Showing papers in "IEEE Transactions on Dielectrics and Electrical Insulation in 2021"

Applications of the Acoustic Method in Partial Discharge Measurement: A Review

Abstract: This paper reviews the main features of acoustic emission technique. First, the characteristics of acoustic signals and their physics are addressed. Then, the structure of acoustic sensors employed for capturing signals is studied. In the next step, the acoustic method of PD measurement is compared with the standard electrical method. Afterward, the applications of AET in PD measurement in different equipment are summarized. All acoustic method and combined acoustic-electrical method for PD localization in transformers are discussed. Moreover, the applications of AET in the monitoring of GIS systems are explained, along with the acoustic behaviors of moving particles in these systems. Finally, the general considerations in the signal processing of the acoustic signals are reviewed.

Insulation Materials and Systems for Power Electronics Modules: A Review Identifying Challenges and Future Research Needs

Abstract: This manuscript critically reviews recent research on electrical insulation materials and systems used in power electronics devices and focuses on electrical treeing in silicone gel, PD modeling, and mitigation methods. For mitigation methods, electric field grading techniques, such as 1) various geometrical techniques, and 2) applying nonlinear dielectrics are discussed. Alternatives for silicone gel, such as liquid dielectrics, are also highlighted. The drawbacks of reported research and technical gaps are identified. In particular, we show that the investigations carried out to date are in their infancy regarding the working conditions targeted for next-generation WBG power devices. This review will provide a useful framework and point of reference for future research.

Effect of Crystallization Regulation on the Breakdown Strength of Metallized Polypropylene Film Capacitors

Abstract: In this work, polypropylene (PP) film samples doped with an organic phosphorus nucleating agent under three cooling processes are examined for the effects of regulating the crystallization The conductivity and DC breakdown strength of the film samples were tested at 25, 55 and 85 °C The average breakdown strength with 001 wt% nucleating agent increased by approximately 25% compared to un-nucleating samples and the DC conductivity decreased slightly For the three cooling methods in these tests, the slow process increased the crystallinity of the film samples and stabilized the electrical properties of the PP samples It is concluded that improving the insulation performance through crystallization control is feasible, and this method shows great potential for the modification of PP films

Gas Convection Affecting Surface Charge and Electric Field Distribution around Tri-post Insulators in ±800 kV GIL

Abstract: A multi-physics simulation model of the tri-post insulator in ±800 kV GIL laid horizontally is established to study the effects of gas convection on surface charges and electric field distribution. Results show that gas density together with the dielectric strength around the upper post is reduced over 11% under gas convection generated by the load current. The larger temperature gradient (29 K) along the lower post accelerates more surface charge injection through insulator buck conduction and distorts the electric field around the bottom of the lower post, which is 19.4% higher than that around the bottom of the upper post. The insulation performance of SF 6 /N 2 filled GIL has the most striking spatial distribution characteristics compared with the insulation performance of SF 6 and C 4 F 7 N/CO 2 . Though the flashover voltage is improved with high gas pressure, the flashover is most likely triggered near the lower post of the insulator according to the stream theory. It is hoped that this study can contribute to a useful guide for the design and manufacture of DC-GIL.

High Temperature Insulation Materials for DC Cable Insulation — Part I: Space Charge and Conduction

Abstract: Fundamental studies of potential candidates for DC electric power transmission in high temperature environment, including ETFE, FEP, PTFE, PI and PEEK, are carried out and presented in form of the series papers containing space charge and conduction as part I, partial discharge as part II, and the degradation and surface breakdown as part III. In this part, the space charge at 20kV/mm was measured at 25 °C and with a thermal gradient at 50 °C. The electrical conductivity was measured at electric fields ranging from 10kV/mm to 30kV/mm in temperatures ranging from 25 °C to 200 °C. The experimental results showed that considering the effect of thermal condition and electric field, FEP has the lowest total amount of space charge accumulation and electric field distortion among these materials at the measured conditions. PI and PEEK have the lowest amount of trap-controlled mobility at 25 °C due to deeper average trap level because of the aromatic rings in the structure. PTFE and PI have the lowest amount of thermal activation energy and temperature-dependent electrical conductivity due to the more uniform morphological phase comparing to ETFE, FEP, and PEEK. The outcomes of this paper serve as a benchmark for the fundamental researches over high temperature materials for DC applications and lay a basis for Part II and Part III.

Electric Field Simulation of Permittivity and Conductivity Graded Materials (ε/σ-FGM) for HVDC GIS Spacers

Abstract: This paper discusses the application of the novel technology - functionally graded material (FGM), which combines both spatial permittivity (e) and conductivity (σ) distributions with the aim to control the electric field around DC-GIS spacer. Some distribution types of e and/or σ in the spacer bulk, such as U-type and graded to higher conductivity (GHC) are investigated through electric field simulation, in comparison to the uniform spacer with constant e and σ distributions. The electric field relaxation effect of each distribution type of e/σ-FGM under DC steady state, DC polarity reversal, DC-on, and lightning impulse voltage superimposed on DC steady state condition are obtained. The results show that e/σ-FGM with U-type permittivity and GHC-type conductivity distribution of which low σ is applied near the high voltage side of the DC-GIS spacer is the most effective in reducing the maximum electric field under all stated conditions.

Field Grading Composites Tailored by Electrophoresis—Part 3: Application to Power Electronics Modules Encapsulation

Abstract: A series of three articles presents an innovative way to build advanced functionally graded materials (FGM) based on polymer/ceramic (epoxy/SrTiO 3 ) composites tailored by electrophoresis for field grading in power electronics. In Part 3, this method is applied in the context of power modules for DBC substrate encapsulation. An evaluation of the FGM performances is reported based on electrostatic simulations and breakdown voltage measurements on encapsulated DBC substrates. The results show a significant mitigation of the electric fringe field at the triple point while breakdown is largely increased by a factor 2 for FGM composites compared to neat epoxy. The process enables the use of electric field reinforcements of HV electrical systems (e.g. tips coming from the design), and thus potential weak points, to locally ‘self-heal’ them in-situ. Such an electrophoresis process used to build FGM composites paves the way for the next generation of functionalized polymer composites used in high voltage power applications for improving the electrical aging of insulating materials and power system reliability.

Identification and Application of Machine Learning Algorithms for Transformer Dissolved Gas Analysis

Abstract: Power transformers represent one of the most abundant and expensive components in the electric power industry. Dissolved gas analysis (DGA) of transformer is the most widely accepted diagnostic tool across the globe to understand insulation incipient failures. Nevertheless, DGA fault gas interpretation is a remarkable challenge for transformer owners and utility engineers. Several computational techniques have been adopted for DGA fault classification along with offline methods. However, limited data availability, high ambiguity in DGA interpretation, suitability, and model accuracy are critical challenges in the DGA fault classification using computational techniques. In this work, highly diverse and large DGA data samples of in-service transformer fleets from five different utilities have been used to develop an efficient fault classification methodology. A total of 4580 DGA samples and IEC TC 10 database are used for training and testing, respectively, for various machine learning algorithms. Discussions on performance indicators and evaluation of several algorithms to verify the most suitable class algorithms are also the focus of this work. Furthermore, a best-performing model is identified based on various performance indicators. The hyperparameters of the best model are further tuned to achieve a most precise fault classification. It is inferred that non-parametric methods and non-linear SVM are best suitable for transformer DGA fault classification. Importantly, the rankings in the present study suggest that transformer DGA fault prediction is better with ensemble learning methods.

Breakdown Strength of TiO 2 /Epoxy Nanocomposites Using Centrifugation Agglomerate Removal

Abstract: In this study, TiO 2 /epoxy nanocomposites are evaluated for impulse and AC breakdown strength when agglomerations of TiO 2 nanofillers are removed by centrifugation It is found that the nanocomposites with micrometric agglomerates exhibit lower breakdown strengths than that of unfilled epoxy resin However, with the removal of agglomerates larger than 05 µm for 04 vol% filler concentration, the breakdown strengths are higher than that of unfilled epoxy resin Micrometric agglomerates of TiO 2 nanoparticles behave as defects, thus lowering the breakdown strength of the nanocomposites When such agglomerates are removed prior to curing, an improvement of the breakdown strengths is observed with very low filler concentration

Aging Assessment Model of Transformer Insulation Based on Furfural Indicator under Different Oil/Pressboard Ratios and Oil Change

Abstract: Oil change, as an effective method, could improve the performance of liquid insulation. However, due to the furfural loss in oil after the oil change, the aging assessment results of cellulose insulation based on the traditional furfural-degree of polymerization (DP) model is not accurate. Besides, the different oil/pressboard mass ratio of the transformer also has a significant impact on the aging assessment results. Current research only focuses on the aging assessment based on furfural analysis under the single oil/paper mass ratio and oil change conditions. In view of this, the aging assessment model for transformer insulation based on furfural indicator under different oil/pressboard ratios and oil change conditions was studied. First, oil/pressboard insulation systems with different oil/pressboard mass ratios were performed for total oil changes. Then, the initial furfural-DP equation and the furfural corrected equation were obtained. Finally, the modified furfural-DP model extended to different oil/pressboard mass ratios under oil change was established. The proposed model is expected to promote the accuracy of the aging assessment of transformer cellulose insulation under different oil/pressboard mass ratios and oil change conditions.

Aging Behavior of Flame-retardant Cross-linked Polyolefin under Thermal and Radiation Stresses

Abstract: In this study, flame-retardant cross-linked polyolefin, which is an important polymeric insulation material for electric power cables, is aged under severe thermal and radiation stresses After aging, the samples are analyzed mechanically, thermally, and electrically The degradation of this polymer is found to become very severe when it is aged thermally than when aged by a combination of heat and gamma irradiation The formation of densely cross-linked structures, induced by the gamma irradiation, appears to make the polymer harder and more resistant against oxidation and against its resultant scission of chemical bonds

Capacitance Loss Mechanism and Prediction Based on Electrochemical Corrosion in Metallized Film Capacitors

Abstract: For the metallized film capacitors (MFCs) designed to maximize the self-healing performance in the event of overvoltage, electrochemical corrosion is the major capacitance loss (CL) mechanism especially under high temperature and/or high humidity. In this paper, the electrode corrosion evolution is analyzed by dividing the process into anode and cathode reaction. In order to investigate the CL characteristics, three types of MFCs are stressed with 305 VAC under different temperature and humidity. Based on the corrosion mechanism and distribution analysis, a staged CL prediction method is proposed. The results indicate that the typical CL curve can be divided into 4 stages which are characterized by the reaction rate coefficient and the capacitance loss exponent derived from the experimental data.

Charge Transport and Trapping of Surface Modified Stator Coil Insulation of Motors

Abstract: This paper presents the surface modification and charge transportation mechanism of thin polyimide films. Here, we demonstrate that the surface modification of these films, by using the fluorine gas-phase, improves the charges mitigation and, thereafter, suppress the space charges. The experimental results indicate that the thermal step current for fluorine coated films reduce significantly in comparison to pure polyimide films. The thin layer of fluorine coating helps to release the surface charge, reduce the charge injection from the electrode, provide hindrance for charges to penetrate into the bulk of the films and the higher degree of charge disappearing helps to reduce the space charge growth.

Calculation of Electric Fields in HVDC Cables: Comparison of Different Models

Abstract: This paper presents a comparison of different models used for calculations of electric field distributions in HVDC cable insulation. Calculated electric field and space charge distributions for a loaded 320 kV HVDC cable are compared with the experimental characteristics reported in the literature. Additionally, for selected models, a sensitivity of the results to variations of the model parameters is analyzed. It is noticed that the calculations utilizing the empirical expressions for the field and temperature dependent electric conductivities (Le., macroscopic approach) allowed for predicting so-called field inversion phenomena under DC stresses. It is found, however, that such models are not capable of predicting the magnitudes of the maximum field stress and polarity of the accumulated space charges accurately. Models based on bipolar charge transport (i.e., microscopic approach) in addition to the effects described by the macroscopic models, predict the experimentally observed accumulation of homo- and hetero-charge in the insulation reasonably well. It is concluded that the microscopic approach has been used with proper descriptions of the rates and parameters of physical processes of the transport, generation and losses of charged species in the bulk of the insulation, as well as of its interfaces, provides convincing electric field estimations for HVDC cables. Because of the uncertainty in model parameters, the accuracy of DC field calculations is less than the high accuracy of AC calculations for HVAC cables that we are used to.

Space Charge Characteristics in Epoxy Resin Impregnated Paper under AC and AC-DC Superimposed Electric Fields

Abstract: In this paper, charge distributions in three-layered epoxy resin and epoxy resin impregnated paper samples under AC and AC-DC superimposed electric fields are measured by the improved high-speed pulsed electro-acoustic system. To exclude the interferences caused by the discontinuity of relative permittivity, a method to distinguish between net dipolar charges and free charges is proposed. Through analysis, the influences of frequency, AC component and DC component on the charge transport and trapping characteristics in multi-layered epoxy composites are obtained, which could provide important reference to the structural design and optimization of converter transformer bushings.

BNNS Encapsulated TiO 2 Nanofillers Endow Polypropylene Cable Insulation with Enhanced Dielectric Performance

Abstract: In this work, nano-titanium oxide (TiO 2 ) is encapsulated into boron nitride nanosheets (BNNS) by the calcination process to fabricate core-shell structured TiO 2 @BNNS nanofillers. The effects of TiO 2 @BNNS fillers on the DC conductivity, space charge and breakdown strength of polypropylene (PP) composites are investigated. Experimental results show that PP/TiO 2 @BNNS composites have a lower temperature-dependent conductivity with an activation energy of 0.64 eV, which is much lower than that of PP, 0.95 eV. The DC breakdown strength of PP/TiO 2 @BNNS samples is 22.2% higher than that of PP at 90 °C. The average space charge density and electric field distortion rate of PP/TiO 2 @BNNS are reduced by 6.6 C/m3 and 23.3%, respectively, compared with PP, under a polarization electric field of 40 kV/mm at 90 °C. It is proven that TiO 2 @BNNS nanofillers form numbers of electrical potential wells for charge carriers, thereby, inhibiting the hot electron transport and their damage to polymer molecules. This work provides an effective method to improve the dielectric performance of PP cable insulation.

Optimization of the Temperature-Dependent Electrical Resistivity in Epoxy/Positive Temperature Coefficient Ceramic Nanocomposites

Abstract: Nano-size positive temperature coefficient (PTC) ceramic particles and epoxy composites doped with 0, 0.5, 1, 5, 10 and 20 phr of PTC nanoparticles are prepared to suppress the decrease of electrical resistivity with temperature. Electrical resistivity under 30, 50, 70, 90, and 110 °C, DC breakdown strength, and trap characteristic are measured. DC electric field distribution under radical temperature gradient is simulated using a simplified bushing model insulated with prepared materials. The mechanism for the regulating of electrical resistivity-temperature characteristic of epoxy composites is discussed. The electrical resistivity at 110 °C, DC breakdown strength at 110 °C, and electric field distortion of the epoxy composite with 1 phr PTC nanoparticles is 454, 131 and 74%, respectively of the neat epoxy resin, which exhibits potential to be used as insulating materials in high voltage DC electrical systems.

Influence of Liquid Type, Filler and Coupling Agent on the Interface Resistivity Between Silicone Rubber and Fiber Reinforced Plastic

Abstract: The interface between silicone rubber (SR) and fiber reinforced plastic (FRP) is a weak point in composite insulator, which is easy to be affected by moisture. However, the corresponding evaluation methods and detection means are not complete yet. Based on the newly proposed method for measuring the interface resistivity with a four-electrode system, the influence of liquid type, filler and coupling agent on the interface resistivity are studied in this paper. It is found that the interface resistivity reduced by 2–4 orders of magnitude after permeation by deionized water, saline and low concentration nitric acid, but the variation trend of interface resistivity is specific when permeated in high concentration of nitric acid solution (1 mol/L). The experimental results of samples with and without silicone coupling agent (SCA) are compared and it is found that the interface without silane coupling agent is easier to be penetrated by liquid, and the change of interface resistivity is more obvious. Besides, the influence of moisture-sensitive Alumina Tri-hydrate and SiO2 fillers, which could provide paths for water and hydrated ions to permeate, on interface resistivity are analyzed. The experimental results are of great significance for the selection of the appropriate formula system and the prevention of liquid penetration for composite insulators.

Application of Deep-Learning via Transfer Learning to Evaluate Silicone Rubber Material Surface Erosion

Abstract: In this letter a deep learning-based model is proposed for online inspection of silicone rubber outdoor insulators. The inclined plane tracking and erosion test is used as per ASTM D2303 in order to simulate standard erosion on silicone rubber insulation composites. Photos taken for the tested composites are used as training and testing inputs for a convolutional neural network topology in the proposed deep learning model, thereby classifying the degree of erosion damage into light, moderate and severe. The remarkable classification accuracy obtained shows the potential of utilizing the proposed framework for online monitoring of outdoor silicone rubber insulators in the transmission and distribution grid.

Comparative Properties of HTV Silicone Rubber for Composite Insulators—ATH and Silica Fillers

Abstract: This paper presents a comparative evaluation of high temperature vulcanizing (HTV) silicone rubber consisting of ATH and silica fillers for use as the housing of outdoor composite insulators. Various volume filler fractions (vol%) of alumina trihydrate (ATH) and silica (SiO 2 ) and several median particle sizes are incorporated into HTV silicone rubber and evaluated for erosion and tracking resistances using the inclined-plane tracking and erosion tests (IPT), in accordance with IEC 60587 at several AC test voltages. Test durations, eroded masses, erosion depths, and leakage currents are reported. The dry-band arcing temperatures of the various samples are recorded during the tests. The results show that at 30 vol% filler fractions, ATH filled samples show lower erosion than the corresponding silica filled samples; however, at 15 vol% fraction, the silica filled samples show lower erosion. For most tested conditions, the maximum dry-band temperatures of the silica filled samples are lower than the corresponding ATH filled samples.

Effects of Hindered Phenolic Antioxidants on Space Charge and Breakdown Properties of Polypropylene

Abstract: This paper focuses on the effects of hindered phenolic antioxidants AO300 and AO736 with similar molecular structure on the electrical properties of isotactic polypropylene (iPP) Experiments of conductivity, space charge distribution, and DC breakdown strength after the hetero-polarity DC prestress are performed The experimental results show that these two antioxidants can increase the conductivity-temperature coefficient, reduce the accumulation of space charge and improve the DC breakdown strength after the hetero-polarity DC prestress of iPP By calculating the molecular orbitals of these two antioxidants, it is found that the antioxidants introduce local states in the forbidden band of iPP matrix to capture high-energy charges Besides, the bond dissociation energy (BDE) of the hydrogen-oxygen bond in AO736 is lower than that in AO300, representing a stronger ability of proton transfer, which scavenges the radicals, inhibits the growth of microvoids and prevents the process of breakdown It is concluded that the improvement of space charge and breakdown properties is attributed to the coupling effects of deep trap sites and radicals scavenging introduced by antioxidant This research shows that AO736 has potential applications in PP insulation

Dissolved Gas Analysis in Mineral Oil and Natural Ester Liquids from Thermal Faults

Abstract: This work addresses the gas formation mechanisms for paraffinic-type mineral oil and soybean-based natural ester with a predominantly linoleic fatty acid under thermal faults. Based on a literature review, possible decomposition paths due to pyrolysis are proposed. A case study of hot spots of different temperatures is presented and its results are compared and analyzed with the gas profiles of the IEEE C57.155-2014 guide. The results obtained for both cases are consistent with the proposed mechanisms. For the same thermal fault, the gas profiles are different. The natural ester presents significant proportions of ethane and carbon dioxide concerning mineral oil. For temperature faults above 700 °C, the behavior of the chemical structures of the natural ester is similar to the carbon structures of the mineral oil and is subject to similar reactions. Therefore, the major differences in the gases generated for thermal faults are at low temperatures.

The Possibility of Fault Location in Cross-Bonded Cables by Broadband Impedance Spectroscopy

Abstract: Broadband impedance spectroscopy (BIS) provides a new method for cable fault location, however, this method has not been properly used in locating cable fault of cross-bonded (CB) cables due to the geometric discontinuity of impedance spectroscopy measurement loop. This study addresses the possibility of fault location by BIS in CB cables. First, this paper proposes a fault location method by transforming BIS in frequency domain into the location function in space domain with the aid of integral transformation (IT). The kernel function $\mathrm{e}^{j{2}\beta x}$ is used to obtain fault location function. Second, this paper demonstrates an impedance spectroscopy measurement loop in CB cables. BIS in CB cables is observed having a generalized orthogonality with the kernel function $\mathrm{e}^{j{2}\beta x}$ . Given this, the IT method is applicable for fault location of CB cables. Finally, this study carries out the location experiment of cable joints on a 110 kV CB cable in cable trench. The experiment shows that BIS in CB cables could be obtained by the loop proposed in this paper. Besides, the results verify the possibility of fault location in CB cables by IT method using BIS.

Dielectric Property Improvement of Polypropylene Films Doped with POSS for HVDC Polymer Film Capacitors

Abstract: This paper focuses on the effects of the nano Polyhedral oligomericsilsesquioxanes (POSS) with three similar molecular structures on the thermal and electrical properties of polypropylene (PP) films, aiming to propose a comprehensive modification method. After doping with micro amount of nano POSS, the crystallinity and regularity of films are elevated. Correspondingly, PP with optimized microstructure has the lowest conductivity, due to the interaction between the nanoparticles and the matrix, as well as the strong binding force of the crystal region to the carriers. The loss of the sample is restricted, especially in the low frequency region, since the combined effect of polarization and conduction. The DC breakdown strength of the modified PP is higher than that of the PP, with an increase of above 20% at 25–85 °C. Based on quantum chemical calculations, the energy level distributions are analyzed. By selecting appropriate additives, deep trap sites can be introduced to catch carriers, preventing the breaks of polymer molecular chains. This modification of PP films shows huge potential in the improvement of dielectric properties in the field of DC capacitors.

Microscopic Reaction Mechanisms of Formic Acid Generated During Pyrolysis of Cellulosic Insulating Paper

Abstract: Cellulosic insulating paper is the essential component of the insulation in power transformers. Under the thermal stress inside the transformer, the cellulosic insulating paper degrades and generates formic acid. Meanwhile, the presence of formic acid further accelerates the aging process of cellulosic insulating paper. This study takes the cellulose molecule composed of D-glucose as the research object. The ReaxFF reactive force field is applied to simulate the high-temperature thermal aging process of cellulosic insulating paper. During pyrolysis process, the number of formic acid molecules presents short-term fluctuations and continuous increases. Though simulation, the main reaction pathways of cellulose pyrolysis to generate formic acid are obtained. In addition, an accelerated thermal aging experiment of oil-paper insulation is designed. The formic acid in the experimental samples is detected by the silanization derivatization method. Through the combination of simulation and experimental results, the feasibility of formic acid as an aging indicator for cellulosic insulating paper is further demonstrated.

Gaussian Process Multi-Class Classification for Transformer Fault Diagnosis Using Dissolved Gas Analysis

Abstract: Dissolved gas analysis (DGA) is widely used for oil-immersed power transformers as a conventional fault diagnosis tool. However, interpretation criteria from DGA assessment often depends on empirical discrimination from a specialist, which can render unreliable or ambiguous diagnoses. Intelligent fault classification algorithms can be implemented to conquer uncertainty in conventional methods, and which require feature learning of transformer condition information data rather than expert experience. In this paper, a Gaussian process multi-classification (GPMC) method is proposed, which uses multiclass recognition with a Gaussian process (GP) and renders an output with a probabilistic interpretation rather than a deterministic guess. The method is investigated using large-scale DGA field datasets to improve diagnostic accuracy and presents reliable incipient fault diagnosis ability. A kernel-based learning algorithm and versatile artificial intelligence (AI) methods, (support vector machine (SVM), artificial neural network (ANN), K-nearest neighbors (KNN), decision tree and logistic regression (LR)) have been used to obtain comparative classification accuracy in comparison to the proposed method. Additional comparison is demonstrated between conventional DGA and AI methods. The effectiveness and robustness of the proposed GPMC method are confirmed by experimental accuracy >95%, which illustrates that the proposed method is able to provide superior and reliable diagnoses for operational transformer faults.

Analysis of Disordered Dynamics in Polymer Nanocomposite Dielectrics for the Realization of Fractional-Order Capacitor

Abstract: In this work, analysis of disordered dielectrics is performed in light of partially conductive graphene nanosheets (GNS)/P(VDF) polymer nanocomposites The analysis shows that the polymer composites with minimal conductivity can be employed as dielectrics for fractional-order capacitor (FOC) GNS weight percentage is varied from 3 to 13 wt% in P(VDF) matrix to track the onset and saturation of conductivity Impedance spectroscopy of the samples is carried out in the frequency range of 100 Hz to 20 MHz along with XRD and Raman spectroscopy The phase angle of the impedance varies from −84° to −46° with a maximum phase ripple of ±5° over a frequency range of ~ 3 decades The fractional-order ‘α’ of the complex impedance varies from 093 to 051 Nyquist and Bode plots for all the samples are fitted using Cole-Cole model for non-ideal dielectric response and the electrical equivalent circuit based on Cole-Cole model is obtained The deviation from ideal capacitive behavior is monitored using the equivalent circuit data

Estimation of Moisture Content in Oil-impregnated Pressboard through Analyzing Dielectric Response Current under Switching Impulse

Abstract: This paper proposes an advanced technique for estimating the paper moisture content within oil-impregnated pressboard by analyzing its dielectric response current signature under switching impulse voltage. The present work is oriented towards development of a method which can assess the condition of transformer insulation through measuring the dielectric response current during its switching on or off operation to the grid. In order to investigate the effectiveness of the method, the dielectric response current of a few oil-impregnated pressboards containing different moisture contents, are measured using switching impulse in laboratory. Based on the applied switching impulse and corresponding response current, the branch parameters of the equivalent circuit of insulation are evaluated which have been employed for formulating its transfer function. The zero and pole calculated from the transfer function are found to be sensitive to the paper moisture of insulation. Correlation of transfer function zero and pole with the paper moisture of insulation are derived using the experimental results. These correlations are validated through the experimental investigation on few other test samples.

Effect of Thermal Ageing on Physico-Chemical and Electrical Properties of EHVDC XLPE Cable Insulation

Abstract: This paper presents the influence of thermal ageing on the physico-chemical and electrical properties of 500 kV extra high voltage direct current (EHVDC) XLPE cable insulation. EHVDC XLPE cable sections are placed in a thermal oven and thermally aged at 106 °C (melting peak temperature) for 250, 500 and 1000 h. After ageing, the inner, middle and outer layers of the cable insulation are characterized by Fourier transform infrared (FTIR) spectroscopy, oxidation induction time, X-ray diffraction (XRD), space charge, DC conductivity and DC breakdown measurements. The physicochemical analysis results show that the cross-linking by-products in the cable insulation are difficult to be completely removed during the ageing process, whereas the thermal ageing cannot completely consume the antioxidants in the materials. The space charge measurement results show that the hetero charges are accumulated inside the unaged middle layer samples. However, the space charge accumulation and electrical conductivity of the samples are significantly reduced after 1000 h ageing. Moreover, the DC breakdown strength of the inner and middle layer samples is continually increased with the increase of the ageing time. It is elucidated that the thermal ageing leads to the re-crystallization and re-degassing of samples, improving the space charge behavior and the charge carrier trapping phenomenon. It is proposed that the temperature of 106 °C could be used as a suitable degassing temperature for the EHVDC XLPE cable production.

High-Temperature Performance of Dielectric Breakdown in BOPP Capacitor Film Based on Surface Grafting

Abstract: In this paper, a novel modification method for biaxially oriented polypropylene (BOPP) membranes is proposed based on surface-grafting technology. The experimental results show that, compared with the pristine samples, the surface-grafted films show better breakdown strength under different voltage stress especially at higher temperatures (85 °C). The DC withstand strength of the surface-grafted PP membranes grows by 42.7% from 386.4 to 551.2 kV/mm. The breakdown strength climbs by 63.5% from 305.9 to 500.2 kV/mm under DC superimposed pulse voltage. Under DC superimposed harmonic stress, the breakdown strength is increased from 175.6 to 587.4 kV/mm by 234%. The influence of surface modification on charge transmission behavior is proposed based on the Density Functional Theory (DFT) simulation. This research provides a theoretical and methodological reference for performance optimization by surface modification for BOPP capacitors.