| In recent years, every country in the world has widely put attention to the integration research and development of Distributed Generation (DG) micro-grid into the main grid, offering great practical significances to all sectors of society. Micro-grid (MG) is a controllable independent power generation system composed of distributed generations, loads, energy storage devices, power conversion devices, and protection devices. Micro-grid technology is put forward to realize the issues as the flexibility of DG, efficient application, solution of the large number and various forms of grid-connected operation of DG. Adopting a good grid-connection control technology is an important way to improve the penetration of the distributed generations. This thesis adopts the relative advanced control technology based on Virtual Synchronous Generator (Virtual Synchronous Generator-VSG) control technology, making the micro-grid inverter process external characteristics and the function of regulating voltage and frequency of the synchronous generator, and establishing the mathematical model of the micro-grid inverter. A harmonic suppression control method of combining Active Power Filter with Static Var Compensator is proposed to effectively suppress harmonics in the micro-grid and guarantee the stability of the system. The main research contents are as follows:(1) The concepts as well as the background of the micro-grid in and out of China were introduced. The micro-grid experimental platform of various countries in the world and relative demonstration projects were summarized, as well as the characteristics and development status of three kinds of the micro-grid control structures. Moreover, the virtual synchronous generator technology and its development process are also presented.(2) The fundamental features of models of a variety of distributed generations in the micro-grid (mainly including photovoltaic solar cells, wind power, and battery energy storage) are analyzed and studied. The micro-grid’s operation characteristics and technical requirements are summarized, and the control of the inverter is presented to be the key factor to its stable operation. Moreover, several traditional inverter control methods such as droop control, constant voltage-frequency control, and constant power-frequency control etc are comparatively analyzed. After comparing the advantages and drawbacks of these methods, the Virtual Synchronous Generator control strategy was proposed.(3) The control strategy of virtual synchronous generator (VSG) is proposed with reference to the advantages of synchronous generator such as self-built voltage, large inertia, and high output impedance etc. Based on the VSG and the mathematical model of the electrical part of the synchronous generator stator, their equivalent output impedance characteristic, external characteristic, dynamic characteristic, and the power-angle characteristic are compared in detailed. The proposed virtual synchronous generator model is classified and summarized to two aspects of mathematical model and control strategy. Moreover, its characteristic is also be verified by simulation.(4) The frequency and voltage control strategies of the VSG-control micro-grid are designed with reference to the basic principles of the speed governor’s electric power system and excitation controller. The frequency and voltage of the micro-grid are analyzed focusing on the relevant provisions of the frequency and voltage deviation. Afterward, the three main control sections of the VSG control technology (virtual governor, virtual excitation controller, virtual synchronous algorithm) are analyzed and designed, laying a foundation of utilizing virtual synchronous generator control technology to control micro-grid frequency and voltage. Based on the Matlab/Simulink platform, the proposed virtual synchronous generator control technology is analyzed by simulation of micro-grid in islanding and grid-connected modes, focusing on the frequency and voltage stability. The simulation result shows that, the proposed voltage control method offers a fast stable voltage in relative short period of adjusting time; the proposed frequency stability control method could recover the power grid frequency rapidly after connection of the micro-grid.(5) Aiming at solving the harmonic suppression problems of micro-grid system, the combined operation control method of Active Power Filter (APF) and Static Var Compensator (SVC) is proposed. This method can compensate reactive power and suppress harmonic simultaneously. Moreover, the control strategy of the APF and SVC is analyzed deeply and simulated in Matlab/Simulink. The simulation results show that after injecting combined system of the APF and SVC, the power quality is improved obviously. The combination of APF and SVC in micro-grid has excellent reactive power compensation and harmonic suppression performance, satisfying the real-time requirements of power quality conditioner. |
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