Modelling and Passivity Analysis of VSC Output Impedance under SRF and αβ Control Frames

Passivity property of input impedance of grid connected Voltage Source Converter (VSC) is a good measure for ensuring small signal stability of the power system composed of numerous interconnected dynamical system. It is well known that a pure passivity for a VSC is impossible to obtain, however, if the frequency range of the negative admittance from VSC terminal can be minimized, the possibility instability due to grid side impedance interacting with the VSC system can be reduced. For a VSC, there are two major design alternatives for the current controller;a decoupled dq current controller and αβ frame Proportional Resonant controller. This paper compares these two current controller alternatives for VSC, based on their passivity property. The small signal model of LCL filter equipped VSC with the two types of current controller is also derived. Some design recommendations using the considered current controllers, for reducing non-passive frequency range is also presented.