To improve the capability of microgrid in coping with new energy output and load uncertainty, an optimal control strategy for microgrid considering flexible resources is proposed. According to the source-storage-load characteristics of various flexible resources, a two-layer optimal scheduling model for microgrid is established. In the user layer, user-side flexibility resources are introduced, with an optimization goal of minimizing the difference between user costs and net load and decision variables of electric vehicles and output power of translatable loads. In the source-storage layer model, flexible resources are added to the energy storage and power generation sides, with an optimization goal of minimizing the microgrid operators’ cost and load loss rate and decision variables of gas turbines, main network tie-line and output power of energy storage unit. A case study based on scenario-reduced seasonal typical daily data is simulated, and the improved MOEA/D(multi-objective evolutionary algorithm based on decomposition) algorithm is used to solve the two-layer optimal scheduling model. Results show that the average annual user cost is reduced by 6.85%, the average annual total cost of operators is reduced by 14.68%, and the average annual load loss rate is reduced by 6.65%. The results verified the correctness and effectiveness of the proposed method.

The dual-active-bridge (DAB) converter is a key device in a bidirectional power transmission system. In this paper, its fundamental operation principle and topologies are reviewed at first. Then, four basic modulation strategies for the DAB converter are introduced, including single-phase-shifted, dual-phase-shifted, extended-phase-shifted and triple-phase-shifted strategies. Moreover, the modeling and optimization methods based on these four modulation strategies are compared and analyzed. Finally, some problems faced by practical applications and the corresponding solutions are discussed. Along with the development of DC power distribution, energy storage and distributed energy resources, DAB converters will have broad application prospects.

In view of the recent situation in which power sources gradually reach their service terms in China, an evaluation method based on improved fuzzy analytical hierarchy process (FAHP) and entropy weight method (EWM) is proposed. First, an appropriate hierarchical structure is constructed based on the analytic hierarchy process to form a judgment matrix. Then, FAHP is used to process the judgment matrix between various layers, and the importance-oriented weight vector is obtained. At the same time, the Delphi survey method is used to form an evaluation matrix for the last sub criterion layer and the target layer. After normalization, EWM is used to obtain the value-oriented weight vector. The two weight vectors are synthesized to form a comprehensive weight vector. Finally, the final weight vector of the scheme layer to the target layer is formed, and the best scheme is given. The result of an example shows that the proposed evaluation method has strong flexibility and wide applicability. In addition, it also has a clear and reasonable process, as well as intuitive and accurate results.

The development of industry and economy has caused a huge consumption of energy, which brings serious energy crisis and environmental pollution. Therefore, building a safe and clean energy interconnection network is a way to solve the relationship among social development, environment and energy at present. Nowadays, different countries have proposed their policies for the development of new energy electric vehicles (EVs). As the core component of EVs, lithium-ion batteries are directly related to the driving performance and safety of EVs. The state-of-charge (SOC) estimation is a core parameter of lithium-ion batteries used in various industries, and the estimation accuracy is directly related to the service life and efficiency of batteries. In this paper, the problem of battery SOC estimation accuracy in EV applications is studied, and an SOC estimation method based on the extended Kalman filter (EKF) optimized by the whale optimization algorithm (WOA) is proposed. On the basis of constructing the covariance matrix of system noise and observation noise, the improved and optimized WOA-EKF algorithm is used to optimize the noise covariance matrix under dynamic conditions, thus improving the SOC estimation accuracy. The model parameter identification and comparative simulation verification are carried out in MATLAB/ Simulink. Results show that the SOC estimation of lithium-ion batteries based on the WOA optimized EKF algorithm can control the SOC estimation error to be within 2% under different working conditions, which is of significance to the promotion of develop- ment of batteries in the new energy field.

The control system for hybrid distributed energy storage virtual synchronous generator (VSG) composed of a battery and a supercapacitor is improved. A principle for the power allocation of hybrid energy storage is proposed, and a frequency division sliding filter method is designed according to different frequency bands. In view of the power fluctuations of different frequencies, the virtual moment of inertia is piecewise improved to achieve the coordination between the battery and the supercapacitor. Aimed that the problem that the traditional VSG cannot suppress the frequency oscillations, the virtual damping coefficient is modified, and adaptive virtual damping is used to improve the regulation capability of the control system when the frequency oscillations occur. MATLAB is used to verify the proposed control strategy through simulations. Results show that the hybrid energy storage VSG can be controlled in terms of frequency, i.e., when the load fluctuates irregularly, the supercapacitor responds to the high-frequency power fluctuation in time while the battery suppresses the low-frequency power, thus realizing the coordination between the battery and the supercapacitor. When faced with a sudden decrease or increase of 10 kW load, adaptive virtual damping can be used to deal with the frequency oscillations of active output caused by load power fluctuations of hybrid energy storage, and the frequency overshoot is controlled within 0.06 Hz. As a result, the proposed control strategy can adjust the small deviation of frequency regardless of the sudden increase or decrease of power.

The flying capacitor clamped three-level converter has many advantages, e.g., it can reduce the voltage stress of a switch and the volume of a filter inductor. Under its operation, it is necessary to stabilize the flying capacitor voltage at half of the high-voltage side voltage, so a control strategy of adjusting the duty cycle is often used. However, this method has the problem of coupling control between flying capacitor voltage and output voltage, resulting significant fluctuations of inductance current in the process of flying capacitor voltage regulation. To solve this problem, the advantages of using the phase-shifting control strategy to realize the decoupling control of flying capacitor voltage and output volt-age are analyzed, and the corresponding control characteristics are also studied. Through the establishment of a harmonic model of flying capacitor voltage, the relationship between flying capacitor voltage and phase-shifting angle is given. A low-order harmonic function relationship is constructed, which indicates that the flying capacitor voltage is affected by the switch duty cycle D and phase-shifting angle ∆φ. The effective duty cycle interval of phase-shifting control and the duty cycle that optimizes the performance of phase-shifting control are delimited by combining with a time-domain model. A simulation model was established, and an experimental prototype with 3.6 kW was built. The control strategies of adjusting flying capacitor voltage based on phase-shifting angle and duty cycle are compared to verify the decoupling advantages and control characteristics of phase-shifting control.

Silicon carbide metal-oxide-semiconductor field effect transistor (SiC MOSFET) has attracted attention from the industry owing to its excellent characteristics such as high voltage, high frequency and low conduction loss. However, compared with the silicon-based IGBT, the problem of gate oxide reliability caused by the high defect density at the SiC/SiO2 gate oxide interface has become a key bottleneck restricting the large-scale applications of SiC MOSFET devices. By sorting out and analyzing the research results of the gate oxide reliability of SiC MOSFET at home and abroad in recent years, the causes of the gate oxide reliability problems at present were elaborated upon, and various commonly-used gate oxide reliability evaluation methods were summarized and compared. Finally, the gate oxide reliability of SiC MOSFET under extreme operating conditions and the development status of technologies for improving its performance were discussed.

In an energy storage system, the current-fed dual-active-bridge converter has a large current stress and the corresponding soft-switching range is limited, which limits the converter’s efficiency and power density. To solve these problems, combined with the coupled inductor technology, a current-fed dual-active-bridge converter with a low current ripple on the energy storage side and a wide soft-switching range is proposed. Two current-fed full bridges are connected in parallel on the energy storage side, thus effectively reducing the current stress of switches therein. By adjusting the phase shift angle between the two parallel full bridges on the energy storage side, the current ripple is reduced. By reasonably designing the coupling filter inductance, the obtained mutual inductance current is large enough to satisfy the soft-switching conditions for switches. The working principle and steady-state analysis of the converter were given in detail, and a 400 W experimental prototype was designed to verify the superiority and feasibility of the proposed converter.

To address the issue that a dual-active-bridge DC-DC converter will produce large current stress when voltages mismatch and result in a large reduction of its efficiency, a combined dual-phase-shifting (CDPS) control strategy is proposed, which combines dual-internal-phase-shifting (DIPS) and interlaced-dual-phase-shifting (IDPS). First, the working principles of the two control strategies are analyzed, and the mathematical models of transmission power and current stress are established. Second, with the minimum current stress as the objective, the optimal phase-shifting ratios are solved by using the Lagrange multiplied method under the Karush-Kuhn-Tucker condition. Third, the optimization methods under the two control strategies are combined according to different voltage ratios and transmission power. The CDPS control is used to obtain the optimal solution of current stress, which is compared with those obtained under the existing single-phase-shifting and dual- phase-shifting control strategies. Results show that the proposed control strategy can further reduce the current stress and reactive power under the condition of high voltage ratios and improve the efficiency. Finally, an experimental prototype was built to verify the feasibility of the proposed control strategy.

Limited by the switching frequency, the frequency-controlled LLC resonant converter is difficult to achieve a wide output voltage range. To solve this problem, an expandable variable-mode interleaved parallel LLC resonant converter is studied. The secondary-side of this converter adopts a voltage doubling rectifier circuit, which can work in a parallel or series mode according to different switch combinations of two half-bridges on the primary-side, and it can adapt to the wide output voltage range of 1-3N times. A fixed-frequency PWM control method is proposed. In the middle region between the parallel and series modes, the fixed switching frequency is taken as the resonant frequency, and the duty cycle of one bridge arm is changed to realize voltage control. PSIM simulation results show that the wide output voltage range of 1-3N times can be realized by expanding 2N resonator cavities. The experimental results of a 100 W prototype demonstrate that the wide output voltage range of 1-3 times can be achieved with two half-bridges and two resonant cavities, and the effectiveness of the proposed converter and its control strategy was verified.