Research On Control Strategy Of Virtual Synchronous Generator For Microgrid Inverter

Under the background of"dual carbon",it is necessary to establish a new power system with new energy as the main body and the integration of wind,solar and storage and multiple energy sources.The optimization of grid-connected control technology of inverters is one of the major difficulties in promoting the absorption of new energy.The control strategy of Virtual Synchronous Generators(VSG)can improve the problems of inertia and lack of damping in new power systems by simulating the operating characteristics of synchronous generators.This thesis conducts a series of analysis and research on VSG control strategies.Firstly,the underlying control logic of the inverter is analyzed with the three-phase bridge inverter as the research object,and the virtual active speed control unit and virtual reactive power excitation control unit corresponding to VSG are designed by referring to the rotor motion equation and the stator electromagnetic characteristic equation of the traditional synchronous generator,and a perfect VSG topology is established.The influence of perturbation of control parameters on VSG output characteristics is analyzed in detail,and the weight of the active-frequency droop coefficient k_ωcompared to the damping D is used to coordinate and optimize the frequency and output power fluctuations of the system after disturbance.Based on the power angle characteristic curve and the system small signal model,the virtual inertia,damping and active droop coefficients are tuned,and the cooperative adaptive control function is constructed,and the simulation results show that the proposed adaptive control strategy has better dynamic stability than the conventional VSG control strategy.Secondly,for the problem of frequent switching of operating states faced by microgrid systems in practical engineering applications,the voltage amplitude,frequency and phase angle of VSG output terminal running in isolated islands are consistent with the grid side as much as possible by introducing a pre-synchronization unit,and enter the synchronous operation state before the grid connection is switched on and off.The simulation results show that the introduction of the pre-synchronous unit can effectively avoid the current surge and voltage distortion caused by the voltage mismatch between the two sides of the grid connection point during direct grid connection,and ensure the success of grid connection.Finally,aiming at the power coupling problem of VSG parallel system,the principle of power decoupling realized by virtual impedance method and rotational coordinate system method is analyzed and different applicable occasions,and the decoupling effect simulation verification is carried out by taking the parallel system as an example.On this basis,an improved virtual impedance control strategy based on quasi-PR resonant controller is adopted to reduce the influence of system circulation and further improve the distribution accuracy of reactive power in parallel systems.By analyzing the influence of key control parameters on the distribution of active power and reactive power of the parallel system,the parameter tuning of the active control loop and reactive power control loop of the parallel system is completed,and the output of each unit is realized according to the set proportion.Taking the dual-machine system as an example,the power allocation strategy is simulated and verified in the Simulink platform,and the results show that the parallel inverter can output power according to the rated value under the same and different capacity conditions,and realize the accurate distribution of active power and reactive power.