A three-phase Vienna rectifier is taken as the research object in this paper. In view of the difficulty in selecting the weight factor when the finite control set model predictive control realizes the neutral-point potential balance control, as well as the problem of large current ripple on grid-side caused by the action of one single vector during the sampling period, an unweighted factor model predictive control strategy based on double vectors is proposed. First, a single-objective cost function based on power prediction is constructed, and the efficiency of single optimization is improved by sector division and unweighted factor. According to the fluctuation of neutral-point potential on DC side, the redundant small vector is selected to realize the unweighted factor neutral-point potential balance control. Then, based on the obtained optimal vector, the double-vector fixed switching frequency control is realized by combining with the zero vector. Finally, the proposed control strategy is verified on a hardware-in-the-loop platform based on RT-LAB from the aspects of steady state, transient state and neutral-point potential fluctuation, and results effectively prove its correctness and effectiveness.

The problem of high-order nonlinearity in the transient process of a wireless power transmission system is a difficulty in load measurement and identification. When dealing with beat-frequency signals due to the high-order nonlinearity, there are some problems in the existing analog envelope detection method, such as signal distortion, poor circuit flexibility, and limited measurement range. Aimed at these problems, a digital envelope detection method was proposed based on Hilbert transform, and a digital envelope detector was designed to extract the envelope of the voltage beat-frequency signal. In addition, simulations and experiments were conducted to validate the proposed method. Results show that this method can accurately extract the envelope of the voltage beat-frequency signal with different loads, and it achieved an error of less than 5% in the load resistance range of 1-2 000 Ω. The proposed method extends the contactless load measurement range based on characteristic parameters in the transient process, providing a new idea for measuring the internal resistance of batteries in aircraft such as unmanned aerial vehicles.

Aimed at the particularity of an isolated DC/DC converter when it is applied in a space radiation environment, the isolated magnetic feedback circuit is usually used to improve the feedback accuracy and stability. The working principles for several commonly used amplitude modulation magnetic feedback circuits are introduced, and a forward-flyback amplitude modulation bidirectional magnetic feedback circuit is introduced and optimized to solve the problem of the need for an additional secondary power supply voltage. Under the premise of realizing the same function, a two-winding transformer is used to replace the three-winding transformer, which effectively reduces the volume of the magnetic core and simplifies the circuit structure. The working principle for the circuit and the design method for a pulse sampling circuit and a magnetic feedback transformer are analyzed in detail. In addition, based on the magnetic feedback circuit, a DC/DC converter prototype with 100 W output was built. Simulation and experimental results show the effectiveness of the proposed isolated magnetic feedback circuit, providing a theoretical guidance for engineering design.

The conventional Buck-type power factor correction (PFC) converter has a disadvantage of high harmonic current, which limits its applications. A Buck-type single-switch integrated PFC converter is proposed and analyzed. This converter is composed of a Buck-type PFC converter and a Buck-Boost PFC converter, and they are integrated by only one active switch, thereby simplifying the control. Under the constant on time (COT) control, the dead zone of input current in the Buck-type PFC converter is eliminated. With advantages of Buck and Buck-Boost converters, this converter can achieve a high power factor and a high efficiency in universal-input applications. The circuit structure, working principle, steady-state characteristics and design considerations of the proposed converter are introduced and analyzed. Finally, a 56 W prototype was built, and experimental results verified the analysis results.

In this paper, the modulation scheme for a dual active bridge (DAB) bidirectional DC/DC converter is studied. The main advantage of the DAB converter is that it has characteristics such as symmetrical structure, bidirectional power flow capability, wide soft switching range and flexible control capability. The simplest way to control this topology is to control the direction and magnitude of power transmission by adjusting the phase shift angle between the primary and secondary bridges. However, when the input or output voltage of the converter varies widely, a large amount of reactive power will be generated under light load conditions. Meanwhile, the zero voltage switching (ZVS) operation of part of switches cannot be maintained, which directly leads to a low conversion efficiency. Therefore, to improve the efficiency of the DAB converter, a hybrid phase shift modulation (PSM) scheme is proposed, which can reduce the inductor root-mean-square (RMS) current and extend the soft switching range on the basis of keeping the control simple, thereby improving the performance of the converter. First, by making the controllable variables in the extended phase shift(EPS), dual phase shift (DPS) and triple phase shift (TPS) modulation schemes equal, four different PSM schemes are obtained. Then, the steady-state characteristics of these modulation schemes are compared and analyzed, including their transmission power capacity, inductor current level and soft switching performance. On this basis, a hybrid PSM scheme is formulated. Finally, an experimental platform was built to verify the effectiveness and correctness of the proposed modulation scheme.

The circuit topology of a novel three-phase quasi-Z source AC-AC converter is proposed, and the basic working principle and structure of the circuit are analyzed. In addition, the relationship between input voltage and output voltage is also derived. This circuit topology is controlled by a pulse width modulation method, which can achieve the effect of changing the output voltage. MATLAB/Simulink is used to build a simulation model, and the simulation results are analyzed. Finally, an experimental circuit was built on the basis of the simulation model, and experimental results verified the feasibility of the proposed circuit topology and the correctness of circuit analysis.

To improve the performance of modern unipolar power diodes and further break through the "Silicon limit", by increasing the junction depth of P+ region in traditional JBS diodes and introducing a super junction structure to reduce the chip thickness, the contradiction between on-state voltage drop and reverse blocking voltage of traditional unipolar devices is alleviated and the conduction current density of devices per unit area is improved. The effects of P-pillar concentration, N-pillar width and N-pillar concentration of super junction JBS diode on the forward conduction and reverse blocking characteristics are analyzed using a numerical method. The forward conduction and reverse blocking mechanism of super junction JBS diode is analyzed using the theory of electric field coupling effect, and a super junction JBS diode with 300 V is designed.

The diode-rectifer-based high voltage direct current(DR-HVDC) system is a promising low-cost scheme for the connection of offshore wind power, which can deliver wind energy from remote offshore wind farms to onshore power systems. However, the increasing number of offshore DR-HVDC systems may lead to a growing difficulty in controlling wind turbine(WT) converters and a poor system stability. On this basis, a novel grid-formation control method for the connection of DR-HVDC to offshore WT converters is proposed. Two positive-sequence control loops are used to adjust the active power output from WTs and maintain the frequency and voltage of offshore AC grid, in which the first controller can adjust the active power error of each WT to the voltage angle deviation and thus cause frequency changes, and the second controller counteracts frequency changes by adjusting the AC voltage amplitude of WT. The converter's internal current control loop is used to limit the fault current and eliminate high-frequency resonance in the system. Finally, the effectiveness and superiority of the proposed control method are verified by electromagnetic transient simulations in terms of fault crossing, WT power change, reactive power disturbance and WTs shutdown.

The phenomenon of dynamic avalanche occurring during the IGBT turn-off process is one of the important reasons for its failure. To study the dynamic avalanche failure mechanism of IGBT, the Silvaco software was used to simulate and analyze this mechanism. Through the simulation and analysis of the breakdown mechanism, current density distribution and temperature distribution of dynamic avalanche, it is concluded that dynamic avalanche can generate moving current filaments and dead filaments which are either moving slowly or fixed. However, the failure of the device is caused by the dead filaments formed by dynamic avalanche. The dead filaments will lead to a sharp increase of local temperature in the IGBT, and the IGBT will eventually fail because the local temperature is too high to burn the device. On this basis, the causes of dead filaments are analyzed, and specific measures to prevent the dynamic avalanche failure of IGBT are also put forward.

Since the existing distribution network technology increases the active power loss of distribution network system, problems such an increase in the network loss value and a lower penetration rate will arise. To solve these problems, the research on the optimal control method for the flexible soft switch in distribution network considering demand-side response is proposed. First, a demand-side response model and an optimal control model of flexible soft switch are established. Then, the particle swarm optimization algorithm is used to solve these models, and the design of load transfer in distribution network is completed. Finally, the constraint conditions such as the operation of flexible soft switch, power flow in distribution network system and output from renewable energy power generation are set, thereby realizing the optimization of the control of flexible soft switch in distribution network. Experimental results show that after the application of the proposed method, the load of distribution network system can be effectively transferred, the active power loss of distribution network system and the overall network loss can be greatly reduced, and the penetration rate of distribution network system and the consumption of renewable energy power generation can be improved. As a result, the operation optimization of distribution network system is realized, providing a guarantee for its safe and economic operation.