Pulsed Electro-acoustic Method Research Articles

Distinctive electrical properties in sandwich-structured Al2O3/low density polyethylene nanocomposites

The sandwich-structured Al2O3/low density polyethylene (Al2O3/LDPE) nanocomposite dielectrics consisting of layer-by-layer with different concentration Al2O3 loading were prepared by melt-blending and following hot pressing method. The space charge distribution from pulsed electro-acoustic method and breakdown strength of the nanocomposites were investigated. Compared with the single-layer Al2O3/LDPE nanocomposites, the sandwich-structured nanocomposites remarkably suppressed the space charge accumulation and presented higher breakdown strength. The charges in the sandwich-structured nanocomposites decayed much faster than that in the single-layer nanocomposites, which was attributed to an effective electric field caused by the formation of the interfacial space charges. The energy depth of shallow and deep traps was estimated as 0.73 eV and 1.17 eV in the sandwich-structured nanocomposites, respectively, according to the thermal excitation theoretical model we proposed. This work provides an attractive strategy of design and fabrication of polymer nanocomposites with excellent space charge suppression.

Effects of DC prestressing on partial discharge in oil-impregnated pressboard insulation

The AC (50 Hz) partial discharge (PD) characteristics of oil-impregnated pressboard (OIP) insulation are severely influenced by the DC voltage component in the converter transformer. Therefore, this study investigates the effect of different DC prestressing voltage (DCPV) amplitudes, prestressing time, and repetitions times of the applied AC voltage on the OIP insulation AC PD characteristics. Meanwhile, the space charge characteristics of distribution and dissipation are measured by pulsed electro-acoustic method. The experimental results show that the PD inception voltage (PDIV) and PD extinction voltage (PDEV) after DCPV were lower than that under AC voltage without DCPV. The PDIV decrease with higher DCPV at prestressing time from 30 to 60 min. But it increase with higher DCPV at prestressing time from 90 to 120 min. However, PDIV initially increase then decrease with the increasing repetition times of the applied AC voltage. The space charge distribution in 1 mm OIP insulation takes 60 min to reach a balanced condition. The residual space charge is relatively higher at a longer prestressing voltage time and higher DC voltage when the DC voltage was turn off. The PDIV and PDEV after DCPV is improved by Joule heating with longer prestressing time and higher DCPV. The space charge had negative effect on them.

Charge transport in thermally and electrically stressed oil-impregnated paper insulation

The relation between ageing and space charge is a hot area, but there is little direct evidence. In this paper, oil-paper samples were exposed to 20-kV/mm DC electrical field, 130°C temperature or both 20-kV/mm DC electrical field and 130°C temperature separately. Charge transport through them was analyzed at intervals by the pulsed electroacoustic (PEA) method. We find that the quantity of trapped charge increases with stressing durations, no matter what kind of stress oil-paper is exposed to. That indicates that energy release through space charge movement, such as charge trapping/detrapping and recombination, is another reason for traps generation besides deterioration caused by thermal ageing. However, because the difference in cellulose degradation mechanism among them, charge transport dynamics are thereby different under different stressing conditions. Space charge accumulates faster inner electrothermally stressed oil-paper than that inner another two kinds of oil-paper, and the sign of trapped charge is positive if oil-paper is electrically or electro-thermally stressed. The latter behavior is discussed in terms of electromagnetic dynamics due to cavity within oil-paper once there is external electric field.

Calibration of the laser induced pressure pulse method when using a semiconducting electrode as the laser target

Space charge distributions can directly be qualitatively estimated from a laser induced pressure pulse (LIPP) signal. To obtain precise quantitative space charge density, calibration of the LIPP method is necessary which requires an ideal measured signal. However, when a carbon loaded polymer electrode is used as the laser target, a long signal tail can be found. The cause of the signal tail is analyzed in this study and two methods are proposed to correct the experimental signals. According to the calibration procedure introduced, space charge density is obtained respectively by using the two correction methods. Finally, the calculated results are compared and the validity of the calibration and correction methods is confirmed. The correction and calibration procedures proposed can be extended to piezoelectric induced pressure pulse and pulsed electroacoustic methods.

Negative differential mobility for negative carriers as revealed by space charge measurements on crosslinked polyethylene insulated model cables

Among features observed in polyethylene materials under relatively high field, space charge packets, consisting in a pulse of net charge that remains in the form of a pulse as it crosses the insulation, are repeatedly observed but without complete theory explaining their formation and propagation. Positive charge packets are more often reported, and the models based on negative differential mobility(NDM) for the transport of holes could account for some charge packets phenomenology. Conversely, NDM for electrons transport has never been reported so far. The present contribution reports space charge measurements by pulsed electroacoustic method on miniature cables that are model of HVDC cables. The measurements were realized at room temperature or with a temperature gradient of 10 °C through the insulation under DC fields on the order 30–60 kV/mm. Space charge results reveal systematic occurrence of a negative front of charges generated at the inner electrode that moves toward the outer electrode at the beginning of the polarization step. It is observed that the transit time of the front of negative charge increases, and therefore the mobility decreases, with the applied voltage. Further, the estimated mobility, in the range 10−14–10−13 m2 V−1 s−1 for the present results, increases when the temperature increases for the same condition of applied voltage. The features substantiate the hypothesis of negative differential mobility used for modelling space charge packets.

Preparation, microstructure and properties of polyethylene/alumina nanocomposites for HVDC insulation

In the recent decades, the phenomena of space charge accumulation in the high voltage direct current (HVDC) insulation have been attracted more attention. In this paper, low density polyethylene (LDPE) nanocomposites filled with alumina nanoparticles (nano-Al2O3) were prepared employing melting blend method. Morphologies of nanoparticles and LDPE/Al2O3 nanocomposites were performed by scanning electron microscopy (SEM). Electrical properties of the LDPE nanocomposites were also investigated. Results shown that the nano-Al2O3 particles modified with vinyl silane coupling can effectively enhance the breakdown strength of LDPE nanocomposites. With the nano-Al2O3 particles loading, the volume resistivity of the LDPE nanocomposites was increased, while dielectric permittivity of the nanocomposites was decreased. Space charge of the LDPE nanocomposites was measured by pulsed electro-acoustic (PEA) method. The charge profiles indicated that space charge suppression of the LDPE nanocomposites was better than that of pure LDPE. The excellent insulation properties of the LDPE nanocomposites were attributed to the better interfacial adhesion between the surface-treated nano-Al2O3 particles and the LDPE matrix.

Surface flashover characteristics in polyimide/ZnO nanocomposite under DC voltage in vacuum

It is well known that the solid materials flashover phenomenon is very complicated and influenced by many factors, such as surface conditions, electrodes and traps, etc. In this paper, dc flashover experiments of polyimide (PI) and PI/ZnO nano-composite are studied in vacuum to reveal the surface flashover mechanism. The trap levels and space charge distributions were obtained by thermally stimulated current (TSC) and pulsed electro-acoustic (PEA) methods respectively. Further researches on surface states of flashover tested sample, such as observation by scanning electron microscopy (SEM), and measurement of secondary electron emission (SEE) coefficient, were carried out as well. Experimental results showed that the dc surface flashover voltage of PI/ZnO nano-composite is significantly enhanced compared to that of pure PI. The experiments results can be explained as that the doped ZnO nano-particles introduced deeper traps, which can reduce the level of charge carrier mobility, secondary electron emission and space charge accumulation. Meanwhile, the gas desorption efficiency of nano-composite is much lower than that of pure PI. It suggests that the doped ZnO nano-particles plays a critical role in affecting the surface flashover performance under dc voltage in vacuum.

Space charge dynamics at the physical interface in oil-paper insulation under DC voltage

Space charge has received much attention recently because an understanding of space charge in oil-paper insulation is useful for the design of converter transformers. This paper presents a study of space charge dynamics at the physical interface using the pulsed electroacoustic (PEA) method. After sample inversion, space charge in the vicinity of semiconductor electrode could be clearly observed when the sample is under depolarization. When an unexposed sample was placed next to a previously polarized one, negative charge appeared at the dielectric interface. These results indicated that there were other reasons in addition to acoustic loss that led to space charge distribution distortion. As regards the dielectric interface, it was found that an electrical double layer was formed at the interface, and the space charge sign was dependent upon the polarity of applied voltage. Space charge behavior at the interface depended on several aspects of the status of the interface, and during voltage polarity reversal, the interface was a weak point of the insulation.

Space charge behavior evolution with thermal aging of oil-paper insulation

Space charge is a threat to insulation materials and oil-paper insulation gradually degrades in service. The degradation of cellulose affects space charge formation, accumulation and dissipation, and the presence of space charge affects the performance of dielectric. In this paper, the effect of thermal aging on space charge behaviors and other properties including permittivity, conductivity, and tensile strength was investigated. Oil-paper samples were aged at a hot spot temperature (180℃). During thermal aging, space charge distribution was measured at a regular interval at room temperature. And the pulse electro-acoustic (PEA) method was used for space charge measurement. The results showed that the thermal aging at hot spot temperature caused increase of conductivity and tensile strength degradation. The permittivity increased at the beginning but then it decreased, while dissipation factor showed an upward trend. Besides these, trap distribution and carriers characteristics were also affected. The formations of space charge of unaged and aged oil paper were consequently different under low electric field. Due to the generation of traps by the degradation of cellulose, more charge, especially positive charge was trapped, but the increased traps mostly were shallow traps. Based on the Schottky model, a correlation between space charge injection and the permittivity was drawn. This, together with the appearance of positive bulk charge could be used for oil paper aging status diagnosis.

Some aspects of coupled electrical-mechanical effects in dielectric materials

The propensity of electrically insulating materials to generate/store electrical charges leads to a panel of electromechanical phenomena that can be either exploited in applications relevant to electrical engineering, or represent limitations in the performance of insulating materials. The aim of this contribution is to describe various features of these electrical-mechanical coupling phenomena with focus on the field-induced strain measurement of charged polymers, on the charge distribution measurement by pulsed electroacoustic method and on the contribution of electromechanical effects in electrical ageing phenomena.

Space Charge Behavior of Oil-paper Insulation Thermally Aged under Different Temperatures and Moistures

Moisture and high temperature are the most important factors that lead to the ageing of oil-paper insulation, but the research about space charge characteristics of oil-paper insulation does not take the combined effect of ambient temperature, moisture and thermal ageing into account. The pulsed electroacoustic (PEA) method was used to investigate the influence of moisture and temperature on space charge characteristics of oil paper at different ageing stages. The results showed that moisture could speed up formation of space charge in oil paper when water concentration was low, but the formation was restrained if the water concentration was high. At the beginning of thermal ageing, heterogeneous charge accumulation had predominance, but it gradually changed to homogeneous charge injection with ageing. It was believed that moisture concentration could speed up ageing and enhance charge accumulation on one hand, and accelerate or slow down the establishment speed of space charge on the other hand, therefore, charge accumulation type changed with ageing. The more seriously the oil-paper insulation was thermally aged, the deeper the trap energy level was, hence more space charge was trapped, which could be speeded up by increasing the ageing temperature, but the effect of ambient temperature did not fit the Arrhenius law.

Spatial charge distribution in fibrous dielectrics

Abstract In this work, an attempt was made to study the spatial charge distribution in fibrous dielectrics using pulsed electro-acoustic method. The polarized dielectrics displayed in-depth charge polarization and exhibited slow charge decay. The initial charge density was found to increase with an increase in polarization voltage as well as polarization time. The statistical analysis revealed that though the polarization voltage affected the initial charge density significantly, the polarization time was insignificant. A multiple linear regression model of initial charge density was developed and found to predict the experimental results quite satisfactorily.

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.

Space charge characteristics of LDPE nanocomposite/LDPE insulation system

Two kinds of insulation systems with nanocomposite have been fabricated by co-pressing low-density polyethylene (LDPE) and nano-titania doped LDPE (TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -LDPE) films. The three-layer systems considered are TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -LDPE/LDPE/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -LDPE (namely, NLN) and LDPE/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -LDPE/LDPE (namely, LNL). Space charge distributions under different applied electric fields were measured by the pulsed electro acoustic (PEA) method. The charge transport characteristics and the interface space charge formation as well as the effect of nano-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration have been systematically discussed. The results show that heterocharges and homocharges are formed at the LDPE/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -LDPE polymer interfaces in the system NLN and LNL, respectively. The nature of the interfacial charge has been explained in term of the interfacial polarization theory combined with conductivity and permittivity of the materials. Additionally, the recombination between charges influences the amount of the interface space charge. However, the NLN system always stores more charges than LNL, which was explained by the change of the effective electric field caused by the formation of the interface space charges. The results obtained would provide experimental support for optimizing high voltage direct current power cables.

Space charge behavior in thick oil-impregnated pressboard under HVDC stresses

The space charge accumulation can affect the dielectric performance of oil-impregnated pressboard in convertor transformers. In this paper, the space charge behaviour in 1mm thick impregnated pressboards is investigated by the means of pulsed electroacoustic method. The dried pressboards are separately impregnated in fresh oil and service aged oil as comparison. The space charge distributions are investigated under ±15 kV/mm and ±25 kV/mm at room temperature. The homocharge injection can be observed for all the samples, which is much more significant in the aged oil impregnated samples. The amount of injected charges and the depth of the injected charges accumulation in the pressboard bulk are closely related to the oil properties, amplitude and polarity of the applied voltages. The service aged oil shows the dominant effects on the local electric field distortion, about 48% enhanced in the aged oil impregnated samples while about 15% enhanced in the fresh oil impregnated samples. The estimations of the apparent charge mobility and trap depth also indicate the great impacts caused by the service aged oil.

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.

クミルアルコールに浸漬したXLPE中に形成される空間電荷の試料厚依存性

In a bulk of cross-linked polyethylene (XLPE), there usually remain some chemical residues of cross-linking by-products such as cumylalcohol, acetophenone and α-methylstyrene. It is said that such cross-linking by-products are cause of the space charge accumulation under dc stress. Since the space charge accumulation affects the breakdown strength of the insulating materials, we have been investigating the effect of cross-linking by-products on space charge accumulation under high dc stress using pulsed electro-acoustic (PEA) method. In this report, we tried to observe the dependence of sample thickness on the space charge accumulation in XLPE including cumylalcohol under very high dc stress. It is found that the space charge accumulation in the thicker sample induces the breakdown rather than that dose not occur in the thinner sample even under the same stress.

Modeling and Simulation of Pulsed Electroacoustic Measurement Method

Pulsed electroacoustic (PEA) method is a very popular method for measurement of charge in dielectrics. In this method, an acoustic wave proportional to charge is generated by a pulsed HV. The acoustic wave traverses several material layers before being sensed and converted to electrical signal by a piezoelectric transducer. Due to multiple reflections in its path, a direct analysis of traveling acoustic wave would be complex. In this paper, we have proposed simulation of the physical model of the PEA method consisting of electrical and acoustic circuits. Using PSpice model of the method, authors have tried to address some design-related issues, which appear to have not been reported in earlier works. The acoustic circuit has been realized using analogy between acoustic wave and wave propagation in electric transmission line. The piezoelectric transducer is also modeled as a transmission line as in modified Leach's transducer model, except that, in the present case the transducer absorbs acoustic signal instead of generating. Approximate theoretical equations have been used to compare with simulations, wherever a comparison is possible, and found to match well in those cases.

Dissociating space charge processes from orientation polarization in poly(ethylene naphthalate) films

Thermo-stimulated depolarization current (TSDC) measurements and space charge measurements were performed on poly(ethylene naphthalene 2,6-dicarboxylate) (PEN), an aromatic and polar polyester. The aim is to develop an understanding of the dipolar and conduction processes at play in this material and in particular to understand the effects of temperature. For the TSDC measurements, when polarizing at 130 and 170 °C, the sub-glass transition and the glass transition relaxations are observed. However, in the case of a polarization temperature of 170 °C, one more current peak, labelled ρ peak, is observed at temperatures above the glass transition. This peak is not only of dipolar origin and could be associated with charge detrapping in the material. To unravel the mechanisms behind this process, a TSDC was combined with space charge measurements using the pulsed electroacoustic method (PEA) and the partial heating method was used. It is shown that the ρ peak is predominantly associated with the release of the negative charge build-up in the material.

The Effect of the Semiconductive Screen on Space Charge Suppression in Cross-Linked Polyethylene

The space charge distributions of cross-linked polyethylene (XLPE) with Borouge's Borlink™ semiconductive screen type LE0550 and LE0595 from a pulsed electro-acoustic method are obtained. The contact interface morphology at the semiconductive screen and the structure of XLPE near the interface are characterized. The dielectric spectrum and the conductivity current of XLPE with the different semiconductive electrodes are compared. The semiconductive screen changes the structure and the dielectric characteristic of XLPE near the contact interface, which may be the main reason for space charge suppression in XLPE with Borouge's type LE0550 semiconductive screen.