Pulsed Electro-acoustic Measurement Research Articles

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.

Constructing interfacial barrier from tribo-positive shell microcapsules to suppress space charge in thermochromic phase change composites for smart electronics

Thermochromic phase change composites (TPC) exhibit versatile functions under electrothermal stimuli, which are extensively applied as packaging, insulation and circuit components in considerable advanced smart electronics. However, space charge accumulation under DC voltages is still a critical and universal issue for the serious deterioration of functionalities and life-span of TPC and electronics. This study presents a novel strategy of suppressing space charge in thermochromic phase change epoxy composites (TPCE) by constructing interfacial barrier from tribo-positive shell thermochromic phase change microcapsules. The strong tribo-positive melamine-formaldehyde (MF) shell of microcapsules can establish negative charge barriers in the interfaces between microcapsules and epoxy matrix through triboelectrification, which is directly verified by Kelvin probe force microscopy. Pulsed electroacoustic measurement results exhibit the space charge accumulation in TPCE under an extreme electric field strength of 50 kV/mm and an elevated temperature of 70 °C can be significantly diminished by 39.3% compared to that of neat epoxy resin after incorporating merely 1 phr MF shell microcapsules, affirming the effect of interfacial barrier on space charge inhibition. The interfacial barrier could improve the charge carrier dissipation and mitigating the space charge accumulation as confirmed by hopping conduction analysis. Moreover, DC breakdown strength of TPCE can also be enhanced by 12.3% and 6.1% with 1phr microcapsules at 30 °C and 70 °C compared to that of neat epoxy resin owing to the suppressed space charge. The results provide a potential approach of designing high-performance thermochromic phase change composites for smart electrical and electronic devices.

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.

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.

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.

Effects of nanosecond impulse and step excitation in pulsed electro acoustic measurements on signals for space charge determination in high-voltage electrical insulation

The presence of space charges in high-voltage insulation can influence the operational stresses of the dielectric materials; the topic is present in various, even distant domains such as power grids or transportation segments that are represented by electromobility, rail, more-electric aircraft, and space systems. In this article, the effects of nanosecond impulse and step excitation in the pulsed electro-acoustic measurements (PEA) of space charges in high-voltage electrical insulation have been investigated. Various parameters of a pulser (such as impulse width, rise time, and sensor thickness) have been investigated by using a designed programable pulser. The investigations have been performed on a polyethylene sample in one- or double-layer configurations. In the latter, an interlayer sprinkled charge was analyzed. The experimental results have shown the main PEA effect during the transition phase of the applied voltage. It was observed that the pulse transition has a critical importance for stimulating space-charge vibrations and generating elastic waves. Hence, the slope’s rise and fall times are crucial in pulser design. The pulser width also has an impact on a signal’s magnitude as well as on its dispersion and attenuation. The correspondence between a pulse and step excitation was presented. Some advantages are attributed to the step-based excitation in terms of fewer wave reflections and interferences as well as the avoidance of the wavelet cancelation effect for a very narrow pulse width. Hence, step excitation can result in the easier postprocessing and interpretation of individual peaks in the acquired PEA signal. Another advantage may be recognizing the space-charge polarity instead of the net result. Important recommendations refer to the tuning of pulser timing settings with respect to signal attenuation (especially in thicker specimens) and the adequate adjustment of the sensor’s thickness. The presented analysis and experimental results extend the insight into PEA measurement – especially with a focus on diagnostic applications.

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.

Space Charge Measurement Equipment for Full-Scale HVDC Cables Using Electrically Insulating Polymeric Acoustic Coupler

A new, very sensitive device for space charge measurements on full-size cable is proposed. The measurement employs the basis of the pulsed electro-acoustic (PEA) method, in which an acoustic signal is produced due to the interaction between the pulsed electric field and the space charge. A polymeric material was used as an acoustic coupler that also isolates the cable and the measurement device electrically. It was found that the new type of device with polymeric coupler has a four times higher sensitivity than the conventional PEA device. The isolation from the cable body was proven to be advantageous for easily connecting the probing pulse voltage to the measurement section of the cable through its insulation screen (outer screen electrode) while applying a dc high voltage to the conductor. Feasibility of the new PEA device was examined using a cable with 20 mm-thick insulation. It was shown experimentally that PEA measurements performed at a voltage as low as 5 kV provided clear charge patterns. As a result of this, the accuracy of the electric field calculation was estimated to be 0.25 kV/mm or better, which leads to 2% of accuracy at 250 kV operating voltage.

Effect of LiCoO 2 on the positive temperature coefficient effect (PTC) properties of semiconductive composites and its ability to inhibit space charge injection

The semiconductive shield layer is between the conductive core and the insulation layer of high voltage direct current transmission cables and plays an important role in suppressing the accumulation of space charge in the insulation layer. Since the prevalent positive temperature coefficient effect (PTC) of the semiconducting layer leads to cable ageing and failure, this paper proposes the use of lithium cobaltate to modify the semiconducting shield to suppress its PTC effect and improve its ability to inhibit space charge injection. The ionic conductor LiCoO2 was prepared by the sol–gel method. LiCoO2 particles with particle sizes ranging from tens to hundreds of nanometres were obtained by ball milling. The prepared LiCoO2 was used as a filler to modify the carbon black (CB)/ethylene vinyl acetate copolymer (EVA)/low density polyethylene (LDPE) composites. After melt blending and moulding cross-linking, LiCoO2/CB/EVA/LDPE semiconductive shielding specimens were successfully fabricated. Resistivity, Thermally Stimulated Depolarisation Currents (TSDC), and pulsed electro-acoustic measurements were conducted for the prepared specimens. The PTC effect, which is common in polymer composites, was successfully suppressed, and the PTC strength was reduced by 17%. The resistivity of the semiconductive shielding layer was significantly reduced. The ability of the semiconducting shield to inhibit space charge injection from the conductive core to the insulation layer was significantly improved. When LiCoO2 content is 0.5 wt%, the semiconductive shielding layer has the best performance, and the space charge in the insulation layer was reduced by 70% compared to the undoped semiconducting shield.

Model-based processing for the estimation of space-charge distribution from non-contact pulsed electro-acoustic measurements

Estimating the space-charge distribution using the pulsed electro-acoustic method requires a calibration procedure, in which a reference voltage signal is measured and used to divide the output voltage signal of the piezoelectric sensor. Such a procedure is time consuming and generally involves manually setting filter parameters whose values should be chosen to reduce ringing artifacts in the charge profile and, at the same time, preserve the bandwidth of the profile. This paper proposes a model-based estimation method called the matrix pencil that does not require a calibration procedure to retrieve the charge distribution, albeit with normalized amplitudes, and thus it paves the way for reducing data acquisition and processing times. Results on simulated and experimental data confirm the effectiveness of the proposed method in estimating the space-charge distribution in irradiated polymers.

Influence of external electric fields and temperature on the behavior of water and acetophenone molecules in C120H242 chains: A molecular dynamics study

Polymers are widely used as electrical insulating materials, with polyethylene being one of the most commonly used materials. However, the presence of water and acetophenone (AP), which are byproducts of the cross-linking agents used to produce cross-linked polyethylene, can cause space charge formation and electrical degradation. In the present study, the effects of an external electric field and temperature (250–400 K) on the dynamic and structural properties of water molecules (1 wt%) with AP molecules (1 wt%) in C120H242 chains were evaluated by molecular dynamics simulations with the SPC/E water model and the general AMBER force field. The results showed that water molecules diffused into the C120H242 chains under the condition of E = 5 GV/m in the temperature range of 250–400 K. Moreover, the AP molecules diffused into the C120H242 chains under the condition of E = 1 and 5 GV/m in the temperature range of 350–400 K. These results support a previously proposed model for space charge formation in polyethylene, which was based on pulsed electro-acoustic measurements. Thus, we verified a molecular model for the space charge and a suppression mechanism for electrical degradation from the viewpoint of the diffusion of water and AP molecules.

Reliability of PEA Measurement in Presence of an Air Void Defect

This paper deals with the reliability of the Pulsed Electro-Acoustic (PEA) technique in the case of a specimen containing an air layer. The first approach to this study has been proposed by the authors in previously published works. In these papers, the mathematical description, the PEA cell simulation model, and some experimental tests have been reported. In this work, a more accurate description of the acoustic wave behavior within the PEA cell and specimen with and without an air layer is given. Moreover, the comparison between simulation and experimental tests for both cases (specimen with and without air layer) allowed the validation of the previously developed PEA cell simulation model. The latter was previously validated only for a single layer specimen, here the good performances of the model have also been confirmed in the case of a multilayer specimen, also with an air layer. Experimental and simulation results show that the air layer acts as a barrier for the acoustic signal, due to the different acoustic impedance between the air and the solid dielectric material which constitute the specimen. Therefore, the aim of the present work is to demonstrate that in the case of a three-layers specimen, composed as dielectric-air-dielectric, the PEA cell is not able to provide the complete profile of the entire specimen under test.

An equivalent investigation of space charge for plane samples and coaxial cables under a temperature gradient

In this research, the equivalence of space charge for coaxial cables and plane samples are analyzed through simulation. Bipolar charge transport models of coaxial cables and plane samples are established with the same material-dependent parameters, the bulk charge and total space charge distributions are simulated and comparatively researched under isothermal and temperature gradient conditions. The presented results indicate that the space charge behavior shows a basic equivalence in terms of charge variation laws and distribution. In addition, few discrepancies in the heterocharge amount and charge waveform amplitude are observed and mainly ascribed to the sample geometry, mobility gradient distribution and the spatial resolution of the measuring system, respectively. Investigations of space charge characteristics with a plane pulsed electro-acoustic measurement system may be accepted as a practice for providing the space charge characteristics of coaxial cable insulation, unless it is necessary to investigate the cable space charge distribution under actual operating conditions.

Influence of the Pulsed Voltage Connection on the Electromagnetic Distortion in Full-Size HVDC Cable PEA Measurements.

Nowadays, with the widespread use of High Voltage Direct Current (HVDC) cables in power systems, the measurements of space charges in full-size cables are becoming more relevant. One of the most common methods used for space charge measurements is the Pulsed Electro-Acoustic (PEA) method. This paper analyzes two factors that influence the electromagnetic interference on the piezoelectric signal. These factors are the connection of the injected pulsed voltage at the PEA test cell and the grounding of the PEA test cell. The influence was analyzed by means of experimental tests to compare different configurations and the electromagnetic distortion created in each one of them. It was observed that the physical location of the pulsed voltage at the electrode has a very important impact on the magnitude of the electromagnetic distortion. Moreover, it is shown that the physical connection of the grounding and the existence of a parasitic capacitance at the PEA test cell are also an important source of distortion.

Investigation on space charge and charge trap characteristics of Al–epoxy nanocomposites

Optimised quantity of nano-aluminium (Al) filler mixed with epoxy resin using percolation threshold criteria was adapted to fabricate nanocomposite with desired space-charge and charge-trap properties. Surface potential variation due to charge deposition on to the insulating material under transient voltages was investigated, and it showed the improved decay rate with reduced trap depth for the nanocomposites with nanofillers addition in the range between 0.5 and 5 wt%. Threshold electric field calculated based on space-charge density variation by pulsed electroacoustic measurement was found to be between 7–9 kV/mm and maximum electric field calculated through polarity reversal test is about 9 kV/mm, for 5 wt% (for the optimised value of nanofiller) epoxy nano-Al composites.

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.

A comparison of space charge behaviors in coaxial cable and film sample under temperature gradient

Space charge in DC cables affect the long-term reliability of the cable during operation. A temperature gradient (TG) develops across the cable insulation due to Joule heat in the conductor. Extensive researches on space charge in film insulations under TG have been reported. However, up to date, it is still questionable whether the measurement results of film samples can directly reflect the space charge characteristics in an actual coaxial cable. In this study, the space charge behavior in a 10 kV XLPE cable was investigated under different temperatures and electrical stresses using an improved pulse electro-acoustic (PEA) measurement system. The results show that the coaxial geometry and the TG both enhance hetero-charge accumulation near the cable outer semi-conductive layer. Space charge in film samples sliced from the same cable under the same conditions were also measured and compared to that in the cable. It is found that the space charge behaves similarly in terms of charge variation laws and distributions under different electrical stresses and TGs regardless of the type of sample structure, the two space charge measurement methods show well equivalence.

Experimental and simulation study on space charge characteristics of epoxy resin filled with graphene oxide

Graphene has attracted much attention due to its advanced properties. In previous work, the relationship between temperature and the conductivity of epoxy-based nanocomposite filled with graphene oxide has been experimentally studied. To understand the charge transport behaviour, the space charge characteristics at different temperatures and field strengths are measured with an improved pulsed electroacoustic measurement system, which is designed for high-temperature space charge measurement. From the measurement results, epoxy resin (ER) filled with multi-layer graphene oxide (MGO/ER) shows suppressed charge migration while ER filled with single-layer graphene oxide (SGO/ER) shows a hetero charge accumulation, which is assumed to be caused by the partial reduction of graphene oxide to graphene. Compared to pure ER, the apparent mobility of the nanocomposites is smaller while the trap depth and number of trapped charges are larger, which is probably caused by the introduction of traps. In the simulation, the relationship between mobility, trap depth, trap density and charge distribution are firstly studied. Then the calculated values of mobility and trap depth are given to the parameters to compare the simulated charge profiles with experiment results. The simulation results agree well qualitatively with the experimental results, giving some support for the proposed explanations.

Effects of electric field and temperature on space charge characteristics in double-layered polytetrafluoroethylene films

As the core component of composite insulation for dry-type high-voltage equipment, polytetrafluoroethylene (PTFE) films are subjected to high temperatures and complex electric fields. Therefore, this study explores the effects of electric field and temperature on the space charge characteristics of double-layered PTFE films based on the measurement of space charge distribution in double-layered PTFE films under different DC voltages and temperatures using the pulsed electro-acoustic measurement system. Results show that more charges accumulated in the PTFE sample subjected to a positive electric field during polarization and the space charges dissipated more rapidly during depolarization compared with those subjected to a negative electric field. The existence of a PTFE/PTFE interface increased the density of trap and deepened the trap. The positive charges that accumulated at the PTFE/PTFE interface during polarization gradually held a dominant position with the increase of temperature. Finally, the formation of charge in the PTFE/PTFE interface mainly depended on two factors. When the applied DC voltage was not large, the charge formation was mainly dominated by the Maxwell-Wagner polarization effect. When the voltage increased to a certain threshold, the charge formation was mainly caused by the charge injection and migration characteristics.

Experimental demonstration of deep traps in silica-based polyethylene nanocomposites by combined isothermal surface potential decay and pulsed electro-acoustic measurements

The ability to suppress space charge accumulation at high electric fields makes nanocomposites attract significant research interest as potential insulation materials in high-voltage direct current cable development. At present, the deep trap introduced by nanoparticles is frequently applied to be responsible for the observed space charge suppression in nanocomposites. However, the experimental results that support deep-trap formation have not been rigorously examined. We therefore propose herein a simple and more direct approach based on isothermal surface potential decay combined with pulsed electro-acoustic measurements to verify the presence of deep traps in silica-based blend polyethylene nanocomposites. The results indicate that the deep traps are indeed introduced by filling nanosilica and the space charge suppression observed in the nanocomposite with a low loading ratio is caused by deeply trapped charges in the sub-surface region of specimens.