Research On Stability Analysis Of VSC System Based On Impedance

VSC-HVDC is a new kind of transmission,different from traditional high voltage dc transmission,it has some advantages,such as current self-closing capability,Supply power to passive network,suitable for new energy access,etc.At present,with the development of flexible direct current transmission technology in power industry application,the system stability problem is becoming more and more highlighted.This paper analyzed the stability of VSC-HVDC problems from the Angle of the impedance stability.Stability of impedance is applied in the analysis of traditional ac power field,discussed the relationship between the weak power grid and grid inverter.Long distance overhead transmission line makes the power grid become weak power grid and the system contain weak power grid and the grid inverter,resonance will happen under certain conditions,have an affect on power grid like the harm of harmonic pollution on The harmonic pollution caused by the impedance different from the conventional harmonic pollution,conventional harmonic pollution is produced by some nonlinear load.Impedance harmonic problems caused by the processing influence on the system of power quality,also affect the grid inverter running state,when this influence reaches a certain extent will affect the stability of the grid inverter.With the continuous development of flexible HVDC,the stability problem is more and more important.HVDC-VSC system of grid inverter application is very wide,long flexible overhead lines dc transmission lines to the power grid is introduced into an equivalent impedance,makes the grid is of weak power grid.For this type of interactive system,the power grid impedance and the interaction between inverter output impedance will produce resonance,thus cause harmonic pollution to power network,this is different from the traditional sense of nonlinear load harmonic pollution to electric network.Caused by the interaction between impedance harmonic not only affect the distributed generation system of power quality,and affect the stability of the grid inverter.The impedance of the stability of vsc-hvdc similar to the impedance of the traditional ac power system stability problems,because existing in use of flexible HVDC system is mostly employ the long distance overhead transmission lines,and the problem of the stability flexible HVDC mostly happenend in converter and the process of the interconnection of power grid.In this paper the research content mainly includes derived the basis of the impedance of the flexible HVDC system stability analysis method with the small signal model,named the Nyquist stability criterion.And put forward a kind of practical engineering application of directly impedance measurement method.Propose the vsc-hvdc stability criterion on the basic concept of Nyquist stability criterion.Finally,researched the modeling of two terminal two level vsc-hvdc and the basic operation,and run on the simulation software PSCAD/EMTDC,and get the measurement impedance values in MATLAB for analysis,validate the correctness of impedance stability analysis in this paper.