Since lithium-ion batteries have been widely applied in energy storage systems and electric vehicles, the accurate estimation of their state-of-health(SOH) is a necessary condition for ensuring the reliable and safe operation of the system. SOH is analyzed from the perspective of capacity, with seven health indicators which are extracted from the constant current-constant voltage charging voltage and temperature curves as input. Based on the data-driven method, a sparrow search algorithm-back propagation neural network(SSA-BPNN) SOH estimation method for lithium-ion batteries is proposed, and data enhancement is applied to further improve the model's robustness. Finally, this method is verified on the NASA Randomized Battery Usage Dataset. Compared with the traditional BP neural network without data enhancement, the SOH estimation accuracy of the proposed method is significantly improved. The maximum absolute error and root mean square error of SOH estimation on the test set are less than 3% and 1.32%, respectively. Experimental results show that this method has advantages of small error, fast convergence, global search capability and adaptation to different characteristics of battery aging.

With the increasing attention to environmental issues, more and more distributed energy systems represented by microgrids are appearing in the power system, which also poses some challenges to the traditional power systems. For example, the delay in digital control system, variations in grid impedance of weak grid and the interaction between parallel converters in microgrids will cause adverse effects on the stable operation of microgrids. On this basis, a novel type of grid-forming control method for microgrid considering control delay and variation in grid impedance is put forward to enhance the stability of microgrids under uncertainties. First, the above problems are modeled, and a delay compensation method is proposed to improve the robustness of the control system with respect to the variation in grid impedance. Then, a feedforward loop is introduced into the control system to protect it from the interference of parallel inverters in microgrids. Finally, experimental results demonstrate the effectiveness and superiority of the proposed control method.

To improve the efficiency of a dual-active-bridge (DAB) converter, a control strategy of minimum current stress with varying switching frequency control is proposed. First, the minimum-current-stress-optimized control method when the voltage changes in a wide range is analyzed, the expressions for the conduction loss and switching loss of the DAB converter under light load conditions are established, and the conduction loss and switching loss at different switching frequencies are further compared. Then, the implementation method for closed-loop control and the power transmission range in each mode are introduced in detail. Under light load conditions, the proposed method can significantly reduce the current stress while improving the efficiency of the DAB converter. Finally, simulation and experimental results verified the advantage of the proposed control method.

An evaluation platform for the CM noise suppression characteristics of high-frequency transformer was established, which was suitable for batch applications in engineering. The conduction mechanism of common-mode (CM) noise in a transformer was analyzed, and the conduction characteristics of CM noise along the coupling path in the transformer was investigated for evaluating the transformer's capability of suppressing the CM noise. First, a function generator was used to generate a high-frequency voltage pulsation signal, which was assigned on the primary winding of the transformer to simulate the transmission characteristics of CM noise. Then, an oscilloscope was used to capture the voltage drop generated by the CM signal on the sampling resistor to judge the suppression effect of the transformer on the CM noise, so as to analyze the influence of the sampling resistor selection on the evaluation results. The effectiveness of the proposed evaluation method for the CM noise suppression characteristics of transformer was verified by comparing the evaluation results with the test results of conducted electromagnetic interference spectrum.

In view of the serious current harmonics in a doubly-fed induction generator (DFIG)-DC connection system and the large loss of a dual-voltage source inverter (VSI) connection system, a novel dual-converter connection system which connects a three-phase DFIG to DC microgrid is designed. First, the topology, pulse-width modulation (PWM) measurement and DFIG model of the dual-converter connection system are described in detail. Different from the traditional connection systems, the proposed connection system adopts an open-end winding structure and uses a three-bridge arm rectifier on each side of the stator winding. Considering that these arms are usually composed of insulated gate bipolar transistors (IGBTs), a diode is used instead of the IGBT in the rectifier to decrease the number of control switches and reduce the cost. Second, the control strategies for a stator-side converter (SSC) and a rotor-side converter(RSC) under the new topology are given. Third, a comparison with the DFIG-DC connection system and the dual-VSI connection system is performed through simulations, and results show the advantages of the proposed method in terms of current harmonic distortion, torque ripple and semiconductor loss. Finally, experimental verification was carried out on a 0.56 kW DFIG, and results also verified the advantages of this method in terms of loss, harmonics and torque ripple.

With the rapid development of power generation by renewable energy and the grid-connection technology, the microgrid dominated by power electronic converters has attracted more and more attention in recent years. Owing to the low inertia and high nonlinearity of power electronic converters, an islanded microgrid under large disturbances is more likely to lose its transient stability. Considering the interactions between grid-forming and grid-following converters in the microgrid, a transient stability criterion based on the equal area criterion(EAC) and an improved control strategy for transient stability are proposed. First, the simplified second-order dynamic model of the islanded microgrid is established, which contains a nonlinear damping term relying on the power angle. Then, the impact of the nonlinear damping term on the acceleration and deceleration areas is revealed from the energy perspective. Considering the distribution characteristics of nonlinear damping, a transient stability criterion is formulated for the positive damping region. In addition, according to the stable boundary conditions, an improved control strategy for transient stability based on voltage feedforward is also put forward. Finally, simulations are carried out with MATLAB/Simulink to verify the effectiveness of the proposed stability criteria and the improved control strategy. The results show that the microgrid transient stability criterion and the improved control strategy proposed can provide a theoretical basis for the parameter optimization design of power electronic converters and the improvement of the stable operation capability of microgrid.

The new generation of wide bandgap power semiconductors such as SiC and GaN are driving the rapid high-frequency, high-efficiency, and small volume development of power electronic equipment. However, they are also more likely to interfere with sensitive loads, affect wireless communication, and even endanger their own safety and reliable operation, which poses great pressure and challenges to the electromagnetic compatibility (EMC) performance of power electronic equipment. In recent years, the radiated frequency(RF) characteristics of power switches, wideband electromagnetic models of magnetic components, electromagnetic radiation mechanisms of switched mode power supplies, near-field characteristics of wireless power transmission(WPT), and the new designs of electromagnetic interference(EMI) filters have become current research hotspots and received continuous attention from academia and industry. The Journal of Power Supply has specially released the album "Electromagnetic Compatibility in Power Electronic Systems" to promote the exploration of difficult and hot issues in the field of EMC analysis and design of power electronic systems.

To improve the dynamic response performance of a dual active bridge (DAB) converter with dual phase shift(DPS) control during load switching and reduce the current stress, a novel dual phase shift (NDPS) control method is studied. By changing the shifting direction of the internal phase shift angle in traditional dual phase shift (TDPS), the relationship between the transmission power and phase shift ratio is reconstructed, and the adjusting range of phase shift ratio is extended. The solving method for the optimal phase shift ratio of DPS control under the condition of current stress minimization is studied, and the dynamic response characteristics of NDPS and TDPS control under load switching conditions are compared and analyzed. Finally, an experimental platform of DAB converter was built to verify the theoretical analysis, and experimental results show that the optimal phase shift ratio combination based on current stress minimization can effectively reduce the current stress under light load conditions. At the same time, NDPS control has better dynamic response characteristics than TDPS control.

To improve the reliability and efficiency of a T-type three-level inverter, a discontinuous pulse width modulation (DPWM) strategy is proposed to reduce the common-mode voltage while reducing the switching loss, which is also named as the RCVDPWM strategy. According to the mechanism of switching sequence in the T-type three-level topology which acts on the common-mode voltage, five DPWM clamping methods for common-mode voltage reduction are summarized. It can be found that there is at least one clamping method for common-mode voltage reduction at any modulation ratio or phase angle. For those phase angle regions where multiple clamping methods exist, the switch tube of the phase with the largest absolute value of current is preferentially selected for clamping to reduce the switching loss. Meanwhile, the proposed strategy can ensure that the DC component of neutral-point voltage is zero, and thus the neutral point shows a self-balancing capability. Experimental results verify the feasibility and effectiveness of the proposed RCVDPWM strategy.

To improve the state-of-charge(SOC) prediction accuracy of lithium battery, a prediction method based on the fusion model of Attention mechanism and convolution neural network-long short-term memory(CNN-LSTM) is proposed. This model uses one-dimensional CNN and LSTM neural network to learn the nonlinear relationship between SOC and lithium battery discharge data, as well as the long-term dependence existing in SOC sequences. At the same time, it adopts a "many-to-one" structure and establishes a mapping relationship between the SOC at the present moment and the discharge data at multiple historical moments, and pays attention to the historical discharge data which has a greater influence on the SOC at the present moment through the Attention mechanism, thus further improving the SOC prediction accuracy. The SOC prediction experiments under dynamic conditions show that the average prediction error of the proposed method is 0.89% under different temperature conditions, which is 81.2%, 66.7% and 56.5% lower than those of SVM, GRU and XGBoost algorithms, respectively. In addition, this method is also superior to LSTM and CNN-LSTM models that do not combine the Attention mechanism, showing a higher prediction accuracy and higher application values.