Research On SSO In Wind-farm VSC-HVDC Connected System

In this wind energy widespread using era,grid stability problem progressively occurs for more and more wind farms connected to the grid,especially in the system with series capacitor compensation or HVDC.This paper is meant to study the sub-synchronous problem in the DFIG-HVDC system as follow:（1）A DFIG-HVDC SSO model is built with considering blocks like wind turbine system,DFIG model,wind farm converter,HVDC model and RLC branch.Basically theory of sub-synchronous oscillation is supplied by this mathematical model of the system.（2）A SSO mechanism is derived in the DFIG-VSC-HVDC system by using complex torque coefficient methodology.According to the derivation of the damping characteristic,we can divided the factor to two parts,which one factor affects like RSC control coefficients the rotor side electric characteristics while others impact the grid side characteristics like HVDC control coefficients and its strategies,DC line length,RLC compensation ratio,ect.（3）Infects of SSO in the DFIG-VSC-HVDC system is analyzed according to control parameters and control strategies of the converter in the system.Firstly,a larger proportional coefficient and a smaller time constant of the converter RSC and VSC1 can entails strong negative damping characteristics.What’s more,it shows that rigid negative damping region and strong negative damping characteristic emerges clearly when the constant P-Q strategy is adopted in the sending end of the HVDC system,while wider negative damping region but weaker negative damping characteristic presents when the constant P-u_acstrategy is adopted（4）Infects of interaction between the VSC-HVDC and the series capacitor is analyzed according to compensation ratio、the DC line length and the control strategies of the converter in the system.A higher compensation ratio of the RLC branch in the system can induce strong negative damping characteristic and other frequency oscillation,which entails the system more unstable.However,a longer DC cable can release the electric oscillation in the system to some extent,but it doesn’t totally mitigate the oscillation in this way.And when the constant AC voltage strategy is used in the converter near the RLC branch,it can decrease the risk of SSO that high compensation ratio brings.（5）SSO influences of small disturbances and voltage droop strategies under faults in the DFIG-MMC-HVDC system is analyzed by time scale simulation.AC side disturbance shows stronger influence on system stability than DC side disturbance,while compensation capacitance put into the system shows least influence of system stability characteristics.Moreover,the voltage droop control strategy put into after fault occurs in the system,we can conclude that sending end AC fault disturbs stronger than receiving end AC fault,and DC system fault can induce system frequency characteristic change,especially receiving end frequency characteristic.At last,DFIG-MMC-HVDC system is tending to the positive damping system,which is more stable than DFIG-VSC-HVDC system.