Study Of Sub-synchronous Torsional Interaction With HVDC Transmission System

Distribution of energy-rich areas and energy-consuming areas in China is extremely uneven,long-distance large-scale transmission meets the actual needs of power system development.In recent years,Line-Commutated Converter high voltage direct current(LCC-HVDC)system has developed rapidly in China's power system,and more and more LCC-HVDC projects have been put into operation.However,when there are thermal power units near the converter station of the LCC-HVDC sending-end system,there is a potential risk of unstable shafting torsional vibration interaction under certain conditions,i.e.subsynchronous shafting oscillation of the generator,which may damage the large shafting of the generator and seriously affect the stability of the power system.In this paper,the mechanism and characteristics of torsional vibration interaction between LCC-HVDC sending-end system and thermal power unit shafting are analyzed.Firstly,the typical scenario topological structure of LCC-HVDC system and the working principle of its equipment are introduced,including the composition and working principle of the generator electrical part and shafting part,and the working principle of LCCHVDC,and the typical control strategy of LCC-HVDC is analyzed.Then,the concepts of motion equation and time-scale are introduced,and the relationship between DC current control timescale and sub-synchronous frequency band is analyzed.Based on the idea of unified amplitude-phase motion equation,the small signal model of synchronous machine and LCC-HVDC motion equation model are established respectively under DC current control timescale.By integrating electrical part of generator and network of LCC-HVDC sending-end system and the common AC network,a complete linear model of the system is obtained.Under DC current control timescale,the accuracy and correctness of the small signal model of synchronous machine and LCC-HVDC motion equation are verified by time domain simulation method.Finally,based on the interaction analysis method of self-stabilizing and en-stabilizing,the transfer function of generator self-stabilizing and en-stabilizing is deduced from the perspective of synchronous generator,and then the expression of generator self-stabilizing and en-stabilizing damping is obtained.The influence of network strength of LCC-HVDC sending system,DC transmission power and controller parameters of LCC-HVDC sending system on the interaction between LCC-HVDC feeder system and shafting torsional vibration is analyzed,and the analysis conclusion is verified by time domain simulation method.