Analysis Of Interaction Characteristics And Receiving Terminal Voltage Stability Of Multi-outfeed And Multi-infeed UHVDC System In Similar Path

Large capacity trans regional UHVDC transmission system is of great significance in China’s power grid strategic layout of "west to East power transmission,north to south mutual supply and national interconnection".With the intensive operation of HVDC/UHVDC,the power grid structure of multi-outfeed and multi-infeed UHVDC system in similar path has gradually become prominent.There is a complex AC-DC coupling relationship in such system,the strong DC and weak AC operation characteristics are significant,and the interaction of AC and DC faults and chain risk are difficult to control.Therefore,the in-depth study on the interaction characteristics and voltage stability of multi-outfeed and multi-infeed UHVDC system in similar path is conducive to accurately grasp the potential risks of power grid operation,which is of great significance to the stable operation of power grid in China.Firstly,taking Tianzhong UHVDC transmission system of Henan power grid as an example,this paper makes an in-depth study on the primary equipment,mathematical model,control mode and coordinated control strategy of transmitting and receiving converter station of LCC-UHVDC.At the same time,PSCAD/EMTDC equivalent model is built based on the actual engineering parameters of Tianzhong,and the further Tianzhong-Qingyu double circuit multi-outfeed and multi-infeed UHVDC system.The transient process of the system under normal state and three-phase short-circuit fault is simulated to verify the accuracy of the model,which is used as a follow-up research tool in this paper.Secondly,this paper analyzes its structural characteristics based on the topology of the multi-outfeed and multi-infeed UHVDC system in similar path,and studies the AC and DC fault propagation mechanism of the system.At the same time,the existing evaluation indexes of DC interaction MIIF and MOVIF are discussed,and the shortcomings of their application in the multi-outfeed and multi-infeed UHVDC system are summarized.The voltage interaction factor MOMI＿IF suitable for evaluating the different DC interaction characteristics of multi-outfeed and multi-infeed system is proposed,prove its accuracy and analyze its influencing factors.Based on MOMI＿IF further defines the effective short circuit ratio MOMI＿ESCR of the system is used to characterize the support ability of AC system to DC system.Finally,the static voltage stability of the receiving terminal is analyzed based on the effective short circuit ratio method.The voltage support ability of the receiving terminal is evaluated by the threshold of different range division of MOMI＿ESCR.The optimization of VDCOL and the mechanism of installing synchronous dimmer to improve the voltage stability of the receiver and suppress the commutation failure are studied.It is proved that the DC control optimization means and the installation of SC can stabilize the voltage of the receiver and accelerate the recovery of system failure.Furthermore based on the electromagnetic transient model of Tianzhong-Qingyu double circuit multi-outfeed and multi-infeed UHVDC system,the above research results are verified.The simulation results show that the evaluation indexes of interaction characteristics of the same sending and receiving system（MOMI＿IF and MOMI＿ESCR）proposed in this paper are accurate and reliable,and more accurate and effective than the existing indexes.At the same time,it is verified that the optimization of VDCOL and the installation of SC can effectively improve the voltage stability of the receiving terminal and inhibit the commutation failure to a certain extent.The conclusion of this paper has strong theoretical significance and practical value in the related research of the multi-outfeed and multi-infeed UHVDC system in similar path.