Nanodielectrics, which are concentrated in polymer matrix incorporating nanofillers, have received considerable attention due to their potential benefits as dielectrics. In this paper, short-term breakdown and long-term failure properties of nanodielectrics have been reviewed. The characteristics of polymer matrix, types of nanoparticle and its content, and waveforms of the applied voltage are fully evaluated. In order to effectively comment on the published experimental data, a ratio k has been proposed to compare the electric properties of the nanodielectrics with the matrix and assess the effect for nanoparticles doping. There is evidence that the short-term breakdown properties of nanodielectrics show a strong dependence on the applied voltage waveforms. The polarity and the cohesive energy density (CED) of polymer matrix have a dramatic influence on the properties of nanodielectrics. Nanoparticle doped composites show a positive effect on the long-term failure properties, such as ageing resistance and partial discharge (PD) properties of nanocomposites are superior than microcomposites and the matrix. The larger the dielectric constant and CED of the matrix become, the more significant improvements in long-term performance appear. Based on the reported experimental results, we also present our understandings and propose some suggestions for further work.

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Short-term breakdown and long-term failure in nanodielectrics: a review

Digital control system analysis and design

The paper proposes a procedure for life estimation of high voltage AC cables in real operating conditions, i.e. subjected to voltage and load cycles, so that electrothermal stress is the dominant aging factor of cable insulation. As possible alternatives for representing the effects of the electrothermal aging of insulation, three life models wellknown in literature are considered, namely the Zurkov, Crine and Arrhenius-IPM models, each within the probabilistic framework needed for associating time-to-failure with reliability. The cumulation of loss-of-life fractions over load cycles is evaluated through Miner's law. The thermal transients that affect insulation as a consequence of cyclic current variations are simulated via the CIGRE two-loop thermal network analog. The procedure is applied to XLPE-insulated high voltage AC cables, subjected to two typical stepwise-constant daily load cycles differing as to the load severity. The application shows that cable life is very sensitive to load cycles, thermal transients and electrothermal synergism, aspects that all deserve attention for estimating accurately the life expectancy of high voltage AC cables in service. The three life models employed, though weighing differently the electrothermal synergism as a consequence of their own functional expressions, give a concordant indication about the possible life extension of cables already in service.

The combination of electro-thermal stress, load cycling and thermal transients and its effects on the life of high voltage ac cables

The two objectives of this study are to compare the performances of different data fusion techniques for the enhancement of urban features and subsequently to improve urban land cover types classification using a refined Bayesian classification. For the data fusion, wavelet-based fusion, Brovey transform, Elhers fusion and principal component analysis are used and the results are compared. The refined Bayesian classification uses spatial thresholds defined from local knowledge and different features obtained through a feature derivation process. The result of the refined classification is compared with the results of a standard method and it demonstrates a higher accuracy. Overall, the research indicates that multi-source information can significantly improves the interpretation and classification of land cover types and the refined Bayesian classification is a powerful tool to increase the classification accuracy.

https://www.tandfonline.com/doi/pdf/10.1080/19479830903562041

Fusing high-resolution SAR and optical imagery for improved urban land cover study and classification

A novel model based on wavelet LS-SVM integrated improved PSO algorithm for forecasting of dissolved gas contents in power transformers

This work the combined electrical and thermal stresses of the 154 kV XLPE power cable used in the underground power transmission in Turkey. The dependence of aging on electrical and thermal stresses is given by a straight line in a certain range of electric field (E) values which indicates that the aging is a thermochemical process. Furthermore, cable tests are conducted in oil, and the results are valid for immersed insulation but might not be completely representative of XLPE used in service in a "dry" environment. Therefore, each cable insulator has to be tested and the parameters must be determined for each cable separately.

Aging of 154 kV underground power cable insulation under combined thermal and electrical stresses

This paper focuses on the realistic remaining service life prediction and monitoring system of power transformers, using an algorithm developed by the authors. The power transformer is one of the vital components of power transmission and distribution substations. It is important to know the loading capacity and remaining service life of a power transformer to ensure its economical and secure operation. This information is directly related to the condition of cellulosic insulation. According to classical method of prediction, the power transformer is considered to be operated under constant load for the remainder of its lifespan. However, in reality the load factor varies continuously, and this is a key factor affecting the service life of power transformers. In this study, a realistic method for monitoring and predicting the service lifetime of power transformers is presented by considering annual load factors. The proposed method is applicable to any power transformer and can be updated by considering relevant standards for different liquid and solid insulation materials. As a case study, life-loss calculation and the remaining service life predictions are carried out using a thermal model that has been created considering a 250 MVA mineral oil?immersed power transformer.

Lifetime estimation and monitoring of power transformer considering annual load factors

We present two optical system designs using aspherical lenses for beam circularization, collimation, and expansion of semiconductor lasers for possible application in lidar systems. Two different optical lens systems are investigated; namely, two aspherical lens and single aspherical lens systems. Software package programs of ZEMAX and MATLAB to simulate the optical designs are used. The beam reshaping results are presented for one specific laser beam output.

Optical beam-shaping design based on aspherical lenses for circularization, collimation, and expansion of elliptical laser beams

In this paper, we present experimental results of surface flashover characteristics of epoxy resin and epoxy resin cast with powdered Al2O3 (nano/micro-composite dielectric) samples in partial pressure of nitrogen. The flashover studies are conducted separately using DC and 20 kHz AC signals. The voltage, current, and light emission waveforms are obtained. The optical data is collected by a video camera and a photomultiplier tube (PMT). The nano/micro composite dielectric samples used in the experiments are produced in-house by mixing 1:10 weight ratio of AI2O3 powder with epoxy resin of known properties. Throughout this study, copper electrodes are used and are placed over the dielectric samples to initiate surface flashover events. The results of surface flashover voltages as a function of pressure are presented. Additionally, optical emission characteristics along with the voltage and current waveform data are also presented.

Nano/Micro Dielectric Surface Flashover in Partial Vacuum

A speckle reduction algorithm, the Edge Map-Directed Adaptive Mean (EMDAM) filter, is studied in this paper. It adapts the ordinary mean filter according to the scene heterogeneity. Edge-crossing maps determined by an edge detector are used to find the largest homogeneous subregion in the moving filter window. Then, the mean filter is adapted only to this homogeneous part of the moving filter window and applied if no edge crossing is found. We compared some filters in the literature to the EMDAM filter using two examples: a 1997 JERS-1 synthetic aperture radar (SAR) image of Tuzla, Istanbul and a computer-simulated SAR image. The filter performance was assessed both quantitatively and qualitatively. We found that the EMDAM filter preserves textures and details while reducing speckle to a desired level. A new testing quantity, the Quality Factor (Q), is also introduced.

Hulya Kirkici

Papers

Aging of 154 kV underground power cable insulation under combined thermal and electrical stresses

Lifetime estimation and monitoring of power transformer considering annual load factors

Optical beam-shaping design based on aspherical lenses for circularization, collimation, and expansion of elliptical laser beams

Nano/Micro Dielectric Surface Flashover in Partial Vacuum

Edge and fine detail preservation in SAR images through speckle reduction with an adaptive mean filter