Pulsed Electroacoustic Technique Research Articles

Deconvolution without calibration measurement of multi-peak charge distributions in dielectric materials

The electrical and mechanical properties of a dielectric material are significantly influenced by its charge distribution, necessitating an understanding of how charge accumulation occurs in order to determine the suitability of use for specific applications. This research presents a novel approach to deconvolving charge distributions with multiple peaks while dispensing with calibration measurement required by the non-contact pulsed electro-acoustic technique. Unlike traditional deconvolution methods that rely on reference signal measurements, the proposed approach reconstructs the charge distribution through parameters directly estimated from the piezoelectric transducer output voltage, effectively eliminating the need for such measurements altogether. Simulations show that even when closely spaced peaks exist within multi-peak charge distributions, our method can accurately reconstruct their profiles. In addition, the charge distributions obtained from experimental data of PTFE, FEP, and PEEK indicate that the proposed approach outperforms traditional methods in predicting penetration depth while reducing fluctuations and spurious peaks. We expect this method to be a valuable tool for the design and optimization of electronic systems in harsh environments through offering a simple, fast, and accurate alternative to conventional deconvolution methods.

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.

Modification of insulating oils and oil-based titanium dioxide nanofluids for transformers: a review.

In the last decade, oil-based titanium dioxide nanofluids (TiO2 NFs) have gained immense interest due to their unique insulating properties and excellent thermal performance, which endow them with the potential for application in the field of modified insulating oils. A timely comparison, analysis and summary of recent advances in the preparation, characterization, and properties of different oil-based TiO2 NFs for oil-immersed power transformers will contribute to provide a useful reference for the subsequent development of such materials. Preparation methods are reviewed along with their merits and demerits. Characterization techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical microscopy (OM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), dynamic light scattering (DLS), Fourier transform infrared (FTIR) spectroscopy, thermally stimulated current (TSC), pulse electroacoustic technique (PEA), finite element analysis (FEA), fluorescence spectroscopy, Raman spectroscopy and zeta potential analysis are all applied to determine the crystal structure, particle size, surface function, surface charge and stability. Stabilization mechanisms are also discussed in detail. Some critical properties of oil-based TiO2 NFs under the application of different influencing factors such as base oils, crystal structure, size of nanoparticles, surface modifiers, mixing percentage, and aging environment are highlighted. Finally, the existing challenges and perspectives are presented for future research.

Transport Characteristics of Interfacial Charge in SiC Semiconductor-Epoxy Resin Packaging Materials.

The silicon carbide (SiC) wide bandgap (WBG) semiconductor power device has been widely applied for its excellent properties. However, the charge accumulated in the interface of SiC semiconductor-related insulation packaging may lead to serious material performance degradation and failure, threatening the reliability and operation life of power devices. In this research, interface charge accumulation characteristics of SiC–epoxy resin double-layered material were investigated, and space charge injection, transport, and accumulation mechanisms, as well as the related temperature effect for the SiC–epoxy resin under polarization and depolarization conditions, were studied by the pulsed electro-acoustic (PEA) technique. The results show that a charge peak appears between the SiC–epoxy resin interface, which shows the same polarity as the SiC side electrode. Charge injects from the SiC electrode, transports along with the SiC semiconductor bulk due to the high mobility, and is blocked by the interface barrier. In addition, under high temperature and high electrical stress conditions, obvious charge accumulation occurs inside the epoxy resin bulk, which was captured by the deep traps. The charge accumulation of the SiC-insulation packaging material can influence the operation of the power device and should attract more attention.

Space Charge Behavior of Quantum Dot-Doped Polystyrene Polymers

This paper deals with the role played by the interface and bulk volume of the nanofiller about affecting the electrical properties of a nanocomposite material. For this purpose, a simple and completely amorphous matrix, polystyrene (PS), is used as base material, and core-shell quantum dots are exploited for simulating the structure of nanocomposites: CdSe core and CdSe-ZnS core-shell semiconductor quantum dots (QDs) are added into a PS matrix. The latter is to highlight the effect of the ZnS interface and as contrast to the core material. Dispersion and distribution of QDs are first microscopically observed and optimized by including isopropyl alcohol in the manufacturing phase as an additional solvent. Among electrical properties the focus is on space charge accumulation, tested by means of the pulsed electroacoustic technique at 10 kV/mm and 50 kV/mm on CdSe and CdSe-ZnS doped PS composites. Results are then compared with a reference PS without QDs. Trap depth and density are also obtained by space charge measurement results. When CdSe QDs are added to PS, the trap density increases with respect to the baseline values measured on the unfilled polymer. In contrast, the ZnS shell around the CdSe core creates an additional trap level with lower trap depth, which increases charge mobility, thus turning homocharge into heterocharge accumulation. Therefore, the surface shell-structure of QD nanocrystals appears to significantly influence the space charge behavior of the nanocomposite, independently of the polymer.

Understanding the thermal ageing performance of epoxy aluminium nitride nanocomposites through space charge studies and by LIBS analysis

Epoxy aluminium nitride (AlN) nanocomposites were prepared to understand the influence of thermal ageing on dielectric and space charge performance of the material. It is observed that the real relative permittivity values of epoxy AlN nanocomposites are higher than epoxy resin when wt% of AlN is >1 wt%. Also, the epoxy AlN nanocomposites depicted an increase in dielectric loss compared to the neat epoxy specimen. The space charge variations in the test specimens are analysed by adopting a pulse electro-acoustic technique. It is observed that the nanocomposites have lesser space charge in the bulk volume of insulating material compared with pure epoxy resin. Also, it is realised that the local electric field is less with nanocomposite material in both virgin and thermally aged specimens. The threshold electric field is observed to be lesser with epoxy resin compared to nanocomposites. Laser-induced breakdown spectroscopy (LIBS) analysis clearly indicates the reduction in plasma temperature with epoxy AlN nanocomposites compared to pure epoxy resin. Principle component analysis (PCA) has classified the nanocomposite materials having different filler concentrations from the pure epoxy resin, qualitatively based on their LIBS spectra. Thermally aged epoxy AlN nanocomposites are successfully classified from unaged nanocomposite specimens, using PCA.

Understanding the impact of space charge variations with UV‐ and water‐aged epoxy alumina nanocomposites adopting pulsed electroacoustic techniques

Epoxy alumina nanocomposites were prepared to understand the space charge variations with ultra violet (UV)- and water-aged specimens. Addition of alumina nanofiller to epoxy resin had suppressed the space charge formation. A pulsed electroacoustic technique was employed to measure the space charge accumulation in epoxy–alumina nanocomposites. A marginal increase in space charge with enhanced electric field in the bulk volume of insulation was observed with UV- and water-aged nanocomposites. It was observed that 3 wt% sample showed better performance in all ageing conditions, which was confirmed through space charge studies and by the measurement of the dielectric parameters especially dielectric constant and tan(δ).

Review of space charge measurement by pulsed electro-acoustic technique

<p>Pulsed electro-acoustic (PEA) is well known as a non-destructive technique for testing and describing space charge profiles in the insulation. Degradation due to moisture, ageing, external flashover, partial discharge and space charge are considered as insulation defects that is an issue in high voltage insulator. Space charge may distort the internal electric field that may lead to an electrical breakdown. For instance, a method based on an acoustic phenomenon can be used to describe the space charge profiles. This paper compares several earlier researches on different specimen of PEA method and reviews their results to find the space charge distribution profiles. The scope of the review includes types of methods used. The acoustic technique is considered for this study. It has been observed that the PEA technique successfully gives the same result as theoretical that is based on the obtained profile. Linear low-density polyethylene (LLDPE) composite demonstrates about 89 % lower charge density value compared to cross-linked polyethylene (XLPE) and polypropylene (PP) composite. While the electric field value of ethylene-vinyl acetate (EVA) sample is 75 % lower than low-density polyethylene (LDPE) sample. This study found that the time interval, specimen thickness and the electric field sway the space charge profiles. </p>

Impact of accelerated aging of epoxy‐Ni nanocomposites on space charge variation adopting pulsed electro acoustic technique

AbstractEpoxy‐Ni nanocomposites of different weight percentages are subjected to accelerated aging by water aging, thermal aging and UV irradiation to understand the influence of aging conditions on their space charge behavior. Homo‐charge accumulation is observed for unaged epoxy‐Ni nanocomposites. The magnitude of accumulated space charge is found to be lesser in 0.5 wt% epoxy‐Ni nanocomposite specimen compared to other unaged specimens. Water diffusion coefficient of epoxy‐Ni nanocomposites decreased with increasing weight percentage of filler addition. Space charge accumulation tends to increase with water aging. Increment in the space charge accumulation of 0.25 wt% epoxy‐Ni nanocomposite specimen is lesser compared to other water aged specimens. Exposure to thermal stress resulted in deterioration of the molecular structure of insulating material leading to an increase in trap‐controlled mobility, further leading to the reduction in space charge density with aging period. UV irradiation marginally assisted the charge transportation phenomenon, which resulted in the slight decrease of space charge density of epoxy‐Ni nanocomposites after UV irradiation. Hetero‐charge formation is found for all the three aging procedures. Overall, the specimen S1 reflected better space charge characteristics after being subjected to different aging conditions.

Dielectric characterisation of epoxy nanocomposite with barium titanate fillers

High permittivity materials are currently in use for mitigation of electrical stress in high-voltage apparatus and energy storage systems. In this work, epoxy-based high permittivity nanocomposites with Barium titanate (BaTiO3) nanofillers are considered, for the purpose of stress mitigation. Uniform dispersion of the fillers in the polymer up to 10% by volume is achieved. Apart from the use of as-received fillers, the effect of using surface-functionalised nanoparticles (with 3-glycidoxypropyltrimethoxy-silane) before use is also investigated. The nanocomposite is characterised in terms of its complex permittivity, DC conductivity, short-term AC breakdown strength and space charge accumulation, to gauge its suitability for use in high-voltage insulation. Complex permittivity is measured using broadband dielectric spectroscopy over a broad frequency range of 1 mHz to 1 MHz. DC conductivity is studied from polarisation–depolarisation current measurements. Short-term AC breakdown strength tests are performed at power frequency (50 Hz). Space charge density along the sample thickness is obtained using pulsed electro-acoustic technique. A computational case-study is presented to show the feasibility of using the high permittivity nanocomposite for electric stress control in high-voltage equipment (viz., at mounting flanges of 69 kV bushings).

Analysis of space charge and charge trap characteristics of gamma irradiated silicone rubber nanocomposites

Silicone rubber is widely used for electrical insulation and may be exposed to a harsh environment. The present study envisaged to improve insulation properties of silicone rubber by adding an optimised quantity of nanofillers. The fundamental space charge and charge trap characteristics were studied by adopting the pulsed electroacoustic analysis technique and through surface potential measurement. The dielectric properties of the materials were analysed through measurement of permittivity and loss factor of the material at different frequencies and temperatures. The influence of gamma irradiation on variations in fundamental properties of the material was characterised. The results of the study indicate that 5 wt.% alumina added nanocomposites had better space charge performance under gamma irradiation compared with virgin silicone rubber.

Bipolar electronic charge transport model under trap density Gaussian spatial distribution: application to dielectric polymer interfaces

Our new approach consists of applying the Gaussian spatial distribution function to study the trapping process bipolar electronic charge transport at dielectric–dielectric as well as dielectric-electrode interfaces. Indeed, the application of the Gaussian spatial distribution function, to the traps spatial density distribution as well as to the corresponding trapping coefficients, has not been investigated so far for bipolar transport modeling in insulating and dielectric materials. In fact, this approach takes into account the non-uniformity of the trap density spatial distribution, which constitutes the main contribution that presents a more realistic aspect taking into account the existence of the inhomogeneities in the dielectric materials. Several numerical resolution techniques have been applied to the theoretical equations formulated with associated assumptions and physical conditions. The obtained results show several physical phenomena, such as charge accumulation and Maxwell–Wagner behavior. In this paper, to validate our approach, we have focused our investigations on the accumulation phenomenon of trapped charges. Our results are in good agreement with those obtained by the pulsed electro-acoustic experimental technique, for the example, epoxy–polyethylene interfaces.

Space charge behavior in silicone rubber from in-service aged HVDC composite insulators

Space charge behavior in sheds removed from service aged 500 and 800 kV HVDC silicone rubber insulators and compared to a stored reference insulator is reported in this paper. The aim of this investigation is to evaluate the material aging. The investigations are made under electric field of ±20 kV/mm at room temperature using pulsed electro-acoustic technique. Based on the results, the electric field distribution and mobility of charge carriers, considering the contributions of de-trapping process from both shallow and deep traps, are deduced. It is concluded that space charge concentration in the aged material increases compared to that in the reference material and the dynamics of the charge decay changes. The understanding of this behavior provides a means of possibility assessing insulator material aging.

Multilayers oil and oil-impregnated pressboard electric field simulation based on space charge

Space charge build-up has been considered as the major issue in the development of HVDC apparatus such as the converter transformer. The existence of space charge can distort the local electric field, which could lead to the degradation and even breakdown of insulation materials. Therefore, it is vital to investigate factors that can affect space charge formation and dissipation characteristics such as the temperature, moisture, ageing, thickness, multi-layered structure and electric fields. This paper mainly focuses on the effect of multilayers and thickness on space charge behaviour of oil and pressboard insulation system. Space charge was measured using the pulsed electroacoustic technique (PEA) method. The space charge results are quantitatively analysed to establish the relationship between interfacial charge density and different pressboard and oil thickness ratios. A new space charge interpolation methodology is utilized to input space charge into the multilayers oil and pressboard model using COMSOL software. The local electric field of multilayers oil-impregnated pressboard and oil could be simulated, with the emphasis on the electric field after the polarity reversal operation. From space charge results, they indicate that the increased thickness of pressboard could prohibit the interfacial charge increase while the increased oil thickness could facilitate interfacial charge increase. Moreover, from the electric field simulation results, they indicate that there is the electric field gradient caused by the space charge for multilayers oil and oil-impregnated pressboard structure. After the polarity reversal, the maximum electric field of the oil caused by the space charge is higher than electric field calculated based on the Maxwell-Wagner theory.

Effects of UV radiation on the charge trapping capability of PET

Poly(ethylene terephthalate) (PET) is able to retain space charge in traps, as is usual in dielectric materials. This allows the material to attain an almost permanent polarization when space charge is displaced by an external electric field or is injected from an electrode. We have studied the influence of UV irradiation on the charge trapping capability of PET in samples exposed for different periods of time, up to 10 weeks. The pulsed electro-acoustic technique (PEA) has been used to determine the charge profile. The injected charge that the material is able to retain on the irradiated surface increases with irradiation time. This indicates the formation of new traps. An extensive characterization of these localized states has been performed by the technique of thermally stimulated depolarization currents. Parameters of charge relaxation kinetics have been obtained by fitting spectra of the space charge relaxation current of a semicrystalline polymeric material peak (), related to injected charge, to the general-order kinetics model. A relaxation map analysis shows that relaxation times become more distributed and the activation energy decreases as irradiation time is increased. The activation energy decreases by approximately 10% after 10 weeks of exposure. These results show that UV irradiation creates additional traps on the treated surface, which agrees with PEA results, and that the energy depth of these traps is shallower and it is more distributed than in the case of pre-existing traps.

Comparison and analysis of three pulse injection methods in the pulsed electroacoustic technique used for long cables

With the advantages of high transmission capacity, low loss, resistance to aging, long lifetime, and unlimited transmission distance, high voltage direct current (HVDC) cable has many advantages in the field of DC power transmission [1]–[3]. However, space charge accumulation in the insulation of DC cables still presents a problem [4]–[9].

Epoxy-hBN nanocomposites: A study on space charge behavior and effects upon material

The emergence of nano dielectrics for specialized high voltage applications sparked off a variety of research activities, which proved that nano-fillers are capable of improving the electrical, thermal and mechanical properties of polymers. This paper primarily investigates the effect of addition of hBN (hexagonal boron nitride) nanoparticles into an epoxy polymer base by increasing fill-grade, from 0.2 to 5 % by volume, from two different standpoints: (a) characterizing the electrical space charge (S.C.) accumulation threshold under DC electrical fields, and, (b) demonstrating the alterations in material properties of the modified polymeric materials, from the unfilled polymer. Objective (a) is experimentally investigated by the pulsed electro-acoustic (PEA) technique, well known for determining spatial charge distribution in dielectrics. Objective (b) is investigated by determining the ultrasonic velocity response of the modified composites and unfilled polymer. The obtained results suggest a relation between electrical threshold fields for S.C. accumulation & fill-grades, as well as the fact that incorporating stiff filler materials into brittle polymer bases leads to a tougher composite (capable of withstanding greater breaking stress levels), but with reduced ductility.

Space charge characteristics in oil and oil-impregnated pressboard and electric field distortion after polarity reversal

The formation of space charge in the oil/paper insulation under HVDC condition could influence the electric field distribution. In the case of HVDC converter transformer, the distortion of the electric field may affect its performance. It is, therefore, important to analyze factors that can affect space charge formation and dissipation characteristics such as temperature, moisture, electric stress and ageing process. This paper mainly focuses on the effect of electric stress and oil ageing on space charge characteristics in oil/paper insulation system. The pulsed electroacoustic technique (PEA) method was used to monitor space charge dynamics. This data was further incorporated into a model established using COMSOL software to estimate space charge impact on electric field distribution, with an emphasis on the electric field after polarity reversal operation. The results show that the electric field arising from the space charge formation is much higher compared to the average electric field after the polarity reversal operation. Moreover, the electric field caused by space charge decreases faster for the aged oil-impregnated pressboard compared to the fresh one, which could be related to different space charge dissipation characteristic related to the condition of the oil.

Space charge behavior in oil gap and impregnated pressboard combined system under HVDC stresses

The reliability of oil-pressboard insulation system used in high voltage direct current (HVDC) convertor transformers can be affected by the presence of space charge. In this paper, the space charge behaviors in a 0.5 mm thick oil film combined with a 1mm thick impregnated pressboard have been investigated by the pulsed electroacoustic (PEA) technique under 12 kV/mm and 20 kV/mm at room temperature for both short term and long time tests. Two types of oil with different aging status were used for comparison. The results show that a charge peak is quickly formed at the interface between oil and pressboard with the same polarity as the electrode at the oil film side. However, the dynamics of the interfacial charges are very different for fresh oil and aged oil samples. The maximum electric field occurs in the middle of the pressboard, which is significantly enhanced in the aged oil samples. Compared with the Maxwell-Wagner (MW) polarization, more space charges are formed in the insulation system. It needs much longer time to reach steady state, depending on the status of the oil. The interfacial and bulk effects result in a significant electric field enhancement in the middle of pressboard, especially in aged oil samples.

Dependence of space charge dynamics in LDPE on history of voltage application

The space charge phenomenon in PE (polyethylene) particularly in LDPE (low density polyethylene) is being widely researched for more than two decades. Despite various theories proposed by researchers, the charge dynamics in general and charge packets in particular, inside the polymeric insulation are yet to be completely known. The authors have investigated the dependence of charge dynamics on electric field using PEA (pulsed electroacoustic) technique. The experiments were carried out at 25 °C on 100 μm LDPE film samples under applied electric fields of 100–150 kV/mm. External current measurements were also performed under similar conditions. The analysis of charge profiles and external currents shows that apart from other factors, charge formation and polarity of charge also depend on history of electric field application. Interesting observations were made on the consequence of such results. Experiments were performed repeatedly on virgin samples, taken from same lot, and the results were found to be almost same. It is believed that the effect of history of voltage application on charge formation, presented in the paper, is useful for understanding practical behavior of LDPE which, if used in power cables, is expected to serve as a dielectric for several years — accumulating a lot of history of voltage application.