| Energy problem has always been the focus of attention in countries.Microgrid composed of distributed renewable energy provides a better solution for the comprehensive utilization and economic operation of energy,and it has been widely used in practical industrial production.As the interface device between the power generation unit and grid,the inverter can quickly realize AC and DC energy interaction.However,after a number of inverters are connected to the grid,the inertia of grid will be reduced,which will affect the safe and stable operation of grid.The Virtual Synchronous Generator（VSG）technology makes the inverter have inertia and damping characteristics,and improves the inverter’s transient steady-state performance and anti-interference ability.In view of the advantages of VSG technology in the parallel and grid-connected applications of inverters,this thesis applies VSG technology to three-phase inverters,and research the on/off-grid switching and power sharing control strategy for paralleling of the inverters.The main research contents are as follows:（1）The research background of the subject is briefly introduced,at the same time two common control strategies of voltage-source inverter are introduced,and the research status of VSG on/off-grid switching and power sharing strategy is summarized.（2）The working principle of VSG is introduced in detail,after the inertia and damping coefficients of the active power-frequency loop are selected reasonably by using the small signal modeling method combined with the root locus method.At the same time,the stability of the voltage and current double closed-loop control strategy in the stationary coordinate system and the rotating coordinate system is also analyzed.（3）Since the output frequency of VSG based on the primary frequency control cannot reach 50 Hz,and the frequency will further decrease with the increase of the load,in order to avoid the output frequency beyond the stable range,a secondary frequency control strategy is proposed to make the frequency reach the rated value in the steady state.In order to avoid the impact current phenomenon of VSG during off-grid switching,a frequency and voltage presynchronization strategy is proposed to enable VSG to smoothly switch from on/off-grid to grid-connected mode,and to automatically switch to off-grid secondary frequency control mode when the grid faults.（4）Because traditional VSG in parallel is difficult to achieve accurate reactive power sharing,this thesis proposes a power sharing method of non-interconnection line based on small voltage signal injection.After injection of the small voltage signal,the disturbance of the power system is formed to compensate the amplitude of the output voltage,so that the sharing of active and reactive power can be realized in steady state.The VSG parallel system achieves accurate power sharing after small voltage signal injection,but the fundamental frequency cannot reach 50 Hz.Therefore,the secondary frequency control strategy is added into the VSG parallel system,and the secondary frequency control and reactive power sharing are realized at the same time in steady state,but the active power is not sharing.Therefore,the method of double small voltage signal injection is adopted,and the fundamental frequency is kept at 50 Hz in the steady state and the active and reactive power achieve sharing.The parameters such as the amplitude and frequency of the injected small voltage signal are selected reasonably,and the stability of the system is also analyzed.Finally,the proposed method is verified by simulation.（5）A three-phase VSG experimental platform is built,and the correctness and effectiveness of the proposed control strategy is verified. |
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