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Abstract The problem of non‐uniform voltage distribution in the internal element of metallized film capacitors in flexible HVDC transmission systems exists, the influence of electrical parameters of dielectric and metal copper row are not considered in the existing metallized film capacitor equivalent circuit model. In this paper, the conductance of biaxially oriented polypropylene film (BOPP) for metalized film capacitors is measured at different electric field intensity and dielectric properties at different frequencies. The current density distribution and voltammetric properties of the metalized film capacitor elements under AC and DC superimposed voltages are calculated by the finite element method, and the voltammetric properties of the copper row considering the skin effect are derived. The equivalent circuit model of the metalized film capacitor under the AC‐DC superposition voltage is proposed. The simulation results of the capacitor equivalent circuit model show that the voltage distribution of the internal elements of the capacitor is uniform at low frequency AC voltages (

Abstract The strain clamps and leading wires are important components that connect conductors on overhead transmission lines and conduct current. During operation, poor contact between these components can cause abnormal overheating, leading to electric failures and threatening power system reliability. Recently, the use of unmanned aerial vehicles equipped with infrared thermal imagers for strain clamp and leading wire maintenance has become increasingly popular. Deep learning‐based image recognition shows promising prospects for intelligent fault diagnosis of overheating faults. A pre‐treatment method is proposed based on dynamic histogram equalisation to enhance the contrast of infrared images. The DeepLab v3+ network, loss function, and existing networks with different backbones are compared. The DeepLab v3+ network with ResNet101 and convolutional block attention module added, and the Focal Loss function achieved the highest performance with an average pixel accuracy of 0.614, an average intersection over union (AIoU) of 0.567, an F1 score of 0.644, and a frequency weighted intersection over union of 0.594 on the test set. The optimised Atrous rates has increased the AIoU by 12.91%. Moreover, an intelligent diagnosis scheme for evaluating the defect state of the strain clamps and leading wires is proposed and which achieves a diagnostic accuracy of 91.0%.

Repetitve nanosecond impulses in gas-insulated metal-enclosed switchgear (GIS) are likely to trigger inside flashover. Interface charges on the spacer in GIS are considered one of the main factors damaging insulation performance and may be induced by overvoltage. For good understanding of insulation failures, accumulation characteristics of charges between SF6 and epoxy spacers under repetitive nanosecond impulses are investigated. It can be found under nanosecond impulses, the charge source in gas volume contributes to interface charge accumulation predominantly. Interface charges will be promoted by impulse number and amplitude. Accumulation processes are analyzed based on runaway electrons mechanism. When impulse amplitude exceeds a threshold value, discharge in the gas volume turns to a runaway mode. A runaway electron leads to the interface charge accumulation. Affected by motion of the runaway electrons, the potential peak gradually moves close to the grounded electrode when impulse amplitude is raised. Meanwhile, increasing impulse number can enhance surface potential. Surface potential will reach saturation eventually. However, memory effect of the repetitive impulse discharge makes the half-peak width of the surface potential at the interface change little. Design of GIS gas-solid insulations can refer to this research.

A recurring challenge of a DC SF6/N2-filled GIS/GIL apparatus is the charge accumulation at DC stress. The conventional design rules and knowledge of AC spacers may not be applicable for this new type of apparatus. A novel design rule is proposed considering the effect of accumulated charge on the threshold of electric field strength being resistant to the superposed voltage. A surface charge accumulation simulation model is introduced, and the key parameters in the simulation model are measured. In addition, an experimental platform for a 100 kV spacer flashover test is established. Spacer flashover tests under superimposed voltage with opposing polarities are carried out, and the withstanding voltage of the spacer is obtained. Finally, based on the proposed model, the threshold of the surface electric field strength (tangential component) on the DC spacer in SF6/N2 mixed gases is discussed. For a reliable insulation design of a DC GIS/GIL apparatus filled with 0.7 MPa SF6/N2, the threshold of surface electric field strength on the DC spacer is 12 kV/mm. The insulation design rule can be referenced in the design of a high-voltage DC SF6/N2-filled GIS/GIL apparatus.

Gas-insulated equipment (GIE) that utilizes the most potent greenhouse gas sulfur hexafluoride (SF6) as insulation and arc-quenching medium has been widely used in the power industry. Seeking eco-friendly insulating gas with advanced performance for next-generation SF6-free GIE is significant for the “net-zero” goal and sustainable development. In this paper, the utilization, emission, and reduction policies of SF6 around the world were summarized first. Then, we systematically reviewed the latest progress in comprehensive performance evaluation of eco-friendly insulating gas in terms of molecular design, dielectric insulation, arc-quenching, stability and decomposition, materials compatibility, biosafety, etc. Further, the representative applications of eco-friendly insulating gas in medium-voltage, high-voltage GIE as well as relevant maintenance-related technologies were highlighted. Accordingly, the existing challenges and future perspectives were proposed, presenting a roadmap to hopefully steer the development of eco-friendly insulating gas and GIE.

Abstract To meet the requirements of power grid operation control, constructing an extensive sensor network in the power grid is a future development trend. However, due to the high cost, the difficulty of continuous power supply for a long time, the large amount of energy consumption, and the environmental problems caused by abandoned batteries, the traditional power supply mode based on batteries for sensors is inconsistent with China's goal of carbon peak and carbon neutrality. It cannot meet the needs of the future grid. Therefore, it is significant to solve the energy supply problem of numerous sensors to promote the construction of new power grid and improve the utilization rate of renewable energy. As an emerging energy harvesting device, triboelectric nanogenerators (TENG) are suitable for developing self‐powered sensors and powering low‐power sensors due to their mall size, high efficiency, low cost, and environmental friendliness. At the same time, due to the abundant, stable, and widely distributed magnetic field around the equipment in the grid, it can provide continuous excitation for the TENG. Therefore, applying magnetic field energy‐harvesting TENG in the future power grid has a broad development prospect. This paper analyzes the development trend and the crucial problems facing the power grid in the future. We review the research and application of TENG based on the magnetic field in recent years and explain the mechanism of TENG in detail. Finally, the application prospect of TENG in the future power grid has been prospected. This paper can provide a reference for applying magnetic field energy harvesting triboelectric nanogenerators in future power grids.

Abstract There are a lot of quantitative performance characterisation methods for composite insulators. However, how to apply the characterisation methods with different dimensions to comprehensively assess the health state of composite insulators remains a big problem for operators and researchers. A comprehensive health assessment model for composite insulators based on the Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method is proposed in this study. Fourteen composite insulators from four regions in Yunnan Province, China, and five quantitative characterisation indicators of the insulators, namely hardness, hydrophobicity, loss of hydrophobicity, recovery of hydrophobicity, and trap charge, are selected to verify the model. The results show that the Pearson correlation coefficient of the TOPSIS‐based comprehensive health score with operating time is 0.72, which is better than the other single indicator. By introducing the operating time into the TOPSIS‐based model, we can also obtain the ageing resistance performance of the composite insulator. This work provides a new idea to assess the health state and ageing performance of composite insulators.

Abstract When high‐temperature superconducting (HTS) tapes in high voltage DC HTS devices are quenched, amounts of thermal bubbles are generated in liquid nitrogen (LN2) that will reduce the insulation strength greatly. Especially when resistive‐type superconducting fault current limiters meet a short‐circuit fault, the insulation strength of the LN2 decreased significantly due to the thermal bubbles and electrothermal pressure. Solid insulating barriers can improve the insulation strength of the LN2. The objective of this study is to obtain the effect of barrier thickness on the DC dielectric breakdown characteristics of LN2. Insulating barrier Polytetrafluoroethylene with three different thicknesses were tested. Three types of electrodes such as rod‐plane, needle‐plane, and plane‐plane were applied. The results showed that the barriers can increase the negative and positive breakdown voltage both with and without thermal bubbles in a slightly non‐uniform field (SNF) and non‐uniform field (NF). For the SNF, the DC breakdown voltage is higher when the barrier is thinner. The effect of the barrier thickness in the NF is lower than that in the SNF field. For the NF, the positive breakdown voltage is higher when the barrier is thicker.

Abstract Superconducting fault current limiters (SFCLs) are important in improving the stability of electrical power systems. However, the existing resistive‐type DC SFCL has a long recovery time, which cannot meet the requirements of power system auto‐reclosure. Thus, it is necessary to investigate the methods of reducing recovery time of SFCLs. In the simulations on superconducting tapes, there is a lack of investigation about the effects of design factors of superconducting tapes and coils such as material, thickness, coil radius, coil winding method on recovery characteristics. The objective of this study is to obtain the effects of material and geometric properties of superconducting tapes and coils on recovery characteristics and obtain methods to reduce the recovery time. The recovery times under different conducting currents are also compared by simulations and experiments to verify simulation accuracy. The maximum error is below 40% and most of the cases have an error below 10%. Besides, by increasing tape thickness, the recovery time can be reduced by 22%. Tape materials and coil winding methods can be designed according to the auto‐reclosure and current‐limiting requirements of the power systems. The research provided directions for the design of superconducting tape and superconducting coil for resistive‐type DC SFCLs.

In recent years, Unmanned Aerial Vehicle (UAV) technology has been widely used in power transmission line inspection. UAV operating parameters determine the quality of the acquired insulator image, which directly affects the efficiency of transmission line inspection. Regarding the inspection of ceramic insulators, this work establishes a three-dimensional (3D) model of ceramic insulators, and simulates the changes in shooting distance, angle, and height of the UAV through three geometric transformations: rotation, translation, and scaling. The influence of those three UAV operating parameters (the shooting distance, angle, and height) on the integrity of the ce-ramic insulator steel cap is studied. In the infrared zero value detection of 110kV ceramic insulators, It is found that the best setting for obtaining the maximum steel cap area is: shooting distance of 3–7 meters, shooting height of 1 meter below the center of the insulator string, and a shooting depression angle of 0°. The UAV operating parameters setting method proposed in this paper will provide theoretical and practical basis for the in-depth intelligent inspection of transmission line insulators.

Abstract Composite insulators are widely used in the power transmission system. Operating in the outdoor environment, the composite insulator will be inevitably polluted by natural contamination with the increase of service life, and contamination on the surface is one of the main reasons for the abnormal temperature rise of the insulator. In this study, the rated voltage was applied to the kieselguhr sample under a high‐humidity environment, and its temperature rise characteristics and leakage current were tested. The research results show that the temperature rise in contamination falling‐off region is significant and the leakage current is greater than the clean sample. By analysing the phenomenon with the help of the circuit model, it is concluded that the high resistance and voltage in contamination falling‐off region leads to high heating power of the polarisation loss.

The bird's nest on the transmission line tower has a bad impact on the transmission equipment, and even threaten the safe and stable operation of the power grid. In recent years, the number of bird pest in transmission line is increasing year by year, resulting in increasing economic losses. The traditional bird's nest identification method of transmission line is time-consuming and labor-intensive, and its security level is low. Therefore, this paper proposes an automatic detection method of bird's nest on transmission line tower based on Faster_RCNN convolution neural network. This method can automatically identify the location of the bird's nest on the transmission line tower by using the image collected by unmanned aerial vehicle (UAV). The problem of insufficient training samples and overfitting of neural network classifier is solved by enlarging the bird's nest image. The experimental results show that this method can effectively detect bird's nest targets in complex environment, and the highest recall rate can reach 95.38%, the highest F1 score can reach 96.87%, and the detection time of each image can reach 0.154s. Compared with the traditional nest detection method, this method has stronger applicability and generalization ability. It provides technical support for analyzing bird activities and taking effective preventive measures.

Trapped space charges in epoxy composite distort the electric field, which will induce the failure of the insulation system, and nano graphene oxide may inhibit the curing behavior of epoxy resin matrix. This paper analyzes how the two interfaces affect the electron traps of epoxy resin/graphene oxide systems with different nanofiller contents. The electron affinity energy of epoxy resin matrix and nano filler molecules in the epoxy resin/graphene oxide system is calculated based on quantum chemistry. It is found that nano graphene oxide has a strong electron affinity energy and is easier to capture electrons. Then the influence of the interface formed by the epoxy resin matrix and the nano graphene oxide on the electron transfer ability is calculated. The epoxy resin matrix contains the electron transfer ability of interfaces formed by nano graphene oxide and the molecular chain is different from that of unreacted molecules. The results can provide a reference for the modification of epoxy resin/graphene oxide nanocomposites.

Water invasion induced aging and degradation of the unidirectional glass fiber reinforced epoxy resin (UGFRE) rod is inferred as the primary reason for the decay-like fracture of the composite insulator. In this paper, the moisture diffusion processes in the UGFRE from different directions at various test humidities and temperatures are studied. The moisture diffusion of the UGFRE sample obeys the Langmuir diffusion law under the humidity conditions of 53%, 82% and 100% at 40 °C. In deionized water, the moisture diffusion of the UGFRE sample also obeys the Langmuir diffusion law when the invading direction is vertical to the glass fiber. However, when the water invades the UGFRE sample, parallel with the glass fiber, the weight loss caused by composite degradation should not be neglected. A modified Langmuir model, taking Arrhenius Theory and the nonlinear aging characteristic of the composite into consideration, is proposed and can successfully describe the moisture diffusion process. Both the glass fibers and epoxy resin will degrade in the deionized water. The glass fibers show better resistance to degradation than the epoxy resin. The epoxy resin degrades from the glass fiber/epoxy resin interface and become fragments. For composite insulators, the water invasion through the ends should be avoided as far as possible, or the degradation of the UGFRE rod will result in decay-like fracture.

Tower foundations have been used as grounding electrodes to reduce the area of the grounding devices. However, it is difficult to calculate the grounding resistance due to the complex structure of the reinforced concrete foundation. A method for calculating grounding resistance of reinforced concrete foundations is proposed in this paper. The method equates the complex foundation structure into a cylindrical conductor and then calculates the grounding resistance with the help of the method of moments, which simplifies establishment of the simulation model and reduces the extensive computation. In addition, the applicability of the equal cross-sectional area method and the equal cross-sectional perimeter method is analyzed. It shows that both methods are applicable only when the concrete resistivity is close to the soil resistivity. The equal cross-sectional area method is applicable when the concrete resistivity is within twice the soil resistivity, while the equal cross-sectional perimeter method is applicable when the concrete resistivity is approximately same or less than the resistivity of the soil.

With the rise of renewable energy resources, especially offshore wind power plant, the DC transmission concept has attracted more and more attention. The selection of eco-friendly insulating gas, as most critical insulating part of gas-insulated equipment is top priority for developing highly reliable system. Moreover, it is acknowledged that eco-friendly C(3)F(7)CN/CO(2) gas mixtures have become a potential alternative to SF(6) gas due to its excellent performance. This study reviews the basic physical properties of C(3)F(7)CN gas/gas mixtures and insulation properties including the gas gap breakdown and surface flashover performance at DC, as well as lightning impulse voltage. Investigation about the gas stability is presented, including the compatibility of gas with solid materials in gas-insulated transmission line (GIL), and the decomposition characteristics under long-term electrical and thermal stresses. The important progress on the charge accumulation characteristics of gas–solid interface in the novel gas environment under DC voltage stress is analysed. Finally, the key issues that need to be paid attention to and further followed through regarding the application of this novel gas in GIL are summarised and put forward. This study hopefully can provide a complete reference for the development of eco-friendly DC gas insulation equipment with C(3)F(7)CN/CO(2) gas mixtures.

Surface charge accumulation on the spacers is one of the key issues restraining the development of HVDC GIS/GIL. The precise measurement of surface charge properties provides the basis for further study of the surface charge transport mechanism as well as the charge-induced flashover mechanism under DC voltage. In this study, the authors discuss their perspective on the current status, development needs and potential developing orientation of surface charge characterisation techniques. Different surface potential measurement methods and charge inversion algorithms are reviewed regarding the previous studies and future research needs. Drawbacks and outlooks of surface charge measurement techniques are also discussed with the background of laboratory experiment results and on-site measurements. It is hopefully that this study can serve as a useful guide reference for researchers within the same research field. More importantly, it is authors’ hope that this study can inspire some novel ideas for readers into developing of more accurate and scientific interface charge characterisation techniques.