Three-level neutral-point-clamped （3L-NPC） interconnected converters have been widely used in the AC-DC hybrid distribution grids due to their superiorities of large capacity and high power quality. However，their working conditions are always with high power，varying load，and limited heat dissipation，etc.，with a high open-circuit failure rate of power switches. Meanwhile，existing fault diagnosis methods are mostly single mechanism-based or data-based，unable to overcome the problems of complex system model structure and changing operating conditions，resulting in low diagnostic accuracy and speed. To this end，a mechanism-data-fusion-driven fault diagnosis method for interconnected conversion systems was proposed. Firstly，a mechanism-data-fusion model was constructed using a neural network observer to improve the fault diagnosis accuracy. Subsequently，the trajectories of current residuals after open-circuit faults of different devices were analyzed，and a current residual table was summarized，based on which a fast and accurate open-circuit fault diagnosis method was formed. Finally，the experimental and hardware-in-the-loop results verify the effectiveness of the proposed method.

Aiming at the problem that the ripple content of current and voltage in cycloconverter speed control system is large and the ripple period is time-varying， the real-time sampling value is not suited for feeding back to the speed control system directly，a variable period mean sampling algorithm was proposed. Firstly，the algorithm took the interval time of trigger pulse of cycloconverter power device as the mean period to calculate the meanvalue of actual current and voltage. Secondly，the phase lag angle caused by the mean period was calculated according to the real-time frequency and mean period of current and voltage. Finally，the lag compensation and interpolation operation were carried out according to the phase lag angle. More ideal voltage and current feedback values were obtained for speed control system. Compared with the average sampling method，this algorithm is more suitable for the time-varying ripple period of cycloconverter conversion，and it eliminate the sampling lag and improve the performance and stability of the speed control system.

In view of the lack of flexibility in the input and exit of the current metal sheath loop suppression device for high voltage cables，the problems such as excessive induced voltage of the sheath and excessive power loss were not considered when determining the suppression impedance value，a loop suppression device was designed which could dynamically input or exit the loop suppression impedance of the sheath according to the field conditions. Firstly，the structure and operation flow of the suppression device was described. Then，the variation characteristics of sheath circulation，induced voltage and sheath loss when the suppression impedance was pure resistance，pure inductance and combined resistance inductance were analysed. On this basis，an optimization model was established to solve the optimal impedance value which can minimize the power loss when meeting the requirements of sheath circulation and induced voltage. Based on the optimal impedance value，the parameters of thyristors and other components were configured. The results show that the proposed method can obtain the optimal value of cable suppression impedance that meets the engineering requirements，and achieve a good circulation suppression effect.

The flexible DC interconnection of medium and low voltage can improve the flexibility and reliability of the distribution network，and the dual active bridge converter is a key link in achieving voltage conversion and electrical isolation in flexible interconnection equipment.The current source resonant dual active bridge utilizes DC capacitors to participate in resonance and soft switching design，which can significantly improve the power density and efficiency of the dual active bridge. However，there are significant differences in the characteristics between the current source resonant dual active bridge and the traditional voltage source resonant dual active bridge. The resonant cavity is composed of a DC resonant capacitor and an AC side resonant inductor，which leads to complex time-domain modeling of high-frequency current and affects the frequency design of zero current switching （ZCS），making it difficult to obtain analytical expressions for the ZCS switching frequency and resonant frequency. An equivalent circuit modeling method for high-frequency isolation links based on fundamental components from a frequency domain perspective was proposed，established an analytical relationship between ZCS switch frequency and resonant frequency，and verified the effectiveness and accuracy of the proposed method through simulation and experimental results.

Flexible multi-state switch （FMSS） is a new type of power electronic device that replaces traditional tie switches and is applied to modern distribution networks. It can optimize the consumption and regulation of distributed power sources in modern distribution networks. The FMSS controlled by VSG is not affected by the deterioration of phase-locked loop performance in weak power grid environments，it can provide frequency support to the grid at low grid strength，thus enhancing the stability of the system. As a device connected to the distribution network，FMSS，when there is three-phase imbalance in the grid voltage，the output current of FMSS under traditional VSG control will become unstable，and the power will also fluctuate significantly，greatly reducing the efficiency of the grid connection point. To address this issue，based on the modular multilevel converter （MMC） structure and the traditional VSG control strategy，positive and negative sequence current compensation was introduced to achieve the stability of FMSS output current，active power，and reactive power under unbalanced power grids. Finally，a four terminal FMSS model was built using Matlab/Simulink，and the effectiveness of the proposed control strategy was verified through the simulation.

With the continuous promotion of the "double carbon" strategy，the reformation of China's electricity market has entered a new stage，but the overall construction of the electricity market is still in the primary stage，and there is an urgent need to develop a scientific and feasible regulatory prevention strategy. An iterative model of "evolution game—optimized clearing" was established between regulators and risky subjects. A library of market risk and profit indicators was formed. And the risk prevention strategy was quantitatively constructed for the electricity market. The evolutionary stabilization strategy and market risk changes in the process of continuous game interaction between the two participants was explored. Iterative simulations base on real data，the simulation results show that when the market risk is high，after limited rounds of evolutionary game，the regulatory prevention strategy can reduce the degree of transaction risk，which verifies the effectiveness of the regulatory prevention strategy. The proposed method can provide theoretical support for the formulation of electricity market regulatory strategy.

When a large number of energy storage devices are integrated into the power grid through a virtual synchronous generator （VSG），improper selection of control parameters in traditional fixed inertia and damping control strategies can lead to long adjustment times or large overshoot，and fail to fully leverage the flexible advantages of VSG control. To address this issue，an adaptive control strategy for energy storage system based on the crisscross optimization （CSO） algorithm was proposed. Firstly，the VSG model of the energy storage system was established，and the minimum value of the sum of the frequency error of the VSG system and the total harmonic distortion of the voltage was taken as the objective function of CSO. The battery state of energy （SOE） constraint was introduced to solve the optimal inertia and damping. This algorithm has a faster convergence speed and effectively avoids parameter local solutions. On this basis，an improved inertia and damping adaptive control strategy was designed to effectively improve the dynamic performance of VSG. Finally，the effectiveness of the proposed strategy was verified by building a simulation model using Matlab/Simulink.

Up to now，the distribution network planning project optimization method rarely considered the impact of the speed of load growth on the power supply efficiency of the project and the year when the project is selected，and rarely considered the urgency，technical benefit and economic benefit of the project at the same time. Therefore，the optimal model for medium and low voltage distribution network planning projects was established with the goal of maximizing technical and economic benefits. According to the urgency of the project，the technical benefits of the project were rewarded and punished. In order to measure the annual change rate of the power supply benefits of the project，the differential weight method was used to obtain the five-year comprehensive level of power supply benefits after the project was put into operation. An improved multi-objective particle swarm algorithm solution model was proposed，and a series of project combination schemes with mutually advantageous objectives were obtained，and the optimal project combination scheme was obtained by multi-level screening of the project combination scheme set according to the target preference of the power supply enterprise. Taking the distribution network planning project database in a certain region as an example，it show that the proposed model can better consider the change of power supply efficiency and the urgency of the project，and achieve multi-regional and multi-objective comprehensive improvement.

The amplitudes of strong pull-in current and hold-on current passing through the high speed solenoid valve were regulated accurately by using hysteresis control to realize pulse width modulation of the voltage of fast solenoid valve， the duty cycle of the current pulse passing through the high speed solenoid valve was controlled by variable frequency control with variable duty cycle， and the protection functions of input overvoltage， input undervoltage and output short circuit were designed. A high speed solenoid valve control instrument was designed based on hysteresis control method. By using the proposed high speed solenoid valve control instrument during the maintenance of aeroengine high speed solenoid valve and main fuel control accessories， the fault of main fuel system of aeroengine caused by unstable input signal of controller was eliminated. Benefited from strong anti-interference ability of the proposed instrument， instrument damage caused by misoperation in the maintenance process was prevent， maintenance quality of high speed solenoid valve has been improved.

In order to solve the problem of overvoltage and overcurrent in the process of load sudden change for ion thruster screen grid power supply， the phase shift pulse frequency modulation （PS-PFM） was used to stabilize the output voltage of the LLC resonant converter. When the sampling voltage is lower than the lower limit of the set value， it enters into the frequency control mode， and when the sampling voltage is higher than the upper limit of the set value， it enters into the phase shift control mode. The experiment proves that， compared with the common frequency control， the addition of phase shift control can solve the problem of high voltage drift in light load or no-load mode， and through the adjustment of PID parameters， it can achieve stable and fast adjustment when the load jumps， so that the output voltage remains at the set value. Through the PCB layout and magnetic components optimization design， the developed principle prototype efficiency reaches 98.9% under rated conditions， and the power density reaches 6.47 \times {10}^6 \mathrm{ } \mathrm{W} / {\mathrm{m}}^3， which is advantageous in the same level of power supply products.