Frequency Domain Dielectric Spectrum Research Articles

Parameter Extraction for Thermal Aging of Oil-Paper Insulation in Onboard Transformer Based on the Transfer Function and Dielectric Response Test

In this study, the frequency domain dielectric spectrum of oil-paper insulation at various aging states was analyzed from the perspective of system stability. The results of the study revealed that 1) as the aging increased, the closed-loop poles of both the insulation oil and the insulation paper move regularly in the same direction. What’s more, the step response speed of the system was increasing, which corresponded with the step response rise time and regulation time of system gradual shorten; 2) The presence of rise time is a characteristic of insulation paper, which is completely different from that of insulation oil in time-domain step response; and 3) the step response rise time of insulation paper, step response regulation time and closed-loop mean poles of insulation paper, step response regulation time and closed-loop mean poles of insulation oil, the above parameters were revealed to be the characteristic parameters of aging state evaluation.

Evaluation Means of Oil-paper Insulation’s Water Based on DC Conductivity

.The measured spectral line based on frequency domain dielectric spectrum contains multiple superpositions of conductance and polarization processes. In view of the lack of methods to separate the polarization and the conductance processes and the lack of unique methods to characterize the model conductance parameters, the micro conductance processes are studied in depth according to the real and imaginary Kramers-Kronig relation. Firstly, different water content insulation samples were prepared during the laboratory conditions, and dielectric lines in frequency domain were obtained by measurement. Secondly, according to the k-K relation, the relaxation loss spectrum line is obtained by numerical integration of the measured real part of the complex polarization spectrum line, and then the conductivity loss spectrum line is obtained. Finally, the least square method is used to fit the conductivity spectrum line and calculate the conductivity value. The experimental results show that the conductance of the virtual part of the repolarization measurement can be separated from the relaxation loss line by k-K transformation, and the dc conductance can be calculated. At the same time,with the oil-paper insulating sample’s water content increasing, its electrical conductivity is gradually enhanced.

Analysis of low-frequency dielectric loss of XLPE cable insulation based on extended Debye model

Dielectric response technology is an effective method on state detection of dielectric materials. However, when the technology is employed to evaluate the aging extent of the cable, the interpretation of the microscopic mechanism, which leads to changes in the dielectric loss factor spectrum, is lacking in the research. It also does not refine the frequency range with strong polarization loss caused by defects, such as water trees and impurities. In this paper, the loss characteristics of XLPE cable insulation at low frequencies were studied. First, the frequency domain parameters were obtained by the time–frequency domain dielectric conversion principle through the polarization current measurement. Second, the polarization loss and conductivity loss based on the extended Debye model were effectively distinguished. Finally, the frequency domain dielectric spectrum of polarization loss caused by water treeing defects was extracted and discussed. The research results show that in the low frequency range, the conductivity loss and polarization loss of the XLPE cable increase as the aging degree of the cable deepens. Among them, the contribution of polarization loss to the overall loss is gradually increasing. At the same time, the polarization loss (tan δp3) caused by defects, such as water trees and impurities, inside the cable insulation has an obvious effect in the ultra-low frequency band. As the degree of aging increases, the extreme value of tan δp3 increases, and it moves to higher frequencies.

Prediction method of power cable insulation remaining life considering the uncertainty of influencing factors

Due to the complex working environment of marine cables, the prediction of residual insulation life of power cables has become a concern. This study mainly discusses the power cable insulation residual life prediction method considering the uncertainty of influencing factors. The aging time points were 0 h, 72 h, 144 h, 264 h, 432 h, 600 h, 844 h and 1152 h. Finally, the elongation at break, the dielectric loss angle of frequency domain dielectric spectrum and space charge distribution are explored. At the beginning of aging, the decline trend of elongation at break curves was gentle, from 510% to 499.6%, 480.6%, and 470.2%. The results show that the difference value of the life prediction method designed in this study is small, which can be effectively used to predict the residual life of power cable insulation.

Onboard EPR Cable Aging Evaluation by Rectangular-SPP-CNN Based on LMMGS Processing Method

Onboard cable is important electrical equipment of an electric locomotive. In order to effectively evaluate the aging state of the cable, this article proposes a method to evaluate the aging state of the cable based on a new method of dielectric spectrum data processing and a new structure model of recognition algorithm. First, the log-min-max gray-scale (LMMGS) method is proposed, which transforms the dielectric parameters based on the frequency-domain dielectric spectrum (FDS) test into the gray matrix. The correlation among different dielectric parameters is retained, and the interference of their order of magnitude difference on feature extraction is reduced. Then, the new structure of spatial pyramid pooling (SPP) is improved by rectangular mesh dividing named Rectangular-SPP, which increases the number of divisible layers of the spatial pyramid, and it is used to optimize convolutional neural network (CNN), so as to remove the limitation of recognition algorithm on the size of input data and expand the scale of feature extraction. Finally, a set of methods for evaluating the aging state of onboard cable is proposed. The experimental results show that this method can effectively distinguish the insulation aging state of onboard cable. Meanwhile, the proposed Rectangular-SPP-CNN algorithm has a better performance than the conventional machine learning algorithms.

The mathematical model of dissipation factor with temperature–frequency effect for oil-impregnated paper bushings

The accuracy of insulation defect diagnosis based on the dielectric spectrum response is severely influenced by temperature for oil-impregnated paper bushing. The dielectric loss (tan δ) mathematical model with a temperature–frequency factor to restrict the application of the frequency domain dielectric spectrum (FDS) for insulation diagnosis is not clear. In this paper, five temperatures are selected to study the frequency temperature characteristic of the FDS and polarization current for oil-impregnated paper bushing. The experimental results show that the temperature has different effects on the lower frequency (0.001 Hz–1 Hz) and higher frequency (1000 Hz–10 kHz) of FDS. With the increasing temperature, the lower frequency tan δ of 1 mHz–1 Hz shows a better pattern of exponential growth, while the higher frequency tan δ at 1 kHz–10 kHz shows the declining law with the rising temperature. With the increasing temperature, the polarization current shows a better pattern of exponential growth, while resistance obviously shows the declining law of the exponential pattern. The FDS curve of oil-impregnated paper bushings can be amended by the mathematical model of tan δ for the dielectric response on site for accuracy diagnosis.

Experimental and simulation analysis of the influence of moisture on dielectric and breakdown properties of insulation pressboard used in power transformer

The influence of water molecules on cellulose has attracted much attention. The influence of water molecules on the dielectric property and breakdown property of cellulose requires in-depth research. In this paper, firstly, the effect of water on the dielectric spectrum in the frequency domain dielectric spectrum and the breakdown voltage of cellulose were studied. Then the parameters such as free volume, hydrogen bond, energy level and state density of cellulose under different moisture content were analysed by molecular simulation. The changes of dielectric properties and breakdown properties of cellulose under different water content were studied and summarized. The variation mechanism of dielectric property and breakdown property of different moisture content was explained by molecular simulation.

A Method for Diagnosing the State of Insulation Paper in Traction Transformer Based on FDS Test and CS-DQ Algorithm

Traction transformer is vital equipment in high-speed railway. Insulation status decides its safety and reliability, and the frequency-domain dielectric spectrum (FDS) test is one of the most effective methods reflecting the changing of insulation status. For field applications, the following problems should be addressed: 1) how to obtain the result of paper insulation from the combined result of the oil–paper insulation system and 2) how to discriminate the defects of insulation paper between aging and damp. In this article, the first problem was transferred to a nonlinear equation set, and a cuckoo search algorithm optimized by the differential evolution algorithm and the quadratic interpolation (QI) method (CS-DQ algorithm) was proposed to solve it. Then, the insulation states were discriminated by establishing a multiclass least-squares support vector machine (LS-SVM) model, in which the CS-DQ algorithm was also used. Finally, a diagnostic approach for the insulation paper in the traction transformer was proposed. The results in the laboratory show that the pure result of insulation paper can be obtained, and the insulation states can be discriminated effectively by using the proposed approach. Meanwhile, the proposed CS-DQ algorithm has a better performance than the conventional CS algorithm. The results of field testing also verify the proposed approach.

Research and Field Application of Oil-paper Insulation Material Aging State Based on FDS

The effects of three factors, such as aging degree, water content and temperature, on the dielectric response characteristics of oil-paper insulating materials were studied by setting up a frequency-domain dielectric response experimental platform. In this paper, the frequency domain dielectric spectrum (FDS) test is carried out for the on-site transformer paper insulated casing, and it is found that the method is more abundant than the traditional power frequency dielectric loss method, which is less affected by the work frequency electromagnetic field, and can realize the nondestructive measurement of the aging state of the oil-grease insulation.

Investigating a New Approach for Moisture Assessment of Transformer Insulation System

With the wide application of vegetable insulating oil in power transformers, it is essential to investigate the influence of moisture on the frequency domain dielectric spectrum (FDS) analysis in order to propose an accurate moisture assessment method for vegetable oil-paper insulation system. In this regard, various vegetable oil-paper samples are prepared in a laboratory environment. FDS curves for all samples are obtained at different moisture contents and operating temperatures. A new method to estimate the moisture content in insulation paper is proposed based on the low-frequency characteristic quantity. Also, the effect of operating temperature and water migration between the oil and paper insulation on the FDS analysis is investigated. Based on this analysis, a compensation factor is introduced to eliminate the influence of temperature on paper moisture assessment. Furthermore, an improved normalization method for the temperature effect on the FDS of vegetable oil-paper insulation with different moisture contents is proposed. The proposed moisture assessment and temperature normalization methods are validated thorough their application on an operating power transformer.

Dielectric properties of epoxy resin impregnated paper insulation in different stages of partial discharge development

AbstractTo investigate the variation of dielectric properties of epoxy resin impregnated paper insulation in different stages of partial discharge (PD) development, a PD experimental platform was established and epoxy resin impregnated paper insulation samples were prepared. According to the discharge amount, the number of discharges and discharge phenomena during the PD process, the PD process is divided into an initial stage, development stage, burst stage, and near‐breakdown stage. The variations of dielectric constant and dielectric loss factor of the sample during different PD development stages were measured by frequency domain dielectric spectrum tester. Scanning electron microscopy was used to observe the microscopic morphological changes of the samples at different stages of PD development. The surface characteristic functional groups at different stages of PD development of the sample were determined by Fourier transform infrared spectroscopy. The results show that: as the PD progresses, the insulation performance deteriorates; under continuous bombardment by high‐speed electrons, the network structure of the fiber surface and the epoxy molecular chain are destroyed, causing the weakening of the binding force between the molecular chains, and the short‐chain structure formed by cracking has a stronger steering ability. Under the continuous action of PD, the inside of the long‐chain molecule breaks and oxidizes, and a large number of small molecules, such as free radicals and monoglucose, are formed. The decrease in the molecular weight of the polymer leads to the weakening of the interaction between the molecules, resulting in an increase in polarization, a large dielectric constant, and an increase in the dielectric loss factor.

Insulation Defect Diagnostic Method for OIP Bushing Based on Multiclass LS-SVM and Cuckoo Search

Frequency-domain dielectric spectrum (FDS) is an effective testing method to reflect the changes of internal insulation status of oil-impregnated paper (OIP) bushing. In field application, the results of FDS test can be both affected by aging defects and damp defects, then the internal insulation defects of OIP bushing cannot be diagnosed in detail, which is an issue of discrimination of multiclass classification. To solve this problem, a method to diagnose the internal insulation defects of OIP bushing is proposed based on multiclass least square support vector machines (LS-SVMs) optimized by cuckoo search (CS) algorithm. First, the multiclass LS-SVM parameters are optimized by the CS algorithm. Then, the training data set is used to train the multiclass LS-SVM model, and the test data set is used for model testing. The experimental results show that the proposed method can effectively diagnose the internal insulation defects in detail, i.e., aging defects and damp defects. In addition, in the proposed method, the CS algorithm is better than the genetic algorithm and particle swarm optimization algorithm, and the convergence rate of the CS algorithm is faster than the other two algorithms.

Influencing mechanism of phenyl thioether group in the main molecular chain on high frequency electrical properties of the modified polyimide films

Polyimides (PIs) and their nanocomposites may render quite high dielectric loss under high frequency (HF) electrical stresses, which will significantly affect the insulation properties and operating lifetime of the equipment when the composite materials are used in the high-voltage and large-capacity power electronics transformers (HFPTs). In this paper, the phenyl thioether groups of various molar ratios were introduced into the diamine residue as to prepare modified PI films via alternating copolymerization method. Frequency-domain dielectric spectrum and high frequency surface discharge were adopted to study the HF electrical properties. The results indicated that with the increase of the phenyl thioether content, the dielectric loss factor firstly decreased and thereafter rose up again, while the lowest arrived at 9.29 × 10–4 for a PI film incorporating 40% phenyl thioether content, which case also presented the longest endurance lifetime against surface discharge tests at 15 kHz. Based the UV–vis tests on the charge transfer complexation, further analysis verified that for the 40% molar content of phenyl thioether in the PI molecule, it was the weak charge transfer characteristics that resulted in the smallest stacking structure of the ordered molecular chain in the film, which led to the weakest lossy polarization under the HF alternating electric field and manifested the minimal dielectric loss factor accordingly. The proposed research presents a feasible technological approach to regulate HF electrical properties of the modified PI films.

Parameter Identification of Mixed Polarization Equivalent Model of Oil-paper Insulation Based on Frequency Domain Spectroscopy

Based on the extended Debye model, the interface polarization branch is introduced to make the oil-paper insulation mixed polarization equivalent model more suitable for the actual polarization process. The mixed polarization equivalent model parameter identification method using frequency domain dielectric spectrum is studied. Firstly, the relationship between the frequency domain spectrum and the circuit parameters is derived based on the mixed polarization equivalent circuit. Secondly, the objective function of the equations for solving the multivariate parameters is established, and the artificial intelligence algorithm is used to solve the parameters. Finally, the method is used to verify the identification of the method. Feasibility and accuracy of mixed polarization equivalent model parameters. The calculation results show that the mixed equivalent model considering interface polarization can more accurately reflect the relaxation process of oil-paper insulation. Compared with the time domain dielectric spectrum parameter identification method, the identification method based on frequency domain dielectric spectrum has the advantages of simple solving process and accurate and reliable parameter calculation results.

Moisture estimation for oil‐immersed bushing based on FDS method: field application

Temperature has obvious effects on the tested results of frequency domain dielectric spectrum (FDS) method. To estimate the moisture content of field oil-immersed bushing, the effects of temperature must be considered and eliminated. In this study, the frequency-domain dielectric spectrum of 72.5 kV bushing samples with different moisture contents was tested at different temperatures. The law that temperature affects the results of FDS tests was analysed and concluded. Afterwards, a method to transfer the tested results to the reference temperature (15°C) was proposed. Finally, an estimation method of moisture content for field oil-immersed bushing was proposed and testified. The following conclusions can be concluded: (i) the effects of temperature on the FDS results may be divided into two aspects, i.e. one is along the horizontal axis and the other is along the vertical axis and (ii) based on the proposed method, the temperature effects on the results of FDS tests can be eliminated, and the moisture content of field oil-immersed bushing can be estimated effectively and feasibly.

Moisture estimation for oil‐immersed bushing based on FDS method: at a reference temperature

Oil-immersed bushing is one of the most important accessory equipment of a transformer, whose insulation status is crucial. Moisture is an important factor that affects the insulation status of oil-immersed bushing. It has a very important significance to estimate the moisture content of oil-immersed bushing accurately. Here, the 72.5 kV bushing samples with different moisture contents were made. Afterwards, the frequency domain dielectric spectrum (FDS) of samples were tested at 15°C. The Havriliak-Negami equation was modified based on the analysations of complex planes, then it was used to analyse the experimental results. Moreover, the characteristic parameters and method to estimate moisture content were obtained and testified. Finally, the field application of the proposed estimation method was also discussed. The results of the studies shown here indicate that the moisture content of oil-immersed bushing can be estimated based on the proposed method effectively; however, in order to estimate the moisture content of field oil-immersed bushing, the elimination of temperature effects should be considered.

FDS analysis for multilayer insulation paper with different aging status in traction transformer of high-speed railway

The traction transformer of a high-speed railway is long-term subjected to the effects of impulse traction loads, such that the aging status of a multilayer insulation paper between high and low voltage windings is non-uniformly distributed due to the non­uniform distribution of temperature. To enhance the use of the frequency domain dielectric spectroscopy (FDS) method in engineering, this paper studies the effects of non-uniform thermal aging on the frequency domain dielectric spectrum of multilayer insulation paper. Based on the experimental designs and preparation of experimental materials, the frequency domain dielectric spectrum of a multilayer insulation paper with uniform and non-uniform thermal aging distribution was tested. Afterwards, the effects of non-uniform thermal aging on the frequency domain dielectric spectrum of multilayer insulation paper were discussed and analyzed based on the tested results. The results show that: 1) The effects of the non-uniform distribution of DP (cellulose structure) can be ignored. The non-uniform distribution of the aging byproducts (except moisture) has limited effects on the tested results, which is far less than the effects of moisture. The non-uniform distribution of moisture is the dominant factor and affects the results. 2) In terms of insulation paper with a defined aging status, its order in the multilayer insulation paper does not affect the tested results. 3) The tested results of the insulation samples with a non-uniform aging distribution can no longer reflect the moisture content of the wettest section, and the evaluated moisture content is slightly higher than the average moisture content of the multilayer insulation paper.

Frequency domain dielectric response of oil gap in time-varying temperature conditions

The frequency domain dielectric response method has been one of the most effective approaches for the condition assessment of transformers. Prior to conducting the test, the transformer should be taken offline and given adequate time to reach internal temperature equilibrium in order to diminish any temperature-related effects. This can greatly increase the maintenance time; moreover, there is not enough time for some special transformers to reach this internal temperature equilibrium; for example the traction transformers used in the railway industry. The oil-paper insulation in transformers was composed of oil gap, oil-impregnated insulation paper (board). The study related to the dielectric response of the oil gap under time-varying temperature conditions is the basis for studies regarding oil-paper insulation. In this work, the frequency domain dielectric spectrums of oil gap were tested in three cases with different temperature change rates, and the currents in the three cases were tested when the oil gap was applied with low frequency excitations. A current model of the oil gap has been presented here, which is applicable to the time-varying temperature conditions. The current model was verified by comparing the simulated and measured currents. During the verification process, the parameters used in the current model were obtained on the basis of experimental tests. When combined with the established model, a method was developed to obtain the modified results in time-varying temperature conditions; this method was verified by comparing the results calculated from the proposed model with experimentally measured values. In the modification process, the parameters used in the current model were obtained by directly analyzing the tested frequency domain dielectric spectrum of oil gap, rather than through experimental tests, which is more suitable for the maintenance of field transformers. Lastly, the frequency domain dielectric spectrum of oil gap tested in time-varying conditions can be modified based on the proposed model and method.

Time and frequency domain dielectric spectrum of SrxBa1−xTiO3 ceramics

According to the SrxBa1−xTiO3 formula, the ceramics samples were prepared by a common mixed-powder technique. While the strontium level was 0.30, the ceramics show the remarkable relaxation phase transition. The function of the differential time domain dielectric spectrum can be expressed as: The time domain parameters (τn and αn) have different values in ferroelectric phase transition region, ferroelectric phase, and paraelectric phase respectively. The fast component is the contribution of the lattice polarization. The slow components are the contribution of space charges that are in various bound state. They are in correspondence with the results of frequency domain measurement that the reciprocal of e’ dependence of lgfare in different slope line segments.

THREE KINDS OF ELECTRIC RESPONSE MECHANISM IN CERAMICS

Using the method of time domain and frequency domain dielectric spectrum, the response signals of applied field for grains, intergranuhr boundary layer, and barrier layer in a ceramics may be dissociated. So as to give the contribution of each mechanism in total resistance and total reactance of a sample. The effect of conductivity relaxation due to hopping conductance of polaron has been observed. This effect makes the impedance of the sample appear with inductance character. The properties of intergranular boundary in three kinds of typical ceramics of zinc oxide, strontium titanate, and barium titanate are compared.