Frequency Based Synchronization Control Investigation Of VSC-HVDC Under Weak AC Grid Condition

With the widely used vector current control(VCC),the Voltage Source Converter based HVDC(VSC-HVDC)system may experience oscillations and even dynamic instability under large Phase-Locked-Loop(PLL)gains due to the close interactions between PLL and VCC control loop when it is connected to a weak AC grid.Thus,a control method which can effectively improve the small-signal stability of the VSC system under weak AC conditions and can also limit the fault current is needed.This paper investigates the impact of Frequency-based Synchronization Control(FSC)approach on VSC-HVDC under weak AC grid condition.First,this thesis introduces basic working priciples of FSC.Compared with two commonly used control strategies for VSC-HVDC,which knowns as VCC and power synchronization control(PSC),the differences between FSC and these two control strategies are dicussed and the mechanisim of FSC is further explained.Then,operating characteristics of FSC based VSC-HVDC system connected weak AC grid has been analyzed in three aspects,small signal static stability,dynamic stability and transient stability.The research ideas are shown as follow:First,nonlinear state space equations and electromagnetic transient simulation model of FSC based VSC-HVDC system connected weak AC grid has been built in MATLAB and PSCAD/EMTDC respectively.To verify the electromagnetic transient simulation model,the small signal dynamic model is derived by linearizing the nonlinear state space equations in small range and the simulation results under the same parameters are compared using both electromagnetic transient simulation model and small signal dynamic model.Based on small signal dynamic model,the damping coefficient and synchronous coefficient are analyzed with eigenvalue locus and participation factor methods.Then it investigates the dynamic stability after large disturbance like system frequency and load step in FSC based MMC-HVDC system.Finally,the influence on transient stability under three phase short circuit fault has been disscussed using simulation.The simulation results have indicated FSC not only promotes small signal stability of VSC connected weak AC grid,but also enhances the system performance of frequency tracking response and overcurrent limiting capability.Furthermore,this thesis also look into the small signal stability of FSC based Modular Multilevel Converter based HVDC(MMC-HVDC)system connected with weak AC system.Similarly,small signal stability model of FSC based MMC-HVDC system is built in MATLAB and simulation results are compared with electromagnetic transient model in PSCAD/EMTDC to verify the small signal stability model.Then,the damping coefficient and synchronous coefficient are analyzed with eigenvalue locus and participation factor methods in MMC-HVDC system to direct the selection of FSC control parameters.The research indicates FSC can promote small signal stability and operate with high performance in both VSC-HVDC and MMC-HVDC system.Finally,this thesis summerizes the system characteristics of FSC based VSC-HVDC and MMC-HVDC system.It concludes that the increased damping and synchronization control brought by FSC can improve the small signal stability effectively when VSC and MMC-HVDC connected to weak or extreamely weak AC grid,and FSC has deeper influence on system small signal stability with bigger damping factor and smaller synchronization factor.