Pulsed Electro-acoustic Measurement Research Articles

Space charge behavior evolution of thermal-aged double-layered polyimide films

Polyimide film (PI) is widely used as dry-type smoothing reactor insulation for ultra-high voltage direct current (UHVDC) projects. In practice, the temperature of a central conductor can reach 80 °C to 100 °C at the rated power flow. Thus, the inner insulation composed of PI sustains these high temperatures. In this study, the space charge characteristics of PI at different temperatures (40 °C, 60 °C, and 80 °C) and thermal aging degrees (336, 500, 1000, and 1500 h) were examined with an improved pulsed electro-acoustic (PEA) measurement system to investigate the effects of temperature and thermal aging on dielectric performance. The apparent trap depths corresponding to various aging degrees were obtained according to the space charge decay dynamics at the interface. Results indicate that positive charges likely accumulate within the bulk of the material because of the thermal aging of homogeneous materials. The polarity of space charge at the interface also changes from positive to negative as aging degree increases. The space charge decay dynamics at the interface further shows that the trap energy increases as the thermal aging degree increases. The amount of residual space charge also increases remarkably as the electric strength and aging degree increase. The electric field distortion is closely related to temperature, thermal aging degree, and electric strength.

Electrical characterization of extruded DC cable insulation — The challenge of scaling

HVDC cable technology with extruded insulation systems have been growing rapidly in the recent years. Different insulation concepts including crosslinked or thermoplastic polymers with or without particle fillers have been studied intensively. The DC conduction in the insulation systems is one of the most important mechanism in dielectric physics; therefore reliable and representative methods are needed to characterize it. In the development process of HVDC cables from small scale plaque sample experiments to full scale cable testing, high field DC conductivity measurement and space charge measurement with the pulsed electro-acoustic (PEA) method are common. These two methods provide two different views into the conduction physics in the insulation and provide different types of information. But both of these methods have their own drawbacks and limitations which are important to keep in mind when choosing measurement methods in different stages of the development process. Another important aspect is the inherent differences between the different types of test samples. Thin pressed plaque samples are easy to produce and require less complicated testing equipment than experimental cables but there are major differences which should not be ignored. In this paper, the principles, advantages and limitations of DC conductivity and PEA measurements on samples of different scales are described and their relevance to the evaluation process is discussed. Some of the less discussed challenges of these measurement methods on different objects are discussed in more details and recommendations are made for obtaining more useful results.

Packet-like charge formation in cable insulating materials at polarity reversal

Investigation of space charge accumulation processes in a chemically cross-linked polyethylene for ac usage (XLPE) and a modified XLPE for dc usage (DC-XLPE) at voltage polarity reversal was carried out using pulsed electro-acoustic (PEA) measurement system. We found that a huge amount of so called packet-like charge, some of which partially increased the electric field in the material to more than twice of the average applied stress, generated in XLPE at the polarity reversal of DC stress. It enhanced the electric stress locally in the sample by almost twice of the applied average stress. On the other hand, we also found that the packet-like charge was not observed in DC-XLPE under the same experimental condition.

Characteristics of Shallow Traps in the Dielectric Surface and Their Effects on Diffuse Dielectric Barrier Discharge in Air

Diffuse Townsend dielectric barrier discharge (DBD) was realized in 3-mm air gap at atmospheric pressure using special Guangdong Zhuhai Yueke Tsinghua Electronic Ceramics Company, Ltd (GTT) ceramics as the dielectric while discharges with dielectric of other ceramics or quartz were always filamentary. It is supposed that there exist shallow traps ( $E eV) in dielectric surface. In order to experimentally study how the shallow traps influence the generation of diffuse DBD in air and to explore the mechanism of extraordinary extinguishment in Townsend DBD, thermally stimulated current measurements were performed. It was found the trap level of two types of ceramics was 0.34–0.36 eV, lower than 0.61 eV of quartz, which made it more easily to desorb trapped electrons for the ceramics. There was 280.8 nC of trap charge in GTT ceramics, which was much more than that in common ceramics (38.3 nC) and quartz (15.12 nC). Pulse electroacoustic measurement also indicated that GTT ceramics could acquire more seeds electrons with a longer decay time than the other two dielectrics. Scanning electron microscope observations clearly demonstrated denser distribution and better dispersion of grains in GTT ceramics surface, as was supposed to be the reason why it behaved better than other dielectrics. The results prove that there exist shallow traps in dielectric surface, which could offer seed electrons for generation of diffuse DBD. Lower trapping energy and higher density in the material would make it more easily to generate diffuse DBD.

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.

Research progress on space charge characteristics in polymeric insulation

Due to their excellent electrical insulation properties and processability, polymer materials are used in many electrical products. It is widely believed that space charge plays an important role for the electric field distribution, conduction, ageing, and electric breakdown of polymeric insulation. This paper reviews measurements and characteristics of space charge behavior which mainly determined by the pulsed electro-acoustic (PEA) measurement technique. Particular interests are the effects of the applied voltage, the electrodes, temperature, humidity, microstructure, additives, and filler materials on accumulation, distribution, transport, and the decay of space charge in polymeric materials. This review paper is to provide an overview on various space charge effects under different conditions, and also to summarize the information for polymeric materials with suppressed space charge and improved electrical behavior.

Effect of Nanoparticle Surface Modification and Filling Concentration on Space Charge Characteristics in TiO2/XLPE Nanocomposites

This paper focuses on the space charge characteristics in TiO2/cross-linked polyethylene (XLPE) nanocomposites; the unmodified and modified by dimethyloctylsilane (MDOS) TiO2 nanoparticles were added to XLPE matrix with different mass concentrations (1 wt%, 3 wt%, and 5 wt%). The scanning electron microscope (SEM) showed that the MDOS coupling agent could improve the compatibility between TiO2 nanoparticles and XLPE matrix to some extent and reduce the agglomeration of TiO2 nanoparticles compared with unmodified TiO2 nanoparticles; the volume resistivity testing indicated that the volume resistivity of TiO2/XLPE nanocomposites was higher than Pure-XLPE and increased with the increase of filling concentrations. According to the pulsed electroacoustic (PEA) measurements, it was concluded that the space charge accumulation was suppressed by filling TiO2 nanoparticles and the distribution of electric field in samples was improved greatly. In addition, it was found that the injection of homocharge was more obvious in MDOS-TiO2/XLPE than that in UN-TiO2/XLPE and the homocharge injection decreased with the increase of filling concentration.

Experimental Study of Space Charge Characteristics in Thin Films of Polyvinyl Chloride Nanocomposites

This paper aims to provide a view of a lively space charge accumulation in new polyvinyl chloride (PVC) nanocomposite thin films. Space charges in thin films of polyvinyl chloride nanocomposite insulation materials are done to distort the local electric fields and affect high-field conduction and breakdown phenomena, whatever, adding nanofillers to polyvinyl chloride changes their electrical ages. Thus, space charge characteristics have been investigated by pulsed electroacoustic (PEA) measurement system for thin thickness of polyvinyl chloride nanocomposite materials which filled with different separately nanoparticles like ZnO, TiO2, and Al2O3. This paper depicts that space charge accumulation in polyvinyl chloride nanocomposites differs with respect to type and concentration of nanoparticles.

Surface fluorinated epoxy resin for high voltage DC application

Charge accumulation under high voltage DC is a major concern in the transmission system as its presence distorts the local electric field. By performing chemical treatment on polymeric insulation via fluorination process, the charge transport characteristics of the material can be modified. In doing so, excellent surface properties of fluoropolymers can be attained without compromising the bulk properties of the original polymeric insulation. The change in chemical components at the surface of polymeric insulation should lead to a corresponding change in electrical properties at the surface and so suppress charge accumulation. In this paper, epoxy resin samples were formulated and treated with various surface fluorinating conditions. The samples then were characterised by scanning electron microscope (SEM), Raman spectroscopy and DC surface conductivity measurements. The surface potential decay measurement was performed and the result shows that there is a significant change in surface potential decay measurement with the introduction of surface fluorinated layer. The pulsed electroacoustic (PEA) measurement was used to further explain the decay mechanisms responsible for the observed phenomena. Surface DC flashover test using a pair of finger electrodes had also been conducted. It has been found that the introduction of fluorinated surface layer on epoxy resins plays an important role in improving the surface dielectric properties as apparent from the experimental results.

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.

Observation of the charged defect migration that causes the degradation of double-Schottky barriers using a nondestructive quantitative profiling technique

The migration and neutralization of charged defect ions during the degradation of a double-Schottky barrier are observed by performing nondestructive pulsed electroacoustic measurements on ZnO bicrystals. This offers the possibility to experimentally access the predicted defect migration behavior and provides a solid foundation to validate the theoretical aging model for electroceramics. Theoretical modeling of the acoustic attenuation effects during measurements is also discussed to understand and validate the experimental results, gaining insight into the widely used acoustic technique.

Investigation on heterocharge accumulation in crosslinked polyethylene: Experiment and simulation

This paper presents the pulsed electro-acoustic (PEA) measurement results of the space charge behavior in as-received and degassed crosslinked polyethylene (XLPE) plate samples. A modified bipolar charge transport model, taking into account the formation of ionic charges and the extraction barrier for electronic charges, is introduced to explain the origin of the observed heterocharge accumulation. The results show that in degassed sample, the formation of heterocharge at the anode is due to delayed extraction of injected electrons. In as-received sample, the heterocharge near the cathode is positive ionic charges, whereas that observed near the anode is caused by the combined effect of negative ions and injected electrons. It is also found in the simulation that the heterocharge amount increases with the increasing of byproducts content and extraction barrier height. The extraction barrier height at the electrodes only relates to charge amount, but has no effect on the threshold for heterocharge formation.

Effect of humidity in charge formation and transport in LDPE

Abstract Charge distribution and transport have been investigated in LDPE films with different humidity content under electric fields up to 130 MV/m. Pulsed electroacoustic measurements showed that, as water content increases, positive charge packets formation in the anode is enhanced and they propagate toward the cathode with higher transit speeds. Fits of surface potential decay measurements showed that charges in dry samples are injected directly into the volume, but the presence of moisture generates new trap centers in the surface of the material. This new trap level causes a charge accumulation on the surface, that gradually passes into the bulk. The observed behavior in development and propagation of charge packets are explained according to these results. Thermally stimulated depolarization current measurements showed a non-distributed relaxation associated to the new trap levels on the surface of the wet samples.

Effects of paper-aged state on space charge characteristics in oil-impregnated paper insulation

Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performance. The paper insulation degrades under the combined stresses of thermal, electrical and chemical stresses during routine operations, resulting in the reduction of its degree of polymerization (DP) and hence its mechanical and electrical strength. Space charges in a dielectric material have a close relation to its electrical performance. Therefore, in this work, the effect of paper-aged states (with different DP values) on the space charge behavior in oil-impregnated paper insulation is investigated, and the pulsed electro-acoustic (PEA) method is used for the space charge distribution measurement. In order to achieve this purpose, new oil-impregnated paper samples are thermally-accelerated aged for different time interval to obtain different DP values, and then the residual oil in each sample is removed, and the Kraft paper samples are dried in a vacuum dry oven. After that, all paper samples are impregnated with new oil, and PEA measurements are performed at room temperature. The experimental results show that, when the external dc voltage is applied on the oil-paper sample, homo-charge injection occurs for all samples. The samples with different aging states exhibit quite different space charge characteristics for both voltage-on and voltage-off conditions. When the aged states are worse, for voltage-on condition, more charges are induced at cathode and anode, as well as more space charges are accumulated in the bulk. For voltage-off condition, the space charge decay rate shows no obvious change for different aged samples.

Study of signal treatment for a pulsed electro-acoustic measurement cell: a way of improving the transfer matrix condition number

This work is focused on the improvement of the condition number of the transfer function matrix in a pulsed electro-acoustic (PEA) cell. A numerical electro-acoustic model is developed with the software COMSOL. This model is one-dimensional and the system of equations with partial differential functions is solved using a finite element method in non-stationary situations. Using this model, we can establish the output voltage of the piezoelectric sensor and acoustic pressure at each point of the calculation domain. Our approach consists in recovering the charge distribution within the sample using a deconvolution method between the simulated output voltage and the transfer function of the PEA cell. Results show why some changes of the PEA cell such as the nature of materials or sensor geometry involve an ill-posed problem or ill-conditioned problem, and why other arrangements lead to a well-conditioned problem, more amenable to giving the appropriate solution.

Relationship between space charge and nonlinear characteristics of ZnO varistor

In order to reveal the conduction mechanism of ZnO varistor, the pulsed electroacoustic (PEA) measurement is employed to investigate the space charge distribution characteristics of ZnO varistors under different current densities in this paper. Experimental results show that the relationship between the space charge and the current density is consistent with the voltage-current characteristics, and can reflect the nonlinear characteristics of ZnO varistor. The space charge increases linearly with the increase of the electric field strength and the current density in the small current region, this reflects the decrease of the surface states of the grain boundary and changes of the Schottky barrier. There exists a transition region between the small current and middle current regions, where the current is composed of the current by the emission of thermally activated electrons and the tunneling current. In the middle current region, the space charge will decrease as the current increases, and will disappear with the depletion layer decreasing to zero.

A new dielectric response model for water tree degraded XLPE insulation - part a: model development with small sample verification

Water tree degradation in underground XLPE insulated cables is a growing, worldwide problem. This form of degradation is ultimately fatal for affected cables, and therefore the detection of damaging trees in power cable insulation is vital for distribution companies to avoid catastrophic failure. Dielectric response measurements, in both the time and frequency domains, can generate valuable information about the condition of the cable. However, the interpretation of how these dielectric response measurements relate to water tree density and length is a difficult task. This paper will present a new dielectric response model for water tree degraded XLPE insulation. The model is based on finite element analysis to determine the electrical behaviour of water tree degraded insulation. Preliminary simulations will verify the model development by comparing the results to small sample pulsed electro-acoustic (PEA) measurements performed by other researchers. The importance of a strong non-linearity mechanism for accurate modelling will also be elucidated.

Measurement of the surface potential decay of corona-charged polymer films using the pulsed electroacoustic method

In this paper, the pulsed electroacoustic (PEA) technique that allows the determination of space charge in a dielectric material has been used to monitor the electrical potential decay of corona-charged polyethylene films of different thicknesses. To prevent possible disturbance of the surface charge during the PEA measurements, two thin polyethylene films were placed on both sides of the corona-charged sample. Charge profiles measured at different times were used to calculate the potential across the sample. The obtained potential decay was compared with the potential measured using the conventional method. Good agreement has been obtained. More importantly, the charge profile obtained using the PEA technique indicates that bipolar charge injection has taken place.

Space charge measurements on multi-dielectrics by means of the pulsed electroacoustic method

The pulsed electroacoustic (PEA) method is now widely accepted as one of the most simple and effective techniques for the measurement of the dynamic space charge distribution in solid dielectrics. Recently, the PEA method has been applied also to laminar test objects composed of two or more layers of different dielectrics (multi-dielectrics). However, when a multi-dielectric is tested by means of the PEA method, the different acoustic and electric properties of the materials affect the detected space charge signal. In this paper, the principle of the PEA technique is reviewed in case the test object is a multi-dielectric. The generation, transmission and reflection of electrically-induced acoustic waves are described. Based on the proposed approach, results of PEA measurements performed on various kinds of multi-dielectrics are presented and discussed.