lowered BD strength by adding nano-Al2O3 fillers. Above experiments were carried out for nanocomposites and nano-micro-composites. It was clarified that the nano-fillers can improve the PD resistance of composites obviously, while nano-fillers improve the BD strength of composite slightly. Such a phenomenon was found that the BD strength of composites was increased with the dispersion state of nano-fillers.
TL;DR: In this article, an investigation and condition assessment of 152 MVA, 512 kV, 1-phase, 2-winding generator step-up transformer having a history of combustible gases is presented.
Abstract: Power transformers are designed to withstand system abnormalities such as over voltages and external short-circuits. Transformer monitoring and diagnostics are the effective techniques in preventing the eventual failures and contribute to ensure the plant's reliability. Transformer life mainly depends on the integrity of its solid insulation (cellulose). The solid insulation in transformers degrades with time at rates which depend on the temperature and the amount of moisture, oxygen and acids in the insulation system. Moisture and oxygen cause the paper insulation to decay much faster than normal. Moisture in a transformer accelerates the aging process and causes severe deterioration in the mechanical and electrical properties of insulation system. The aging of paper insulation is irreversible. The degree of polymerization value is a main relation between insulation deterioration and formation of aging products. To predict a failure or an unusual behavior, an accurate interpretation of monitoring, diagnostics and maintenance data is required. The monitoring of insulation system and verification of core and winding mechanical integrity allow optimization of the lifecycle management of an asset. This paper presents an investigation and condition assessment of 152 MVA, 512 kV, 1-phase, 2-winding generator step-up transformer having a history of combustible gases.
TL;DR: In this article, a numerical simulation method dedicated to design assistance of high reliability machines fed by PWM inverters is proposed, which allows the computation of winding turn-to-turn maximum voltage stress for any combination of the wire positions in the stator slot.
Abstract: This paper proposes a numerical simulation method dedicated to design assistance of high reliability machines fed by PWM inverters. It allows the computation of winding turn-to-turn maximum voltage stress for any combination of the wire positions in the stator slot. With such a tool, it is possible to design coils with the best wire arrangement for any fast-fronted pulses. An equivalent circuit is used to simulate the voltage distribution among the turns of the winding. A finite-element analysis simulation package is used to estimate the high-frequency distributed-circuit parameters of the winding. In order to validate the simulation results, they are compared with experimental results.
TL;DR: In this article, the authors present experimental studies on streamer and breakdown of synthetic and natural ester liquids with the presence of parallel pressboard interface in a divergent pointplane field under lightning impulse voltage.
Abstract: This paper presents experimental studies on streamer and breakdown of synthetic and natural ester liquids with the presence of pressboard interface in a divergent point-plane field under lightning impulse voltage. Based on the current, light signals and streamer images, characteristics of both positive and negative streamers on pressboard surface were described, which resemble the features of streamers in open liquid gaps. Introducing pressboard in parallel does not influence the streamer stopping length and thus not weaken the oil gap breakdown voltage, under both positive and negative polarities at gap distances up to 75 mm. However, presence of parallel pressboard significantly reduces the acceleration voltage in mineral oil but seems have no influence on ester liquids under positive polarity, while it tends to accelerate the streamer velocity at larger gap distance and under higher voltage in both ester liquids and mineral oil under negative polarity. Generally, streamer on ester/pressboard interface propagates further than on mineral oil/pressboard interface at the same voltage level; both breakdown voltage and acceleration voltage on ester/pressboard interface are lower than those on mineral oil/pressboard interface at the investigated gap distances up to 75 mm.