Impact Of Synchronous Condenser On The Dynamic Behavior Of LCC Based UHVDC System Hierarchically Connected To AC System

Hierarchical connection(HC)is very attractive mode for ±800 kV line commutated converter(LCC)based Ultra High Voltage Direct current(UHVDC)system,which is connected to different AC voltage levels to reduce the scale factor of converter transformer.With the construction and widespread use of ultra-high voltage alternating current(UHVAC)and ultra-high voltage direct current(UHVDC)transmission technology,the multi-infeed HVDC system in load bases of China will become the prominent problem in the development of China power grids.To overcome the technical problems related to multi-infeed HVDC system,LCC based UHVDC system hierarchically connected to AC system has been acquired as the fundamental connection mode in China.Some advantages of hierarchical connection mode are that it can increase voltage support capability of receiving AC systems and distribution of power flow can be guided by utilization of specific control techniques and Project investment cost can be reduced by the full use of available equipment.Faults in the HC-UHVDC system can cause commutation failure(CF),hence in this thesis impact of Synchronous Condenser(SC)to analyze the dynamic behavior of LCC based HC-UHVDC system is analyzed.For that purpose,a ±800kV HC-UHVDC system along with Synchronous Condenser is built in PSCAD/EMTDC.Transient performance analysis of HC-UHVDC,for single and three phases to ground faults are investigated.Commutation failure immunity index(CFII),Commutation failure probability index(CFPI),Fault Recovery Times(FRT)and Transient Overvoltage(TOV)are used as measures to analyze the effects of SC at HC-UHVDC system design.The simulation results show that SC can make the HC-UHVDC system less susceptible to CF,effectively improve the fault recovery performances of the designed system and effectively reduces the transient overvoltage when single or multiple converters are blocked.The results of this research can provide technical assistance in real-world HC-UHVDC projects.