Search

The Kerr electro-optic technique is used to investigate electric field and space charge effects using large Kerr constant propylene carbonate as the dielectric liquid with Kerr constant B≈1.22×10-12 m/V2. However, improved accuracy of this technique to measure space charge distributions remains an important goal. This paper describes inversion algorithm research on obtaining accurate space charge distributions from Kerr measurements. The original charge-coupled device (CCD) image of light intensity was converted into grayscale values for improved calculation. The grayscale CCD image was transformed into the frequency domain through a two-dimensional Fourier transformation. Butterworth filtering techniques were employed for edge enhancement algorithms. The most important parameter for calculation of space charge distributions are the integer fringe numbers n in the sequence of light and dark fringes measured by the CCD camera. Comparison of the improved algorithm and traditional methods for calculating electric field and space charge distributions between a pair of stainless steel parallel plate electrodes was investigated. The space charge dynamics were also calculated after processing the CCD images with some improvements. The results showed the electric field was essentially uniform at early times of pulsed high voltages. This uniformity indicated the absence of space charge during the early transient interval, while a bipolar charge injection was found in the central region between electrodes at later times. For longer times the electric field and space charge distributions slowly became uniform again. Grayscale processing and Butterworth filtering played an important role in the improvement of the algorithm for better image quality and edge enhancement. [ABSTRACT FROM PUBLISHER]

The electrical performance of composite insulators with adhered ice is less than that of porcelain or glass insulator strings, which is mainly caused by the shed configuration differences. In this paper, the shed con-figuration optimization for ice-covered Extra High Voltage (EHV) composite insulators is discussed in order to improve the electrical performance of ice-covered composite insulators. The effects of the position and length of the air gaps on the electric field distribution of ice-covered composite insulators are investigated and analyzed based on the electric field calculation model. The basic optimization principles of the shed con-figuration for ice-covered composite insulators are based on the electric field calculation results. Moreover, the artificial icing flashover tests on the ice-covered composite insulators with different shed configurations were carried out in artificial climate chamber, and the flashover path was recorded by a high speed cam-era. According to the flashover voltage and the flashover path of composite insulators with different shed configurations, it can be concluded that the composite insulators with large shed spacing and various shed diameters have better electrical performance and can be used in the transmission lines across the icing area [ABSTRACT FROM AUTHOR]

• A targeted self-healing composite was fabricated under capsule load as low as 4 wt%. • The magnetic guiding method achieves a fourfold rise in local capsule concentration. • Self-healing mechanisms were analyzed by a kinetic and a molecular dynamics models. One-component self-healing epoxy-matrix composites are needed urgently in the electronics industry because of their ability to repair surface damage autonomously. However, their practical applications are limited by unsuitable external stimuli and heavy microcapsule loads. Here, we report a magnetic-field and ultraviolet dual-responsive microcapsule to endow epoxy composites with a highly targeted self-healing capability under low microcapsule loads. Specifically, 5 wt% Fe 3 O 4 @SiO 2 nanoparticles that were incorporated into the core material served as magnetic targets and helped the microcapsules to navigate to the near-surface region of a high-failure probability, to form magnetically gradient-distributed microcapsule/epoxy composites. This resulted in a more than fourfold increase in the local microcapsule concentration. The successful targeted self-healing with a microcapsule doping percentage as low as 4 wt% was confirmed by laser-scanning fluorescence confocal microscopy and scanning electron microscopy observations, and electrochemical testing. In addition, a kinetic model based on isothermal photo-differential scanning calorimetry measurements and a molecular-dynamics simulation model were both established to analyze the self-healing mechanism. Because of the facility and universality of this magnetic control method, these magnetic microcapsules could be guided to concentrate in vulnerable parts of other electrical insulators as desired, which highlights the good potential of this targeted self-healing method in energy fields. [ABSTRACT FROM AUTHOR]

Under operational conditions of high electric fields and elevated temperatures, the accumulation of space charges at multilayer insulation interfaces is often considered as an important factor affecting insulation performance. This study experimentally explored the influence of different thermal aging degrees (110 °C for 0, 720, 1600, 2100, and 2900 h) on physicochemical characteristics. The space charge dynamics in two-layered thermally aged PET-PET films were measured using the pulsed electro-acoustic (PEA) method and simulated on the basis of a one-dimensional modified bipolar charge transport model. The parameterization for key parameters involved in the model was analyzed through parameter sensitivity. Results indicated that the molecular structure, crystallinity, and dielectric spectra of the PET films are affected by thermal aging. The thermalization process also has noticeable effect on the surface state characteristics, which are characterized by deeper trap depth and larger trap density. Several experimental phenomena measured by the PEA method were observed on the basis of numerical simulation. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) high-voltage direct current (HVdc) transmission is a promising method to integrate renewable energies. However, limiting the dc-link fault current and accelerating the fault-current clearance are critical issues for MMC-HVdc systems. In this article, we propose an air-core-transformer-based fault-current limiter (ACT-FCL) in cooperation with the dc circuit breaker to limit fault-current rising and decrease fault-clearing time. Compared with the conventional dc reactor, hybrid current-limiting circuit, current-commutation-based fault-current limiter (FCL), and magnetic coupling FCL, the ACT-FCL can increase the equivalent inductance during the dc fault stage to reduce the rising rate and peak of fault current. The ACT-FCL can also decrease the equivalent inductance during the fault-current-clearing stage to reduce the fault-clearing time. The simulations and laboratory experiments verify the effectiveness of the proposed ACT-FCL in the bidirectional HVdc systems. The ACT-FCL may be extended to more fields, such as dc microgrid and domestic dc transmission. [ABSTRACT FROM AUTHOR]

Despite being discovered more than 20 years ago, nanofluids still cannot be used in the power industry. The fundamental reason is that nano-insulating oil has poor stability, and its electrical performance decreases under negative impulse voltage. We found that C60 nanoparticles can maintain long-term stability in insulating oil without surface modification. C60 has strong electronegativity and photon absorption ability, which can comprehensively improve the electrical performance of insulating oil. This finding has great significance for the industrial application of nano-insulating oil. In this study, six concentrations of nano-C60 modified insulating oil (CMIO) were prepared, and their breakdown strength and dielectric properties were tested. The thermally stimulated current (TSC) curves of fresh oil (FO) and CMIO were experimentally determined. The test results indicate that C60 nanoparticles can simultaneously improve the positive and negative lightning impulse and power frequency breakdown voltage of insulating oil, while hardly increasing dielectric loss. At 150 mg/L, the positive and negative lightning impulse breakdown voltages of CMIO increased by 7.51% and 8.33%, respectively, compared with those of FO. The AC average breakdown voltage reached its peak (18.0% higher compared with FO) at a CMIO concentration of 200 mg/L. Based on the test results and the special properties of C60, we believe that changes in the trap parameters, the strong electron capture ability of C60, and the absorption capacity of C60 for photons enhanced the breakdown performance of insulating oil by C60 nanoparticles. [ABSTRACT FROM AUTHOR]

The multi-chamber arc-extinguishing structure (MAS), which consists of a lot of semi-closed short-gap arc-extinguishing chambers (SSAC) in series, can be used in parallel gap lightning protection devices to improve the ability to extinguish power frequency follow current. The arc-extinguishing ability of single SSAC directly affects the arc-extinguishing performance of the whole MAS. Therefore, the arc-extinguishing performance of MAS can be improved by optimizing single SSACs. A two-dimensional model of the arc plasma in a SSAC is built based on the magneto-hydrodynamic (MHD) theory. The motion characteristics of an arc in the SSAC are simulated and analyzed. An optimization method of the SSAC structure is proposed. Finally, an impact test platform is built to verify the effectiveness of the optimized SSAC structure. Results show that the short-gap arc forms a high-speed airflow in the SSAC and the arc plasma sprays rapidly to the outlet until the arc is extinguished at its current zero-crossing point. The amplitude of airflow velocity in the optimized structure can be increased to about 8-fold the velocity in the basic structure. Experiments also show that the dissipation time of an arc in the optimized SSAC is 79.2 μs, which is much less than that in the original structure (422.4 μs). [ABSTRACT FROM AUTHOR]

We prepared nano-C60 based insulating oil, which has the potential to overcome the application barriers of nanomodified insulating oil. We find that nano-C60 based insulating oil has an excellent stability. Its electrical performance increases by 17.9%, 9.3%, and 8.3% for AC and positive/negative lightning impulse voltage, respectively. We believe that C60 molecules have a strong capacity to absorb electrons and can capture photons in a streamer, which may weaken photoionization in the streamer and thereby improve the electrical performance of insulating oil. [ABSTRACT FROM AUTHOR]

This paper deals with the influence of microscopic morphological disturbances on the surface stability of streamer developing in a dielectric liquid. A dynamic model based on the stresses acting at the streamer channel surface is established, and the influence of different modes of microscopic morphological disturbances on the surface stability of streamer are analyzed. The model shows that disturbances on the streamer body can cause pinch-off when the applied voltage is low. At higher voltage, the short-wavelength disturbance is easier to cause unstable of streamer, such as branching, shrinkage, expansion and pinch-off, than long-wavelength disturbance. The symmetric disturbance grows faster than asymmetric disturbance. The streamer head is also sensitive to disturbances and becomes unstable. Depending on the form of the disturbance, the perturbation on streamer head may cause streamer branching and direction change. [ABSTRACT FROM AUTHOR]

Arc ablation is one of the important inducements leading to the surface insulation failure of epoxy resin (EP) break insulation in a HVDC circuit breaker. As a promising method to enhance the arc ablation resistance of EP insulation, the use of heat-conducting nano-fillers has always been confronted with the contradiction between the enhancement of thermal properties and that of mechanical strength. Up to now, the ablation characteristics of EP nanocomposite under short-time partial arc ablation is still poorly understood, and the synergistical enhancement of arc ablation resistance and mechanical strength of EP insulation is not realized as yet. Focused on these uncharted areas, in this work, we prepared a novel EP nanocomposite by blending method using dopamine-modified h-BN flakes and rigid α-Al 2 O 3 microsphere as binary modifiers. The results indicate that, though the arc ablation resistance of EP insulation can be improved to some extent after introducing functionalized h-BN, however, the tensile strength decreases significantly. In comparison, the h-BN&α-Al 2 O 3 binary nanofillers may not only improve the apparent activation energy of EP insulation in thermal decomposition, but also regulate the interfacial interaction between particle swarms and EP matrix. This is beneficial to relieve the partial carbonization on the insulation surface thereby improving the toughness of the composite, which provides a feasible method to synergistically enhance the multiple properties of EP insulation. [ABSTRACT FROM AUTHOR]

Arc ablation during the operation of HVDC circuit breakers can cause irreversible damage to the epoxy resin (EP) insulation pull rod. This is one of the critical factors that affect the reliability and lifetime of a HVDC circuit breaker. In this work, we built an arc ablation experimental platform and studied the thermal response properties of EP insulation under arc ablation. The surface charge evolution characteristics and the dielectric properties were analyzed to reveal the underlying surface insulation failure mechanism of EP after arc ablation. The results indicate that arc ablation can cause severe carbonization to EP insulation. With increasing arc duration, the EP gradually transforms into carbide and pore products. Arc ablation can decrease the molecular moment orientation polarizability and the relative dielectric constant of EP insulation, and thereby increasing the current density formed by Schottky injection. Therefore, when the external electric field is applied, the charge on EP insulation surface migrates from high voltage electrode to the ablating area, causing the regional accumulation of surface charge and the formation of local failure area. This can decrease the surface electrical performance of EP insulation and lead to insulation failure finally. [ABSTRACT FROM AUTHOR]

Polymer insulation materials are subjected to a combination of electromagnetic fields, mechanical stresses, long-term light exposure, ambient temperature and other operating conditions that produce mechanical and electrical tree damage which lead to insulation failure. Self-healing technology is a fundamental solution to this problem. However, effective long-term self-healing of mechanical and electrical tree damage to insulating materials is extremely challenging. To address this issue, this work reports a magnetic field dual responsive blueberry-like microsphere (MFDR-BLMS) that makes a breakthrough in the long-term self-healing of mechanical and electrical damage. The microspheres are attracted to the vulnerable area of the insulating material by the directional magnetic field. When the damage come into contact with the microspheres, the high thermal conductivity microspheres as a whole rapidly melt and fill the damage channels under the action of electromagnetic induction heating. This method achieves a magnetically dual-action, non-contact, long-term effective self-healing of mechanical and electrical tree damage after photothermal aging of insulation materials. This is expected to eliminate the requirement for human contact during the repair process under live conditions and provides a new technological idea for the industrial application of self-healing materials for electrical insulation. The smart phase change microspheres/EP composite are self-sensing, self-triggering and self-healing, and can repair mechanical and electrical tree damage over long periods of time under photothermal aging circumstances. When the doping concentration is less than 2 wt%, the effect on the matrix properties is minimal. After mechanical and electrical tree damage, the insulation strength of the composite recovered to 97.57 % and 92.38 % of that of the pure epoxy resin, respectively. [Display omitted] • Self-repairing after long term photo-thermal ageing and in the presence of mechanical and electrical tree damage. • The synthesis process does not produce any toxic substances and is environmentally friendly. • Dual action of magnetic fields, i.e. magnetic migration and electromagnetic induction heating. • Non-contact, directional multiple self-repair of insulation damage is achieved. [ABSTRACT FROM AUTHOR]

• A fast single-end relaying algorithm is developed for fault protection using bus-voltage change rate and fault component current. • It solves the problem the protection of the emerging fault between the DCCB and line reactor. • The wall bushing fault can be identified to avoid erroneous openings of healthy lines. • The method is with high selectivity, sensitivity, and reliability. Fault protection for a modular multi-level converter (MMC)-based high voltage direct current (HVDC) grid with dc circuit breakers (DCCBs) is a great challenge, because faults need to be isolated fast to prevent the collapse of the grid and overcurrent damage to the equipment. Before clearing faults, faults must be detected as soon as possible. However, the fast rising speed of fault currents and the emerging fault between the DCCB and line reactor increase the difficulty of fault protection. A fast and reliable fault detection and protection method relying on bus-voltage change rate and fault component current is proposed in this paper. It requires the measurements with local sensors and makes use of a lower number of voltage sensors. The proposed algorithm is applied to a four-terminal MMC HVDC grid. Results show that the proposed method can identify the fault type and discriminate bus faults, line faults, and wall bushing faults. It can detect faults quickly and does not generate erroneous and unnecessary trip signals. [ABSTRACT FROM AUTHOR]

• Method for describing the gradually quicker saturation process of the transformer core; • Approach to compute λ-i data points by several measured magnetizing inductances; • Modified dual reversible model considering the gradually quicker saturation process; • The proposed model can accurately predict the first peak of inrush current. The major nonlinear effects in the transformer core are saturation and hysteresis, in which saturation is the predominant effect for the transients involving saturation of transformers. A detailed representation of the saturation of the core is required. However, the available methods seldom provided experimental methods to represent the transients of the two magnetizing curves of the duality-based transformer model from slight saturation to deep saturation. This paper proposes a method to retrofitting the dual reversible transformer model with the gradually quicker saturation process of the core. Methods are proposed to measure the incremental inductances of the saturated core and using them to compute flux–current data of magnetizing branches of the proposed dual reversible π model. The entire magnetization curve is obtained by combining the unsaturated region by the available methods with the saturated region determined using the proposed methods. A single-phase two-winding transformer with a rated capacity of 500 VA is used to perform the proposed method to extract the required parameters of the proposed dual reversible transformer model. Inrush current tests are carried out and compared with simulations using ATP-EMTP to validate the model. Results show that the proposed model has better accuracy to simulate the first peak of the inrush current compared with the available model, contributing to the simulation involving low-frequency electromagnetic transients. [ABSTRACT FROM AUTHOR]

Wearable electronic devices with multifunctions such as flexible, integrated, and self-powered play a crucial role in the fields of health monitoring, motion monitoring, and human-computer interaction. However, their core basic components, flexible pressure sensors, face challenges including poor long-term stability and insufficient real-time sensing accuracy. In order to solve the challenges of long-term, stable, and accurate sensing of the sensor, this paper prepares polydimethylsiloxane (SHPDMS) with intrinsic self-healing property and designs a high-sensitivity self-healing capacitive flexible pressure sensor with dual microstructures (grating microstructured electrodes and microporous dielectric layer) as the substrate based on SHPDMS. Specifically speaking, the self-healing of the sensor under mild conditions was realized by introducing reversible imine bonds with low bonding energy into the polydimethylsiloxane (PDMS) flexible substrate, which solved the problem of the material's long-term service durability. A grating-like microstructure was introduced into the flexible electrode by using a spotted bamboo taro leaf as a template, and a dual microstructure sensor was constructed by combining it with a microporous dielectric layer doped with single-walled carbon nanotubes. This way reduces the elastic modulus of the dielectric layer, improves the dielectric constant of the sensor under loading, and thus significantly improves the sensor's sensitivity and extends the measurement accuracy in a low-stress range. The prepared self-healing flexible sensor achieves a sensitivity of 3.6 kPa -1 , a minimum detection limit of 5 Pa, a response recovery time of less than 80 ms, and stability over 5000 cycles, which exceeds most previously reported silicone rubber-based capacitive flexible sensors.

High-performance insulating materials are essential for developing lightweight, compact, and green offshore wind power equipment. It has been shown that nanoporous structures can limit the development of electron avalanche, leading to a significant increase in the breakdown electric strength of dielectrics. Hence, we fabricated a polysiloxane nanoporous biopolymer insulating material (PNBIM) with the nanoporous structure that presents exceptionally high electrically insulating properties. Under a CO 2 atmosphere, the breakdown electric strength of the PNBIM was remarkably improved, exhibiting 761.01 % enhancement relative to that of the pure gas gap. Moreover, the PNBIM demonstrates excellent dielectric performance, with a low dielectric constant (1.63) and dielectric loss (0.014), as well as high hydrophobicity and excellent thermal stability. The investigations revealed that the nanopores inside the material can effectively suppress gas discharge and improve the breakdown electric strength. This study widens our understanding of conventional insulating materials and provides a promising approach for exploring high-performance insulating materials provide valuable insights., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Small defects induce concentrated electrical stress in dielectric polymers, leading to premature failure of materials. Existing sensing methods fail to effectively visualize these defects owing to the invisible-energy state of the electric field. Thus, it is necessary to establish a nondestructive method for the real-time detection of small defects in dielectric polymers. In this study, a self-reporting microsensor (SRM) inspired by Noctiluca scintillans is designed to endow materials with the ability of self-detection for defects and electrical stress. The SRM leverages the energy of a nearby electric field to emit measurable fluorescence, enabling defect localization and diagnosis as well as electrical-stress visualization. A controllable dielectric microsphere is constructed to achieve an adjustable electroluminescence threshold for the SRM, thereby increasing its detection accuracy while decreasing the electroluminescence threshold. The potential degradation in the polymer performance owing to SRM implantation is addressed by assembling long molecular chains on the SRM surface to spontaneously generate an interpenetrating network. Results of finite element analyses and experiments demonstrate that the SRM can effectively realize nondestructive visualization and positioning of small defects and concentrated electrical stress in polymers, positioning it as a promising sensing method for monitoring the electric field and charge distribution in materials., (© 2024 Wiley‐VCH GmbH.)

The long-term operation of power equipment and power electronics can cause local overheating and discharges in the insulation material, resulting in irreversible insulation damage. Further development of such damage can eventually lead to equipment failure, but this problem is very difficult to solve. In this paper, inspired by how the petals of morning glory change color with the environment due to the presence of pigmented globules, a dual-function heat alert in the form of a self-healing (HASH) microcapsule with a nested structure is prepared by using microfluidic technology. By combination of the microcapsule with the insulation material, the local overheating in equipment can be detected promptly under live operating conditions without manual external intervention, and the defects that occur can be repaired autonomously. These HASH microcapsules can be pre-embedded in places at which the material is prone to overheating using artificial magnetic targeting. The doping of the matrix material with microcapsules does not cause any deterioration in its electrical or mechanical properties. This technology is expected to be applied to electrical equipment and electronic devices to allow for the early detection of local overheating and the autonomous repair of defects, thereby ensuring the safety of the equipment and improving its service life.

The development of high-precision, non-destructive, and three-dimensional (3D) in situ imaging of micro-scale damage inside polymers is extremely challenging. Recent reports suggest that 3D imaging technology based on micro-CT technology causes irreversible damage to materials and is ineffective for many elastomeric materials. In this study, it is discovered that electrical trees inside silicone gel induced by an applied electric field can induce a self-excited fluorescence effect. Based on this, high-precision, non-destructive, and 3D in situ fluorescence imaging of polymer damages is successfully achieved. Compared with the current methods, the fluorescence microscopic imaging method enables slicing of the sample in vivo with high-precision operation, realizing the precise positioning of the damaged area. This pioneering discovery paves the way for high-precision, non-destructive, and 3D in situ imaging of polymer internal damage, which can solve the problem of internal damage imaging in insulating materials and precision instruments., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Despite the proposal of nanodielectrics in 1994, the impact of nano- and microstructures on composite performance is still not completely understood. A key reason for this knowledge gap is the lack of in situ characterization of micro- and nanoscale structures within materials. In this study, we observed the self-excited fluorescence of a microscale-damaged microchannel inside a composite under the influence of an electric field. Furthermore, we conducted in situ imaging of the internal microstructures and discharge channels in the composite utilizing external laser excitation. The imaging results reveal that the electrical treelike damage in the composites grows with a single channel under the guidance of the nanoskeleton embedded in the matrix, which demonstrates that the three-dimensional (3D) nano-order skeleton hinders the development of electrical trees. Furthermore, we analyzed the nanoskeleton intervention's enhancement mechanism on the insulation properties of the composites. This work aids in the precision imaging-guided structural design of nanodielectrics.

During the long-term operation of solid insulation materials, strong electric fields and mechanical stress cause electrical trees and cracks that are undetectable and irreversible, leading to the failure of electronic and electrical devices. A promising means of protecting against these problems is to endow the insulating materials with some self-healing capability alongside their excellent intrinsic properties. However, this has proved extremely challenging. In this paper, we describe an ultraviolet light, moisture, and magnetic field triple-response microcapsule that enables epoxy resin materials to heal themselves against various forms of damage without affecting the intrinsic performance of the matrix. In particular, microcapsules wrapped inside functional shells containing Fe 3 O 4 nanoparticles are precisely controlled by a targeted magnetic field and distributed in the vulnerable area of the insulation materials, resulting in a high healing rate at low doping concentrations. Using the in situ ultraviolet light emitted by the electrical trees, artificial ultraviolet light, and moisture in the operating environment, it is possible to induce active or passive curing of the healing agent, thus realizing the intelligent, non-contact, and targeted self-healing of mechanical cracks and electrical tree damage. This method opens an avenue toward the development of self-healing insulation materials for electrical and electronic applications.