Dielectric Response Measurements Research Articles

An Electrode Setup for Non-contact Dielectric Response Measurement

The dielectric response measurement is a widely used technique for characterizing dielectric materials. However, the contact problems between samples and electrodes existing in the use of conventional electrode setup limit the accuracy of the measurement. This paper studies an electrode arrangement that avoids direct contacts of the sample with the electrodes on both sides. The edge effect of this arrangement is calculated by the FEM model. The equations to derive the complex permittivity are presented. The measurement instrument is described and the influence of the small voltage between its measure and guard terminals is analyzed. The error sensitivities of the non-contact and contact methods are compared. The results show that this electrode arrangement can be used to perform non-contact measurements, the edge effect of it is not significant, and the guard voltage is not an obstacle to obtain results with high accuracy. Overall, the non-contact electrode arrangement combined with the IDAX 300 can potentially improve the accuracy of dielectric response measurements although the non-contact methods can increase the sensitivities to errors.

Time domain dielectric response of field grading sleeves subjected to high humidity and temperatures

It is experienced that MV XLPE cable links including heat shrink cable joints can have a high electrical conductivity compared to the XLPE cable insulation. The field grading sleeve included in the joint design is proposed to be responsible for the reduced insulation resistance and is therefore studied in this paper. New field grading sleeves are aged in the laboratory at temperatures up to 150 °C in both dry and humid conditions. The degree of ageing is determined by DSC measurements, the mechanical properties are determined by tensile strength testing, and the electric properties are characterized by time domain dielectric response measurements. The results show that after only a few days above the melting point of 104 °C the tensile strength of the material is strongly reduced. The electrical conductivity is strongly dependent on the degree of thermal ageing of the sleeve material but also the water content. The combination of significant thermal degradation and a high water content results in particularly high conductivity values. It is shown that the increase of the conductivity after ageing at 150 °C for 15 days is five orders of magnitude for wetted field grading sleeves measured at 30 °C.

Analysis of Dielectric Strength of Virgin Coconut Oil as an Alternative Transformer Liquid insulation

This paper presents the experimental results of a study, which has been carried out to analyze dielectric strength of virgin coconut oil in order to check the suitability as liquid insulation in power transformers.In this study dielectric strength response measurements after five times breakdown voltage test both mushroom electrode and spherical electrode slightly decreased, but the value of breakdown voltage is still high enough. While the measurement of the water content decrease is not stable, viscosity increase almost linear, the same thing also happened linear increase in measurement of flash point.

Accelerated ultraviolet weathering investigation on micro‐/nano‐SiO 2 filled silicone rubber composites

This study attempts to elucidate whether the addition of micro and/or nano-silica (SiO2) particles can enhance the resistance of pure polydimethylsiloxane against synergistic effects of UV, temperature and high-voltage stress. Four types of composites (U-SR, M-SR, MN-SR and N-SR) are fabricated by adding micro and/or nano-silica particles and then subjected to multi-stress degradation in a test chamber. Results show that there is an apparent surface discoloration in the form of yellowish pale tint and a significant resistance to hydrophobicity reduction is offered by N-SR and MN-SR followed by M-SR and U-SR. Scanning electron microscopy and surface roughness findings proclaimed that N-SR and MN-SR offer excellent resistance against filler exposure and an increase in surface roughness. There is a minor reduction in absorbance level of Si (CH3)2 and Si–O–Si functional groups of composites but interestingly, hydrophilic hydroxyl group absorbance level is found higher in the U-SR comparatively. Furthermore, dielectric response measurements indicate considerable sensitivity to weathering with N-SR and MN-SR giving the lowest dielectric loss. Results indicate that the addition of nano-silica to pure and micro-silica filled SR can enhance its UV weathering resistance considerably by building an effective UV shielding layer.

Experience with transformer diagnostics based on dielectric response measurements in time domain

Experience with transformer diagnostics based on dielectric response measurements in time domain

Dielectric response measurement of oil-paper insulation based on system identification and its time-frequency-domain conversion method

Dielectric response testing is a non-destructive insulation testing method that is widely used for assessing insulation properties. Time-and frequency-domain dielectric spectra are closely dependent on insulation conditions. In comparison with the time-domain spectrum, the frequency-domain dielectric response under sweep sinusoidal excitation has a stronger anti-interference capability and a more comprehensive insulation information. However, the wide application of the measurement using the frequency-domain dielectric response under sweep sinusoidal excitation is hindered by its long testing time. A possible solution is obtaining the frequency-domain spectrum by transforming the tested time-domain spectrum, such that the time- and frequency-domain spectra can be obtained in one measurement. Polarization depolarizing current measurement under DC excitation is a common time-domain testing method for time-frequency-domain conversion. However, such conversion is valid only at extremely low frequencies (less than 0.05 Hz), indicating a slight overlap with the frequency range of the measurement using the frequency-domain dielectric response under sweep sinusoidal excitation. Moreover, the effective conversion frequency range cannot be controlled manually, and the measurement using time-domain dielectric response under DC excitation is hindered by the anti-interference capability due to micro polarization-depolarization current. In addition, scholars have found nonlinearity in a partial insulation system. As the bridge of the traditional time-frequency-domain conversion, a linear extended Debye model cannot accurately reflect insulation materials that exhibit nonlinear characteristics, thereby further limiting the application scope of time-frequency-domain conversion. This study introduces the system identification theory to demonstrate the problems existing in the traditional time-frequency-domain conversion and proposes an improved time-domain testing method based on the previous work. The improved method extends the effective frequency range of time-frequency-domain conversion and emphasizes the comprehensive analysis of the insulation by combining the measured time-domain spectrum and the frequency-domain spectrum conversed from time-domain data.

Use of chirp excitations for frequency domain spectroscopy measurement of oil-paper insulation

Frequency domain spectroscopy is a non-invasive, offline test performed to estimate moisture content, conductivity and aging status of the composite oil-paper insulation of a transformer. In conventional frequency domain spectroscopy test, sinusoidal excitation voltage is applied on the insulation under test for two cycles starting from 0.1 mHz to 1 kHz. One important aspect of conventional dielectric response measurement is the choice of excitation voltage. The insulation in real life operation is often stressed by voltage waveforms, which are non-stationary in nature. Therefore, modification of excitation voltage waveform is required for reliable assessment of insulation condition. Another important factor is the measurement time. Conventional frequency sweep from 0.1 mHz to 1 kHz, makes frequency domain spectroscopy measurement inadvertently lengthy, hence development of new measuring technique is necessary to reduce measurement duration for diagnosis of insulation condition. Considering these two facts, this contribution reports the feasibility of conducting dielectric response measurement in frequency domain using chirp excitation voltage waveforms. Instead of applying sinusoidal excitation at discrete measurement frequencies, chirp signals (band limited from 0.1 mHz to 1 Hz) were used to conduct frequency domain spectroscopy measurement on three laboratory prepared test samples and on two transformers. The dielectric dissipation factors evaluated using chirp waveforms showed a very good agreement when compared to that obtained by using sinusoidal excitation. Besides, investigations also revealed that the proposed method offered least amount of measurement time in comparison with conventional measurement as well as with some existing state of the art methods.

A Parameterization Approach for the Dielectric Response Model of Oil Paper Insulation Using FDS Measurements

To facilitate better interpretation of dielectric response measurements—thereby directing numerical evidence for condition assessments of oil-paper-insulated equipment in high-voltage alternating current (HVAC) transmission systems—a novel approach is presented to estimate the parameters in the extended Debye model (EDM) using wideband frequency domain spectroscopy (FDS). A syncretic algorithm that integrates a genetic algorithm (GA) and the Levenberg-Marquardt (L-M) algorithm is introduced in the present study to parameterize EDM using the FDS measurements of a real-life 126 kV oil-impregnated paper (OIP) bushing under different controlled temperatures. As for the uncertainty of the EDM structure due to variable branch quantity, Akaike’s information criterion (AIC) is employed to determine the model orders. For verification, comparative analysis of FDS reconstruction and results of FDS transformation to polarization–depolarization current (PDC)/return voltage measurement (RVM) are presented. The comparison demonstrates good agreement between the measured and reconstructed spectroscopies of complex capacitance and tan δ over the full tested frequency band (10−4 Hz–103 Hz) with goodness of fit over 0.99. Deviations between the tested and modelled PDC/RVM from FDS are then discussed. Compared with the previous studies to parameterize the model using time domain dielectric responses, the proposed method solves the problematic matching between EDM and FDS especially in a wide frequency band, and therefore assures a basis for quantitative insulation condition assessment of OIP-insulated apparatus in energy systems.

Influence of Conductive and Semi-Conductive Nanoparticles on the Dielectric Response of Natural Ester-Based Nanofluid Insulation

Nowadays, studies of alternative liquid insulation in high voltage apparatus have become increasingly important due to higher concerns regarding safety, sustainable resources and environmentally friendly issues. To fulfil this demand, natural ester has been extensively studied and it can become a potential product to replace mineral oil in power transformers. In addition, the incorporation of nanoparticles has been remarkable in producing improved characteristics of insulating oil. Although much extensive research has been carried out, there is no general agreement on the influence on the dielectric response of base oil due to the addition of different amounts and conductivity types of nanoparticle concentrations. Therefore, in this work, a natural ester-based nanofluid was prepared by a two-step method using iron oxide (Fe2O3) and titanium dioxide (TiO2) as the conductive and semi-conductive nanoparticles, respectively. The concentration amount of each nanoparticle types was varied at 0.01, 0.1 and 1.0 g/L. The nanofluid samples were characterised by visual inspection, morphology and the dynamic light scattering (DLS) method before the dielectric response measurement was carried out for frequency-dependent spectroscopy (FDS), current-voltage (I-V), and dielectric breakdown (BD) strength. The results show that the dielectric spectra and I-V curves of nanofluid-based iron oxide increases with the increase of iron oxide nanoparticle loading, while for titanium dioxide, it exhibits a decreasing response. The dielectric BD strength is enhanced for both types of nanoparticles at 0.01 g/L concentration. However, the increasing amount of nanoparticles at 0.1 and 1.0 g/L led to a contrary dielectric BD response. Thus, the results indicate that the augmentation of conductive nanoparticles in the suspension can lead to overlapping mechanisms. Consequently, this reduces the BD strength compared to pristine materials during electron injection in high electric fields.

Grey Relational Analysis for Insulation Condition Assessment of Power Transformers Based Upon Conventional Dielectric Response Measurement

Conventional dielectric response measurement techniques, for instance, recovery voltage measurement (RVM), frequency domain spectroscopy (FDS) and polarization–depolarization current (PDC) are effective nondestructive insulation monitoring techniques for oil-impregnated power transformers. Previous studies have focused mainly on some single type of dielectric measurement method. However, the condition of oil paper insulation in transformer is affected by many factors, so it is difficult to predict the insulation status by means of a single method. In this paper, the insulation condition assessment is performed by grey relational analysis (GRA) technique after carefully investigating different dielectric response measurement data. The insulation condition sensitive parameters of samples with unknown insulation status are extracted from different dielectric response measurement data and then these are used to contrast with the standard insulation state vector models established in controlled laboratory conditions by using GRA technique for predicting insulation condition. The performance of the proposed approach is tested using both the laboratory samples and a power transformer to demonstrate that it can provide reliable and effective insulation diagnosis.

Prediction of dielectric constant and loss for some polypropylene - additive compounds

In order to influence properties of polymers, several different type of additives can be used. Often, time consuming experimental work is needed to assess performance and suitable concentrations for the chosen polymer-additive system. Therefore, it is preferable to be able to estimate the properties by means of theoretical work. In the present work, dielectric constant and loss for polypropylene-meso-erythritol compounds were studied. The experimental results were compared to the theoretically assessed properties.The compounds were prepared in three different concentrations. The obtained samples were pressed to plates which were characterized by dielectric responsemeasurements. The dielectric constants were also assessed using Maxwell-Garnett estimation and molecular dynamics simulations. For the later, bothatomistic model and a more coarse multiscale model were used. The Maxwell-Garnett equation gave a good estimation of the dielectric constant level whereas with the molecular dynamics simulations the obtained dielectric constant were in exact accordance to the measured values. With the molecular dynamics simulation, also the loss could be estimated. As expected, the atomistic model gave better correlation to the measured loss levels than the multi-scale model.

Temperature and Field Dependence of Field Grading Tubes for Medium Voltage XLPE Cable Joints

In Norway it has been observed that many medium voltage (12 and 24 kV) cable sections with heat shrink joints have suffered from overheating often causing failure during service. The overheating is due to a bad metallic connection between the aluminium conductors in the joint. This is especially observed for XLPE cables installed in the 80’s, which constitutes a significant part of the installed cables in the network.This paper is part of a work to elucidate the mechanisms causing a low insulation resistance of MV cable joints. As a bad metallic connection can result in local high temperatures during normal operation, the properties of a heat shrink stress control tubes commonly used in such joints has been characterized at temperatures around the melting peak of the material that is just above the maximum service temperature of 90 °C for XLPE cables. The main purpose of this paper is to examine the effect of these temperatures on the electrical properties of the field grading material. Here, the electrical properties have been characterized by time domain dielectric response measurements at different temperatures and axial electrical fields. The results show that the conductivity of the stress control tube is temperature dependent. Also, a polarity change in the depolarization current at higher electrical field strengths was observed.

Fundamental study on dielectric relaxation characteristics of cementitious materials

Dielectric frequency response measurement is utilized for a microstructure analyses of materials in various fields. The movements of the positive and the negative charges cause the response, which is so- called dielectric relaxation, and its measurement enables to clarify the interfacial composition of material. For example, the dielectric relaxation analysis of the human body cells is popular in the medical area. Gener- ally speaking, the indices of dielectric relaxation are capacitance, conductance, and susceptance. If the die- lectric relaxation measurement is applied to the concrete structure, there is a possibility to perform analyses of the concrete microstructures, because the interfacial compositions of aggregates, cement matrix, or pore solution dominate the dielectric relaxation responses of concrete theoretically. The correlations between concrete properties and dielectric relaxation responses are the significant information of concrete internal microstructures. This paper presents the results of the experimental examinations which were conducted to grasp the influence of the pore solution composition and the electrode interval on the dielectric relaxation properties. As a result, it became apparent that the behaviors of conductive electric charge, electric response charge, and displacement charge explain how sodium chloride and the distance between electrodes affected not only the magnitude of dielectric relaxation properties but also frequency spectrum.

Aging assesment of transformer pressboard insulation by micro-strip ring resonator at GHz frequencies

Condition assessment of insulation is widely used to identify the state of aging of power transformers. Traditionally, condition assessment is done using dielectric response measurements conducted at low frequencies. However, due to long testing time required for these methods, attention has now been paid towards measurements at microwave (above 1 GHz) frequencies. In this paper, we establish a correlation between dielectric properties and the state of aging of transformer pressboard samples. This is useful as it paves way to develop techniques to assess the state of aging by measuring dielectric properties at high frequencies. Dielectric measurements are made by using a microstrip ring resonator. The state of aging is determined by measuring the tensile index. Tests were carried out on dry and wet oil-impregnated pressboard samples thermally aged at laboratory conditions for durations up to 45 days. The results were compared with field aged samples collected from a 33 kV sealed type distribution transformer of 18 years in service. It was found that the permittivity values had a good correlation with respective tensile index values whereas loss tangent values showed limited correlation for both dry and wet samples. The results of field aged samples matched well with the established correlation curves. It can be concluded that the permittivity of pressboard samples estimated from the ring resonator can be effectively used to evaluate the state of aging of transformer insulation.

Identification of microscopic domain wall motion from temperature dependence of nonlinear dielectric response

It is known that the permittivity of ferroelectric polydomain films and single crystals in weak electric fields is strongly enhanced by the reversible movement of pinned domain walls. Two mechanisms of the movement exist: first, the bending of free segments of the domain wall and second the planar movement of the domain wall as a whole. In this work, we theoretically demonstrate that it is possible to determine the dominant mechanism of the reversible domain wall movement by means of a temperature measurement of a nonlinear macroscopic dielectric response. In addition, we demonstrate that using this approach, it is possible to obtain quantitative information on the microscopic distribution of the pinning centers. Thus, we suggest that this concept may serve as a simple and useful characterisation tool in the process of development of high-permittivity materials.

Modeling of relaxation phenomena in transformer oil-paper insulation for understanding dielectric response measurements

Dielectric response measurements in either time or frequency domain are being assessed in modern times for condition monitoring of transformer oil-paper insulation. Proper interpretation of the dielectric test results requires understanding of the physical processes taking place in the insulation structure during such tests. It is thus necessary to establish proper mathematical and analytical formulations of the insulation behavior. Dielectric response functions of various forms based on the classical “single relaxation time” Debye model have been proposed by various researchers to model the experimental data. In complex dielectrics, it seems however, that the overall response be represented by a distribution of relaxation times (or frequencies), rather than a single “average” response characteristic. The present contribution provides in brief, the mathematical background for estimating the dielectric response functions used so far for modeling dielectric response measurement data. Function representing distribution of relaxation frequencies has also been estimated. Experimental results on laboratory test samples and field transformers have been presented to study and understand the effects of insulation status and operating conditions on test data.

Dielectric response characterization of in-service aged sheds of (U) HVDC silicone rubber composite insulators

Investigations of bulk dielectric properties of specimens extracted from in-service aged and aged UHVDC composite insulator sheds are reported in this paper. The program of the study included measurements of DC conductivity and complex permittivity at frequency range 10−4–103 Hz at various temperatures in dry and wet (water immersion) conditions. The obtained results are analyzed by considering a combination of relations describing various dielectric relaxation modes in dielectrics, included three Cole-Cole types of responses and a hopping process. The hopping has the strongest impact on the responses in the whole frequency range. Two of the Cole-Cole responses appearing in the low frequency range are attributed to Maxwell-Wagner-Sillars relaxations at macroscopic and mesoscopic levels; first one representing interfacial polarization at the border between specimen and air gap in the electrode arrangement, while the second one is attributed to the charge relaxation at filler boundaries and internal cavities in the polymeric materials. The third Cole-Cole process, active at higher frequencies, is considered as a sign of polymer degradation, as it shifts with the ageing to higher frequencies. Based on the results of this study, it is claimed that dielectric response measurements on specimens of in-service aged composite insulator sheds can provide useful information on the degree of material degradation. In addition, a water immersion test in combination with such measurements may be a good way to enhancing the observed changes.

Influence of moisture on dielectric condition of high power Insulated Gate Bipolar Transistor modules

Moisture in a high power Insulated Gate Bipolar Transistor (IGBT) module raises serious concern on component and system level reliability. Employing diagnostic test methods on high power IGBT modules might provide an opportunity in detection and estimation of moisture, however has not received wide attention. This paper investigates on understanding the influence of moisture in high power IGBT modules through time and frequency dependent dielectric response measurements and partial discharge analysis. The moisture in the main insulation of an IGBT module under DC excitation produces a sharp rise in its response current introducing deviation in their respective time dependent dielectric properties. Alternatively, the frequency dependent dielectric response measurements under wide frequencies provide more information than their DC counterpart. This method enables discrimination of various defects such as moisture, physical defects, etc., thereby revealing a clear picture about the actual status. In particular, the lower frequencies reveal longer range ion movements, hence could be used for more accurate estimation of moisture. The higher frequencies though not accurate in moisture estimation reveal deviation in the material properties. These deviations alter the apparent dielectric characteristics such as loss factor, permittivity, etc., of the IGBT samples which becomes clear through simple comparative analysis. In conclusion, the findings in this paper might help in understanding the influence of moisture over the dielectric integrity of a high power IGBT module. Additionally, the permissible or acceptable level of moisture which might act as a threshold to initiate necessary precautionary measures could be subsequently developed.

Extrinsic response enhancement at the polymorphic phase boundary in piezoelectric materials

Polymorphic phase boundaries (PPBs) in piezoelectric materials have attracted significant interest in recent years, in particular, because of the unique properties that can be found in their vicinity. However, to fully harness their potential as micro-nanoscale functional entities, it is essential to achieve reliable and precise control of their piezoelectric response, which is due to two contributions known as intrinsic and extrinsic. In this work, we have used a (K,Na)NbO3-based lead-free piezoceramic as a model system to investigate the evolution of the extrinsic contribution around a PPB. X-ray diffraction measurements are performed over a wide range of temperatures in order to determine the structures and transitions. The relevance of the extrinsic contribution at the PPB region is evaluated by means of nonlinear dielectric response measurements. Though it is widely appreciated that certain intrinsic properties of ferroelectric materials increase as PPBs are approached, our results demonstrate that the extrinsic contribution also maximizes. An enhancement of the extrinsic contribution is therefore also responsible for improving the functional properties at the PPB region. Rayleigh's law is used to quantitatively analyze the nonlinear response. As a result, an evolution of the domain wall motion dynamics through the PPB region is detected. This work demonstrates that the extrinsic contribution at a PPB may have a dynamic role in lead-free piezoelectric materials, thereby exerting a far greater influence on their functional properties than that considered to date.

Condition diagnosis of transformer oil-paper insulation using dielectric response fingerprint characteristics

The life expectancy of a transformer is closely related to the condition of its oil-paper insulation. In the present study, oil impregnated pressboard samples with different aging states and water contents were prepared under controlled laboratory conditions to analyze the oil-paper insulation condition of the power transformer by the dielectric response method. Dielectric response measurements, i.e., polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS), were conducted on the pressboard samples at different temperatures. Results show that the dielectric responses of oil impregnated pressboards are highly sensitive to aging, water, and temperature, but the influence of the temperature on FDS can be effectively eliminated by using the master curve technique. Meanwhile, dielectric characteristic parameters, which are sensitive to aging state and water content, were extracted to form a fingerprint database to indicate the various states of the oil impregnated pressboard. A grey correlation diagnosis model was first introduced to assess the aging state and water content of oil-paper insulation. Finally, FDS measurements were performed on a field transformer to verify the effectiveness of the new evaluation technique proposed in this paper for the condition assessment of the transformer oil-paper insulation.