Research On Key Technology Of Paralleled Virtual Synchronous Generators Based On Virtual Impedance In Microgrid

Inverter plays a vital role in the performance of microgrid.Among control strategies of inverter,virtual synchronous generator(VSG)control could simulate the inertia and damping characteristics of synchronous generator,which helps to guarantee the stable operation of microgrid and reduce the negative impact of new energy resources' access.Moreover,inverters in parallelled pattern could expand the capacity and improve the reliability of microgrid,so the control methods of paralleled VSGs are studied in detail in this paper.With reference to the operation characteristics of the synchronous generator,this thesis explains the feasibility of using inverter to simulate the characteristics of synchronous generator.On that basis,virtual speed controller and virtual excitation controller are designed and the VSG control model is obtained.In addition,the influence of key parameters in the VSG control is also discussed.In order to make the output impedance of inverter to present inductive,a virtual impedance loop based on second order generalized integrator(SOGI)is introduced to the VSG control and the impact on system output characteristics after the introduction of virtual impedance loop is analyzed.The implementation method of virtual impedance loop based on SOGI is deduced and the system control block diagram is shown in this thesis,which improves the VSG control.The control methods of paralleled VSGs based on virtual impedance loop are studied in detail: The principle of generation of circulating current in paralleled system is analyzed and the method of current sharing is studied;the allocation strategy of active power and reactive power of VSGs is proposed on consideration of the line impedance;the principle of no-difference frequency modulation mode is also studied and a self-adaptive no-difference frequency modulation control method is proposed to enhance the stability of the system.Finally,the paralleled VSGs are simulated in the microgrid to verify the operating performance of power sharing and frequency modulation in off-grid mode,toghther with pre-synchronization interconnection and active power support in on-grid mode,the results validate the presented theories and strategies are effective.