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Epoxy resin (EP) is a typical and widely used insulating material for medium-frequency transformers (MFTs). The dielectric loss of EP is one of the key variables in the design and operation of MFT. In this article, six types of epoxy/anhydride systems are prepared, and the dielectric loss of MFT is calculated by finite element method (FEM) analysis based on broadband dielectric spectroscopy measurements. The material dependence of the dielectric loss of MFT is investigated using dielectric spectroscopy and molecular dynamics simulations over a wide temperature and frequency range. The results show that the variation of dielectric loss with output power is attributed to the temperature characteristics of the imaginary part ($\varepsilon ^{\prime \prime }$) of complex permittivity. Compared to the dielectric loss at no-load state, the one at rated power is reduced by 5–10 W. The anhydride hardener containing benzene ring or a methyl substituent has an inhibitory effect on dielectric loss, and the EP cured by methyl nadic anhydride (MeNA) has the lowest dielectric loss (16.2 W) at rated power. The dielectric loss and overload characteristics of MFT are closely related to the $\beta $ -relaxation and $\alpha $ -relaxation of EP, respectively. The acid anhydride curing agent containing benzene ring or a methyl substituent reduces the temperature corresponding to the $\beta $ -relaxation peak by more than 50 °C compared to the one without. The increased rigidity of the chain segment of EP makes the $\beta $ -relaxation move to the low-temperature side, which is beneficial to suppress the dielectric loss. This article not only serves as a bridge among dielectric loss of MFT, dielectric spectroscopy, and molecular structure of EP but also lays a solid foundation for the selection of insulating materials for MFT. [ABSTRACT FROM AUTHOR]

In this article, a joint measurement of dielectric polarization, depolarization, and potential recovery is carried out to investigate the instantaneous and relaxation polarization characteristics of epoxy resin in high magnetic field. The dielectric polarization of epoxy resin is converted to a combination of weakened instantaneous polarization and the relaxation polarization with reduced amplitude and increased speed. Changes in dielectric polarization processes leads to the decrease of relative permittivity. Furthermore, partial discharge (PD) of internal defects is harder to be initiated in high magnetic field. However, the discharge frequency is increased and the phase distribution becomes wider in the case of ac. In the case of dc, discharge lasts for longer time in high magnetic field. The experimental results are caused by the coupling effects of high-speed polarization and abnormal charge accumulation at the gas-solid interface due to the effects of Lorentz force on charges generated by PD. [ABSTRACT FROM AUTHOR]

Frequency-domain dielectric spectroscopy (FDS) is widely used in the insulation assessment of oil-immersed power equipment, and the moisture in insulating paper could be qualitatively predicted by mathematical approximation. However, due to the still lots unexplored underlying mechanisms, until now, there is still no convincing and physical method for the diagnosis of moisture content. With the aid of spectral decomposition methods, this article aims to explore the effects and mechanism of moisture on the independent dielectric response spectra. First, the measured dielectric response curve is unfolded and reconstructed by derivation of the real part curve. Second, the independent polarization and conductance spectra of damp samples are extracted, and then, the relationships between moisture content in insulating paper and spectral parameters are established quantitatively. The results show that moisture introduces a new polarization in the midfrequency region at room temperature, and both polarization and conductance increase with the moisture content. The multiple adsorption model and the proton exchange model of moisture are employed to illustrate the changes of the polarization and the conductance spectra, respectively. This research provides a methodological reference for the moisture assessment of oil–paper insulation. [ABSTRACT FROM AUTHOR]

Despite the technical popularity of the dielectric response (DR) methods used for the nonintrusive diagnosis of the cellulose insulation condition, it is still considered a challenging and tough task so far to trustworthily interpret the details of the measurement results for further understanding and tracing the insulation changes behind. From a novel perspective of parametric study on the circuital DR equivalence of cellulosic insulation—extended Debye model (EDM), the new dielectric characteristic quantities and the quantification approach to determine the cellulosic moisture condition are investigated in the present study. First, a kind of tailor-made scale-down bushing model is prepared, with its cellulosic condenser body absorbing differently controlled quantities of water to simulate the dampness of real-life insulation, of which the frequency-domain spectroscopy (FDS) is measured under controlled temperatures. Then, the EDM is employed for a parameterized study on the FDS details with the moisture changes, by identifying the model parameters against the FDS curves, whereby the two time-domain dielectric spectra—polarization/depolarization current and return voltage—are transferred from the FDS so that a set of hybrid time–frequency characteristic quantities is proposed, which is revealed to be indicative of the dampness variations of cellulosic insulation. Finally, the principal component analysis and the feature recognition algorithm based on minimum distance are applied to determine the moisture content of the condenser body, which verifies the holistic effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Dielectric response measurement is widely used to characterize oil/oil-impregnated pressboards in order to evaluate the condition of transformer insulation. Insulation systems with oil can exhibit voltage dependence at low frequencies. This is due to the limited number of mobile charges in the liquid, rather than to the dielectric relaxation processes that may, in some cases, be of more interest to study. This work investigates the voltage-dependent properties of oil/oil-impregnated pressboards under ac voltages with regard to the complex permittivities. Frequency-domain spectroscopy (FDS) measurements are made with a dc voltage added to the ac voltage stimulus. Considered influences are the voltage amplitudes and polarity as well as the dependence on temperature. Properties of the space-charge polarization in the oil and oil-impregnated pressboard are calculated and discussed based on the dc-biased measurements. It is seen that using the dc bias in the FDS measurements of oil/oil-impregnated pressboard can significantly decrease the voltage dependence of the results that are caused by ion drift and consequent depletion of ions from the bulk liquid. Based on the dc-biased measurements, the complex permittivity due to the linear and nonlinear polarization can be separated. [ABSTRACT FROM AUTHOR]

Epoxy resin (EP) insulations are widely considered as potential candidates for various applications. However, it is not suitable for high frequency (HF) and high temperature (HT) applications due to its low thermal conductivity. This research presents the tuning of thermal and electrical properties of EP via resin stoichiometry and incorporation of surface-modified hexagonal boron nitride (h-BN) nanofillers. In this research, the curing agent concentration for pure EP is optimized first based on high thermal stability and sufficiently reliable electrical properties. Second, the optimized pure EP is used as a base material to prepare epoxy/h-BN nanocomposites. Various physicochemical, thermal, and electrical characterizations are performed to analyze the properties of prepared materials. Results show that the higher curing agent concentrations offer a good balance between the thermal and electrical performance of the EP. However, the HF and HT stress exhibits detrimental effects on the breakdown strength and dielectric properties of the materials. Also, the addition of nanofillers further enhances the thermal properties while exhibiting sufficient electrical properties. By the control of resin stoichiometry and nanofiller addition, suitable composites for the HF and HT environment can be realized. [ABSTRACT FROM AUTHOR]

It is shown that the concentration of single-walled carbon nanotubes in polyethylene can be estimated using a spectral-correlation method, which consists of statistical processing of random intensity distributions (speckles) formed by scattering on optical inhomogeneity of the material. The optimal conditions for performing measurements are established, possible causes of errors are determined, and the statistical errors of measurements are estimated. Based on experimental data, expressions are obtained for approximating the dependence of the nanotube concentration on the half-width of the normalized cross-correlation function of the intensity distributions of backscattered radiation at different wavelengths. The possibility of using extrapolation of calibration dependencies for two different physical models (linear and exponential) is considered and the expediency of using a linear model at nanotube concentration above 2% is shown. The concentration of nanotubes in one of the elements of the high-voltage coupling design is evaluated and the possibility of detecting inhomogeneity of the filler concentration in the material is demonstrated. [ABSTRACT FROM AUTHOR]

Simultaneous measurement at various temperatures of space charge and relaxation current is performed on cross-linked polyethylene (XLPE)/ethylene-propylene-diene monomer (EPDM) double layers with and without silicone grease. The simultaneous measurement correlates the charge distribution and current information for the same sample and avoids discrepancy, due to the separate measurement on different samples. It is found that the polarity of interfacial charges change to the opposite with the increase in temperature in most samples, which is related to both Maxwell-Wagner polarization and enhanced charge injection and migration at higher temperatures. Bipolar charges appear in the vicinity of interface in some XLPE/EPDM double layers with silicone grease. This phenomenon can be explained by Maxwell-Wagner polarization at two interfaces of XLPE/silicone grease and EPDM/silicone grease. The addition of silicone grease decreases the conductivity of XLPE/EPDM double layers. Based on the proposed three-component exponential decay model, it is found that the interfacial charge movement affects the polarity of induced relaxation current, and the addition of silicone grease decreases the interface trap depth. [ABSTRACT FROM AUTHOR]

In this work, a finite element-based model is developed for the computation of effective dielectric constant and/or conductivity of single layer, multi-layered, and FGM composites. The computed values are validated against experimental data, and are seen to provide estimates better than all theoretical models. Thus, this computational model is expected to be useful in the design of new insulation systems. [ABSTRACT FROM AUTHOR]

In this paper, a model of the Kelvin probe is established in COMSOL Multiphysics based on electrostatic field theory, and it is verified by measuring the surface potentials. The influence of the probe is also investigated. An improved charge inversion method based on surface potentials measured online is proposed. The surface charge density is directly obtained by the Gauss's theorem to avoid the calculation of the inversion matrix. The improved method is verified by comparing the surface charge density calculated by the analytical method. When considering the effect of the probe, the maximum difference in surface charge density is 0.3 µC/m2 comparing to that when ignoring its effect, and the percentage of the difference is as high as 41%. The results indicate that the effects of the voltage of the probe under online measurement conditions must be considered. [ABSTRACT FROM AUTHOR]

This paper focuses on the effect of unipolar and bipolar square, sine and triangle voltages on space charge characteristics inside solid insulation based on the bipolar charge transport model. According to the model solution process and the polarization characteristics, it is found the algorithms for the model should be chosen with regard to the direction of the electric field at each subdivision inside the calculation sample. Further comparison shows that the charge amount under different conditions is mainly determined by the input power of the imposed voltage, while the migration depth of the injected charge is less than 20 μm from the sample surface. Correlation between the current injection and the charge amount indicates that the variation trends of them follow basically the same. The results under unipolar conditions are not affected by the frequency provided it is larger than 10 Hz, while the charge amount under bipolar ones is negatively correlated to the frequency. Meanwhile, the impact of the initial phase of bipolar polarization decreases rapidly with the increased simulation time. Hence, for space charge measurement inside dielectrics under such conditions, a reliable system should be established according to the polarity and waveforms of the polarization. [ABSTRACT FROM AUTHOR]

This paper determines the possibility of generating a correlation or correction factor for the dielectric properties between in vivo and ex vivo tumor tissue measurements. The experiments performed include injecting the backs of C57BL/6 mice with colonic fibroblasts, which are derived from colonic tumors developed in the same species of mice. After the tumors grew to at least 9.5 mm in diameter, dielectric readings of the tumorous tissues and surrounding tissues are gathered for analysis. Data are collected using a network analyzer of the series E5071C and dielectric properties probe (85070E) over the microwave frequency range between 1-5 GHz. The results conclude that a general correction coefficient could be found ranging from 12.30 to 4.0 in terms of relative permittivity and from 0 to 1.44 in terms of effective conductivity. [ABSTRACT FROM AUTHOR]

In this paper, the space charge behavior of polyimide at cryogenic temperatures is studied. The pulsed electro-acoustic (PEA) and thermally stimulated current (TSC) methods are exploited to evaluate the space charge distribution, quantification of space charge accumulation and trap characteristics of polyimide. The results show that the change of space charge accumulation at 273 to 123 K is divided into three temperature regimes. At 273–223 K, the space charge accumulation decreases significantly with decreasing temperature, which is mainly due to the suppression of charge generation and migration. At 223–173 K, the negative space charge gradually increases with decreasing temperature and dominates the space charge distribution. Analysis shows that this is due to the significant decrease of trap level in the sample, which improves the carrier mobility and promotes the electron injection and migration. According to quantum chemical calculation and dielectric spectroscopy measurement, the rotational displacement motion of water molecules related to γ relaxation in polyimide produces more shallow electron traps, which may be the main reason for the decrease of trap level. When the temperature is below 173 K, the space charge accumulation decreases with decreasing temperature. This study provides guidance for low temperature insulation design. [ABSTRACT FROM AUTHOR]

High strength hollow ceramic spheres are incorporated into epoxy resin to prepare insulating composite materials. The characterization shows simultaneous enhancement in thermal conductivity, mechanical strength and dielectric permittivity. A thermal conductivity of 1.36 W/(m·K) (7.5 times of that of pure epoxy) is measured from the sample prepared from hollow spheres with high alumina content. The compressive strength and impact strength measured from the same sample are 104.04 MPa and 3.61 kJ/m2, corresponding to an increase of 18.3 and 31.4 %, respectively, from pure epoxy. The permittivity of the sample measured at low frequency is 9.85 which decreases slightly as frequency increased to 106 Hz. The electrical breakdown data are analyzed with Weibull model and the decreased breakdown strength is explained. [ABSTRACT FROM AUTHOR]

In this study, epoxy resins incorporated with nano-alumina (nano-Al 2 O 3) are prepared with different concentrations of 0, 1, 3 and 5 wt%. The obtained epoxy resin/Al 2 O 3 nanocomposites are subsequently fluorinated using F 2 /N 2 mixture (F 2 /N 2 20% v/v) at 0.05 MPa and 40°C. The nano-Al 2 O 3 is treated with silane coupling agent γ-aminopropyltriethoxysilane (KH550) to restrict aggregation. The chemical bonding is examined by Fourier-transform infrared spectroscopy. The cross-section and surface morphological analyses are performed using scanning electron microscopy. In addition, the DC surface resistivity is measured using a resistivity test fixture. The variations in electrical properties induced by fluorination are characterized based on dielectric measurements. The experimental results show that chemical bonds are formed between nano-Al 2 O 3 and the coupling agent (KH550). Moreover, the results confirm that fluorination significantly alters the morphology and chemical structure of the sample surface. Gas-phase fluorination produces shallow traps, which consequently reduce the surface resistivity. The treatment of nano-Al 2 O 3 increases the trap energy level to yield higher surface resistivity. Furthermore, the treated Al 2 O 3 nanoparticles show improved distribution and dispersion in epoxy resin, exhibiting good dielectric performance of the nanocomposite. [ABSTRACT FROM AUTHOR]

In this work, eleven commercially available nanoparticles are benchmarked against each other according to their potential for an application in a polypropylene (PP) based nanocomposite for thin film power capacitors. Biaxially oriented films containing a fixed concentration of 5 wt%. Al 2 O 3 , TiO 2 , SiO 2 , or POSS nanoparticles with different surface functionalization were analyzed by dielectric spectroscopy and dielectric breakdown measurements. Two particle systems emerged to increase both the dielectric breakdown strength and the reliability of the films combined with low dielectric losses. The effects of the particles were in agreement with the multicore model of Tanaka et al. The particle dispersion was analyzed on SEM images. A correlation between agglomerates and zeta potential in isopropanol was found yielding a quick analysis tool for the tendency of nanoparticles to form clusters during processing with PP. To assure comparability between the films, the real particle content was determined by incineration. The surface roughness of the film has a large influence on the breakdown behavior and was, therefore, also monitored. [ABSTRACT FROM AUTHOR]

Dielectric response measurement based on the polarisation and depolarisation current (PDC) method is a diagnostic tool commonly employed to the insulation condition assessment of power apparatus. In this paper, experimental studies are carried out on short sections of service-aged XLPE cable. Daniel's normalisation is applied to estimate the difference between the permittivity under DC excitation and that at optical frequency. Physical explanations of various polarisation processes at low and high frequencies are interpreted through the clustering model. Based on transforming the PDC currents into the frequency domain equivalents, dissipation/loss factor (tan δ) measurement and modeling over the range of frequency from very low to power frequency are presented. The analysis reveals the relationship between individual loss factors and the overall loss factor. Also, measurements of the activation energy and testing under varying electrical stresses and ambient conditions provide further information for interpretation. [ABSTRACT FROM AUTHOR]

This study aims at elucidating the correlations between structural changes and the variation of dielectric properties of cross-linked polyethylene (XLPE) during thermal aging. For this purpose, thermal aging experiments were carried out on XLPE samples at 80, 100, 120 and 140 °C for different aging times for a maximum total duration of 5040 hours. Chemical changes were assessed by Fourier transform infrared spectrum (FTIR). Besides, the crystallinity ratio and the melting temperature were evaluated using differential scanning calorimetry (DSC). Frequency domain dielectric spectroscopy was used to investigate the evolution of the dielectric constant and losses of XLPE during aging. The results show that aging at 80 and 100 °C could help to improve the crystalline state of XLPE which leads to the decrease of the dielectric constant. However, for higher thermal stress, the main aging mechanism of XLPE is thermal oxidation which leads to deteriorate the morphology and increase both the polarization and losses, mainly at low frequencies. It has been found also that in some cases, the non-homogeneity of the color of XLPE seems to indicate the increase of interfaces in the material which could lead to increase the dielectric losses due to the interfacial polarization. [ABSTRACT FROM AUTHOR]

High-performance antennas operating at ultra-high frequency (UHF) are typically designed and fabricated using specialized radio frequency (RF) materials with low loss and consistent properties. This paper proposes and investigates the use of recycled polyolefin (high-density polyethylene and polypropylene) which is not specifically manufactured for RF applications and thus with unknown RF properties, as dielectric substrate for high-performance UHF patch antennas. As a demonstrative application, a polarization-reconfigurable patch antenna with dual feeds is designed and fabricated using adhesive copper foil on recycled polyolefin substrate. Depending on the feed's magnitude and phase, different types of linear and circular polarizations can be achieved. The point of resonance for both ports is centered at 915 MHz with 10 dB return loss and bandwidth of 28 MHz. Port isolation is as low as −17 dB with peak linear and circular gain of 7.0 dBi, and 7 dBiC, respectively, observed in all polarizations. The resulting antenna not only performs well, but is also lightweight, low moisture absorbent, mechanically sturdy, inexpensive and ecologically friendly arising from its use of recycled materials. [ABSTRACT FROM AUTHOR]

This paper deals with the interaction of nanoparticles (NPs) in transformer oil and water in the oil and its effect on the electrical properties of the nanoffuid (NF). The breakdown strength and dielectric spectrum measurements under AC for various concentrations of natural SiO 2 and water in mineral oil were studied. Mineral oil-based SiO 2 NFs exhibit significantly higher breakdown voltage than that of base oil under high water content. However, there is an optimal range of NP concentration and water content for maximum breakdown voltage. The results of dielectric spectrum indicate that the dielectric constants of NFs and base oil are greatly affected by water content. The real relative permittivity of oil samples increases exponentially with water content at decreasing frequency (f≤10 Hz). The imaginary relative permittivity of NFs is less than that of the base oil in all cases. Based the experimental results reported, a model on the interplay between NPs and water in transformer oil is presented. The model can broadly explain the influence of the coexistence of NPs and water on the dielectric properties of transformer oil. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The superconducting gas insulated transmission line (S-GIL) is a new design being proposed for helium gas (GHe) cooled high temperature superconducting (HTS) power cables. The S-GIL design utilizes GHe as both cryogen and insulation medium for the HTS cable. This paper builds upon previous proof of concept measurements of the S-GIL. Through optimization of the S-GIL design parameters increase of 20–30% in electrical breakdown voltages was observed for the proof of concept measurements. The paper also contains details on a prototype S-GIL design which was adopted from a commercial coaxial air dielectric cable. The prototype S-GIL cable was successfully demonstrated at 77 K using both GHe and 4 mol% H2 balanced with GHe as the cryogen/insulation medium. The values obtained by the prototype S-GIL cable are higher than the voltage rating currently capable of other GHe cooled HTS cables which verifies the potential of the S-GIL design concept. [ABSTRACT FROM AUTHOR]

In the present study, polyaniline (PANI) conducting polymer was mechanically doped with volcanic basalt rock (VBR), sodium metasilicate (Na 2 SiO 3), and sodium tetraborate (borax) minerals at 10 to 50 wt%, to improve the dielectric response. The influence of these mineral additives on the complex dielectric, electrical modulus, and AC impedance spectra of the PANI was investigated. It was revealed that among the mineral additives, the highest s' is achieved for 50 wt % sodium metasilicate doped PANI composite. In addition, the appearance of peaks in the imaginary component of the electric modulus plots, for especially borax and sodium metasilicate doped samples, indicates the existence of a relaxation process. In addition, the Cole-Cole plots in the complex electrical modulus plane suggests a non-Debye type of relaxation. The single semi-circle arcs observed in Nyquist plots for PANI/Na 2 SiO 3 composites indicates a dominant grain (bulk) contribution. Moreover, the bulk resistance and capacitance values for PANI/Na 2 SiO 3 composites were found to be in the order of MΩ-GΩ and pF, respectively. While the AC conductivity spectrum of the PANI/VBR composites obeys the Nearly Constant Loss model, the PANI/Borax and PANI/Na 2 SiO 3 composites were found to obey the Super Linear Power Law model. In conclusion, the low cost Na 2 SiO 3 additive with different weight percentages for PANI shows remarkable effects on the dielectric and electrical properties. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Knowledge of the dielectric properties of biological tissues is fundamental in the design of novel electromagnetic-based medical devices. Tissue dielectric properties are typically measured using the open-ended coaxial probe technique, which is designed for homogeneous samples. Histological analysis may be conducted to associate the measured dielectric properties to different tissue types within heterogeneous samples. However, the histology radius (i.e., the radial extent of the tissue sample that undergoes histological analysis) has not been consistently defined in the literature; therefore, this parameter may be a source of error in dielectric data. For this reason, we investigate the histology radius of various heterogeneous samples. Dielectric measurements were conducted over the frequency range of 0.5 to 20 GHz on radially heterogeneous tissue-mimicking materials and biological tissues, with different dielectric properties and contrasts. The experimental results were validated with numerical simulations and indicate that: i) the histology radius does not exceed the probe radius; ii) the dielectric properties of radially heterogeneous tissues depend on the spatial distribution of each material within the histology radius; and iii) the bulk dielectric properties of concentric heterogeneous tissues highly depend on the properties of each constituent material within the histology radius. This study supports consistent identification of the histology radius and provides a basis for rigorous interpretation of the dielectric properties of heterogeneous tissues. [ABSTRACT FROM AUTHOR]

A new high temperature ultra-thin polymer film based on Polyetherimide (PEI) chemistry has been developed and electrically characterized and compared with other dielectric capacitor films. More specifically, intrinsic and extrinsic dielectric breakdown strength is compared and reported for multiple resins thus demonstrating feasibility of the new polymer film. In addition, results from a large area electrode test are presented and the importance of this test method in the development of high temperature capacitors for DC applications is discussed. [ABSTRACT FROM AUTHOR]

In this paper, the effects of coagulation and temperature on the dielectric properties of human blood are investigated over the frequency range of 400 MHz?20 GHz using freshly extracted blood samples. The dielectric properties are measured using blood in four different sample collection tubes (bottles): one containing pure whole blood, two containing different anticoagulant agents, and one containing clot activator and serum separator. The collected data indicates that additive agents can have a significant impact on the measured dielectric properties of blood, both immediately after the sample is taken, and over longer time periods. This is an important finding as it suggests that measurements of blood properties conducted on sample repositories, or tissue banks, may not be representative of natural blood properties. Further, the results demonstrate that the dielectric properties of normal blood vary over time due to coagulation. Different clotting rates lead to dielectric properties of female and male blood samples that vary distinctly over time. The results also show that the relative permittivity of the anti-coagulated blood decreases with increasing temperature, up to the cross-over point around 10 GHz where the trend reverses. [ABSTRACT FROM PUBLISHER]

To investigate dielectric property variations induced by electric pulses, equivalent doses of conventional irreversible electroporation (IRE) and high-frequency IRE were used to treat potato slices. The dielectric property of the potatoes were measured before and after treatment in the frequency band of 0.1 Hz-10 MHz, and then the Cole-Cole model was used to fit the experimental data in the frequency band of 1 kHz-10 MHz to explain the ? dispersion mechanism. The Cole-Cole model parameters showed that under an equivalent dose, the conventional IRE pulses were more efficient than the high-frequency IRE pulses for electroporation. After treatment with conventional IRE pulses, the DC conductivity greatly increased at time 0 after the pulses and showed little recovery over time. The electroporation effect of the high-frequency IRE pulses improved with increasing pulse duration. This study presents a potential new method for evaluating the effects of IRE during and immediately after the treatment process. [ABSTRACT FROM AUTHOR]

Frequency Domain Spectroscopy (FDS) is proposed as a suitable diagnostic method in this paper for implementation on energized oil-paper insulation systems. In present FDS tests, the measurement method is only designed for offline applications. To overcome the deficiencies with the offline testing, this work investigates the feasibility of performing FDS measurements on energized transformers with the view to developing an online diagnostic tool using neutral connections as injection and measurement paths. In the proposed technique, a low-pass filter system is designed to remove the high frequency noise as well as the power frequency signals present in the measured neutral currents. The technique has been successfully applied in a laboratory environment on transformers with oil-paper insulation systems with different operational ages and is feasible for single-phase as well as 3-phase transformers with the accessible neutral points. Compared with the conventional FDS results, a nonlinear dielectric phenomenon is observed from the energized insulation systems as higher dissipation factor Tan5 values primarily due to the increased losses under power frequency voltage excitation. The degree of the nonlinearity is found to correlate well with the degradation status of insulation systems under test. [ABSTRACT FROM PUBLISHER]

Gelatin-based material can be used in phantom modeling for microwave imaging applications, but unfortunately, this type of phantom has a short life span. Formaldehyde has been used previously to preserve gelatin-based material, but it might be hazardous. This paper presents a life-span study for gelatin-based material that has been preserved by vinegar in different amounts, observed through the conducted dielectric measurement from 1 GHz to 6 GHz. The Debye parameters are derived to define a Debye relaxation model of the acid contents in a solution of water and vinegar. The study is then focused on dielectric trends prior to and after the addition of different amounts of vinegar into a gelatin-based material. The life-span study concentrates on changes in the sample's dielectric properties over a storage period of six weeks. The obtained findings in this study are useful to validate the suitability of vinegar to be used as preservative in gelatin-based material for microwave imaging purposes. [ABSTRACT FROM AUTHOR]

Dielectric response (DR) measurement is commonly used for condition assessment of insulation systems of power components. A method for measurement of dielectric response using the stimulus of natural transients such as lightning and switching impulses was proposed in earlier work. Its desirable features include the ability to make measurements online over a range of frequencies, without requiring a voltage source. This article presents a laboratory demonstration of the method on a 150 kV service-aged transformer bushing, where the stimulus was a standard lightning impulse voltage superimposed on an AC voltage. Several aspects of the measurement and data processing that affect the results are studied experimentally and numerically. The results are compared with low-voltage frequency domain spectroscopy (FDS). Reasonable accuracy for monitoring changes in DR can be achieved by suitable choice of measurement circuit and data acquisition. The study suggests an approach for online monitoring of dielectric properties of power transformer bushings, and diagnostics of defects that affect the high frequency region of DR, such as moisture content. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The pulsed electro-acoustic (PEA) method is used for thirty years for measuring space charge distribution in dielectric materials [1]. However, this method was limited to at least 150 μm thick films characterization. For some applications, in spatial industry for example, 50 μm thick outer space dielectric materials must be analyzed. A high resolution optimized measurement system is discussed in this paper in order to expand performances and ranges of this method. To design it, a new pulse generator has been conceived and an optimized acoustic detector, key of the PEA method, has been realized and validated. The optimized High-Resolution PEA apparatus allows to obtain calibration signal with 50 μm thick dielectric materials that can be used to obtain the characteristic transfer function of the PEA system, essential step to use the data processing to find space charge distribution to characterize thinner dielectric materials. [ABSTRACT FROM AUTHOR]

As a key influence factor on the lifetime of pulsed capacitors, the repetitive frequency (fR) of charge/discharge is studied in this paper based on the theory of space charge. A fact that in the absence of heat accumulation, from 0.1 to 10 Hz, the lifetime of capacitors increases with the fR is found. The measurement of space charge induced by charge/discharge at various frequencies is conducted. The measurement result indicates that the amount of space charge in capacitors experienced charge/discharge at 0.1 Hz is 29% higher than that of 10 Hz, which can reasonably explain why the lifetime of capacitors tested at 10 Hz is twice of that at 0.1Hz. Besides, a simplified TSDC is carried out to qualitatively measure the trap density of capacitors experienced lifetime tests, and found that the traps formed during charge/discharge tests at 0.1 Hz is more than 7x that of 10 Hz. The dielectric loss angle tangent (tan?) of capacitors which have experienced charge/discharge at various frequencies is also tested, and from the tan? tested at 0.001 Hz, (3.7 when fR is 0.1 Hz and 2.4 when fR is 10 Hz), it is revealed that the aging of pulsed capacitors can be slowed by enhancing the fR of charge/discharge, which matches well with the results of lifetime test. [ABSTRACT FROM AUTHOR]

This contribution discusses thermally stimulated current (TSC) measurements on low density polyethylene (LDPE) in the form of thin films and virtually without additives. The TSC responses are divided into different parts attributed to capacitive currents, currents due to release of trapped carriers, and normal conduction currents. The three contributions dominate at low, intermediate, and high temperatures, respectively. Dielectric spectroscopy measurements at different temperatures indicate the relevance of the capacitive currents for the measurement of thermally stimulated polarization currents (TSPC). The separation of the conduction current, by subtracting reference curves from the total current, allowed the investigation of trap and conduction properties for different experimental parameters (e.g., excitation levels, fields, heating rates). [ABSTRACT FROM PUBLISHER]

Dielectric strength testing of selected Polyjet-printed polymer plastics was performed in accordance with ASTM D149. This dielectric strength data is compared to manufacturer-provided dielectric strength data for selected plastics printed using the stereolithography (SLA), fused deposition modeling (FDM), and selective laser sintering (SLS) methods. Tested Polyjet samples demonstrated dielectric strengths as high as 47.5 kV/mm for a 0.5 mm thick sample and 32.1 kV/mm for a 1.0 mm sample. The dielectric strength of the additively manufactured plastics evaluated as part of this study was lower than the majority of non-printed plastics by at least 15% (with the exception of polycarbonate). [ABSTRACT FROM AUTHOR]

The correlation between structural and electrical properties of cadmium titanate (CdTiO3) ceramic was established through X-ray diffraction combined with electrical and optical characterization techniques. The compound was prepared using a hightemperature solid-state reaction technique. Preliminary study on some structural aspect of the material using X-ray diffraction pattern showed the formation of singlephase compound in orthorhombic crystal system. Some characteristics of molecular structure of the material were studied by using Fourier transform infrared spectroscopy and ultraviolet visible spectroscopy techniques. The size and distribution of grains in the scanning electron micrograph revealed the polycrystalline nature of the material with size anisotropy and small number of voids. Detailed analysis of temperature-frequency dependence of dielectric parameters (i.e. dielectric constant and tangent loss) has revealed many interesting and important results of the material. The impedance studies of CdTiO3 over a wide range of temperature and frequency in the complex plane formalism with suitable equivalent circuit have shown some correlation between microstructure and electrical properties of the material. From the impedance and dielectric measurements some electrical parameters such as bulk resistance, capacitance, relaxation time and relative dielectric constant were calculated. The variation of DC conductivity with inverse of absolute temperature follows the Arrhenius relation. The study of frequency dependence of AC conductivity suggests that the material obeys Jonscher's universal power law. [ABSTRACT FROM AUTHOR]

Lead zirconate titanate (PZT) samples were prepared near morphotropical phase boundary (MPB) (i.e., Pb(Zr0.52-xMnxTi0.48)O3 (x = 0, 0.02, 0.06, 0.10)). These samples were prepared using solid-state reaction technique. Formation of compounds was confirmed by X-ray diffraction (XRD), as a part of our preliminary structural analysis. We observed a significant change in grain size and density on the sample surface due to substitution of Mn. This microstructural analysis of the sample surface was done by scanning electron microscopy (SEM). Tc shift, dielectric constant improvement and reduction in the disordering of the material were observed while doing the detailed studies of the dielectric properties. [ABSTRACT FROM AUTHOR]

Different wood-fibre based papers were characterized by dielectric spectroscopy, mechanical testing and microscopy. The data obtained were utilized to investigate the relationship between the chemistry, morphology and density of a paper and its permittivity and dielectric loss. The density strongly influences the dielectric response, but the response is not affected by the way the density has been achieved; by pressing the paper during drying or by mechanical treatment of the fibres before sheet preparation. The chemical composition of the pulp influences the polarization, dielectric loss and charge transport. It was found that paper-vacuum and paper-oil combinations can be represented by series-equivalent circuits. The permittivity of paper made from electrical grade kraft pulp, used in e.g. high voltage transformers, without any porosity is estimated to be 5.3 and tan δ to 0.01- 0.02 at 50 Hz and 70 °C. The lignin and hemicellulose content of the kraft pulp do not affect the real part of the permittivity significantly, but the dielectric losses increase with increasing lignin and hemicellulose content in both oil and vacuum at 50 Hz and 70 °C. [ABSTRACT FROM PUBLISHER]

Transformer development is limited to smaller sizes and larger capacities due to two major shortcomings: first, the solid insulation that is used in an oil-immersed transformer, which is generally cellulosic paper, cannot resist high temperatures, and second, the significant difference in the dielectric constants between the cellulose paper and the transformer oil will lead to an uneven electric field. In light of this situation, in combination with the requirements of oil-immersed transformers for the dielectric, physicochemical, and mechanical properties of solid insulations, in this study two kinds of polymeric materials are chosen as the research objects - Polycarbonate (PC) and Polyester (PET) - that can resist high temperature, and then, a comparative analysis is made with cellulose paper. They are put in the transformer oil for electro-thermal aging for 300 days at 90, 110, and 130°C, respectively. The variation characteristics of permittivity and the dielectric dissipation factor with frequency were examined in stages (30 days, 140 days, 180 days, 250 days, and 300 days). This paper studies the characteristics of polymer materials so as to analyze whether they can be used in an oil-immersed transformer by considering the materials? physical properties, electron microscope scanning (SEM), and degree of polymerization (DP) at different stages during aging. This valuable exploratory work has never been studied before. If these two new materials can be applied, it will be of great significance to the innovation of the insulation structure of a power transformer. [ABSTRACT FROM AUTHOR]

Frequency Domain Spectroscopy is a popular non-invasive method that is widely used for condition assessment of oil-paper insulation in real-life power transformers. Conventional Frequency Domain Spectroscopy is done using sinusoidal excitation over a wide frequency range. In this work, a different approach has been taken to investigate some more possibilities of Frequency Domain Spectroscopy using non-sinusoidal excitation for a better condition assessment of oil-paper insulation. For that, triangular waveshape was chosen as non-sinusoidal excitations whose slope can be changed to vary the harmonic content and amplitudes of constituent components. Samples with preset moisture contents that emulate the oil-paper insulation of real-life transformer have been prepared in the laboratory for the assessment of insulation condition. For a comparative study, both sinusoidal and triangular excitations having same peak as well as time period are applied to the prepared samples under controlled environmental condition and corresponding dielectric response currents are measured. From the applied voltages and corresponding dielectric response currents, dielectric dissipation factors are calculated and compared. It has been observed that use of nonsinusoidal excitation provides better results than sinusoidal excitation in the estimation of paper moisture content. It has also been observed that among the applied non-sinusoidal excitations, a particular waveform provides more accurate information about the paper moisture content. [ABSTRACT FROM AUTHOR]

Underground low voltage (LV) power cables have a long life span, and so far minimum effort was put in methods for condition assessment. However, a slow but steady increase in the amount of faults is being observed and future de-central power generation may further put pressure on the reliability of the LV grid. Therefore, condition monitoring for LV cable systems is under investigation. With the occurrence of intermittent faults, fast transients from discharges may be observed before an outage occurs. To estimate if such signals are measurable, a model of the cable valid up to 50 MHz was derived. Numerical simulations have been used to obtain the parameters of the multi-conductor transmission line model, with special consideration of the copper screen configuration. Measurements on a 60 m cable segment have been performed to verify the model, showing good agreement. This model will be used to investigate signal propagation through a LV grid with focus on the feasibility to base LV condition monitoring on signals which may originate from upcoming cable faults. [ABSTRACT FROM AUTHOR]

Samples of a dried composite oil-paper system were thermally aged in the laboratory under N2 in a sealed container and analysed using their dielectric response. The contributing loss processes were separated using an equivalent circuit model containing constant high frequency capacitances, dc-conductances, a dispersive capacitance, and a low frequency quasi-dc (q-dc) dispersion process as circuit elements. This low frequency dispersion in oil-paper samples is a bulk property of the system that is often mistaken for a dc conduction process when only the imaginary capacitance or ac-conductance is measured. The Dissado-Hill response function is proposed for the frequency dependence of the dispersive capacitance in the mid-frequency range rather than the Debye function because its cluster concepts correlate with the sample morphology of a matrix of oil-filled cavities. The quasi-dc dispersion is assigned to the transport of mobile charges between charge-clusters associated with locally connected oil filled cavities, which is a process that can be represented on a global scale by the form of hierarchical circuit system that leads to constant phase angle responses, and is here modelled by the Dissado-Hill q-dc function. It was found that the two dispersions moved together to higher frequency with thermal ageing of the composite material. The characteristic frequency common to the dispersions had an activation energy that decreased with thermal ageing, whereas their amplitude increased. This behavior is discussed in terms of possible changes to the oil-paper composite caused by the ageing process. [ABSTRACT FROM AUTHOR]

The magnetoelectric composites (x)Ni0.75Zn0.25Fe2O4-(1-x)Pb0.9(La0.96Sr0.04)0.1(Zr0.65 Ti0.35)0.975 O3 (for x variations 0, 0.20 and 1.0) where Ni0.75Zn0.25Fe2O4 i.e. NZF is ferrite phase and Pb0.9(La0.96Sr0.04)0.1(Zr0.65Ti0.35)0.975O3 i.e. PLSZT is ferroelectric phase, were synthesized via solid state reaction method. The presence of both the constituent phases was confirmed by X-ray diffraction studies. The microstructural analysis and grain size determination was carried out using field emission scanning electron microscope (FESEM). Variation of dielectric constant and dielectric loss with temperature at various frequencies (1 kHz and 10 kHz) was studied. The good ferroelectric behavior and ferromagnetic behavior of the composites were confirmed by Polarization versus Electric field (P-E) and Magnetization versus Magnetic field (M-H) measurements, respectively. The coupling between ferroelectric and ferromagnetic domains was confirmed through the observation of magnetic fieldinduced relative change in dielectric constant (Magnetocapacitance). The value of magnetocapacitance is found to be 2.6 % for the synthesized composite (with x=0.20). The fractional change of magnetic field induced dielectric constant can also be expressed as Δϵ~γM2 and the value of γ is found to be ~8.16?10-2 (emu/g)-2 for composite with x=0.20. [ABSTRACT FROM AUTHOR]

The dielectric properties of two amine cured bisphenol-A epoxy resin systems, Araldite CY1301 and Araldite CY1311 have been characterized using dielectric spectroscopy over the frequency range 1 mHz to 100 kHz. These two epoxy resin systems were chosen to allow the dielectric response to be studied from above and below the glass transition, as Araldite CY1311 is a modified version (with added plasticizer) of Araldite CY1301. The dielectric response was found to comprise both bulk and interfacial features. Above the glass transition temperature, two processes were identified, a low frequency process usually ascribed to interfacial polarization is shown to be a bulk process termed Quasi-DC (QDC) conduction and a dielectric dispersion usually taken to be a bulk process has been shown to be an interfacial effect. [ABSTRACT FROM AUTHOR]

High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular electrical insulation at the instant of interaction with a high voltage transient would determine a level of the degradation of an electrical insulation. This work investigates how high voltage impulses would change Phase Resolved Partial Discharge (PRPD) patterns in test samples consisting of a cavity deliberately introduced between the sheets of oil-impregnated paper used in power transformer bushings. It also investigates how the change in PRPD patterns is related to the degradation level of oilimpregnated paper. In order to accomplish these objectives, the experiments were set in such a way that the effect of HV impulses and an early stage PD activity at an elevated AC stress, HV impulses in combination with a prolonged PD activity at an elevated AC stress, and a prolonged PD activity alone at an elevated AC stress could be investigated separately. The experimental results presented in this paper indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity would not cause a significant change in PRPD patterns, and would not damage oil-impregnated paper to a level which can be noticed by a visual observation. On the other hand, a prolonged AC PD activity alone can cause the change in PRPD patterns, but cannot quickly damage the oil-impregnated paper as it would do when it is combined with HV impulses. However, the combination of both, HV impulses and a prolonged AC PD activity caused a high drop in the PD parameters (total PD charge and a repetition rate). The study found that the decrease of the PD parameters can be considered as a sign of severe degradation of oil-impregnated paper. [ABSTRACT FROM AUTHOR]

Search for a reliable and efficient insulation diagnostic tool has always been the interest of power utilities. Today a large number of methods are available that can be used for insulation condition monitoring. These methods include both traditional and newer techniques. However due to complex aging process of oil paper insulation under the influence of different types of stresses, insulation condition assessment is generally performed by experts after carefully evaluating different measurement data. Furthermore, measurement data are influenced by various factors (like conductive aging byproducts, furanic compounds, paper and oil-moisture) in addition to measurement error (if any). This makes prediction of insulation condition based on single type of measurement rather difficult. This paper presents an Expert System designed to perform insulation diagnosis. The Expert System considers measurement data obtained using both traditional and newer techniques in order to come to a definitive conclusion. The Expert System extracts insulation condition sensitive information from data obtained using different techniques and then uses these to devise an optimized insulation model. This optimized model is used to predict papermoisture content and other insulation condition sensitive parameters. Since these values are predicted using optimized model, they are not dependent on a single type of measurement and hence are less likely to be affected by error of any specific measurement. The performance of the developed Expert System is first tested on a laboratory sample and then on several real life power transformers belonging to NTPC Ltd. [ABSTRACT FROM AUTHOR]

Plain epoxy resins or resin impregnated cellulose have found application as electrical insulation in power equipment. In the past, their performance was improved by the use of inorganic oxide fillers of microscopic dimensions. In the recent past nano-particle doped epoxy insulation came into use with a view to further enhance the dielectric properties. This paper reports dielectric investigations into epoxy nano-composites based on a class of metal oxides, Al2O3 and SiO2. In particular, consideration has been given to the partial discharge performance and electrical breakdown under different voltage profiles as a function of the volumetric composition of the nano-particles in epoxy resin. [ABSTRACT FROM AUTHOR]

Measuring the water volume and the rock textural information downhole in wells with a logging tool requires an accurate knowledge of the dielectric properties of the borehole fluid. A small error on these properties can generate significant misinterpretation of the data, especially in the presence of resistive oil-based mud (OBM) or synthetic-based mud (SBM), both of which are also called nonaqueous drilling fluid (NAF). A model predicting the complex permittivity of NAF as a function of its density and the oil/water (O/W) ratio was developed in the 1970s, but since then, the NAF compositions have changed, which modified their dielectric properties. To revisit this model, a measurements campaign was launched in July 2012 to characterize the dielectric properties of NAF. NAF that results from an emulsion of water in oil consists of many components such as emulsifiers, salt, lime, weighting material, and surfactants. Different NAF formulations were prepared and the interactions between key parameters such as the O/W ratio (90/10 to 60/40) and density (9 to 15 lbm/gal) were evaluated using a central composite design (CCD).The complex permittivities of the selected NAF compositions were measured using two set-ups covering a wide frequency range, from 1 MHz up to 20 GHz. A statistical analysis of the measurements shows that the complex permittivity of NAF depends on two main influences: brine volumetric fraction and weighting material volumetric fraction. These two parameters can respectively be expressed as a function of the O/W ratio and density. The surface quadratic model responses developed during this study are validated by measuring the complex permittivities of different NAF samples from the field. [ABSTRACT FROM AUTHOR]

This paper describes the unique characteristics of the oil-paper insulation system utilized in power transformers, their influence on the dielectric response and means for proper modeling of these characteristics. Dielectric response measurement between windings of a power transformer is used to estimate the water content of the solid insulation. By measuring the dielectric response over a wide frequency range different properties of the insulation materials can be determined. These properties can be modeled in a laboratory so that measured response can be compared to modeled response with known properties. This comparison enables the estimation of properties of the materials such as water content and conductivity. The influences of temperature, aging and insulation geometry are described. Case studies are presented which demonstrate the validity of the test method. The influence of frequency range, temperature, aging byproducts, oil conductivity and insulation material type on the accuracy of the moisture estimation is discussed. [ABSTRACT FROM PUBLISHER]