Research On The Control Strategy Of Virtual Synchronous Generator For Microgrid Inverter

Facing the dual pressures of fossil energy exhaustion and environmental pollution brought about by economic production,the energy structure is deepening reform.The distributed generation is booming,and gradually takes a place in the traditional power system.However,the microgrid based on distributed power generation does not have the inertia and damping of the traditional power system,and the large-scale new energy connection greatly challenges the stability of the system.Virtual synchronous generator technology enables the microgrid,which is mainly composed of power electronic devices,to have the inertia characteristic of synchronous generator.In this way,not only the stability of the microgrid is improved,but also the perfect compatibility with the traditional grid can be better realized.Therefore,the application of virtual synchronous generator control in microgrid is studied in this thesis.The mathematical model of the main circuit of the inverter is established.According to the characteristics of the synchronous generator,the basic principle of the virtual synchronous generator is analyzed,and the speed regulator and excitation regulator of the outer power loop are constructed,and the core algorithm of the virtual synchronous generator is realized.Double closed loop control link is designed to remove the coupling between the coordinate systems and stabilize the output voltage of the inverter.Based on the small signal model,the influence of the key parameters on the dynamic performance and stability of the system is analyzed,which provides a theoretical basis for the further research of the virtual synchronous machine.The virtual synchronous machine can deliver power to the grid based on the value of the command.In order to avoid the impact current at the moment of grid connection,a direct voltage pre-synchronization control is proposed,which eliminates the traditional PLLs.The pre-synchronization control can be achieved by directly using the grid voltage and the output voltage of the virtual synchronous machine,realize a smooth switch from island to grid connection.Considering that the virtual synchronous machine does not have real physical inertia and its anti-disturbance ability is not comparable to that of synchronous motor,a parameter adaptive control is designed.Fuzzy control method is used to adjust the change of adaptive parameters flexibly,and limit the value range of parameters from two aspects of stability and dynamics,so as to effectively restrain the oscillation of the system in the transient process.Aiming at the problem of line impedance inconsistency of parallel virtual synchronous machine system,the power transmission characteristics and distribution mechanism are analyzed.The virtual impedance method is used to remould the impedance of the system,so as to complete the decoupling of power.Parameter design of parallel system with arbitrary capacity ratio is carried out to realize power distribution according to the ratio and restrain the circulation between inverters.Finally,the control algorithm of virtual synchronous machine studied in this thesis is simulated and verified by experiments using Matlab software and experimental platform.