Research On Feasibility And Adaptability Of VSC-HVDC In Urban Power Grid

Energy resources in China are reversely distributed with load centers.In order to optimize resources allocation,power from western and northern China energy bases should be delivered to load centers in Middle Eastern China.High-Voltage Direct Current(HVDC)transmission technology has advantages in interregional transport electricity with large capacity over a long distance.Due to the advantages,development of HVDC is the basis for clean energy accomplishment as well as the key for global resources network establishment.However,if HVDC directly connects with load centers,once there is a DC fault due to the concentration of receiving ends and large transmission capacity,a large-scale of power flow swing will happens,which leads greatly threaten to the security stability of receiving end grid.Voltage Source Converter based on High Voltage Direct Current(VSC-HVDC)has the ability to overcome some shortcomings of traditional HVDC.VSC-HVDC has obvious strengths in power flow regulation,stabilize voltage and etc,it's the best way for providing power to the city center.It's meaningful for VSC-HVDC to solve most recently severe problems in city grid such as short-circuit current over limitation.This thesis mainly focuses on the feasibility and adaptability of VSC-HVDC application in city grid.AS VSC-HVDC is self-commutated devices,it doesn't need to receive the commutation current from receiving end grid,failures in commutation would not exist.Theoretically,VSC-HVDC could be applied to any level of grid,but the influences of grid on the security and stability should be considered.Based on the analyses of the performance characteristics for weak AC grid after VSC-HVDC applied after connected with VSC-HVDC,numerical models for VSC-HVDC connecting with weak AC grid are established,maximum output power equations are derived and the term Short-Circuit Ratio(SCR)is invited.With the researches on the affect factors for AC grid connection,the critical SCR for VSC-HVDC connected with AC grid is obtained.Sample models with actual grid data from PSD-BPA help validate the correctness of theoretical analysis and provide the theoretical guidance for industrial application.Meanwhile,VSC-HVDC is flexible to regulate power flow and can regulate active power and reactive power individually.VSC-HVDC can reverse the flow trend without changing the voltage polarity and control mode.After the voltage drops or collapses,VSC-HVDC can activate its reference voltage,quickly restore power and black start.Based on the derived power flow models for city grid AC/DC system with VSC-HVDC,the optimal power flow optimization for VSC-HVDC application in city grid faults is calculated.Moreover,an actual city grid model in PSD-BPA is used to validate and analyze the optimal power flow optimization.VSC-HVDC does not need reactive power compensation from AC grid,since VSC-HVDC can send or receive reactive power according to grid's reactive power requirements,dynamically compensate reactive power on AC bus bar and stabilize AC bus voltage.With actual grid models in PSD-BPA,fault analysis on the influences of VSC-HVDC on system voltage is performed.