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.

The development of power modules towards high temperature, high power and high density raises higher requirements for the packaging structures of modules. Compared with the traditional wire-bond structure, the double-sided structure has attracted more and more attention owing to its characteristics such as high heat dissipation capacity and low parasitic inductance. However, the mismatch of thermal expansion coefficient between materials used in the double-sided structure makes the structure suffer tremendous thermo-mechanical stress, thus reducing the reliability of power module. Therefore, to develop double-sided bi-directional modules with low thermo-mechanical stress, the effects of chip layouts on the heat dissipation performance of modules and the parasitic inductance were analyzed by simulations at first. Then, a flexible buffering spacer with low Young's modulus is proposed accordingly. The feasibility of reducing the thermo-mechanical stress and improving the reliability of the module was preliminarily proved by simulation and experimental results.

Aimed at the problem that the influencing mechanism of configuration size of a differential pressure channel in proton exchange membrane fuel cell(PEMFC) for the cell's electrochemical performance is unclear, the influences of channel height and rib width on the oxygen concentration, water concentration distribution characteristics, current density, power density, and pressure drop of a differential pressure channel and a straight channel are studied, and a comparative analysis of the two kinds of channels is performed. Results show that channel height has little effect on both channels, but the differential pressure channel has an obvious advantage when its rib width is 1.25 mm or 1.5 mm. The influence of pressure converter area on the performance of the differential pressure channel is further studied, and results show that its peak power density is the highest when the height and length of the pressure converter area are 0.05 mm and 1.5 mm, respectively. With the comprehensive consideration of influences on power density and pressure drop, the differential pressure channel with a height of 0.4 mm, width of 1.25 mm, rib width of 1.25 mm, and a pressure converter area with a length of 1.5 mm and height of 0.05 mm is selected. In this case, its peak power density is 0.366 1 W/cm², which is 6.3% higher than that of the straight channel.

A modeling data and optimization algorithm driven electrothermal behavior model of gallium nitride high electron mobility transistor (GaN HEMT) is proposed to facilitate the quantitative analysis of problems caused by high speed switching, such as turn-on overvoltage, false turn-on, oscillation and EMI noise. Compared with the traditional behavior models of GaN HEMT, the proposed model can precisely depict the electrothermal characteristics of GaN HEMT in a wide temperature range in both the first and third quadrants by only two compact equations. Meanwhile, the nonlinear parasitic capacitances of GaN HEMT can be accurately modeled by one compact equation. In addition, an optimization algorithm combing the genetic algorithm and Levenberg-Marquardt algorithm is put forward, and a one-step extraction of modeling parameters is realized based on this optimization algorithm and modeling data, which can reduce the modeling time and work load to a certain degree. Results show that the proposed modeling method can precisely model multiple types of GaN HEMT devices manufactured by different companies. Finally, the correctness and effectiveness of the proposed modeling method was verified by the well-matched simulated dynamic waveforms and experimental measurement data.

The junction temperature monitoring of power devices in a solid-state power controller(SSPC) plays a vital role in the SSPC reliability. The thermal model method is widely used owing to its contactless measurement and simplicity. However, the aging of the power chip will lead to the degradation of the thermal path, and the junction-to-case thermal resistance of the device will increase. As a result, the actual junction temperature may far exceed the value estimated by the thermal network model, leading to an optimistic estimation of the device's state-of-health. The failure of the solder layer is considered to be one of the main reasons for the aging failure of SSPC power devices. In this paper, the device's aging state is monitored in real time during the life of SSPC, and the thermal model of the power device is adaptively updated online. The thermal impedance taken as an update basis is calculated by measuring the temperature-sensitive electrical parameters which are not affected by the degradation of the solder layer, and the thermal impedance information is associated with the aging state of the solder layer to update the thermal model. Using the proposed method, the thermal model can be updated in real time without affecting the normal operation of SSPC, and experimental results verified the effectiveness of this method.

The traditional nearest level modulation (NLM) has advantages of simple control and low switching frequency. However, when the number of output levels is small, the harmonic distortion rate of output waveform from a modular multilevel converter(MMC) under the NLM modulation will be large. In this paper, a full-bridge auxiliary sub-module is added to each arm of the traditional three-phase six-leg MMC topology, and the capacitance voltage of the auxiliary sub-module is half of that of the half-bridge sub-module. Aimed at this MMC topology, an improved hybrid modulation strategy is proposed, which can realize the balance control of half-bridge and auxiliary sub-module, reduce the harmonic distortion rate of MMC AC-side output current, and reduce the switching frequency and system loss of the full-bridge sub-module. A detailed simulation model is built in MATLAB/Simulink, and simulation results verify the effectiveness of the proposed MMC topology and the improved hybrid modulation strategy.

The HT-6M Tokamak Reconstruction is an international project of cooperation between China and Thailand for responding to the Belt and Road Initiative. The function of pulse power supply for heating field is to breakdown and produce plasma, and the corresponding power supply scheme adopts the form of capacitor energy storage pulse discharge. To calculate the parameters of power supply that meet the requirements, the discharging process of pulse power supply for heating field is analyzed mathematically, and the core devices are designed and developed according to working parameters of power supply equipment. To verify the theoretical analysis of discharging process, a set of small capacitor energy storage pulse power supply was developed. At the same time, a turn-off experiment on a high-power solid-state circuit breaker was carried out.

The parameter identification method for the equivalent circuit model of lithium-ion battery has a great impact on the model accuracy. To solve the problems of low convergence accuracy and slow convergence speed in a satin bowerbird optimization(SBO) algorithm, an improved satin bowerbird optimization (ISBO) algorithm is proposed. The inertial weights, Cauchy mutation, Gaussian mutation and greedy selection strategies are used to improve the convergence accuracy of the ISBO algorithm, and its convergence performance is verified by standard test functions. Based on the battery charging and discharging data, the proposed ISBO algorithm is applied to the parameter identification of the equivalent circuit model of lithium-ion battery. Experimental results show that compared with the SBO and adaptive weight particle swarm optimization algorithms, the ISBO algorithm has a higher accuracy when it is used in identifying the model parameters and the identification accuracy is not affected by the working conditions of battery.