Modular multi-level matrix converter（M3C）is a kind of new topology which can convert one specified electric frequency to another directly，it is applicable to high voltage and large capacity scene. In the fractional transmission system，the complete flow was introduced to achieve power frequency grid fault ride-through. The arm power state equation of M3C in double α-β frame was introduced when power frequency voltage and fractional frequency voltage is unsymmetrical. The control strategy of switching on braking resistor in the fractional frequency side based on the average voltage of all sub-module capacitor was introduced，in order to consume redundant power in the fractional frequency side. Positive and negative sequence reactive current based on the power frequency positive voltage dips and negative voltage component were output. In addition，when power frequency positive sequence voltage is too low，diagonal balancing control was converted to the fractional frequency side from the power frequency side，which can realize the balance of all arm sub-module capacitor voltage in the process of power frequency grid fault ride-though. At last，a 3.3 kV/3.3 kV/3 MV·A M3C model was built in Matlab/Simulink，the effectiveness of power frequency grid fault ride-through strategy was validated.

In order to improve the accuracy of intelligent diagnosis of reactor mechanical fault，according to the correlation characteristics between reactor vibration signal and mechanical state，a vibration diagnosis method of reactor mechanical fault based on stacked auto-encoder（SAE） was proposed. Firstly，the original vibration signal of reactor was decomposed by wavelet packet decomposition algorithm，and the time-frequency energy matrix of the signal was extracted. Then，the diagnosis model of reactor mechanical fault based on SAE was built，the deep feature mining of the time-frequency energy matrix was completed through unsupervised self-learning，and the identification of reactor mechanical fault was realized through supervised fine-tuning. Finally，vibration data of 10 kV oil immersed reactor under different mechanical states was used to train the fault identification model and optimize the super parameters. The numerical results show that the proposed method can identify reactor mechanical fault better than the traditional vibration signal identification method，and the accuracy can reach 98%.

Compared with large photovoltaic systems with isolation transformers，distributed photovoltaic systems have small power and volume. In order to improve efficiency，non-insulated inverters are often used in design，but photovoltaic leakage current endangering system safety is also generated. Taking the single-phase full-bridge photovoltaic system as an example，the generation mechanism of photovoltaic leakage current was analyzed in detail through in-depth discussion of the problem of photovoltaic leakage current. Subsequently，the waveform characteristics of the photovoltaic leakage current entering the distribution network were analyzed，and a photovoltaic leakage current monitoring method based on Hausdorff distance was proposed，which simply and efficiently solved the problem of photovoltaic leakage current monitoring and protection. Finally，the feasibility of the proposed method was verified by simulation analysis.

A rotor position detection strategy based on virtual neutral point was proposed to realize position sensorless control of BLDC motor. Through the three-phase terminal voltage with the same value of three resistances star connection to construct a virtual neutral point，the relationship between the virtual neutral point voltage and commutation time of BLDC motor was analyzed. A conclusion was drawed that the moment when the virtual neutral point voltage through the DC bus midpoint is the zero-crossing time of the back electromotive force （BEMF），and the operation of the motor in each state，the effectiveness of zero-crossing signal of BEMF was further analyzed. The scheme was further verified by experiments，and the output of zero-crossing signal was realized by comparator. Compared with the conventional terminal voltage detection method，the system reduced two comparison detection circuits and has no phase shift. Experimental results show that the position of zero-crossing signal can be accurately determined by using this strategy，and the phase error caused by traditional BEMF method can be avoided.

Active power filters（APF）are widely used for dynamic compensation of power system harmonic. However，under weak grid conditions，the interaction between APF，nonlinear loads and grid impedance can easily lead to the stability issue of harmonic oscillation around the compensation frequency，resulting in the fault of harmonic compensation and further deterioration of the system's power quality. Through small-signal modelling of the loop，the harmonic oscillation mechanism in APF system under weak grid conditions was revealed，and an active damping scheme based on inductor current feedback was proposed to suppress harmonic oscillation according to the conclusion of stability analysis，which ensures the stable harmonic compensation capability of APF under weak grid conditions. The effectiveness of the proposed scheme was verified by PLECS time-domain simulations and hardware platform experiments.

When the permanent magnet assisted synchronous reluctance motor（PMaSynRM）runs at high-speed with flux-weakening control，the DC bus voltage utilization is not high，and the efficiency and torque output capacity of the motor are low. Therefore，a flux-weakening control strategy of permanent magnet assisted synchronous reluctance motor based on hexagonal trajectory was proposed. Firstly，based on the d-q axis equivalent circuit of the permanent magnet assisted synchronous reluctance motor，the voltage and current constraints of the flux-weakening process were derived，and the root cause of the flux-weakening control to improve the speed regulation ability of the motor was proved. Secondly，in order to give full play to the advantages of high-power density under high-speed operation of permanent magnet assisted synchronous reluctance motor，the over-modulation algorithm was derived. It was applied to the flux-weakening operation of permanent magnet assisted synchronous reluctance motor to achieve higher DC bus voltage utilization. Finally，the effectiveness of the proposed method was verified by simulation.

In response to the complex working environment of pumped storage energy units，where disturbances such as temperature variations are prominent，and considering the insufficient control precision of traditional vector control for brushless doubly-fed machine（BDFM），a study was conducted on a control strategy based on linear active disturbance rejection control（ADRC）for power decoupling of BDFM.This strategy aims to mitigate disturbances，enhance control precision，improve operational characteristics，and consequently elevate the energy conversion efficiency within pumped storage energy systems.The mathematical model for BDFM in the unified coordinate system and two-phase rotating coordinate system was established.The coordinate system was determined using the power winding orientation method.The current relationship under the power winding orientation was derived，and based on this，the control winding was used to control the active and reactive power of the power winding，thereby achieving power decoupling control.To improve control performance，the speed loop and current loop were rewritten，disturbances were analyzed，state equations were formulated，an extended state observer was designed，and the control performance of the machine was enhanced using the active disturbance rejection control technique.Based on the analysis of harmonics，it is concluded that the main harmonic in the synchronous coordinate system is the 6th harmonic.To suppress specific frequency harmonics，the repetitive controller was employed.Through Simulink simulation，dynamic responses and Fourier analysis of the d-axis winding current were observed，confirming the effectiveness of the proposed active disturbance rejection power decoupling control method and harmonic suppression method.

In order to optimize the current stress of dual active bridge（DAB）converter，an improved dual phase shift （IDPS）modulation was proposed based on the traditional dual phase shift （DPS）modulation. Firstly，the working principle of IDPS modulation was introduced，its steady-state working characteristics were analyzed，and the mathematical model of current stress on transmission power was established. Then，the optimal value of current stress of IDPS modulation was obtained by using Lagrange multiplier method，and compared with traditional single phase shift（SPS） and DPS modulation methods. Finally，an experimental platform was built to verify the effectiveness of the proposed control strategy in optimizing current stress，meanwhile，the efficiency of the converter was improved.

Under digital control，the inner current loop of the LC-type inverter is affected by the control delay，which leads to the weakening of its active damping effect. Increasing the sampling control frequency can effectively reduce the negative influence of digital control delay. Furthermore，in order to suppress the high order harmonic disturbance caused by strongly nonlinear load，multiple resonant controllers are needed in the outer voltage loop. For reducing the time complexity of the outer voltage loop and increasing the quantity of the controlled harmonics，an optimal sampling digital control strategy was proposed for the current and voltage double loop control of the LC-type inverter. Specifically，the inner current loop was set to the double sampling control frequency and the outer voltage loop was set to the single sampling control frequency. The optimal sampling digital control strategy enhance the suppression ability of nonlinear load disturbance and reduce the total harmonic distortion of the output voltage while maintaining the stability margin of the inner current loop. Finally，the feasibility and effectiveness of the proposed optimal sampling digital control strategy were verified by experiments.

In traditional current protection，the effective value of current is compared with the setting value to identify the fault which is the typical protection scheme for the transmission lines of the power distribution systems. The current is calculated by the fast Fourier transform algorithm. As the calculation of the effective value of current requires a long time，the speed of the current quick break protection is reduced. In this regard，a fast current protection algorithm based on the discrete setting-value was presented to solve this problem. The main emphasis was placed on the construction of the discrete sequence of setting-value and the protection criterion. The protection startup criterion and action criterion were constructed based on comparisons between discrete sequence of setting-value and sampling-value of the current fault component. The operation characteristics and reliability of the fast current protection were analyzed. The performance of the novel fast current protection algorithm was compared with that of the conventional current protection through simulations based on PSCAD/EMTDC. The results verify that the novel fast current protection algorithm has good operation characteristics under different fault conditions.