Research On IC Control Strategy Of Hybrid Microgrid Based On VSG Technology

AC/DC hybrid microgrid,which solves the problem that AC/DC converters are frequent and inefficient,has the characteristics of strong adaptability,wide power access and high energy utilization rate.In hybrid microgrid,interfacing converter(IC)is the hub of power flow between AC and DC subnetworks and the research of its control strategy has important theoretical and practical significance.Aiming at the problem of noninertia,vulnerable to load and power fluctuation in hybrid microgrid,this thesis introduces virtual synchronous generator(VSG)technology and studies the control strategy of interfacing converter.Virtual synchronous generator technology makes the inverter have the external characteristics of synchronous generator(SG).Firstly,the rotor mechanical process and the stator voltage equation of synchronous generator are analyzed and simulated as the algorithm of virtual governor and virtual excitation controller.Secondly,the small signal model of VSG is established.Loop analysis shows that the active and reactive loops of VSG are approximately decoupled and can be designed separately.Finally,the VSG modeling and simulation results show that the VSG algorithm can increase the inertia and stability of the system in the case of load fluctuation.In the control of interfacing converter,VSG technology is introduced.According to the normalized algorithm of double droop control,the AC side frequency and DC side voltage are taken as control objectives.A control method of IC based on VSG technology is proposed.The small signal model of active power loop is established,and the influence of parameters on the steady-state performance and dynamic performance of the system is analyzed through the step response of the open-loop transfer function of the system.Models are built on the platform of Matlab/Simulink to simulate the hybrid microgrid under different operation modes and different operating conditions.The results show that the proposed control strategy is effective.Compared with the traditional double droop control strategy,the strategy increases the inertia of the system,suppresses the frequency and voltage fluctuations,effectively improves the dynamic performance of the system,and verifies the good power sharing effect of the proposed control strategy in parallel with multiple converters.Aiming at the circulating current problem of multi-converter parallel connection,the reason of circulating current is analyzed,furtherly,virtual impedance is added to the control of VSG reactive power loop to suppress circulating current,and the traditional PI controller is changed to PR controller to track sinusoidal signal.The transfer function of reactive power loop is established.The comparison of output impedance of tie-in converter before and after adding virtual impedance shows that adding virtual impedance can offset the influence of output impedance of tie-in converter.The control method before and after adding virtual impedance is modeled and simulated.The simulation results show that the voltage tracking effect of the improved control method is good,the circulating current is effectively suppressed,and the current sharing effect is improved.