Small Signal Stability Of The AC/DC Hybrid Power System Connected With The VSC-HVDC

Voltage source converter based HVDC(VSC-HVDC)is considered as a technically favorable solution for the renewable energy delivery.Stability of the AC/DC hybrid power system has been one of major issues of concern and investigated actively for the stable operation of power systems.In this paper,small signal stability of the AC/DC hybrid power system separately adopted virtual synchronous generator(VSG)and vector control strategy is studied and compared.The interconnected model of the AC/DC hybrid power system is derived and the impact of the VSC-HVDC on the stability of the AC power system under multiple frequency ranges is analyzed.Moreover,how VSC and current flow controller(CFC)impact DC voltage stability of the DC power system is clarified and the method to properly tune control parameters is proposed.Also,the propagation of AC dynamic transients through the DC network,comparing master-slave control and DC voltage droop control,are examined.In final,analysis are demonstrated and evaluated by results of the nonlinear simulation and modal analysis.Main conclusions and novelties are summarized as follows.(1)Interconnected model of the AC/DC hybrid power system is established.Herein,the concerned subsystem is as the feedback loop and the rest of the AC/DC power system is as the feedforward loop.Based on the interconnected model,how interactions between these two subsystems impact the stability of the AC/DC hybrid power system can be clearly revealed.(2)Stability analysis methods of the AC/DC hybrid power system are proposed.It is indicated by virtual damping analysis that the growing oscillation is induced by the provided negative damping of the feedback loop.In addition,it is verified theoretically by Taylor's formula that when two open-loop oscillation modes are approximated to each other on the complex plane,open-loop modal resonance occurs and leads to instability of the power system.(3)Impact of the DC power system,separately adopted VSG and vector control strategy,on the small signal stability of the AC power system is studied.It is concluded that the DC power system adopted VSG control and vector control may result in electromechanical oscillation and sub-synchronous oscillation,respectively when open-loop modal resonance occurs.Methods of tuning parameters are proposed to improve the stability of the power systems.(4)Stability within the DC power system is studied.How DC equipment impacts the DC voltage oscillation mode is clarified.Meanwhile,the method of tuning parameters is proposed based on virtual damping index.Moreover,it is indicated that under DC voltage droop control,the propagation becomes bidirectional between the VSCs adopted the DC voltage droop control,and the AC/DC power system has a risk of the open-loop modal resonance.However,under the master-slave control,the AC dynamic transients can propagate only unidirectional through the DC connection,and the open-loop modal resonance cannot occur between two AC power systems.