Research On Seamless Switching Control Strategy Of Microgrid Vsg Inverter Between On-grid And Off-grid

Under the background that the drawbacks of traditional energy-based power generation technology gradually become prominent,distributed power generation technologies which is based on clean energy such as photovoltaic and wind power are booming.In order to realize the consumption of large-scale distributed generation systems by the grid,the concept of Microgrid(MG)was born.Virtual Synchronous Generators(VSG)technology has become one of the mainstream control strategies for microgrid inverters because it can simulate the external characteristics and operating characteristics of the synchronous generator.Off-grid and on-grid are two operating modes of the microgrid inverters.To achieve seamless switching between this two modes,the corresponding seamless switching control strategy should be designed to suppress the adverse effects such as current impact at the switching moment.In this paper,the problem of how to realize the seamless switching of the microgrid VSG inverter between off-grid and on-grid is studied in detail.Firstly,this paper introduces the concrete implementation of VSG control strategy,including the design of the stator electromagnetic equation,rotor motion equation,governor exciter controller,and the analysis of the selection basis of internal loop feedback variable in voltage and current double closed loop control.Then,analyze the influence of key parameters to the stable operation of the VSG inverter,and then design the variable virtual impedance control strategy to optimize the frequency recovery curve in the transient process of load disturbance,and finally realize the stable operation of VSG inverter both in on-grid and off-grid modes.Secondly,the switching process of the VSG inverter between off-grid and on-grid modes is analyzed,and then the conditions for seamless switching are summarized.Since the phase-locked loop(PLL)can accurately extract the phase of the voltage to be measured,a seamless switching control strategy based on PLL is designed.The strategy mainly includes frequency compensation,amplitude compensation and seamless switching signal formation.It can achieve the seamless switching of a single VSG inverter from off-grid to on-grid mode.Thirdly,since the introduction of the PLL may further increase the complexity of the system and make the VSG control lose its advantage of not requiring a PLL,a seamless switching control strategy based on virtual power is designed.Taking the virtual power is zero as the control target,then designing the seamless switching control strategy of VSG inverter based on virtual power.The pre-synchronized small-signal model is established and stability analysis is carried out.Compared with the VSG pre-synchronous control strategy based on PLL,the suitable pre-synchronous control strategy can be selected according to the actual engineering requirements.Finally,in order to improve the overall capacity and reliability of the microgrid system,multi-machine parallel connection is the most direct solution.The parameter configuration can realize the power allocation of multiple VSG inverters in parallel,and the seamless switching control strategy based on PLL and virtual power are both extended to the multi-VSG inverter parallel operation system to realize multi-machine parallel pre-synchronization.The different working conditions and the circulation problem between the inverters at the moment of grid connection are simulated in MATLAB/Simulink.