Pulsed Electroacoustic System Research Articles

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.

A Simple Method for Determining Shallow Charge Distributions in Dielectrics via Pulsed Electroacoustic Measurements

The understanding of charge dynamics in dielectric materials is paramount in mitigating electrostatic discharge events for spacecraft. The most critical spacecraft charging events are found to result from incident electrons in the energy range of 10 keV to 50 keV. The charge embedded in dielectric materials in this energy range are deposited a distance into the material on the order of a few to tens of microns. One way to measure and understand the deposited charge is via pulsed electroacoustic measurements (PEA). However, the typical PEA spatial resolution of ~ 10 μm is not sufficient to resolve or discern charge deposited at the lower end of this incident electron energy range, where deposited charge distributions are obscured by the superposition of the signal originating from induced mirror charge on the electrode of the pulsed electroacoustic system. A simple method is proposed and demonstrated in which reference measurements from a pristine sample are used to separate the effect of the induced mirror charge from the measured embedded charge to obtain a more accurate determination of the deposited charge distribution.

Space Charge Characteristics in Epoxy Resin Impregnated Paper under AC and AC-DC Superimposed Electric Fields

In this paper, charge distributions in three-layered epoxy resin and epoxy resin impregnated paper samples under AC and AC-DC superimposed electric fields are measured by the improved high-speed pulsed electro-acoustic system. To exclude the interferences caused by the discontinuity of relative permittivity, a method to distinguish between net dipolar charges and free charges is proposed. Through analysis, the influences of frequency, AC component and DC component on the charge transport and trapping characteristics in multi-layered epoxy composites are obtained, which could provide important reference to the structural design and optimization of converter transformer bushings.

Analysis of Attenuation and Dispersion of Acoustic Waves in Low-Density Polyethylene

Acoustic attenuation of a pulsed electroacoustic (PEA) signal is theoretically studied using the Kelvin-Voigt model for viscoelastic materials. Starting with the basics of viscoelasticity, we apply the model to an analysis of acoustic wave propagation in PEA systems. In experiments, low-density polyethylene films with three different thicknesses are used as PEA samples to verify the model. The transfer functions of the polymers in the PEA system are obtained on the basis of the viscoelastic theory with which we can calculate acoustic attenuation in polymer films. The calculated acoustic waveforms show good agreement with the measured one. Furthermore, we also demonstrate the waveform recovery using the transfer function.

Direct Measurement of Accumulated Space Charge Using External Currents

In the past few decades, immense contributions have been put in by researchers to measure the accumulation of space charge in dielectric materials. At the same time, the direct measurement of the amount of space charge in materials has been completely neglected, probably due to the difficulties in measuring currents at high-voltage (HV) electrode. Although the amount can be measured indirectly with the available methodologies, the measured results suffer from accuracy issue. Also, indirect measurements involve complex, post measurement mathematical analysis such as deconvolution and filtering to get the amount of accumulated charge. Keeping the lacunas in mind and contrary to the basic approach of the present methodologies, a novel wireless system is developed which separately monitors the ultralow, volumetric, electrode currents flowing into and out of the material at both HV and ground electrodes. The obtained current data are further processed to observe the accurate temporal variation in the accumulated charge using the fundamental concept of charge conservation law. Some more parameters have also been identified which unmistakably determine the presence of the packet-like-charge phenomenon in dielectric materials such as low-density polyethylene. Furthermore, for comparing the results of the proposed electrode system, space charge measurements have been done using pulsed electroacoustic system. Based on the comparison of the results, it is concluded that the newly developed electrode system monitors the charge accumulation closely but with less effort and complexity.

A new numerical approach to the calibration and interpretation of PEA measurements

In this paper, a new approach for interpreting raw pulsed electroacoustic (PEA) signals is proposed. The theory behind acoustic wave propagation in ideal (no attenuation and no dispersion) materials and in real materials (with attenuation and dispersion) initially free of space charge is described. A simulation model has been developed for the following: (1) the acoustic signal formation in the PEA apparatus, the transmission of the acoustic waves, their attenuation/dispersion and detection; (2) the instrumental effects of having a capacitive piezoelectric sensor driving a 50 Ω input impedance amplifier. The various layers of a PEA system were considered in the model and the effect of each layer on the acoustic wave propagation is analyzed. Since the model allowed raw PEA data to be simulated, it can be used to identify potential sources of error in interpreting real PEA measurement data, such as acoustic mismatch between PEA layers, electrode material effects, pulse voltage, etc. The results showed good agreement between the simulated and experimentally obtained data in the case of space charge free samples.

Crystallization morphology and space charge property of silica/low density polyethylene composites

The accumulation of space charge in polymer insulation limits the development of high-voltage direct current transmission and reduces the service lifetime of power equipment. The electrical properties of polymers are affected greatly by their crystalline morphology. In order to study the effect of silica on the crystalline morphology and space charge characteristics of low density polyethylene (LDPE), nano-SiO2/LDPE composites, micro-SiO2/LDPE composites, and nano-/micro-SiO2/LDPE composites were prepared by melt blending. The silica distributions were characterized by scanning electron microscopy. Fourier transform infrared spectrophotometry was used to determine the chemical structure of the silica, LDPE, and silica/LDPE composites. The crystalline morphology of the LDPE and composites was investigated with an atomic force microscope and by differential scanning calorimetry. The space charge distribution of the LDPE and composites was measured with a pulsed electro-acoustic system. The results show that the introduction of silica particles did not evolve the chemical structure of LDPE. Compared with that of pure LDPE, the crystallinity of the composites increases with the decrease in the grain size. Moreover, the silica/LDPE composites exhibit the ability to inhibit the space charge. The improvement of space charge in the composites was attributed to the traps existing on the interfaces between the silica particles and LDPE (crystalline and amorphous).

An improved pulsed electroacoustic system for space charge measurements under high temperatures and arbitrary waveform voltages

This paper presents the design and calibration method of an improved pulsed electroacoustic (PEA) system for space charge measurements, including a novel nanosecond pulse generator and an improved electrode system, which can be used under various temperatures and arbitrary waveform voltages. The measurement principle of the PEA system is first analyzed, and the calibration method for the applied voltage is proposed to solve the problem of waveform distortion caused by voltages with different frequency components. For the pulse generator, the main parameters of output pulses are an amplitude of ∼700 V, a full width of 4.6 ns, a jitter of 0.24 ns, and a repetition rate of 10 kHz on the 50 Ω load. For the electrode system, the flexible upper electrode design is first introduced to achieve consistently close contact between the sample and the electrode. Besides, the hybrid heating method and the replaceable coupling capacitor and high voltage electrode head are adopted. Space charge measurements are conducted at different conditions using this PEA system, which includes different temperatures and different voltage waveforms. The experimental results verify the effectiveness of the calibration method and the feasibility of the improved PEA system.

Space charge characteristics in 160 kV DC XLPE cable under temperature gradient

A temperature gradient (TG) will be formed across a high voltage cable insulation resulting from Joule heat in the conductor. Therefore, it is important to measure space charge profiles in a full-scale cable under TG. In this study, an improved pulsed electro-acoustic system used for coaxial cables was established by injecting nanosecond pulses into the outer semi-conductive layer. An induced current heating and a water circulation system was used to regulate the temperatures of a cable conductor and the the outer semi-conductive layer. By this method, a TG between the cable conductor and the outer semi-conductor layer was controlled and set at various temperatures and the TG was formed across the radial insulation so as to avoid the influence of heat from the cable conductor. Space charge profiles in a 160 kV DC coaxial XLPE cable was measured under 10 kV/mm stress with polarity reversal. According to CIGRE TB496, the load cycle operation of the cable under 15 kV/mm with temperature difference of 40°C was performed for 48 and 72 h, including cable conductor cooling and heating. The results show that there is little space charge injection in the bulk at room temperature, but some remnant hetero-charges accumulates near the outer semi-conductive layer during voltage reversal. Under TG, hetero-charges appear near the outer semi-conductive layer on the low temperature side and hetero-charges increased with increasing TG. In addition, the hetero-charges increase during cable conductor heating and decrease during cooling.

Analytical Insights Into Parameter Estimation for Wiener Deconvolution

All real-time signals observed from various measurement systems require signal-processing techniques like the deconvolution, to compensate for the effect of transfer function of the systems. Wiener deconvolution is a widely used signal-processing technique for signal restoration. Often, if the power spectral density of signal and noise is unknown, the accuracy of the restored signal depends on an unknown filter parameter. In the literature, time-consuming, iterative, computational methods were reported to estimate the parameter. However, many of these methods give a range of optimum values instead of an unique value and often lead to either over-smoothing or under-smoothing. In this paper, novel analytical expressions are presented through which the unknown parameter can be estimated explicitly. The analytical results of this study are compared with the numerical methods, and they are found to be accurate and robust against numerical evaluation. Further, the results are demonstrated for signals obtained experimentally from the pulsed electroacoustic system and a network, in authors’ laboratory.

High-resolution pulsed electro-acoustic (HR PEA) measurement of space charge in outer space dielectric materials

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.

PEA System Modeling and Signal Processing for Measurement of Volume Charge Distributions in Thin Dielectric Films

This paper discusses our effort to develop advanced pulsed electroacoustic (PEA) measurement system capabilities that incorporate: 1) improved signal processing tools for increased signal/noise ratios and 2) integrated PEA modeling tools. In addition, we emphasize the state-of-the-art system electronic components, integrated environmental controls, and sensor improvements required to achieve high spatial resolution while maintaining reasonable temporal resolution for both ambient and in vacuo measurements of thin dielectrics charged using electron beam injection, which is most applicable for spacecraft charging tests. PEA measurement systems provide an important tool to investigate the spatial extent and dynamic evolution of embedded charge distributions in thin dielectric materials. This knowledge has important applications in spacecraft industries, as well as for semiconductors, high-power electronic devices, high-voltage dc power cable insulation, and high-energy and plasma physics apparatus. The emphasis of this paper is on improved signal processing methods and integrated PEA modeling tools.

Recovery algorithm for space charge waveform under temperature gradient in PEA method

This paper describes a new recovery algorithm for space charge measurement under temperature gradient using the pulsed electro-acoustic (PEA) method. As the temperature difference exists, the density of media becomes inhomogeneous and the waveforms gained by the PEA system are distorted seriously. In this study, the effect of temperature gradient on the acoustic propagation is discussed firstly, and an extended general solution for the acoustic propagation equation is gained. Then, the system transfer function is improved by adding the quadratic approximation to the general solution of the general wave propagation equation. With this new recovery algorithm, the distortion of the waveform can be successfully suppressed. In addition, the PEA waveform recovery method for coaxial cables under temperature gradient is also discussed.

エナメル線の被覆用絶縁材料中に蓄積する空間電荷の分布測定

Space charge distribution in a thin insulating layer of an enameled wire for motor windings was carried out using an improved PEA (pulsed electro-acoustic) measurement system under a dc and a square shape voltage. When a high voltage is applied to the windings of motor, a PD (partial discharge) sometimes occurs. Since the PD gives a physical damage to the insulating layer, many researchers have investigated the mechanism of PD generation. From their results, some researchers have pointed out the effect of space charge accumulation. Therefore, measurements of the space charge distribution in the insulating layer of the winding are required. The space charge accumulation in the insulating material is usually carried out using the PEA method. In this method, we observed the space charge distribution in film shape samples. However, the properties of specially-made film samples are different from those of the insulating layer with the actual configuration of the winding wire, and it is difficult to evaluate the actual influence of the space charge to the motor windings. In this research, we have improved the PEA system which is enable to measure the space charge distribution in the actual insulating layer of the winding wire.

Electrical Breakdown Properties and Space Charge Formation in High Temperature Region in Ultraviolet Ray Irradiated PVC

SUMMARYPolyvinyl chloride (PVC) is the most popular insulating material for electric wiring instruments. However, an exothermic reaction above 150 °C may cause deterioration of the insulating properties of PVC. Therefore, it is important to clarify the heat degradation in PVC, not only to investigate the ignition of electrical wiring products but also to use electrical products safely. It is known that ultraviolet (UV) irradiation causes chemical deterioration of PVC and an increase in its conductivity. Generally, it has been thought that the electrical breakdown properties, electrical conduction, and insulating performance are affected by space charge accumulation in an insulating material. A high temperature pulsed electroacoustic (PEA) system usable up to 250 °C has been developed, and the PEA system can measure the space charge distribution and conduction current in the high temperature range simultaneously. In this investigation, the space charge distribution and conduction current were measured up to electrical breakdown in a non‐UV irradiated sample (normal PVC) and in 353 nm and 253 nm UV‐irradiated PVC samples in the range from room temperature to 200 °C in a DC electric field. In the short wavelength UV irradiated PVC sample (253 nm, 300 h), a deterioration of breakdown strength at 90 °C to 150 °C and negative packet‐like charges were observed at 60 °C and 100 °C, a positive charge accumulated in front of both the anode and cathode above 90 °C, and a higher electric field near the cathode side because the positive charge of the cathode side was greater.

Space Charge Distribution Measurement in Insulation Film with Thickness of Several Tens .MU.m

Since electronic equipment is required to be smaller and lighter, an insulating material for such equipment is consequently required to be thinner. However, under a certain DC voltage, the electric field in it becomes relatively higher according to the insulating material becomes thinner. Therefore, the insulating performance under high electric field must be important in such thin films. On the other hand, it is said that a space charge accumulation in them under DC high electric field is important because sometimes an electrical breakdown occurs in them due to an enhancement of electric field induced by the accumulated space charge. To investigate the characteristics of the space charge accumulation in them, PEA (Pulsed Electroacoustic) method is usually used. However, a spatial resolution of ordinary PEA system is not enough to observe the space charge distribution in thin films with thickness of several tens μm. Therefore, a new measurement system with high positional resolution is developed by making a thin piezo-electric film that is used as a sensor for PEA system. The obtained resolution of the developed system is 4 μm, and it is used for a typical measurement in LDPE film with thickness of 40 μm.

Space charge dynamic at the physical interface in cross-linked polyethylene under DC field

In this paper the behavior of positive charge formed in the vicinity of the Semi-conducting sheet anode at room temperature and under applied dc has been investigated just after the reversal of the sample, and when the sample was combined with another sample without pre-stressing using the pulsed electro acoustic (PEA) method. The results suggest that the PEA system can only show the net charge density, and the space charge distribution is drastically changed when the sample is reversed just after the removal of the applied voltage (depolarization), or when this sample was combined with another without prestressing. A positive charge injected at the dielectric interface XLPE/XLPE under electric field (polarization) and high temperature has been paid much attention. It is found that the positive charge injection takes place under all the test conditions and this charge can be affected by the electric field and/or the temperature. Physical interfaces constituted by the association of two identical dielectrics store the positive carriers for long time.

Morphology Effects on Space Charge Characteristics of Low Density Polyethylene

Low density polyethylene (LDPE) film samples with different morphology were prepared by three kinds of annealing methods which were different in cooling rates in this study. A pulsed electro-acoustic (PEA) space charge measurement system was improved to solve the surface discharge problems for small samples applied with a high voltage. Negative direct current (DC) fields from 50 to above 220 kV/mm were applied to the samples. The influences of morphologies on space charge and space charge packet characteristics were measured by the improved high voltage withstand (HVW) PEA system. Mobility and trap depth of released charges were calculated by space charge decay. It was found that there is a different probability of space charge packet initiation under applied field from -60 to -100 kV/mm. Average velocity and mobility of the space charge packets were calculated by space charge packet dynamics. It was found that the lower cooling rate samples have higher crystallinity, more homo-charge accumulation, lower mobility and deeper trap depth. The mechanism of morphological effects on space charge phenomena have been presumed to give a plausible explanation for their inherent relationships. The morphology in the metal–dielectric interface and in the bulk is convincingly suggested to be responsible for the injection and propagation processes of space charge. A model of positive space charge initiation in LDPE samples was also suggested and analyzed. The mechanism of morphological effects and the charge injection model are well fit with the injection and propagation processes of space charge. The different effects of morphology in the metal–dielectric interface and in the bulk of polymers are stressed.

Morphology Effects on Space Charge Characteristics of Low Density Polyethylene

Low density polyethylene (LDPE) film samples with different morphology were prepared by three kinds of annealing methods which were different in cooling rates in this study. A pulsed electro-acoustic (PEA) space charge measurement system was improved to solve the surface discharge problems for small samples applied with a high voltage. Negative direct current (DC) fields from 50 to above 220 kV/mm were applied to the samples. The influences of morphologies on space charge and space charge packet characteristics were measured by the improved high voltage withstand (HVW) PEA system. Mobility and trap depth of released charges were calculated by space charge decay. It was found that there is a different probability of space charge packet initiation under applied field from -60 to -100 kV/mm. Average velocity and mobility of the space charge packets were calculated by space charge packet dynamics. It was found that the lower cooling rate samples have higher crystallinity, more homo-charge accumulation, lower mobility and deeper trap depth. The mechanism of morphological effects on space charge phenomena have been presumed to give a plausible explanation for their inherent relationships. The morphology in the metal–dielectric interface and in the bulk is convincingly suggested to be responsible for the injection and propagation processes of space charge. A model of positive space charge initiation in LDPE samples was also suggested and analyzed. The mechanism of morphological effects and the charge injection model are well fit with the injection and propagation processes of space charge. The different effects of morphology in the metal–dielectric interface and in the bulk of polymers are stressed.

Effect of Conductive Inorganic Fillers on Space Charge Accumulation Characteristics in Cross-linked Polyethylene

We have observed space charge profiles in cross-linked polyethylene (XLPE) under dc high electric field using the PEA (pulsed electro-acoustic) system to study the relationship between space charge behavior and dielectric breakdown. In our previous research work, we have found that a large amount of, so called, packet-like charge generates in low density polyethylene (LDPE) under high dc electric field of more than 100 kV/mm. The packet-like charge enhances the electric field locally in bulk of the sample, and then finally it leads a breakdown. On the other hand, a new type of XLPE which was made through adding conductive inorganic fillers, shows a good dc dielectric breakdown characteristic and high volume resistivity under dc stress. In this report, we tried to observe the space charge behavior under high dc electric field in this material. From the results, it is found that the charge injection is effectively suppressed by adding only a small amount of conductive inorganic fillers to XLPE.