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There is frequent sandstorm weather in the northwest of China. Roof insulator is exposed to strong wind and sand environment for a long time. A large amount of sand will be deposited on the surface, resulting in a reduction of insulation strength along the insulator surface and a gap between the sheds. Also, affected by the abrasive action of sand, the roof insulator surface hydrophobicity decreases, which is easy to trigger the insulator flashover, threatening the safety of train operation. For this purpose, this article reviews the sand charging mechanism, sand accumulation characteristics, surface discharge characteristics, air-gap breakdown characteristics, flashover characteristics of the insulator, and the effects of surface abrasion on the insulation performance in wind and sand environment. Based on the train operating conditions, future research directions are proposed to be carried out. First, it is necessary to study the sand accumulation characteristics of roof insulators under multifactor coupling and establish a simulation model that reflects the distribution and content of sand. Secondly, it is necessary to carry out research on the flashover characteristics of real roof insulators and higher speed wind and sand environments, and to build the quantitative relationship between wind speed, sand particle size, sand concentration and the gap breakdown voltage. Finally, a method to evaluate the abrasion state of insulator surfaces needs to be proposed, as well as to explore new materials and structures for roof insulators in strong wind and sand environment.

Insulators play a crucial role in electric power systems by isolating high-voltage currents and supporting conductors. However, insulators exposed to outdoor conditions for extended periods are susceptible to faults caused by contamination, aging, and damage, leading to flashovers that may result in grid outages and economic losses. Accurate assessment of insulator condition is paramount for the safe operation of power systems. Noncontact monitoring methods, which do not interfere with the normal operation of the power system, have become mainstream in insulator condition monitoring, contrasting with traditional contact-based detection. Among these methods, optical imaging technology has garnered significant attention due to its noncontact, nondestructive, high sensitivity, and intuitive characteristics. Optical imaging technology captures reflections, transmissions, and absorptions in different spectral bands to gather information about the surface and interior of insulators, thereby determining the type of faults they may exhibit. This article reviews optical imaging detection methods applied in the field of insulator condition assessment, including infrared thermography, ultraviolet imaging, visible light imaging, and some promising optical monitoring technologies. Additionally, the advantages and limitations of these methods under various insulator fault conditions are analyzed, and challenges in industrial applications are discussed. The purpose of this article is to provide a comprehensive and systematic reference for the application of optical imaging technology in insulator condition assessment, aiming to enhance the efficiency and safety of power system operation and maintenance.

As the unique power-discharging channel for high-speed trains, the ‘train-rail’ mobile coupling grounding system plays a vital role in sending the traction power back to the terrestrial substations, through the electrical contact between the rotating wheel and the steel rail. The whole system is composed of two kinds of grounding modes: working grounding is in charge of draining the traction power into the rail track; the protective grounding provides the reference grounding potential for the vehicle-mounted equipment. Different from the fixed terrestrial grounding technique normally implemented in power system, when the relative motion occurs between the train and the rail, both the ‘catenary-to-train’ and ‘train-to-ground’ impedances vary transiently, which may lead to the current flowing through each grounding wheel varying unpredictably. Herein, the ‘catenary-train-rail’ mobile coupling grounding model was built based on the equivalent circuit modeling technique for evaluating the transiently-varying tendency of the grounding impedances and currents flowing through each grounding wheel. To cope with different operational conditions, the adjustable grounding impedance methodology is firstly proposed for adapting the unpredictable variation of the grounding impedance, when the relative motion occurs between the train and the rail. Ultimately, the validity of the novel grounding system applying the adjustable grounding impedance technology is verified under different operational conditions, with the consideration of both TB current and voltage.

Composite insulators are prone to accelerated aging in coastal and industrially polluted environments, leading to flashovers, power grid outages, and economic losses. Traditional detection methods either require power grid shutdown or lack adequate evaluation capabilities. This research introduces a pixel-level assessment of the aging status of composite insulators using hyperspectral imaging (HSI) technology. And proposed a least squares support vector machine (LSSVM) based on Improved Beluga Whale Optimization (IBWO) algorithm to evaluate the aged levels of composite insulators. First, artificial accelerated aged samples are categorized into aging levels I-VI using the static contact angle. Hyperspectral data in the 400 nm–1040 nm wavelength range are acquired using a HSI device, followed by preprocessing steps involving denoising and dimensionality reduction. Subsequently, the IBWO algorithm is employed to identify the optimal parameters (C, σ) for LSSVM. The performance of IBWO was compared with other optimization algorithms on test functions, demonstrating that IBWO exhibits the optimal convergence speed and optimization accuracy, effectively enhancing the classification and generalization ability of LSSVM. In this study, the proposed method was compared with other algorithms using k-fold cross validation, resulting in overall accuracy of 96.83% and demonstrating its superior classification capability. The presented method enables pixel-level assessment of the aging degree and visualizes the distribution of aging states in composite insulators. It provides valuable guidance for studying the aging characteristics and structural design in diverse complex environments, holding significant potential for noncontact evaluation of other electrical equipment.

The detection of defect severity in cable terminations plays a critical role in ensuring the safe and stable operation of high-speed trains (HSTs). However, the partial discharge (PD) characteristics of the same type of defect can appear similar across different severities, posing challenges for accurate insulation defect degree identification. Consequently, this article proposes an image transformation method, named the equivalent bandwidth matrix of relative locations (EBMRLs), coupled with the self-guided transformer (SG-Former) algorithm, which is more effective for fine-grained image recognition, to accurately identify different degrees of defects with similar PD characteristics. In the proposed approach, the original PD signals are first converted into images using EBMRL. This transformation embeds the characteristic and bandwidth information from the original PD data into the images, thereby reducing the similarity of information between classes in the transformed images and enhancing their distinguishability. Subsequently, the local and global features of the transformed EBMRL images are extracted to train the SG-Former model. The model is finally utilized to identify the severity of defects in cable terminations. The results demonstrate that the method proposed in this article achieves better performance compared with some of the state-of-the-art methods.

Background: This paper proposes a defect identification method for vehicle-mounted cable terminals in electric multiple units (EMUs) based on the improved Graham angle field and residual network to address the issue of low recognition accuracy caused by the lack of partial discharge (PD) and identification data for Ethylene Propylene Rubber (EPR) cable terminal defects. Methods: The improved Gramian angular field (IGAF) characteristic transformation method was used to transform the PD one-dimensional time-series signal into a two-dimensional one after cable terminals with four common insulation defects were constructed, and a PD detection platform was built. Finally, an anti-aliasing downsampling module and attention mechanism were added to the residual network ResNet101 model. The Center loss and Softmax loss functions were integrated to increase accuracy for training and recognition classification. Topological feature images improved the distinguishability of defect categories. Results: The test results showed that the diagnostic method has an accuracy rate of 97.3% for identifying PD at the cable terminal. Conclusion: The proposed diagnosis model has higher recognition accuracy and better balance than other conventional fault diagnosis methods, making it suitable for diagnosing high-voltage cable faults in EMU trains.

As a unique reflux path for trains, the ‘train-rail’ mobile grounding system has the double functions – the traction power feedback and the transmission of train's communication signal. With the constant increment of operational velocity and traction power for high-speed railway, the coupling relationship existing in the ‘train-rail’ mobile coupling grounding system becomes complicated, when the train's wheels roll over the insulated joint. It may result in the frequent occurrence of rolling arc and the ablation of rail ends and insulated joints, which might lead to the series connection of the rail circuit or even trigger the misjudgment of operational control system. As the impedances existing in the grounding system vary transiently when the relative distance between the train and the fixed terrestrial grounding installations changes, the dynamic coupling mechanism between the transiently-varying topology of reflux path and the distribution of reflux at each grounding wheel tends to be analyzed. Depending on the built ‘catenary-train-rail’ dynamic coupling grounding reflux model, the correlation between the ‘train-rail’ mobile grounding reflux and the evolutionary process of rolling arc is explored. In order to reduce the risk of ‘wheel-rail’ arcing phenomenon, a novel method of automatically controlling insulated joint is proposed herein, as its feasibility is verified via the comparison with the original type of the insulated joint.

This article reviews the typical faults of and detection methods for transformer bushings and forecasts the future trends of transformer-bushing detection methods.

So far, silicone rubber (SiR) insulators are widely used in the power industry all over the world, due to their outstanding insulation properties and high reliability. In practice, some of the insulators are applied in remote places with harsh environmental conditions such as high altitude, high temperature, high humidity, etc. Herein, the currently existing aging mechanisms are investigated, and some novel insights into these testing and evaluation methods are proposed. Via the investigation, it was found that thermal oxygen aging tends to occur when insulators are exposed to an environment surrounded by hot air. To analyze the different aging mechanisms of SiR insulator sheds, four types of aging testing and evaluation methods are systematically summarized based on the surface’s structural, chemical, optical, and electrical properties. The conclusion is that nondestructive and noncontact methods, such as hyperspectral imaging (HSI), along with the multisource information fusion methods, are becoming increasingly popular in the assessment of SiR insulator sheds aging. These research efforts provide assurance for reducing the incidence of pollution flashover accidents in power transmission lines and ensuring the safe and stable operation of power transmission and transformation equipment.

ABSTRACTDue to the constant pursuit for higher operating velocity, the traction power capacity for high-speed trains needs to be enlarged. When the train passes approach the insulating sections set between the two neighbouring contact lines carrying the traction current with different phases, the vehicle-mounted vacuumed circuit breaker (VCB) needs to be switched off. Generally, the arc may generate when connecting or separating the contacts, when switching on or off the VCB. In order to investigate the characteristics of the reignition overvoltage the ‘train-rail-catenary’ traction power supply model considering the reignition process between the VCB’s contacts is launched in this paper. The impact of the dielectric strength recovery rate between the contacts of the VCB on the number of reignition and the amplitude of the reignition overvoltage is analysed based on the experimental and simulation results. Ultimately, the corresponding restraining measures are proposed for suppressing the reignition overvoltage.

For capacitive energy storage at elevated temperatures1–4, dielectric polymers are required to integrate low electrical conduction with high thermal conductivity. The coexistence of these seemingly contradictory properties remains a persistent challenge for existing polymers. We describe here a class of ladderphane copolymers exhibiting more than one order of magnitude lower electrical conductivity than the existing polymers at high electric fields and elevated temperatures. Consequently, the ladderphane copolymer possesses a discharged energy density of 5.34 J cm−3with a charge–discharge efficiency of 90% at 200 °C, outperforming the existing dielectric polymers and composites. The ladderphane copolymers self-assemble into highly ordered arrays by π–π stacking interactions5,6, thus giving rise to an intrinsic through-plane thermal conductivity of 1.96 ± 0.06 W m−1K−1. The high thermal conductivity of the copolymer film permits efficient Joule heat dissipation and, accordingly, excellent cyclic stability at elevated temperatures and high electric fields. The demonstration of the breakdown self-healing ability of the copolymer further suggests the promise of the ladderphane structures for high-energy-density polymer capacitors operating under extreme conditions.

Strong ultraviolet (UV) radiation is one of the main factors that lead to the aging of composite insulators of transmission lines in high-altitude areas, which has brought security risks to the operation of power systems. The aging status information obtained by traditional insulation surface aging detection methods is indirect and limited, or time-consuming and labor-intensive. In this article, a non-contact non-destructive testing evaluation method based on hyperspectral technology is proposed to quickly detect the UV aging state of composite insulator surface. First, the hyperspectral imaging platform is used to image a number of continuous bands on the insulator surface. Combining the aging calibration data, a locally reserved projection-convolutional neural network (LPP-CNN) model is established to extract the characteristic bands of the aging surface and provide a way to accurately evaluate the surface UV aging degree of insulators. The model has a high resolution of aging degree, which provides a new idea and theoretical basis for realizing non-contact non-destructive online detection of surface aging under UV light.

The mobile distributed grounding system plays an irreplaceable role in discharging power for intercity trains, including two types of grounding settings (GSs): the working GS provides the pathway for discharging the traction current; the protective GS provides the reference grounding potential for the vehicle-mounted equipment. As both working and protective GSs share the rail as the common current-releasing pathway, the traction current released by working GSs might return the carriage through the adjacent protective GSs, forming as the train body (TB) current which threatens the vehicle-mounted electrical equipment. Herein, an equivalent circuit of the mobile distributed grounding system is built to analyze the TB current. For evaluating the influence brought by TB current on the vehicle-mounted equipment, a 3D finite element (FEM) geometry is launched, meanwhile the comparison of the multi-physical distributions is undertaken when applying different grounding layouts, with the emphasis of the highlighted region containing critical devices.

Pollution flashover of insulators has been viewed as a common issue existing in the area of power systems for a long term. However, traditional detection methods such as equivalent salt density and leakage current (LC) can hardly satisfy the requirements of the progressing smart grid. Herein, a novel highly-efficient method is proposed for classifying the pollution degree detection of insulators based on hyperspectral technology. The insulating sheets and actual insulators were set as the research object and their hyperspectral images are obtained. Moreover, the data for describing the characteristic wavelengths and the G color components of the samples were extracted from the images as the characterization of the equivalent salt deposit density (ESDD) and nonsoluble deposit density (NSDD) respectively. The ESDD and NSDD detection model is established based on the multiple linear regression (MLR) algorithm and the random forest (RF) algorithm. Ultimately, the proposed model reaches a detection accuracy of above 84% on the insulating sheets and 75% on the actual insulators, which can be viewed as acceptable. The hyperspectral imaging technique has considerable potential for the non-contact detection of ESDD and NSDD.

As the unique power entrance, the pantograph–catenary electrical contact system maintains the efficiency and reliability of power transmission for the high-speed train. Along with the fast development of high-speed railways all over the world, some commercialized lines are built for covering the remote places under harsh environment, especially in China; these environmental elements including wind, sand, rain, thunder, ice and snow need to be considered during the design of the pantograph–catenary system. The pantograph–catenary system includes the pantograph, the contact wire and the interface—pantograph slide. As the key component, this pantograph slide plays a critical role in reliable power transmission under dynamic condition. The fundamental material characteristics of the pantograph slide and contact wire such as electrical conductivity, impact resistance, wear resistance, etc., directly determine the sliding electrical contact performance of the pantograph–catenary system; meanwhile, different detection methods of the pantograph–catenary system are crucial for the reliability of service and maintenance. In addition, the challenges brought from extreme operational conditions are discussed, taking the Sichuan–Tibet Railway currently under construction as a special example with the high-altitude climate. The outlook for developing the ultra-high-speed train equipped with the novel pantograph–catenary system which can address the harsher operational environment is also involved. This paper has provided a comprehensive review of the high-speed railway pantograph–catenary systems, including its progress, challenges, outlooks in the history and future.

High-frequency (HF) oil-immersed transformers with small size and light weight have been paid wide attention by researchers. Its oil–paper insulation frequently withstands HF over-voltages at frequencies of several kilohertz and above, resulting in the too early failure of oil–paper insulation. In order to investigate the effect of HF on oil–paper insulation, this article investigates the partial discharge (PD) characteristics of oil–paper insulation under HF stress. The experimental results show that a frequency-induced inflection point is existed near 10 kHz and the PD parameters of HF voltages is much higher than that ac voltages. Besides, the theoretical mechanism is expounded to interpret and understand the formation process of frequency-induced inflection points more easily. It can be found that the change in frequency causes the change in the charge decay time constant and the residual charge on the insulating surface, resulting in the formation of the frequency-induced inflection point at HF. These results might contribute to determine the characteristics of PDs between HF voltages and ac voltages. It can provide references for the research and production of HF transformers.

Renewed interest in the reinforced carbon graphite composites has intrigued the community in the advanced materials fields. In this work, we present a simple carbon nanofibers reinforced carbon graphite composites synthetic method by incorporating mixture of coal tar pitch, synthetic graphite, pitch coke and the dispersion liquid of carbon nanofibers via liquid-phase mixing process. The impact of carbon nanofiber utilization on the microstructures and mechanical properties of carbon graphite composites are studied systematically. The covalent surface modification of carbon nanofibers effectively improves its microstructure and thereby promotes the carbon graphite composites’ dispersion behavior. We propose that a small amount of carbon nanofibers could promote the carbonization process of carbon graphite composites, facilitating the densification of carbon graphite composites and reducing the undesired open porosity. The amount of 0.7 wt % of carbon nanofiber concentration allows the enhancement of bend and compressive strength of carbon graphite composites up to 36.50 MPa and 60.46 MPa, increased by 167.9% and 146.9% compared with the pure carbon graphite composite, respectively. Our findings can be rationalized due to the improvement in the mechanical strength of carbon graphite composites could be attributed due to pull-out of carbon nanofibers from the matrix and bridging effect across the crack pores within the matrix.

[Display omitted] • Graphene hybrid aerogel with an anisotropic well-oriented structure was designed. • Aerogel-200 has super-hydrophilicity and outstanding wet-resilience properties. • Aerogel-1500 exhibits superior thermal conductivity enhancement in polymers. Three-dimensional (3D) graphene-based aerogels have significant potential for adsorption, sensors, and thermal management applications. However, their practical applications are limited by their disorganized structure and ultra-low resilience after compression. Some methods can realize a well-aligned structure, however, they involve high costs and complex technology. Herein, a 3D graphene hybrid aerogel with an anisotropic open-cell and well-oriented structure is realized by unidirectional freeze casting, which combines the 'soft' (e.g. graphene oxide, Tween-80) and 'hard' (e.g. graphene assembly) components to realize full recovery after flattening. A graphene aerogel annealed at a moderate temperature (∼200 °C) can possess superhydrophilicity and outstanding wet-resilience properties, including after being pressed under 40 MPa. Furthermore, the graphene aerogel annealed at a high temperature of ∼1500 °C exhibits excellent thermal conductivity enhancement efficiency in polydimethylsiloxane (PDMS). The resultant nanocomposites clearly demonstrate anisotropic thermal conductivity and promising applications as thermal interface materials. This strategy offers new insights into the design and fabrication of 3D multifunctional graphene aerogels. [ABSTRACT FROM AUTHOR]

Along with the operating velocity and traction power increasing continuously, the issue related to the safety and reliability of the high-voltage traction power system for high-speed trains have gradually been exposed during the practical service process. When the train passes a neutral section settled roughly per 15 km between two neighboring substations, the on-board vacuum circuit breaker (VCB) must to be operated for changing the input powers with different phases. At the moment of switching on or off the VCB, the arc between the contacts in the arc-extinguishing chamber may be triggered, which tends to result in the operational overvoltage. Due to numerous inductances and capacitances existing in the traction power supply system, the overvoltage may lead to high-frequency electromagnetic oscillation spreading along the transmission routes. The on-board high-voltage equipments within the power supply system may suffer from the impulse brought from the VCB operational overvoltage frequently, which is possible to cause the insulation aging or breakdown issue. To investigate the characteristic of the operational overvoltage of VCBs, a ‘train-rail-catenary’ power supply model is built based on the measured impedance parameters, which has been verified by the experimental tests. The generating mechanism with the influence factors of the operational overvoltage when switching on or off the VCB has been explored, meanwhile the distribution of the train body overvoltage on each carriage has also been presented.

In recent years, the speed and the current transmission density of high-speed railways are increased. As a result, the rate of the occurrence of pantograph–catenary arc also increases, which directly influences the service performance of the pantograph–catenary system. The detection and identification of the pantograph–catenary arc is of great significance for evaluating the off-line level to ensure the safe operation of the pantograph–catenary system. The currently used conventional optical arc detection method would be distorted by strong sunlight or other environmental lights in the actual operation, which would affect the accuracy of detection. On the contrary, the arc sound signal is not affected by the diurnal variation and can be easily obtained; it can also be distinguished well from other interfering sounds. In this paper, a new method that uses the arc sound is proposed to realize the effective detection of the pantograph–catenary arc. The frequency spectrum and the intensity characteristics of the pantograph–catenary arc sound are analyzed. The results show that the frequency band of the arc sound signal is wide and mainly gets distributed at 5–17 kHz. The curve of the sound waveform is drawn by calculating the short-time average energy, which is used to obtain the arc’s starting time, duration, and intensity, for calculating the off-line rate.

Lithium-ion battery State of Health (SOH) estimation is an essential issue in battery management systems. In order to better estimate battery SOH, Extreme Learning Machine (ELM) is used to establish a model to estimate lithium-ion battery SOH. The Swarm Optimization algorithm (PSO) is used to automatically adjust and optimize the parameters of ELM to improve estimation accuracy. Firstly, collect cyclic aging data of the battery and extract five characteristic quantities related to battery capacity from the battery charging curve and increment capacity curve. Use Grey Relation Analysis (GRA) method to analyze the correlation between battery capacity and five characteristic quantities. Then, an ELM is used to build the capacity estimation model of the lithium-ion battery based on five characteristics, and a PSO is introduced to optimize the parameters of the capacity estimation model. The proposed method is validated by the degradation experiment of the lithium-ion battery under different conditions. The results show that the battery capacity estimation model based on ELM and PSO has better accuracy and stability in capacity estimation, and the average absolute percentage error is less than 1%.

Pantograph arcing is an unavoidable phenomenon in electrified railways, which not only causes damage to the carbon strip and the catenary contact line but also results in voltage surge and electromagnetic interference. In recent years, there are more cases of pantograph arcing due to the increase in speed of trains. Therefore, it is essential to understand the basic electrical characteristics of pantograph arcing at different running speeds of trains. In this work, a pantograph arcing model was proposed, which considers the effects of the speed of a train on the arc-dissipated power. An overall electrical model, concerning the traction power system and the traction drive system, was further established. The results indicated that the running speed of trains significantly influenced the arcing voltage, duration of arcing, and the zero-crossing time. A qualitative relation between the average power of the arc and the speed of the train was also presented. Finally, field tests were carried out, and comparisons between the field testing data and the calculated results were made, which validate the accuracy of the developed model.

Lithium-ion battery (LIB) capacity degradation prediction plays an important role in the prediction of battery health degradation. Accurate prediction of its capacity can guide battery replacement and maintenance, and ensure the safety and stability of the energy storage system. In this paper, a hybrid method based on artificial bee colony (ABC) algorithm and multi-kernel support vector regression (MK-SVR) is proposed to predict the capacity degradation of LIB. Firstly, the capacity degradation prediction model of LIB is established by MK-SVR with few hyperparameters. Then, the hyperparameters of degradation prediction model are optimized by the ABC to improve the accuracy of prediction. Finally, the proposed method is verified by NASA's LIB degradation experiment. Compared with different optimization algorithms, ABC greatly improves the accuracy of capacity degradation prediction. Compared with traditional forecasting methods, the proposed method can predict the capacity degradation of LIB more accurately.

A good contact between the pantograph and catenary is critically important for the working reliability of electric trains, while the basic understanding on the electrical contact evolution during the pantograph–catenary system working is still ambiguous so far. In this paper, the evolution of electric contact was studied in respects of the contact resistance, temperature rise, and microstructure variation, based on a home-made pantograph–catenary simulation system. Pure carbon strips and copper alloy contact wires were used, and the experimental electrical current, sliding speed, and normal force were set as 80 A, 30 km/h, and 80 N, respectively. The contact resistance presented a fluctuation without obvious regularity, concentrating in the region of 25 and 50 mΩ. Temperature rise of the contact point experienced a fast increase at the first several minutes and finally reached a steady state. The surface damage of carbon trips in microstructure analysis revealed a complicated interaction of the sliding friction, joule heating, and arc erosion.

The interfacial interaction between the carbonized matrix and binder directly correlates with the performance of the carbon-matrix composites. The involvement of the reduced graphene oxide (rGO) in composites is considered an effective strategy to enhance the performance of composites via improving the interface property. Herein, the rGO was converted in-situ from the graphene oxide (GO) on the interface between the matrix and the carbonized binder during the sintering process. Compared with the pristine composites, the compressive strength (203.90 MPa ±2.85 MPa) and flexural strength (40.53 ± 1.42 MPa) of carbon-matrix composites doping 0.2 wt% of GO can be increased by 112.1% and 71.2%, respectively. It is attributed to the fact that rGO forms an ideal interface bond with the carbonized matrix and binder, which effectively promotes the deflection of the crack propagation path. Meanwhile, molecular dynamics simulations unlocked an automatic protective reinforcement mechanism of the rGO-enhanced composites. A low-cost yet effective scheme for improving composites interfaces is proposed here, which lays a theoretical foundation for developing the high-strength carbon-matrix composites.

The poor interfacial interaction between carbon fiber (CF) and a matrix is the main factor restricting the enhancement in performance of carbon-matrix composites. Herein, CF was modified via a facile high-speed blending with matrix pitch-cokes particles. AFM force measurements and 3D particle tracking simulations unlocked the blending CF@pitch-cokes assembly mechanism. Inspired by the strong interaction between tree roots and soil, a tree-root-like reinforced structure was designed and developed in carbon-matrix composites by the subsequent carbonization coupling effect between the pitch-cokes adsorbed CF surface and the carbonized matrix. It was found that the tree-root-like structure significantly improved the interfacial interaction and enhanced the mechanical properties of the carbon-matrix composites. Compared with the pristine CF reinforced composites, the compressive strength (158.33 ± 1.95 MPa) and flexural strength (36.65 ± 1.32 MPa) of the carbon-matrix composites with tree-root-like structures increased by 101.4% and 65.8%, respectively. A mechanical interlock mechanism is suggested to explain the enhanced mechanical properties of the composites. This work provides a facile strategy for improving the interface of CF-reinforced carbon-matrix composites and paves the way for scalable, low-cost, and environment-friendly processing of high strength composites.

The inverter-fed motor is a key component of environmental-friendly hybrid cars, high-speed trains, and other industrial applications. After the widespread use of inverter-fed motors, the repetitive impulse surges from inverters were found to cause new insulation problems in inverter-fed motors that were linked to premature insulation failure. The partial discharge (PD) induced by the overvoltage inner stator windings was found to be the root cause of insulation reliability problems. To mitigate this problem, PD detection and life testing at repetitive square voltages should be performed on new types of insulation models and entire motor insulation systems. The obtained partial discharge inception voltage (PDIV) and lifetime of insulation in the tests can be used to evaluate the insulation status of inverter-fed motor insulation systems and thus to improve the insulation designs. At AC voltage conditions, the voltage frequency can significantly affect PD activity. At repetitive square voltage with rapid rise time, the influence of the square voltage frequency on PD activity should be investigated (1) to suggest values for the repetitive square voltage frequency selection for the PDIV and lifetime tests, and (2) to increase the signal-to-noise ratio when extracting PD pulses from strong disturbances generated by the operating switch of impulse generators. Therefore, the main purpose of this paper was to investigate the effects of repetitive square voltage frequency on PD events. Insulation models of both Type I (low-voltage inverter-fed motor) and Type II (high-voltage inverter-fed motor) were used to obtain single-PD and multi-PD statistical features, respectively. Experimental results indicated that higher frequencies caused less PD events with lower PD magnitudes and shorter delay times in one square voltage cycle. This phenomenon was attributed to the different surface charge decay ratios and the initial electron emission probabilities resulting from different square voltage frequencies. Finally, a number of considerations for PD measurements on inverter-fed motors were provided to improve the sensitivity of the PD test system and to obtain high objective insulation lifetime test results, which are used to assess the insulation status of inverter-fed motors.

In this paper, an optimized transmission line model (OTL) for modeling transient behavior of grounding electrodes under lightning currents is presented. The soil ionization effect is considered in OTL, and all electromagnetic couplings between different parts of grounding electrode are also considered by selecting the size of segment conductor properly and calculating the mutual coupling parameters between segment conductors accurately. Comparing with the traditional transmission line model, the optimized model can be used to accurately predict the effective length and transient potential rise (TPR) of grounding electrodes. Transient behaviors of grounding electrodes are simulated by OTL and the results are in good agreement with those of the electromagnetic model proposed by Grcev, and experiment results performed by Electricité de France and Geri. Furthermore, non-uniform discharging phenomenon of grounding electrode under lightning current is discussed, and the effective lengths of horizontal grounding electrode under lightning currents are presented.In this paper, an optimized transmission line model (OTL) for modeling transient behavior of grounding electrodes under lightning currents is presented. The soil ionization effect is considered in OTL, and all electromagnetic couplings between different parts of grounding electrode are also considered by selecting the size of segment conductor properly and calculating the mutual coupling parameters between segment conductors accurately. Comparing with the traditional transmission line model, the optimized model can be used to accurately predict the effective length and transient potential rise (TPR) of grounding electrodes. Transient behaviors of grounding electrodes are simulated by OTL and the results are in good agreement with those of the electromagnetic model proposed by Grcev, and experiment results performed by Electricité de France and Geri. Furthermore, non-uniform discharging phenomenon of grounding electrode under lightning current is discussed, and the effective lengths of horizontal grounding electrode under lightning currents are presented.

Due to overvoltage produced by inverter output, inverter-fed motor insulation systems often experience fast electrical aging process, especially when partial discharge (PD) is incepted. Before putting into use, the PD detection should be performed on inverter-fed motors at repetitive square voltages to avoid the PD caused insulation deterioration when the motors are collected to inverters having specific characteristics. However, unlike PD tests at AC/DC voltages proposed in IEC 60270, the PD detection at repetitive square voltages is much more complex because of serious interference generated by impulse generator. To solve the problem, ultra-high frequency (UHF) method seems recommendable for its preferable signal-to-noise ratio (SNR). The chief aim of this study is to investigate PD pulse and statistical characteristics of turn-to-turn insulation for inverter-fed traction motors. A square-shaped Archimedes antenna, specially designed for the PD detection at repetitive square voltages of fast rise times, was used to perform PD tests on turn-to-turn insulation models. Time and frequency analysis results indicate that energy component of generator disturbance and PD pulses are mainly distributed in the 0–0.5 GHz and 0.6–1.5 GHz range, respectively. Based on the results, suitable filter was designed for power disturbance suppression. Additionally, resorting to the sensor unit (i.e. antenna and filter) and the PD test system, the PD statistical features at square voltages of different frequencies were obtained. Experimental results show that higher frequency will give rise, statistically, to PD of lower magnitudes distributing at smaller phases. A reasonable interpretation of this phenomenon was presented. Lastly, according to the PD statistical features, some suggestions for the PD detection system design, generator parameter optimization and the PD pulse extracting were given. The results of this work would be beneficial to the increase of the sensitivity when performing the PD detection on insulation systems for inverter-fed motors at repetitive square voltages and thus, improving the reliability of inverter-fed motors.Due to overvoltage produced by inverter output, inverter-fed motor insulation systems often experience fast electrical aging process, especially when partial discharge (PD) is incepted. Before putting into use, the PD detection should be performed on inverter-fed motors at repetitive square voltages to avoid the PD caused insulation deterioration when the motors are collected to inverters having specific characteristics. However, unlike PD tests at AC/DC voltages proposed in IEC 60270, the PD detection at repetitive square voltages is much more complex because of serious interference generated by impulse generator. To solve the problem, ultra-high frequency (UHF) method seems recommendable for its preferable signal-to-noise ratio (SNR). The chief aim of this study is to investigate PD pulse and statistical characteristics of turn-to-turn insulation for inverter-fed traction motors. A square-shaped Archimedes antenna, specially designed for the PD detection at repetitive square voltages of fast rise times, was used to perform PD tests on turn-to-turn insulation models. Time and frequency analysis results indicate that energy component of generator disturbance and PD pulses are mainly distributed in the 0–0.5 GHz and 0.6–1.5 GHz range, respectively. Based on the results, suitable filter was designed for power disturbance suppression. Additionally, resorting to the sensor unit (i.e. antenna and filter) and the PD test system, the PD statistical features at square voltages of different frequencies were obtained. Experimental results show that higher frequency will give rise, statistically, to PD of lower magnitudes distributing at smaller phases. A reasonable interpretation of this phenomenon was presented. Lastly, according to the PD statistical features, some suggestions for the PD detection system design, generator parameter optimization and the PD pulse extracting were given. The results of this work would be beneficial to the increase of the sensitivity when performing the PD detection on insulation systems for inverter-fed motors at repetitive square voltages and thus, improving the reliability of inverter-fed motors.

Abstract: Inverter-fed traction motors, which are employed for electric-multiple-units (EMUs), are subjected to Pulse Width Modulated (PWM) impulse voltage. Impulse voltage has fast rise/fall time and high repetition frequency. The failure mechanism of insulation under impulses is different from that of standard 50 Hz sinusoidal condition. Partial discharge (PD) characteristics (discharge number, mean discharge quantity and phase resolved partial discharge (PRPD) pattern) with different frequencies and different rise times of impulse voltage were investigated. The results reveal that discharge number and mean discharge quantity increase, while PRPD pattern moves to the zero of impulse voltage with decrease of rise time. Discharge number and PRPD pattern follow that trend with increase of frequency, the value of mean discharge quantity is similar. Memory effect of the deposited space charges on PD was studied. PRPD pattern moves to the zero of bipolar impulse voltage due to the role of space charges, with increase of frequency and decrease of rise time. It is revealed that available discharge time is prolonged and overvoltage across the cavity increases with augment of frequency, which results in discharge number increasing. Under the condition of shortening the rise time, with the same statistical time lag t L , ΔV would increase if dV/dt is higher, which results in enhancement of discharge quantity. Discharge number increases with reduction of rise time, because the decreased multiple of t d is lower than the increased multiple of dV/dt. Study of PD characteristics and role of space charges is helpful to the exploration of the destructive mechanism of insulating materials under bipolar impulse.