Complex Vector Analysis Of Influences Caused By Voltage Feed-forward And Current Decoupling On Vsc Current Control Stability In Weak Grid

In weak grid, the interactions between the wind generators and the power system are obvious and need considerations. Operation and control of generator-side converter and grid-side converter are independent in full-power wind turbines, therefore only the grid-side converter is included in the current control stability analysis of the interconnected system. The voltage feed-forward and current decoupling are firstly introduced to improve the system dynamic characteristics of current control. Effects caused by them on the stability in weak grid are hardly focused. The problem will be detailedly discussed in this thesis.Complex vector theory, which is used to analyze three-phase ac systems or motor drive, is generalized to study grid-connected converters. Complex vector mathematical model and signal flow graph are derived. Influences caused by the voltage feed-forward and current decoupling on the conductance frequency characteristic are indicated according to the input admittance expressions. Transfer functions are obtained based-on impedance analysis method when converter connected to high impedance grid. Complex vector root locus of interaction system validate the conclusions. In addition, through applying traditional method which develops the converter mathematical model containing the power grid, transfer function representing grid current vector tracking performance is derived. Effects caused by the voltage feed-forward and current decoupling on the current control stability of interconnected system are analyzed according to the complex vector root locus Simulations in Matlab/Simulink are carried out and results validate the root locus analysis.Results show that, the voltage feed-forward and current decoupling worsen the current control stability. Effects of the voltage feed-forward are dominating while effects of the current decoupling are ignorable. Hence the compromise between the high dynamic performance and the stability of the current control loop need to be made. The paper provides guidance to improve and optimize the current control, and generalizes the application of complex vector theory.