Control Strategy Of Virtual Synchronous Generator For Distributed Generation

The capacity of distributed energy resources in power system is increasing rapidly for the past few years, and a high penetration level will bring new influences and challenges to the stable operation of power system. Compared to the conventional power plants, in which the synchronous generator (SG) dominates, the converter-based DER units have either very small or no rotating mass which is beneficial to the grid frequency stability. Virtual synchronous generator is a control scheme applied to the inverter of distributed generator to support power system stability by imitating the behavior of the synchronous generator. Firstly, a grid-connected novel virtual synchronous generator control strategy is proposed based on the analysis of distributed power system’s model and control strategy. Then, an autonomous virtual synchronous generator control strategy is proposed to enlarge the application scope of virtual synchronous generator. The small-signal models of distributed inverter with grid-connected and autonomous working modes are established based on the above proposed control strategies. In addition, the inertia coefficient of the VSG control strategy can be adjusted flexibly with the change of system frequency amplitude. According to this concept, flexible virtual synchronous generator control strategy based on adaptive inertia coefficient is proposed. Finally, a simple micro-grid digital simulation system based on Matlab/Simulink and a physical experiment platform are built to verify the proposed control strategy. The main research contents of the dissertation are as follows:(1) The working principles, mathematical models and equivalent circuits of photovoltaic energy storage system are researched, and the control methods of distributed inverter are analyzed. Then a grid-connected novel virtual synchronous generator control strategy is proposed to improve the stability of power system. The outside control of distributed inverter is constructed by using synchronous generator’s rotor motion equation, primary frequency regulation feature and the delay characteristic of reactive power regulation. The characteristics of active power-frequency and reactive power-voltage for synchronous generator are emulated by this way in the condition of simplifying the control strategy. And the bottom control is adopted by the grid-connected vector relationship of synchronous generator.(2) An autonomous virtual synchronous generator control strategy which can make the distributed inverter work without the support of the grid is proposed after analyzing the application scope and limitations of the proposed grid-conected virtual synchronous generator control strategy. And the droop characteristics of synchronous generator can be emulated perfectly. The active power-frequency and reactive power-voltage controllers are changed. The proportional resonant controller which has better tracking performance of sinusoidal signal is adopted in the bottom non-decoupling voltage and current controller. The principle of proportional resonant control is analyzed, and the design of main control parameters is also analyzed and illustrated.(3) The small-signal models of inverter with grid-connected and autonomous working modes are established. The impact of main control parameters on system stability and dynamic response is analyzed according to the method of state matrix eigenvalue. And the parameters design principles and methods are analyzed and proposed. The important relationship between virtual synchronous generator and improvement of system frequency stability is analyzed in theoretical. In addition, compared with the droop control, the advantages of virtual synchronous generator are illustrated.(4) The inertia constant of the virtual synchronous generator control strategy can be adjusted flexibly with the change of system frequency amplitude. According to this concept, a novel flexible virtual synchronous generator control strategy based on adaptive inertia coefficient is proposed on the basis of research and analysis for the virtual synchronous generator control technology. The inertia coefficient is modified as a function of the frequency change rate in the proposed flexible virtual synchronous generator control, and the virtual synchronous generator control flexibility of distributed inverter is increased. And the proposed flexible virtual synchronous generator control has good practical significance to further improve the system frequency stability.(5) A simple micro-grid digital simulation system based on Matlab/Simulink and a physical experiment platform are built, and the related theory analysis and proposed control strategy are verified in detailed. The results show that the propose control strategies plays an important role in supporting grid system with frequency stability.