Microgrid Inverter Based On VSG And Off-grid Operation Control Strategy Research

In the background of the disadvantages of traditional power generation technology,distributed power generation technology based on clean energy such as photovoltaic and wind power is booming.In order to realize the absorption of large-scale distributed power generation system by power grid,the concept of Microgrid(MG)was born.Virtual Synchronous Generators(VSG)technology has become one of the main control strategies for microgrid inverters because it can imitate the external characteristics and operation characteristics of Synchronous Generators.Off-grid and grid-connected are two operating modes of microgrid inverters.Designing the corresponding seamless switching control strategy can suppress the adverse effects such as current shock at switching time,and finally realize the seamless switching between operating modes.In this paper,how to realize the seamless off-grid switch of micro-grid VSG inverter is analyzed and studied in detail.Firstly,the three-phase bridge inverter is selected as VSG power conversion module,and the mathematical model of the inverter is established.The second-order mathematical model of synchronous generator is introduced into the control of the inverter to simulate the dynamic characteristics of synchronous generator.The frequency control and voltage control of VSG are introduced in detail,the control mechanism of corresponding controllers is analyzed,and the structure of the controller is given.The mathematical model of inner double loop control is established and the corresponding control structure is given.Finally,the control structure of VSG system is designed and the influence of parameter changes on its power-frequency characteristics under different working conditions is simulated.Secondly,the switching process of VSG inverter off-grid is analyzed,and the conditions for realizing seamless switching are summarized.Because Synchronous Reference Frame Phase Locked Loop(SRF-PLL)can accurately extract the Phase of the voltage to be measured,a seamless switching control strategy based on SRF-PLL is designed,which mainly includes frequency,amplitude,Phase compensation and seamless switching signal formation.The final realization of a single VSG inverter off the grid seamless switch.Then,since the introduction of SRF-PLL may further increase the complexity of the system,making VSG control lose its advantage of not requiring SRF-PLL,a seamless switching control strategy based on virtual impedance is designed.To achieve virtual power to zero as the control goal,VSG inverter seamless switching control strategy based on virtual impedance is designed.It is compared with the seamless switching control strategy based on SRF-PLL,so as to select a suitable seamless switching control strategy according to the actual engineering requirements.Finally,in order to improve the overall capacity and reliability of the microgrid system,the parallel connection of multiple inverters is the most direct solution.The power distribution of multiple VSG inverters in parallel can be realized by parameter configuration,and the seamless switching control strategy based on SRF-PLL and virtual impedance is extended to the micro-grid system with multiple VSG inverters running in parallel to achieve seamless switching of multiple machines in parallel off-grid.