Pulsed Electro-acoustic Method Research Articles

A novel simulation of space charge decay dynamics after removing the voltage in epoxy resin composites

Abstract Epoxy resin serves as a critical insulating component in ultra-high voltage dry direct current bushings. However, the accumulation of space charges within the epoxy resin, a byproduct of charge mobility, poses a significant risk to the reliability and operational safety of bushings. Conventional space charge attenuation models, especially after voltage removal, have limited use in simulations aimed at understanding this phenomenon. This study introduces an improved model integrating the bipolar charge transport mechanism with space charge decay based on the hopping conduction mechanism and Schottky’s theorem, establishes a theoretical framework for predicting the space charge behavior following voltage removal, and conducts a simulation to investigate the decay process within the internal structure of epoxy resin and measure the residual charges after voltage removal using the pulsed electro-acoustic method. The experimental data validate the accuracy of the proposed model and theoretical assumptions. The findings show that the remaining negative charges after voltage removal are not enhanced by the field enhancement; the anodic positive and cathodic positive charges are distributed in two-segment discontinuous traps, whereas the negative charges are distributed in one-segment traps. The model identifies two distinct trapping sites for anodic and cathodic positive charges, and one trapping site for negative charges. After voltage removal, when the field strength exceeded 40 kV mm−1, positive charges exist near the upper and lower electrodes, and negative charges exist near the center of the specimen. The consistency between the simulated predictions and experimental data proves the effectiveness of the proposed model in accurately simulating the space charge decay in epoxy materials after voltage removal.

Interface charge accumulation in bi-layer dielectrics between cable and accessory insulation under temperature gradient

Bi-layer dielectrics are a feature of high-voltage direct-current cable and accessory systems, giving rise to the interfaces between the cable and accessory insulation. The interfaces act as preference sites for space charge build-up and are therefore regarded as potential breakdown initiation sites. A further complication is that during the operation of cable, a temperature gradient (TG) will be produced across the bi-layer insulation. In this paper, carbon black (CB) nanoparticles with 0, 0.5, 1, 1.5, 3.5 and 5.5 wt% are doped into silicon rubber (SR), space charge distributions in bi-layer dielectrics composed of cross-linked polyethylene (XLPE) and CB/SR nanocomposites under DC voltage and TG are measured using the pulsed electroacoustic method, and the effects of the doping proportion of CB nanoparticles and the TG on the bi-layer interface charge behavior are investigated. The experimental results show that under the TG, CB nanoparticles doped into SR can effectively suppress the interface charge accumulation, the amount of the interface charge decreases with the increase of the doping proportion from 0 to 1.5 wt% but increases with the increase of the doping proportion from 1.5 to 5.5 wt%, and the CB nanoparticle with 1.5 wt% shows relatively better suppression effect on the interface charge accumulation. With the increase of the TG, the amount of the interface charge increases initially and then decreases.

Macro-dipoles in soft/hard expanded-polytetrafluoroethylene + fluoroethylenepropylene (ePTFE + FEP) fluoropolymer-film systems for high-output piezoelectric ferroelectret-transducer applications

Multi-layer ferroelectrets consisting of fluoroethylenepropylene (FEP) copolymer and open-porous expanded polytetrafluoroethylene (ePTFE) films exhibit stable internal electret charges, high piezoelectric coefficients and heat resistance, making them promising candidates for wearable sensors or nanogenerators in body-area networks. Here, three- and five-layer (FEP/ePTFE/FEP and FEP/ePTFE/FEP/ePTFE/FEP) ferroelectret stacks were laminated and poled in a corona discharge. The resulting charge distributions were measured by use of the pulsed electro-acoustic (PEA) method and revealed that charges of opposite polarity were trapped at the interfaces between the FEP and ePTFE layers. Thus, the existence of one macro-dipole in the three-layer structure and of two macro-dipoles in the five-layer structure was directly shown for the first time. Moreover, electric-displacement-versus-electric-field (D-E) loops revealed that remnant polarization is given by the number of macro-dipoles in the respective stack. Due to the addition of the macro-dipoles, the piezoelectric d 33 coefficient of the FEP/ePTFE/FEP/ePTFE/FEP stack reaches 200 pC/N even under a potentially non-uniform compression of the soft ePTFE layers. The results should be useful for a better understanding and a performance optimization of ferroelectrets in self-powered intelligent devices.

The effect of Li(Ni0.9Co0.05Mn0.05)O2 on improving electrical properties of semi-conductive LDPE/EVA/CB composites

The conductive mechanism of the semiconductor shielding layer between the conductive core and the insulation layer of high-voltage direct current (HVDC) cables includes tunneling and over-diffusion theories; however, the ethylene vinyl acetate copolymer (EVA) matrix in the semi-conductive composite material expands when heated, leading to a break in the carbon black (CB) conductive networks, which results in an increase in resistivity and the formation of the positive temperature coefficient (PTC) effect. The addition of materials with negative temperature coefficient (NTC) properties can weaken the PTC effect while absorbing some free electrons to enhance the suppression of space charge injection into the insulating layer. Li(Ni0.9Co0.05Mn0.05)O2 (NCM) was prepared by sol–gel method, which was melted and mixed with low-density polyethylene (LDPE), EVA, and CB, and pressed and molded to obtain modified semi-conductive composites. The semiconducting composites were measured by resistivity, thermal stimulation depolarizing current method (TSDC), and pulsed electroacoustic method (PEA). The results showed that the composites had the best performance when the NCM doping was 2 wt%.

Improving insulation properties of epoxy filled with surface fluorinated polystyrene nanospheres

<sec>Epoxy resin nanocomposites are widely used in the field of electrical insulation packaging. It is of great significance to regulate the dielectric and insulation properties of composite materials by introducing nano-filler to meet special application requirements. This work proposes a chemical copolymerization method, fluorinated polystyrene nanospheres are synthesized through an addition reaction as filler, and finally the epoxy nanocomposites are prepared. The polystyrene nanospheres have a uniform size and good compatibility with the epoxy resin. The introducing of nanospheres reduces the dielectric constant of the epoxy resin composite material and increases the breakdown strength simultaneously. Although the dielectric loss increases, the composites’ imaginary part remains below 0.04 within 1 MHz frequency. In particular, the fluorinated polystyrene/epoxy composite with a mass fraction of 2% exhibits a decrease in dielectric constant and DC conductivity, while the AC breakdown strength and DC breakdown strength increase by 12.6% and 6%, respectively.</sec><sec>The results of the pulse electro-acoustic method indicate that the charge injection of the epoxy resin filled with non-fluorinated polystyrene nanospheres is evident, while the introduction of fluorinated nanospheres significantly reduces the charge injection level. Calculations based on the depolarization process reveal that the introduction of fillers leads to an increase in trap density and depth of energy levels in the composites. Notably, the epoxy resin filled with fluorinated fillers has the deepest trap levels, providing an explanation for the improved insulation breakdown performance. The research can provide guidance for regulating dielectric properties of epoxy composites and material synthesis for the application of electrical insulation packaging <b>.</b> </sec>

Refining the physical description of charge trapping and detrapping in a transport model for dielectrics using an optimization algorithm

PurposeThe purpose of this paper is to optimize and improve a bipolar charge transport (BCT) model used to simulate charge dynamics in insulating polymer materials, specifically low-density polyethylene (LDPE).Design/methodology/approachAn optimization algorithm is applied to optimize the BCT model by comparing the model outputs with experimental data obtained using two kinds of measurements: space charge distribution using the pulsed electroacoustic (PEA) method and current measurements in nonstationary conditions.FindingsThe study provides an optimal set of parameters that offers a good correlation between model outputs and several experiments conducted under varying applied fields. The study evaluates the quantity of charges remaining inside the dielectric even after 24 h of short circuit. Moreover, the effects of increasing the electric field on charge trapping and detrapping rates are addressed.Research limitations/implicationsThis study only examined experiments with different applied electric fields, and thus the obtained parameters may not suit the experimental outputs if the experimental temperature varies. Further improvement may be achieved by introducing additional experiments or another source of measurements.Originality/valueThis work provides a unique set of optimal parameters that best match both current and charge density measurements for a BCT model in LDPE and demonstrates the use of trust region reflective algorithm for parameter optimization. The study also attempts to evaluate the equations used to describe charge trapping and detrapping phenomena, providing a deeper understanding of the physics behind the model.

Equivalent transmission line characterization and multi‐layer material measurement analysis of the signal conversion process in the pulsed electro‐acoustic method

AbstractThis paper studies the equivalent transmission line model of the pulsed electro‐acoustic (PEA) method with its applications. Based on the consistency of acoustic wave behaviour inside lossy acoustic materials and voltage wave propagation in transmission line, an equivalent simulation model of the PEA system is developed, whose reliability is verified by the output from the transducer and amplifier models and the comparison with measured waveforms. For the problem of acoustic impedance mismatch between different modules, simulation indicates that the unequal impedances of semiconducting electrode and sample can affect the amplitude of the measured signal at the upper electrode side, and the reflected acoustic waves caused by the transducer can affect the charge waveform. Further simulation for multi‐layer materials finds that the reflected acoustic waves of different samples and sound absorbing module can superimpose on the charge signal. Accordingly, a selection criterion is proposed to avoid the effect of the reflected waves at the interface. As for the acoustic reflection caused by internal charge, it needs to be dealt with sequentially in calibration process, starting from the result inside the sample near ground electrode. The research can provide a foundation for analyzing the acoustic properties of the PEA method.

Electrostriction effects in space charge measurements with the pulsed electroacoustic method for ceramics

The pulsed electroacoustic (PEA) method is an established method for space charge measurements in polymeric dielectrics. In view of the poorly understood impact of space charge on the electrical resistivity and the dielectric breakdown behavior of ceramics, it is desirable to adapt the PEA technique to these materials. However, this adaption is non-trivial due to the constitutive properties of ceramics, which are, at least in part, very different from that of polymers. This contribution addresses a particular effect related to the electrostrictive properties of ceramics on the theoretical level. It is shown that these properties may cause an inversion of the sign of the sound wave generated by electrical voltage pulses when compared to typical polymers, which may in turn result in an incorrect interpretation of the measurement results. Using this finding, a reinterpretation of previous experimental results suggests that homo charge forms at the cathode in sheets of alumina ceramics subjected to high voltages.

Relevancy of Pulsed Electroacoustic Measurements for Investigating Spacecraft Charging

The magnitude and spatial distribution of charge embedded in dielectric materials and the evolution of the charge distributions with time are paramount for the understanding and mitigation of spacecraft charging. Spacecraft materials are charged primarily by incident fluxes of low-energy electrons, with electron fluxes in the 10–50 keV range often responsible for the most deleterious arcing effects. While the pulsed electroacoustic (PEA) method can provide sensitive nondestructive measurements of the internal charge distribution in insulating materials, it has often been limited for spacecraft charging applications by typical spatial resolutions of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\le $</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> m, with a 10- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> m range of electrons in common spacecraft materials (e.g., polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), low-density polyethylene (LDPE), or SiO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2})$</tex-math> </inline-formula> at incident energies from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 20 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 40 keV. A series of PEA tests over a range of incident electron energies were devised to investigate the relevance of the PEA method for typical spacecraft charging applications. Thin-film samples of vacuum-baked PEEK were irradiated with 10–80-keV monoenergetic electron beams. PEA measurements of deposited charge profiles determined the peak positions and magnitude of deposited charge. These were used to establish the minimum incident energies for which PEA measurements provided meaningful results and thus to characterize the merits of PEA measurements over energy ranges of relevancy to spacecraft charging issues.

Basic study on calibration using waveform variation in space charge measurement by pulsed electroacoustic method

AbstractThe pulsed electroacoustic method determines the distribution of space charge in an insulation system by applying a pulse voltage and detecting the acoustic response. It estimates the location and magnitude of charges from time‐domain signals. For quantitative analysis, a bias voltage is applied to a specimen with no space charge, and the acoustic signal generated from the electrodes only is used as a reference waveform for calibration. However, a full‐scale, extra‐high voltage insulation system often already contains space charges in it. Therefore, a method of acquiring a reference signal under such a condition was studied. We studied methods to compare voltage variations and response signal variations. We proposed a method that measures the signal difference at different voltages. In addition, we proposed a method of correlating voltage and signal variations that may allo for more rapid signal acquisition. The feasibility was proved using an 11 mm‐thick mimic polyethylene specimen.

Basic Study on Calibration using Waveform Variation in Space Charge Measurement by Pulsed Electroacoustic Method

The pulsed electroacoustic method determines the distribution of space charge in an insulation system by applying a pulse voltage and detecting the acoustic response. It estimates the location and magnitude of charges from time-domain signals. For quantitative analysis, a bias voltage is applied to a specimen with no space charge, and the acoustic signal generated from the electrodes only is used as a reference waveform for calibration. However, a full-scale, extra-high voltage insulation system often already contains space charges in it. Therefore, a method of acquiring a reference signal under such a condition was studied. We studied methods to compare voltage variations and response signal variations. We proposed a method that measures the signal difference at different voltages. In addition, we proposed a method of correlating voltage and signal variations that may allow for more rapid signal acquisition. The feasibility was proved using an 11 mm-thick mimic polyethylene specimen.

Comparison of Pulsed Electroacoustic Measurements and AF-NUMIT3 Modeling of Polymers Irradiated with Monoenergetic Electrons

Successful spacecraft design and charging mitigation techniques require precise and accurate knowledge of charge deposition profiles. This paper compares models of charge deposition and transport using a venerable deep dielectric charging code, AF-NUMIT3, with direct measurements of charge profiles via pulsed electroacoustic (PEA) measurements. Eight different simulations were performed for comparison to PEA experiments of samples irradiated by 50 or 80 keV monoenergetic electrons in vacuum and at room temperature. Two materials, polyether-ether ketone (PEEK) and polytetrafluoroethylene (PTFE), were chosen for their very low conductivities so that minimal charge migration would occur between irradiation and PEA measurements. PEEK was found to have low acoustic attenuation, while PTFE has high acoustic attenuation through the sample thicknesses of 125 and for each material. The measurements were directly compared to AF-NUMIT3 simulations to validate aspects of the code and to investigate the importance of various simulation options, as well as to characterize the PEA instrumentation, measurement methods, and signal processing used. The measurement and simulation values for magnitude of charge deposition, penetration depth, and charge deposition spatial profiles are largely in agreement, though spatial and temporal distributions in incident electron flux and effects of radiation-induced conductivity (RIC) and delayed RIC during the deposition process complicate the process. This work provides an experimental validation of the AF-NUMIT3 deep dielectric charging code and insight into the accuracy and precision of the PEA method.

Characteristic of acoustic waves generated in dielectrics under divergent electric fields

Space charge measurement under divergent electric fields can provide crucial insight into various insulating phenomena, and the space charge reconstruction relies on the relation between the measured acoustic wave and the space charge distribution if the pulsed electroacoustic method is used. However, the relation is still unclear. In this work, the characteristic of acoustic waves generated by a divergent electric field is studied. Three components of electrostatic force leading to acoustic waves, which are surface force, internal Coulomb force, and electrostrictive force, are considered. The relation between the acoustic wave and these three components is provided, and the acoustic wave characteristic is, therefore, investigated. The results show that the acoustic wave exhibits two opposite peaks for each component due to the local distribution of the force, which is different from that with a uniform electric field, and the combined waveform is greatly influenced by the contribution of electrostrictive force, making it challenging to extract space charge information.

Correspondence Between Charge Accumulated in Voids by Partial Discharges and Mapping of Surface Charge With PEA Detection

Partial discharge (PD) dynamics in gaseous inclusions are related to the memory effects that originate from surface charge accumulation. In this article, the correspondence between a deposited charge that is inside a void during the PD activity with the surface potential mapping and space charge quantity that are obtained through the pulsed electroacoustic (PEA) method was investigated. The experiments were performed in the configuration of an asymmetric void that is embedded in a polyethylene (PE) material. Since the air gap in a void blocks acoustic wave propagation, the measurements were focused on one dielectric void wall (opposite to the metallic electrode). It was discovered that the polarity of the last period in a PD sequence influences the polarity of a deposited charge. Two types of readout pulsers were investigated and compared with the PEA method. In the surface charge decay process, a faster rate for negative charges was noticed as compared with positive ones. The experimental results showed a good quantitative correspondence between a real charge that was deposited by a PD during the phase-controlled sequence with an accumulated charge value that was obtained from surface mapping as well as charge detection—both being performed by means of the PEA method on the same layer of a void wall. The presented experimental results and analysis help extend the insight into PD mechanisms and quantitative surface charge accumulations.

Space Charge Characteristics of Natural Ester Oil-Impregnated Paper With Different Moisture Contents

Mineral oil is the most common dielectric liquid for high-voltage direct current (HVDC) converter transformers. However, natural ester oil receives much attention as an alternative dielectric liquid due to its environmental friendliness and low fire risk. Moisture is one of the most dangerous substances, which accelerates the degradation of oil-paper insulation systems. Thus, this article investigates the impact of moisture on space charge characteristics of the natural ester oil-impregnated paper by comparing it with the mineral oil-impregnated paper. Space charge accumulation is a typical phenomenon under dc conditions, and its presence can lead to electric field distortion and accelerate insulation aging. The space charge profiles in oil-impregnated papers with different moisture contents are obtained by the pulsed electroacoustic (PEA) method. It has been found that the hydrophilic characteristic of natural ester oil leads to a slow process of forming heterocharges in the natural ester oil-impregnated paper. On the other hand, the hydrophobic characteristic of mineral oil results in faster heterocharge formation in the mineral oil-impregnated paper.

Charge Carrier Injection Characteristics of Semiconductive and Metal Electrodes in XLPE

In this article, a method using a barrier layer to realize electrode unipolar charge carrier injection is proposed to study the injection characteristics of positive and negative charge carriers by different electrode materials. The space charge distribution in XLPE/polytetrafluoroethylene (PTFE) was measured by the pulsed electroacoustic (PEA) method, and the relationship between the accumulation rate of injected charge carriers and electric field strength at the electrode interface was further established. It was found that the aluminum (Al) electrode had a greater charge injection capability than the semiconductive layer, while both were stronger than that of the gold (Au) electrode. The combined analysis of space charge measurement results and theoretical model verified the accuracy of the relationship between the accumulation rate of injected charge carriers and electrode field to characterize charge injection behavior. Space charge measurement in single-layer XLPE samples verified its validation. This study provides techniques and theoretical guidance for the testing and development of semiconductive materials in XLPE direct-current (dc) cables.

Effects of Oil Flow on Space Charge Behaviors in Oil–Paper Composite Insulation

Although the circulating flow of insulating oil can enhance heat dissipation in converter transformers, it can also influence the space charge distribution in oil–paper composite insulation. In this article, the oil–paper composite insulation between flat electrodes was used as the experimental model. Based on the pulsed electroacoustic (PEA) method, we investigated the effect of uncharged flowing oil on the space charge distribution in composite insulation under a dc electric field and polarity reversal. Under a dc electric field, as the oil flow velocity increased, the amount of interface charge between the oil and paper first increased and then decreased. During the polarity reversal process, the field distortion was aggravated at slower oil flows. These experimental results were explained in terms of the effect of oil flow on ionized charges in the oil.

Comparison of pulsed electroacoustic and thermally stimulated depolarization current measurements of thermally poled PET electrets

We have compared measurements of a set of polyethylene terephthalate (PET) electret samples by means of pulsed electroacoustic method (PEA) and thermally stimulated depolarization current (TSDC) techniques. Samples where the α or the ρ relaxation is activated show uniform polarization and image charge at the electrodes. Different electrode configurations have been tested when external charge carriers are injected into the sample. In these experiments, differences in TSDC spectra are explained by the different blocking behavior of the electrodes. All in all, PEA in combination with TSDC turns out to be a useful technique in the study of thermally poled electrets.

Intelligent model prediction of fluctuant increase of maximum electric field in XLPE insulation using long short‐term memory network algorithm

The electric field distortion due to space charge accumulations plays a significant role in the ageing, degradation and breakdown in failure of HVDC power cables. Currently, limited experimental results of the electric field dominated by space charges are insufficient to diagnose the power cables. This paper proposes an improved long short-term memory network (LSTM) model for predicting the fluctuating maximum electric field (Emax) in cross-linked polyethylene (XLPE) cable insulation. The various Emax data derived from the complex space charge behaviours were measured using the pulsed electroacoustic method. The model uses regularisation and dropout feedback in the LSTM unit, reducing the phenomenon of over-fitting due to the limited data. It enhances the prediction accuracy and ability of long time prediction by improving the prediction of Emax with the non-linear fluctuation. The predicted Emax approaches 190 kV/mm under 150 kV/mm and 60°C after 2 h. The predicted large variation in Emax under 120 kV/mm and 20°C after 4 h ranges from 130 to 160 kV/mm. It indicates high electric stress in the cable insulation during continuous operation. The proposed LSTM model is of great importance to guide the diagnosis of cable degradation in HVDC power cables.

Estimation Accuracy of the Electric Field in Cable Insulation Based on Space Charge Measurement

Space charge measurement accuracy is crucial when assessing the suitability of cables for high-voltage direct current (DC) systems. This study assembled state-of-the-art analysis technologies, including time-domain deconvolution, to mark electric field estimation accuracy, which the present techniques achieve. The pulse electroacoustic method was applied to a 66 kV-class extruded cable, and waveforms were obtained and analyzed to reproduce the electric field distribution. The DC voltage was set to be sufficiently low so that the analysis results can be compared with Laplace’s equation. The statistical analysis of 81 waveforms under a DC voltage of 30 kV showed that the estimation accuracy was −0.3% ± 19.9% with a 95.4% confidence interval, even with the deconvolution parameter optimized. The estimated accuracy using the “reference” waveform is applied to waveforms at higher voltages since similar estimation accuracies were confirmed for waveforms obtained under a DC voltage of 45 kV.