Under the background of limited resources and time，equipment maintenance is considered as an effective way to maintain the stable operation of the system. In order to allocate maintenance resources efficiently，the most critical equipment in the system should be identified first namely those that will cause significant consequences if they fail. Firstly，a new multicriteria decision-making（MCDM） scheme was proposed to identify the critical lines in the distribution network. Different from the previous analytic based hierarchy process，the best-worst method （BWM） was adopted to obtain the weight of the system reliability index according to the knowledge and judgment of experts. In addition，the fuzzy theory was introduced into the traditional best-worst method to overcome the general uncertainty in expert judgment and decision making. Finally，the technique for order preference by similarity to an ideal solution technology was used to prioritize the maintenance of IEEE14 distribution network lines. The proposed method can determine the priority of system maintenance more quickly and accurately.

With the accelerated construction of new power system，the harmonic characteristics of power grid are becoming more and more complex. It is of great significance to study the effective statistical management of harmonic data for evaluating the power quality of power grid. A statistical method of harmonic evaluation index based on maximum entropy principle was proposed. By recording and saving the average value and center distance of harmonic data，the maximum entropy principle was used to fit the probability distribution of harmonics，so as to save and identify the harmonic characteristics and facilitate data storage. Two harmonic evaluation criteria of harmonic 95% probability value and 99% probability value were obtained by using the fitted probability distribution，which ensures the accuracy and consistency of the index. Finally，the effectiveness of the proposed method was verified by the example analysis of harmonic measured data.

The state of the DC system of substation is directly related to the normal operation of the substation. A new method for ground fault detection in substation DC systems，which is a combination of double-tree complex wavelet transform and singular value decomposition，was proposed to achieve fast and accurate location of ground faults occurring in substation DC systems. Firstly，the method constructed a Hankel matrix to decompose the branch current signal through a dual-tree complex wavelet transform（DT-CWT）. Secondly，the Hankel matrix was decomposed by the singular value decomposition（SVD）method with the aim of obtaining a series of singular eigenvalues. Thirdly，the singular value difference spectrum was constructed using the adjacent singular value differences，and the number of singular values was retained by the maximum peak of the singular value difference spectrum. Finally，the low-frequency signal was reconstructed by the retained singular values. The analysis results of the algorithm show that the method can accurately extract the low-frequency AC signal from the branch current signal and achieve the accurate location of the DC system ground fault in the substation，which can largely reduce the influence of the ground capacitance on the detection accuracy.

The power transmission and transformation system is an important link of power transmission. In order to reduce the cost of operation and maintenance and improve the recognition effect，an application method of automatic identification and control of transmission and transformation vulnerable lines was proposed based on artificial intelligence （AI）technology. Using the greedy decision tree algorithm （ID3）of AI to measure the amount of information between different operation characteristics of the line，it got the line characteristics，introduced mutual information feature selection （MIFS）to optimize the decision tree，balanced the redundancy between the line operation characteristics by using the adjustment coefficient and penalty term，and introduced the weighted degree，measured the node weight and focused on the total active power of all connecting lines. Based on the comprehensive analysis of the permittivity and weighted degree，the comprehensive automatic identification index of vulnerable lines was obtained，the identification model was established，and finally the power flow transmission and load change of transmission and transformation lines were changed to achieve control. The experiment shows that the proposed method can accurately identify the location of vulnerable lines，which is effective and feasible，and its control application also effectively reduces the operation and maintenance costs.

In order to improve the debugging and fault monitoring and analysis capabilities of inverter products，the design scheme of network version process data acquisition and analysis software was proposed.The composition and implementation of the software system were discussed，and the technical means such as multi-thread，virtual memory，drawing class library and high-speed fiber synchronization were proposed to solve the technical problems of system operation. The field application of this software system shows that the system runs stably，has the characteristics of low cost，multi-channel transmission，low sampling period，high data accuracy，rich graphics functions，and strong scalability.

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.

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.

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.

Considering that the problem of voltage fluctuations on the DC side caused by the three-phase imbalance of the AC grid of the flexible interconnection device，a DC voltage fluctuation suppression strategy for flexible interconnection device based on the Torricelli point was proposed. Firstly，by defining the Torricelli point of the triangle formed by the voltage vector in the three-phase unbalanced fault system as the "new neutral point"，the voltage amplitude asymmetry and phase angle drift faults in the traditional three-phase coordinate system were transformed into voltage amplitude faults in the Torricelli point coordinate system； secondly，the relationship between the active power oscillation of the three-phase power system and DC fluctuations was analyzed，a new control strategy was established to suppress the voltage fluctuations of the DC bus voltage by suppressing the fluctuation of double frequency component of active power effectively. Finally，the proposed method was proved to be effective by the Matlab/Simulink simulation model.

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.