Pulsed Electro-acoustic Research Articles

Evolution laws of charge mobility of transformer oil and pressboard material with temperature and electric field strength

Abstract Charge mobility, a pivotal parameter in the space charge analysis model for oil-pressboard insulation of converter transformer under DC electric fields, predominantly relies on theoretical or empirical data. The prevalent omission of empirical, measurement-based evaluations, combined with the neglect of temperature and electric field intensity effects, introduces uncertainties in the calculation of electric field and charge dynamics. Grounded on the principles of the electroacoustic pulse technique for space charge measurements, our investigation established a temperature-controlled platform to determine the charge-charge mobility in oil and pressboard insulation mediums. Utilizing diverse oil-pressboard insulation systems, this study identified the temporal patterns of both positive and negative charge mobilities and unravelled the mechanisms modulated by temperature and electric field intensity: 1) With increasing temperatures (from 20°C to 80°C) and intensifying electric field strengths (from 5kV/mm to 25kV/mm), the charge mobility within transformer oil and pressboard exhibits a rising trend. Notably, temperature variations exert the most significant influence, yielding amplifications up to 24.8 times. 2) The intrinsic properties of different transformer oils and oil-impregnated pressboards result in pronounced differences in their charge transport behaviours. Specifically, naphthenic-based oil demonstrates enhanced charge mobility relative to its paraffin-based counterpart. In contrast, transformer oils with higher kinematic viscosities exhibit diminished charge mobility compared to those with lower viscosities. 3) A linear relationship is observed between the oil absorption rate and electrical conductivity, aligning closely with the insulating pressboard's charge mobility. The evaluation of electric field and interface charge dynamics in oil-pressboard insulation under both DC and polarity-reversed conditions revealed that the electric field decay in naphthenic-based oil is more rapid than in paraffin-based oils. Moreover, both the magnitude and rate of interface charge accumulation in naphthenic-based oils exceed those in paraffin-based variants, further validating the accuracy of our charge mobility measurements and ensuing analysis.

Study on the Relationship between Electron Transfer and Electrical Properties of XLPE/Modification SR under Polarity Reversal.

The insulation of high-voltage direct-current (HVDC) cables experiences a short period of voltage polarity reversal when the power flow is adjusted, leading to sever field distortion in this situation. Consequently, improving the insulation performance of the composite insulation structure in these cables has become an urgent challenge. In this paper, SiC-SR (silicone rubber) and TiO2-SR nanocomposites were chosen for fabricating HVDC cable accessories. These nanocomposites were prepared using the solution blending method, and an electro-acoustic pulse (PEA) space charge test platform was established to explore the electron transfer mechanism. The space charge characteristics and field strength distribution of a double-layer dielectric composed of cross-linked polyethylene (XLPE) and nano-composite SR at different concentrations were studied during voltage polarity reversal. Additionally, a self-built breakdown platform for flake samples was established to explore the effect of the nanoparticle doping concentration on the breakdown field strength of double-layer composite media under polarity reversal. Therefore, a correlation was established between the micro electron transfer process and the macro electrical properties of polymers (XLPE/SR). The results show that optimal concentrations of nano-SiC and TiO2 particles introduce deep traps in the SR matrix, significantly inhibiting charge accumulation and electric field distortion at the interface, thereby effectively improving the dielectric strength of the double-layer polymers (XLPE/SR).

Significantly enhanced energy storage performance in multi-layer polyimide films with nano dielectric layer

Polymer-based composites with multilayer structure were emerging as an effective way to solve the contradiction between high breakdown strength and large permittivity in past decades. However, the incorporating layers in conventional multilayer composite films are usually inorganic thick films, which is not beneficial for the retention of excellent mechanical properties of the polymer matrix. In this work, the multilayer films consisting of micrometer polyimide (PI) and nanoscale TiO2 layer were prepared by spin-coating and atomic layer deposition (ALD), respectively. Carrier migration in PI-TiO2 multilayer films was hindered effectively by the interfacial barrier, as indicated by the results of the Pulsed Electro-Acoustic (PEA) tests. Compared to pure PI, both the permittivity, dielectric loss and breakdown strength were optimized in the PI-TiO2 multilayer films. A satisfactory discharge energy density of 6.77 J cm−3 (efficiency >90 %) was obtained in the multilayer films, which is about 5 times higher than that of pure PI. The strategy of constructing heterogeneous interfaces by incorporated nanoscale TiO2 layers is promising for designing high performance capacitors in practical application.

Influence of Thermal Aging on Space Charge Characteristics and Electrical Conduction Behavior of Cross-Linked Polyethylene Cable Insulation.

The aging of cable insulation presents a significant threat to the safe operation of cables, with space charge serving as a crucial factor influencing cable insulation degradation. However, the characteristics related to space charge and conduction current behavior during thermal aging remain unclear. This study focused on the thermal aging of cross-linked polyethylene (XLPE) material and utilizes a combined pulse electro-acoustic (PEA) and conduction current testing system to analyze the space charge and conduction current characteristics in the sample under varying electric fields and temperatures. The average charge density, short-circuit residual electric field, electric field distortion rate, and conduction current were studied. The findings indicate that the space charge in the samples following thermal aging is predominantly governed by the injected charge. The amorphous region of XLPE decreases, while the cross-linking degree increases after aging, thereby facilitating charge carrier migration within the sample and reducing the generation of charge carriers through thermal pyrolysis. The minimum temperature required for charge injection is reduced by thermal aging. Furthermore, modifications in conduction current, residual electric field, and average charge density indicate that thermal aging has the potential to alter the microstructure and trap characteristics of XLPE. This study provides empirical evidence to elucidate the underlying mechanism of cable insulation aging.

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.

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.

Effect of Li1.3Al0.3Ti1.7(PO4)3 on semiconductive composites: Positive temperature coefficient (PTC) performance and inhibition of space charge injection

The semiconductive layer located between the core and insulation layer of a high-voltage DC (HVDC) cable can inhibit the accumulation of space charge in the insulation layer. This helps increasing the electric field inside the cable, thus improving the stress distribution of the electric field on the inner and outer surfaces of the insulation layer and the electrical strength of the cable. An inevitable positive temperature coefficient effect (PTC) in the semiconductor layer leads to aging failure of the insulation layer. To improve this phenomenon, the semiconductive shielding layer was modified for suppressing its PTC effect and reducing the space charge injection into the insulation layer. Li1.3Al0.3Ti1.7(PO4)3(LATP) was prepared via a solid-phase reaction and added to a composite consisting of carbon black (CB), ethylene vinyl acetate copolymer (EVA), and low-density polyethylene (LDPE). Resistivity, thermally stimulated depolarization current (TSDC), and pulsed electroacoustic (PEA) measurements were performed on the modified sample. The results showed that when the LATP content was 4 wt%, the modification effect of the semiconductive shielding layer was most effective, the PTC effect was reduced by 11%, and the space charge injection into the insulating layer was improved significantly.

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.

PEA Signal Simulation Using Multiple Viscoelastic Relaxation Model for Determination of Acoustic Attenuation and Velocity Dispersion

Acoustic attenuation in polymer insulators prevents the accurate determination of space charge profiles measured by pulsed electroacoustic (PEA) measurements. A PEA signal simulation is developed on the basis of a multiple viscoelastic relaxation model, which can determine acoustic attenuation and sound velocity dispersion in polymer insulators. To implement the multiple relaxation model, we use an analogy between the viscoelastic and dielectric relaxations. The Kelvin–Voigt model is used as a single relaxation model, and the Cole–Cole model is used as a multiple relaxation model. The simulation based on the Cole–Cole model as a multiple relaxation model can accurately reproduce measured PEA signals. The distortion of a PEA measured space charge profile due to acoustic attenuation and velocity dispersion is also discussed. The charge profile calculated using the multiple relaxation model is in good agreement with the peak value, width, and tail shape obtained in experiments.

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.

Doping C60(OH) to Regulate the Crosslink Network and Energy Band Structures of Epoxy Resin and Allow for Electronic Directional Drive in C60(OH)/EP

The interfacial effect of nanoparticles can effectively regulate electron migration in epoxy composites. In this article, hydroxylated C60 [C60(OH)] was prepared and added to epoxy resin (EP). The inhibition of electron motion in epoxy composites was realized, and the dielectric strength of the epoxy composites was improved. The results of dynamic mechanical analysis (DMA) showed that C60(OH) regulated the structure of the crosslinked EP network and increased the crosslink density by 16.5%. The results of a density functional theory (DFT) simulation showed that the doping of C60(OH) formed a potential energy barrier at the C60(OH)-EP interface, which was conducive to the directional migration and capture of electrons driven to C60(OH). Electrostatic force microscopy (EFM) and pulsed electroacoustic (PEA) experiments showed that C60(OH) had a strong ability to capture charge, and the charge migration rate of C60(OH)/EP was 43% lower than that of pure EP in the presence of 10 kV/mm when the EP was doped with 0.1wt% C60(OH). The breakdown strength and volume resistivity increased by 17.87% and 222%, respectively. Thus, this article provides a new method to improve the dielectric strength of epoxy composites.

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.

Dynamic Simulation of Acoustic Wave Propagation and Attenuation in Low-Density Polyethylene by Recursive Convolution Finite-Difference Time-Domain Method

The dynamic simulation of pulsed electroacoustic (PEA) signals has been conducted by the recursive convolution finite-difference time-domain (RC-FDTD) method, in which acoustic attenuation in low-density polyethylene (LDPE) is calculated on the basis of viscoelastic theory. This method allows us to quantitatively analyze the distortion of the waveform in an LDPE film resulting from the acoustic attenuation. The RC-FDTD method can easily visualize the propagation, reflection, and attenuation of the acoustic waves in the LDPE film. Furthermore, the attenuated space charge profiles in the LDPE film are also calculated by the RC-FDTD method.

Understanding the Role of Soft X-ray in Charging Solid-Film and Cellular Electrets

Solid-film electrets and cellular electrets are defined as promising insulating dielectric materials containing permanent electrostatic and polarizations. High-performance charging methods are critical for electret transducers. Unlike dielectric barrier discharge (DBD) charging, the soft X-ray charging method, with its strong penetration ability, has been widely used in electrets after packaging and has even been embedded in high-aspect-ratio structures (HARSs). However, the related charging model and the charging effect of the soft X-ray irradiation remain unclear. In this study, the charge carrier migration theory and the one-dimensional electrostatic model were employed to build the soft X-ray charging models. The influence of soft X-ray irradiation under deferent poling voltages was investigated theoretically and experimentally. The conducted space charge measurement based on a pulsed electro-acoustic (PEA) system with a soft X-ray generator revealed that soft X-ray charging can offer higher surface charge densities and piezoelectricity to cellular electrets under the critical poling voltage lower than twice the breakdown voltage.

Space charge characteristics of silicone elastomer for device packaging under positive square wave voltage

AbstractWith the rapid development of power electronic devices towards high voltage and power, the insulation challenges faced by devices have attracted widespread attention. Silicone elastomer is widely used in device packaging as one insulating material, and the study of its insulating properties is helpful to the reliable operation of devices. To this end we obtained the space charge characteristics of silicone elastomer under positive square wave voltage by using the pulse‐electroacoustic (PEA) method. Firstly based on the existing research and IEC standards, the selection criteria of the resistance and capacitance parameters of a PEA system for space charge measurement under square wave voltage are given. Through this experimental system, the space charge distribution under the positive square wave voltage with different waveform parameters is obtained, and the influences of the waveform parameters on the space charge accumulation are analysed. The amounts of charges under the action of square wave voltage with different waveform parameters are calculated, and the influences of waveform parameters on the amounts of charges are analysed. In addition, the reasons for the difference in the accumulation of positive and negative charges are explained through Schottky injection model, the synthesis reaction and the infrared spectrum of the silicone elastomer.

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.