As the demand for flexibility and efficiency in modern industrial equipment increases, motors often operate under variable speed conditions in real-world industrial applications. This poses challenges for traditional time-domain and frequency-domain fault diagnosis methods. These challenges arise primarily due to the non-linear and non-stationary characteristics of signals under variable speed conditions, which can affect fault feature extraction. Single deep learning models generally require training and test data to follow the same distribution, and domain adaptation or multi-source domain generalization methods are difficult to apply in the absence of target domain and multi-source domain data, limiting their ability to enhance the generalization of single-source domain models. To address these challenges, this study proposes a motor rolling bearing fault transfer diagnosis method that integrates angular domain resampling and feature enhancement.

First, to mitigate the issue of time-frequency characteristic offsets in vibration signals under different rotational speeds, angular domain resampling is employed. This technique processes vibration signals at varying speeds, obtaining angular domain vibration signals to minimize the offsets caused by speed changes. Second, to address the generalization limitations of deep learning models, fault data from constant speed conditions are used as the source domain for training the neural network. Covariance loss is introduced to amplify the feature differences among various classes in the source domain data. This allows the network to focus on more informative features for the classification task, thereby improving its generalization capability. Finally, the angular domain vibration signals under variable speed conditions are input into the trained model for fault classification.

The effectiveness of the proposed method is validated through several experiments. Initially, the time-frequency characteristics of vibration signals from an actual bearing inner ring fault are examined before and after angular domain resampling. Before resampling, the vibration signal intervals under variable speed conditions show significant variability. However, after resampling, the variability in the vibration intervals is significantly reduced. Furthermore, using t-SNE visualization, the study observes that networks without feature enhancement show slow gradient updates and minimal changes in feature distribution. In contrast, networks with feature enhancement exhibit continuous changes in feature distribution, even as the classification loss decreases, with increasing feature distances. The study also conducts four cross-working condition fault diagnosis experiments, comparing the proposed method with other methods. The results demonstrate that the proposed method improves fault identification accuracy by 35.04% compared to methods without angular domain resampling, especially in rolling element fault identification. When compared to methods without feature enhancement, the proposed method improves accuracy by 7.45%. Additionally, in transfer diagnosis tasks under different load conditions, the proposed method demonstrates high accuracy, recall, and F1 scores.

In conclusion, the study finds that: (1) Angular domain resampling effectively reduces time-frequency distribution differences caused by speed variations, proving its applicability and rationality in data preprocessing at different speeds. (2) The feature enhancement strategy, by increasing covariance loss between different class features, amplifies feature differences between various health status signals, enabling the network to capture more distinctive features and significantly improving generalization capability. (3) The proposed method, without requiring target domain data, achieves fault identification accuracy of up to 97.29% under variable speed conditions, demonstrating good robustness under variable load conditions.

In distributed residential photovoltaic (PV) power generation systems, microinverters have attracted significant attention due to their benefits, including component-level maximum power point tracking (MPPT), plug-and-play flexibility, and high security. However, most existing microinverters are designed for grid-connected applications and are primarily suited for single-phase two-wire systems. Additionally, the intrinsic double-line-frequency power fluctuation problem greatly limits the increase in power density and reliability of microinverters. This paper proposes a novel voltage-source high-frequency-link (HFL) microinverter with double-line-frequency power decoupling capability, which is compatible with various single-phase distribution grids.

On the primary side of the proposed microinverter, a Boost converter is integrated with the full bridge of the HFL microinverter by sharing the switches. The integration has high voltage gain and additional double- line-frequency power decoupling capability. On the secondary side, a novel structure with three-wire output is proposed to be compatible with single-phase two-wire and single-phase three-wire power systems. Due to its voltage-source-inverter (VSI) characteristics, the proposed microinverter is suitable for grid-connected and islanded applications.

This paper introduces the circuit structure of the proposed microinverter. A soft-switching modulation is proposed along with its logic implementation. Operation modes during a switching cycle and the soft-switching characteristics of each switch are analyzed. The proposed microinverter features a three-wire output, and the two phase-to-neutral output voltages are auto-balanced. Therefore, the specific balancing theory is also analyzed. The double-line-frequency power decoupling principle is presented and analyzed. To meet the required input voltage range and ensure acceptable voltage stress on switching devices, design considerations for key circuit parameters are presented, including the turns ratio of the high-frequency transformer (HFT), the average value of the decoupling capacitor voltage, the inductance of Boost inductor, and the capacitance of the decoupling capacitor.

Moreover, grid-connected and islanded closed-loop control strategies are proposed. Finally, a 600 W prototype is built to verify the proposed topology and strategies. Steady-state and dynamic test results for grid-connected and islanded operations are provided.

The following conclusions can be drawn. (1) The proposed microinverter achieves high gain and double-line-frequency power decoupling, resulting in a 22 V to 55 V wide input voltage and an MPPT efficiency above 99% with a 100μF input capacitance. (2) A special three-wire output is proposed for single-phase two-wire and single-phase three-wire distribution grids, with the two phase-to-neutral voltages auto-balanced without dedicated control. (3) The proposed microinverter operates in both grid-tied and islanded applications, and the closed-loop control strategies ensure stable steady-state operation and fast dynamic response.

In recent years, non-isolated inverters have gained widespread attention in commercial and residential PV grid-connected systems due to their cost, efficiency, and flexibility advantages. However, in practical applications, the output voltage of the PV panel is generally low. Due to the loss of the electrical isolation of the transformer, the high-frequency switching action of the conventional inverter may produce a common mode voltage applied to the parasitic capacitance between the PV array and the ground, resulting in a common mode leakage current, which affects the safe operation of the system. This paper proposes a non-isolated five-level Boost inverter with no leakage current and its dual-mode modulation strategy to enhance the applicability and practicability of the inverter.

Firstly, the circuit structure combines the dual-output Boost converter with the five-level inverter to create a five-level Boost inverter topology. The Boost capability is expanded, suitable for PV power generation applications with low DC voltage on the input side. Secondly, the dual-mode modulation strategy of unipolar carrier level shifted is studied, providing five-level output capability and increasing the equivalent switching frequency under the same carrier frequency. By comparing the PV panel’s DC output voltage and the grid voltage’s absolute value, two working modes of Boost voltage and buck voltage are realized, and the energy transmission efficiency of the converter is improved. In addition, a five-level voltage is output on the side of the bridge arm, and more levels make the output voltage closer to the sine wave, which is conducive to improving the quality of incoming current. Furthermore, the negative polarity of the DC side of the topology is directly connected to the voltage neutral of the AC side to eliminate the common mode leakage current of the stray capacitor to the ground. Finally, the working principle of the inverter circuit and the realization method of the specific modulation strategy are provided, and the key parameters are designed.

An experimental prototype was built. The experimental results show that: (1) The inverter’s two working modes overcome the limitation that the traditional multilevel inverter can only step down, making it suitable for a wide range of input voltage changes. (2) The common ground structure can effectively inhibit leakage current. (3) The output voltage V_AB of the bridge arm presents five voltage levels, and the energy storage capacitor can be charged and discharged at a high switching frequency, ensuring the stationarity of the output voltage of each level. Hence, the output voltage waveform is symmetrical in the positive and negative half cycles. At the same time, the incoming current i_g can accurately track the phase of the grid voltage V_g, producing a smooth output waveform with little distortion, which meets the requirements for grid-connected current quality. (4) The proposed inverter can output reactive power output, which meets the requirements of non-unit power factor operation in IEEE grid-connected standards.

AC-DC converters are key equipment to interface the AC grid, DC loads, and renewable generation sources. Efficiency and power density are the main factors in the design and implementation of AC-DC converters. The two-stage power conversion has low system efficiency and high cost. A single-stage AC-DC converter achieves AC-side current regulation, DC-side voltage regulation, and high-frequency galvanic isolation simultaneously through only one stage of high-frequency power conversion, which has the potential advantages of high efficiency and power density. However, the design and implementation of single-stage AC-DC converters are difficult.

This paper presents a resonant single-stage isolated AC-DC converter based on a fixed frequency pulse width modulation strategy. When the switching frequency of the converter is set to the resonant frequency of the series-resonant tank, the impedance of the resonant tank always features zero impedance. Therefore, in steady-state, the total voltage applied on the resonant tank must also be zero, which means the fundamental voltage generated by the primary-side and secondary-side switching bridges must be equal. Following this idea, the converter can operate in both voltage step-down and step-up modes, and the equivalent voltage gain of the converter is continuously adjustable in a wide range by adjusting the pulse width of the high-frequency excitation voltages applied on the resonant tank. Hence, the voltage and current regulation requirements of the single-stage AC-DC converter can be satisfied. Voltage step-down regulation can be achieved by adjusting the primary-side duty ratio D_p, while the voltage step-up regulation can be achieved by adjusting the secondary-side duty ratio D_s.

In order to realize the soft-switching of all switches within a wide voltage range, the soft-switching characteristics of the converter are analyzed in detail. It is found that the magnetizing inductance L_m and quality factor Z_r of the resonant tank must be small enough within the entire AC voltage range, leading to much higher conduction losses. When the instantaneous AC voltage is low, the switching losses of switches are also low. Therefore, it is unnecessary to achieve soft-switching within the entire AC voltage range, and trade-offs between switching loss and conduction loss must be made to design the converter’s parameters. Therefore, an optimized parameters design method is proposed for the resonant single-stage AC-DC converter.

An experimental prototype is built and tested. The experimental results indicate that through the fixed-frequency pulse-width modulation strategy, step-up and step-down power conversions, the AC and DC side voltage and current regulation, and high power factor can be realized. With the proposed parameter optimization design method, soft switching of switches can be achieved in a wide input voltage range. The efficiency of the converter is up to 94.2%. In addition, experimental results indicate that the converter has excellent dynamic and steady-state performance.

In the application of new energy, energy storage, and emerging power loads, the power supply architecture using the DC bus has more advantages than the AC bus, which is the development direction of the future power supply system. Bipolar DC microgrid systems are often used to connect various renewable energy sources and emerging loads due to their higher reliability, flexibility, and efficiency. This paper proposes a synthesis method for non-isolated bipolar output DC-DC converters. A series of bipolar output DC-DC converter topologies are deduced. In order to further improve the performance of bipolar output converter, a novel high-voltage-gain DC-DC converter with a three-winding coupled-inductor is proposed by introducing coupled- inductor and switched-capacitor step-up technology to Boost bipolar output DC-DC converter.

Based on the characteristics of the bipolar output converter, the topology synthesis principle of the proposed bipolar output DC-DC converter is given. The input ends of a positive output DC-DC converter and a negative output DC-DC converter are connected in parallel, and the output ends are in series. This paper gives a series of bipolar output DC-DC converters by classifying and combining traditional DC-DC converters. However, the boost capacity of these converters is limited, and the positive and negative output voltages are only regulated by the duty cycle of the switch. Thus, a bipolar high-voltage-gain DC-DC converter with a three-winding coupled-inductor is proposed. The bipolar output voltages can be adjusted flexibly by the turns ratio of the coupled inductor and the duty ratio. This paper gives the construction principle, operating mode, voltage gain, and stress derivation of the high-voltage-gain bipolar output converter. Compared with the converters in the literature, the proposed converter has apparent advantages in voltage gain and device voltage stress. An experimental prototype with a rated power of 200 W is designed. Experimental results show that the input current ripple is small, and the actual voltage gain of the converter is 380/32=11.875, slightly lower than the calculated (3+n₁+n₂)/(1-D)=12, which is caused by parasitic resistance and control signal delay. The efficiency of the proposed converter is 95.5% at full load and 96.7% at half load.

The following conclusions can be drawn. (1) High voltage gain can be achieved with low input current ripple and small switching device voltage spikes. (2) Symmetrical bipolar output voltage can be achieved, reducing the need for high voltage gain of power supply and load. (3) The converter power is distributed on two DC bus bars, which makes the system more efficient. (4) Part of the diode realizes zero voltage switching turn-off, reduces the diode reverse recovery loss, and improves the efficiency of the proposed bipolar output converter.

In recent years, DC-DC converters have been widely used and promoted in renewable energy power generation systems, electric vehicles, and aviation power supplies. However, the output DC voltage of renewable energy sources is low. Increasing the duty cycle can improve the high gain but brings problems such as high voltage spikes across semiconductors, high losses, and low efficiency. A high step-up and high-efficiency DC-DC converter is necessary, which can be achieved by busing switched-inductor, switched-capacitor, coupled inductor, and other techniques. Simultaneously, the practical application has imposed stringent requirements on DC-DC converters, including miniaturization and lightweight design. Using magnetic integration technology can partially fulfill the developmental needs of the converter.

Based on the quadratic Boost converter, the switched capacitor and clamping branch combination is simplified using device multiplexing. Subsequently, the coupled inductor is integrated with decoupled magnetic technology, effectively reducing the volume and number of magnetic components. Therefore, a high step-up quadratic converter is achieved with a dual-coupled inductor’s magnetic and switched capacitor. The working principle of the proposed converter is analyzed, the parameters are derived, the calculation methods for loss and efficiency are provided, and the related diagrams depicting loss proportion and efficiency analysis are generated. The structure and parameters of the integrated magnetic component are designed and simulated. The volume of the integrated magnetic component is reduced by about 13.4% compared with the discrete magnetic component. Finally, an experimental prototype is built, and the feasibility of the topology is validated.

The proposed converter’s input voltage is 12 V, switching frequency is 50 kHz, turn ratio is 1, output voltage is 185 V, output power is 200 W, and load is 170 Ω. Different output power can be obtained by adjusting the load size. When the output power is 140, 160, 180, 200, 220 and 240 W, the corresponding efficiency is 91.6%, 91.9%, 92.4%, 93%, 93.3%, and 92.7%, respectively. Under the load of 200 W, the experimental efficiency reaches 93%.

The proposed converter has the following characteristics: (1) the dual-coupled inductors improve the voltage gain. The duty cycle and turn ratio can be adjusted to obtain high voltage gain, and the switch has low voltage stress. When the duty cycle is 0.5 and the turn ratio is 1, the voltage stress is about 25% of the output voltage, and the voltage gain is 16 times. (2) The clamping structure can absorb the leakage inductor of the coupled inductor, which effectively alleviates the voltage spike on the switch. (3) The diodes experience low voltage stress, ranging from 16% to 66% of the output voltage, allowing for the selection of diodes with a low withstand voltage. (4) The decoupled magnetic integration technology is adopted, which reduces the number and volume of magnetic components.

To achieve continuous and real-time stress optimization control of the dual active bridge converter under power transmission or voltage fluctuations, it is crucial to study the patterns between modes and among optimization control variables in modes. However, current research needs depth, and functional expressions of stress optimization control variables are complex.

This paper employs genetic algorithms for stress optimization. The intrinsic laws among the optimization variables in each mode are elucidated through the optimization results. An innovative trigonometric function polar coordinate method is adopted to derive the corresponding optimization control variable function expressions.

Firstly, based on waveform equivalence simplification and the principle of waveform and energy transmission, the four locally optimal modes are identified from the twelve working modes, which exhibit low stress or effective values in different power ranges. It reduces the number of modes that require optimization, which reduces the optimization burden.

Secondly, the stress of the four modes is optimized, and the optimization results are compared to determine the laws governing the optimization variables in different power ranges with different k values. Through systematic analysis, the laws of four local optimal operating modes in the low/high power section can be obtained.

Thirdly, the expressions with optimization variables are obtained by substituting these laws into the corresponding power transfer expression. The optimized variables are converted into trigonometric polar coordinate forms through the trigonometric function polar coordinate method. The expressions for the minimum current stress function and its optimization control variables are obtained by substituting optimized variables into the stress expression to obtain the minimum stress value.

Compared with the current stress in the full power range for four local operating modes, the optimal mode and optimal control variables for each power segment across the entire power range are selected, thereby achieving global optimization control. The innovations in this study are presented.

(1) Analyze and contrast the current stress optimization results for different voltage adjustment rates k to discern the laws among the optimized variables across the four local optimal modes in various power ranges.

(2) The power constraint and trigonometric polar coordinate methods are utilized to derive a precise expression for the optimal stress control variables. The globally optimal control variables are selected by comparing the current stress of four local optimal modes.

The following conclusions can be drawn. (1) Under the buck operation conditions of stress optimization for both forward and reverse power transfer across the entire power range, mode 1.1 is globally optimal during low forward transmission power when 0<P<k²(1-k); mode 1.4 becomes globally optimal during high forward transmission power in the range k²(1-k)<P<k/2. Similarly, for low reverse transmission power, mode 2.1 is globally optimal in the range k²(k-1)<P<0, and mode 2.3 becomes globally optimal during high reverse transmission power when -k/2<P<k²(k-1). (2) The stress optimization control under TPS modulation improves the efficiency of the DAB converter compared to other modulation strategies. Notably, it exhibits a significant enhancement under the low-power segment and high-voltage mismatch scenarios.

This paper proposes a fault diagnosis method based on an improved dung beetle optimization algorithm (IDBO) to solve the problem of low accuracy of mechanical fault diagnosis of high-voltage circuit breakers. Tent chaotic mapping, a golden sine strategy, and adaptive t-distribution perturbation are incorporated to optimize a deep hybrid kernel extreme learning machine (DHKELM).

Firstly, this paper takes a TY-1S-12/630-16 single-phase vacuum high-voltage circuit breaker as the research object and builds a platform for collecting high-voltage circuit breaker closing vibration signals. Five operating conditions are simulated: normal state, cushion spring fatigue, base looseness, insulator looseness, and drive shaft jam. The laser vibrometer’s sampling time and frequency are set to 1 000 ms and 78 125 Hz. 60 groups of samples for each condition of the high-voltage circuit breaker are collected, totaling 300 sets of samples.

Secondly, the successive variational modal decomposition (SVMD) is used to decompose the acquired signals, the seven IMF components with different center frequencies are obtained after decomposition, and the power spectral entropy of each IMF component is extracted to construct the feature vector matrix. Data dimensionality reduction of the feature vectors is carried out using the t-distribution-stochastic neighborhood embedding algorithm (t-SNE) to obtain 300 by 3-dimensional feature vectors. After dimensionality reduction by t-SNE, the samples of the same state show clear clustering characteristics, while the samples of different states are separated in the mapping results of t-SNE. Hence, the problems of information redundancy and high- dimensional data are avoided.

Then, by introducing three optimization strategies-fusion Tent chaotic mapping, golden sine strategy, and adaptive t-distribution perturbation, the improved dung beetle optimization (IDBO) algorithm is proposed. The IDBO algorithm optimizes the parameters of the DHKELM for constructing the IDBO-DHKELM high-voltage fault diagnosis model. The unimodal and multimodal functions from the CEC2005 test suite are selected for performance testing. The improved IDBO algorithm is compared with traditional PSO, WOA, and DBO algorithms, verifying its superior convergence speed, optimization precision, and stability in finding the optimal solution.

Finally, a platform is built to simulate mechanical failures of high-voltage circuit breakers. The fault diagnosis results show that the proposed method’s fault diagnosis accuracy reaches 98.33%, and the average accuracy of the classification of the DHKELM model is improved by 11.67%, 5.83%, and 3.33%, respectively, compared with that of the traditional SVM, ELM, and CNN models. The DHKELM model improves the average classification accuracy by 9.16% and 7.5% compared with PSO-DHKELM and DBO-DHKELM models, and the precision rate, recall rate, and F_1-score are greatly improved.

With the increasing scale of urban power grids and the application of various advanced power equipment, the comprehensive efficiency of the power system depends on the cooperation and synergy between various power equipment. Aiming at the physical entities with dynamic and real-time changes in the state of power equipment, constructing an efficient mathematical model is essential in system-level simulation of power systems and integrated design of large electromagnetic equipment. Taking the magnetically-saturated controllable reactor (MSCR) as the object, this paper proposes a nonlinear dynamic electromagnetic network model, considering the accuracy of theoretical analysis and the efficiency of parameter calculation.

Firstly, according to the structural characteristics and magnetic field distribution characteristics of MSCR, the MSCR solution domain is meshed by domain discretization. Considering the nonlinearity of the iron core, the magnetization curve model of the MSCR iron core is established by the piecewise interpolation method. The nonlinear grid parameters are calculated according to the principle of the flux tube. The MSCR equivalent magnetic network model is generated based on the loop current method.

Secondly, the electromagnetic model of circuit-magnetic circuit separation is established. The electromagnetic coupling equivalent circuit is established using the controlled source to realize the coupling connection between the circuit and the magnetic circuit. The nonlinear dynamic electromagnetic network model of MSCR is generated. Combined with the nonlinear iterative solution of the chord-cut method, the MSCR winding current and the core flux under different magnetic saturations are calculated.

Finally, a three-dimensional finite element model of MSCR is established based on the finite element method, and field-circuit coupling joint simulation is carried out. Experimental measurements are also performed on the MSCR winding current. The MSCR nonlinear dynamic electromagnetic network model is compared with the three-dimensional finite element model and experimental measurements. The MSCR nonlinear dynamic electromagnetic network model is verified.

(1) The proposed model's calculated winding current and core flux agree with the finite element model and experimental results under different magnetic saturations. (2) The calculation speed and the storage space of the proposed model in electromagnetic parameter calculation are approximately 50~240 times and 1/10 000~1/7 000 of the finite element model. The model can improve calculation efficiency and reduce cost while meeting computational accuracy. It has unique advantages in the initial design of controllable reactors and system-level simulation of power systems.

Electromagnetic drive forming technology is a special forming process that uses pulsed Lorentz force to drive a high-conductivity sheet to move, thereby driving a low-conductivity sheet to cause plastic deformation, which can effectively make up for the shortage of traditional electromagnetic forming in forming low-conductivity materials. However, in the existing electromagnetic drive forming, the driver sheet also undergoes plastic deformation, which tends to lead to a serious problem of wastage of the driver sheet, and it is difficult to regulate the forming shape.

To solve this, instead of the traditional circular drive sheet, a solid copper ring with a specific thickness is employed, utilizing the strong electromagnetic force generated in the copper ring to propel it at high speed into collision with a metal sheet. This impact generates a contact force, causing the sheet to undergo plastic deformation. Additionally, an electromagnetic-structural coupling model for the copper ring electromagnetic drive forming process is developed using LS-DYNA software. A series of electromagnetic drive forming experiments are then conducted, using a TA2 titanium plate as the test material, to validate the feasibility of the proposed method. Numerical simulation and experimental results show that under a single discharge (7 kV), a metallic copper ring with a diameter of 80 mm can drive the titanium plate to deform and the forming height can reach 14 mm. Meanwhile, based on strain analysis of the forming sheet and the driven ring, the solid copper ring does not deform and can be reused. In addition, by changing the size and shape of the copper ring, the forming profile of the plate can be flexibly adjusted. For example, when the diameters of the circular driving rings are 65, 80, and 95 mm, uniformly deformed areas with diameters of 58, 72, and 87 mm are observed on the top of the sheet, which is highly consistent with the shape of the rings. Even if the forming height is increased, the forming shape of the center area of the sheet remains a flat-topped profile when enhancing the discharge voltages. On this basis, the dynamic deformation process of the sheet is further investigated through numerical methods, to reveal the deformation behavior and forming mechanism of the titanium plate driven by the copper ring, which demonstrates that the forming velocity approaching 100 m/s and the strain rate is up to 1 000 s^-1. Hence, this forming process belongs to the category of high-speed forming technology.

The obtained results indicate that, since the copper ring is a solid ring with a specific thickness, it does not experience plastic deformation during the electromagnetic drive forming process and can be reused. This effectively addresses the issue of excessive waste of the driver sheet in conventional electromagnetic drive forming. The copper ring also provides shape adjustment capabilities, allowing for the formation of sheets with circular, quadrilateral, and hexagonal flat tops. The height of the flat-topped profile can be controlled by adjusting the discharge voltage, overcoming the problem of limited shape flexibility in existing electromagnetic drive forming methods. These results are of significant practical value for advancing and expanding the applications of electromagnetic drive forming technology.