High-frequency transformers are subject to excitation with a changing duty cycle during operation. Due to magnetic relaxation, the duty cycle of the rectangular wave affects the magnetization time of nanocrystalline alloy for the core material, which affects whether the transformer can reach the saturation operating point. Based on the micromagnetic theory, a three-dimensional model of the nanocrystalline alloy is established, and rectangular wave excitation with different duty cycle D is applied to the micro-model. The influence of D on the magnetization process is analyzed in terms of the hysteresis loss P_v and magnetic moment deflection angular velocity ω. The results indicate that when D = 0.5, P_v is the smallest, and when D increases or decreases, P_v increases. Furthermore, P_v remains the same under the rectangular wave excitation that satisfies the sum of different duty cycles of 1. Regarding ω, the smallest value occurs at the rising edge of the excitation when D = 0.1, while the largest value occurs when D = 0.9. During the falling edge stage, ω is smallest when D = 0.9 and largest when D = 0.1. These results demonstrate that the duty cycle D influences the magnetization time of the material. Due to magnetic relaxation, changing the magnetization time determines whether the material can reach saturation magnetization. Therefore, there is a critical state, which is defined as the critical duty cycle D_c. The results show that for D < 0.5, the range of D_c1 is between 0.2 and 0.21, and for D > 0.5, the range of D_c2 is between 0.8 and 0.81. Increasing the amplitude of the excitation source causes a decrease in D_c, while increasing the frequency causes an increase in D_c.

In the high‐humidity environment, the epoxy resin (EP) insulating materials commonly used in reactors and other equipment are prone to accelerate ageing due to the damp and hot conditions, which affects the long‐term stable operation of the equipment. However, at present, the research on the blocking effect of carbon nano‐modified fillers on the invasion of water molecules into the epoxy resin matrix is still unclear. Based on this, we utilised molecular dynamics simulation technology to establish six EP models of EP materials under different conditions, including water molecule invasion (WI)/non‐invasion (WNI) and carbon nanomaterial modification/no modification. The carbon nano‐modified materials adopted Amido‐Amine Functionalised Carbon Nanotube (AFCNT) and Hydroxyl Functionalised Graphene (HFGNP). We studied the changes in key performance indicators such as relative permittivity, thermal conductivity, and glass transition temperature of six models. Eventually, it was found that the relative permittivity of the EP model significantly increased after water molecule invasion but decreased after doping with AFCNT. The thermal conductivity of the six EP models increased, and the increase in the AFCNT‐EP model was the most obvious, rising by 46.65% at 300 K. The glass transition temperatures of the models all decreased, but the decrease was reduced after doping and modification with carbon nanomaterials. Overall, except for thermal conductivity, the overall performance of the EP model deteriorated to varying degrees after water molecule invasion. However, the performance deterioration trend of the model was alleviated after doping with carbon nano‐modified fillers, and the performance of the model doped with AFCNT was significantly better than that of the EP model doped with HFGNP. This research can provide certain theoretical basis and technical support for the engineering application of epoxy resin materials under extreme conditions such as high humidity.

Interfacial stress concentration induced by curing shrinkage during the manufacturing of epoxy resin is a primary trigger for micro-nano defect formation and electrical performance degradation in power equipment. To address the computational complexity of traditional viscoelastic models and the thermoelastic behavior wherein the stiffness of the epoxy resin varies with temperature during curing, this paper proposes an improved viscoelastic constitutive model incorporating a thermo-elastic factor. By coupling curing kinetics, heat conduction, chemical shrinkage, and mechanical effects, a multi-physics simulation framework is constructed to describe the complete epoxy curing process, thereby revealing the spatiotemporal evolution of curing stress deformation. To verify the model’s accuracy, an in situ monitoring system based on Fiber Bragg Grating (FBG) sensors was established. A temperature compensation method was utilized to effectively decouple temperature and stress within the complex exothermic curing environment. This study reveals a significant strain gradient effect during the resin curing process. Experimental measurements indicate strains of 21,609 με and 5800 με at the interface and surface, respectively, with numerical simulations exhibiting high agreement with the experimental data. This research not only provides an efficient simulation approach for predicting curing stress but also offers a theoretical basis for the crack-resistant structural design of high-performance epoxy-based power equipment.

With the development of the new energy industry in high-altitude regions, epoxy resin insulating materials in electrical equipment face severe challenges from prolonged exposure to strong radiation environments. Strong ultraviolet irradiation induces the generation of free radicals such as alkyl (CH₂), alkoxy (CH₂O), and peroxyl (CH₂OO), which continuously attack the cross-linking structure of epoxy resin, leading to its degradation. This study employs molecular dynamics simulations to evaluate the enhancing effect of graphene quantum dots (GQDs) on the radiation resistance of epoxy resin (EP), proposing cross-linking structural integrity as an evaluation criterion. It compares and analyses pure EP (EP/neat), hydrogen-terminated GQDs (EP/GQD_C₅₄H₁₈), and carboxyl-terminated GQDs (EP/GQD_COOH) under three types of free radicals. The results indicate that the unique sp² hybrid structure and hydrogen-donating ability of GQDs can effectively inhibit the activity of free radicals, and improve the integrity of the cross-linked structure by 8% to 16% compared to EP/neat. While both types of GQDs demonstrate comparable behavior in response to alkyl free radicals, EP/GQD_COOH exhibits superior performance under the influence of oxygen-containing free radicals. This enhanced performance can be attributed to its augmented hydrogen-donating capacity and an increased number of active sites. This study investigates the extent to which GQDs with different structures enhance the radiation resistance of epoxy resins, providing an important theoretical basis for the modification of epoxy resins for applications in high-radiation environments.

Bridge-arm reactors subjected to long-term AC–DC composite currents with multiple harmonics may develop non-uniform winding temperature rise and thermal-expansion mismatch, leading to localized thermal stress concentrations and potential insulation cracking. Unlike previous studies focused mainly on purely AC conditions, this study investigates a ±800 kV dry-type air-core bridge-arm reactor and develops a thermo-mechanical model incorporating AC–DC composite currents and harmonic losses. To mitigate thermal stress concentration, an axially segmented configuration is proposed to relieve the restraint associated with cumulative axial thermal expansion. The results show that a 65% axial segmentation ratio provides the best stress-regulation performance among the investigated cases. Under AC–DC composite conditions containing second- and fifth-order harmonics, the maximum Von Mises stress and maximum first-principal stress decrease by 33.42% and 38.11%, respectively, while the stress distribution becomes more uniform. The analysis is based on a two-dimensional axisymmetric model with one-way thermo-mechanical coupling and excludes long-term cyclic thermal aging and interfacial slip between winding and insulation layers. These findings provide theoretical support for the stress-oriented structural design and reliability assessment of high-capacity bridge-arm reactors.