Research On "source-load" Friendly Grid-connected Control Strategy Based On Virtual Synchronous Machine

Power quality plays the significant role in the fundamental demand of national economic development.Nevertheless,in recent years,with the gradual access of distributed generation system and a large number of controllable loads into the power grid,the penetration rate of power electronics in the system increases significantly,which leads to the continuous decrease of the proportion of traditional synchronous generators in the system,also weakens the frequency modulation and voltage regulation ability of the system,and seriously restricts the renewable energy represented by wind power Grid connected consumption.Aiming at the key bottleneck problem of power quality caused by power electronics,this paper takes the synchronization mechanism of virtual synchronous generator(VSG)control technology as the underlying control mechanism,and uses double-fed induction generator(DFIG)as the bottom control mechanism Generator(DFIG)is used as the "power" side and the controllable load in the AC / DC hybrid micro-grid is taken as the "load" side.A "power-load" friendly control strategy based on virtual synchronous machine is developed,which enhances the survival ability of controllable load and wind turbine in power grid.This research focus and main achievements are mainly reflected in the following three aspects.1)To solve the problem that the dynamic stability mechanism of micro-grid considering load VSG is not clear,a full order dynamic model of single load VSG micro-grid is established.Lyapunov method is used to analyze the dynamic stability of the system at the stable operation point.On this basis,the participation factors of each state variable in the dynamic model are calculated,and the internal influence of each subsystem on the stability of micro-grid is further revealed,which lays a theoretical foundation for the improvement of power frequency control strategy in the second part.2)Aiming at the problem of active power oscillation in AC / DC hybrid micro-grid with multi load VSG parallel operation,the influence of VSG power frequency controller parameters on the local rapidity and stability of primary frequency controller(PFC)and autonomous frequency controller(AFC)is analyzed from both time domain and frequency domain.Secondly,based on the mathematical model of the above controller,a load VSG control strategy with autonomous power frequency characteristics is proposed,which can not only realize the load frequency regulation without error,but also effectively suppress the active power oscillation caused by different VSG parameters of multiple machines.Finally,simulation results verify the accuracy and effectiveness of the proposed method.3)In view of the randomness of grid power fluctuation and the uncertainty of generator parameters during the operation of wind turbine,a nonlinear adaptive robust control strategy based on VSG is proposed in this paper.In order to enhance the comprehensive regulation performance of DFIG,an improved dynamic model of rotor motion equation in VSG is established,and a nonlinear robust adaptive controller with disturbance suppression ability and adaptive learning ability for uncertainties is designed to realize the optimal regulation of DFIG considering robustness and self adaptability.Secondly,the improved control strategy of DFIG grid interface converter is constructed by using voltage and current double loop control and VSG.The strategy can not only keep the excellent dynamic response ability of voltage and current regulation,but also improve the inertia and damping of the system.Finally,the simulation results show that the proposed control strategy is superior to the traditional control strategy,and reveals the robustness and adaptability of the proposed control strategy in dealing with external random disturbances and internal uncertain parameters.