Contributions to the DC-bus voltage controller of back-to-back voltage source converters

A very interesting feature of VSC based back-to-back converters connected to the grid is their capacity to work in the four quadrants, and consequently their capability to change the converter power flow direction eventually. Variable speed wind turbines represent one of the main applications of these devices. The objective of this article is the study of the influence of R_L and C_DC value over a back-to-back converter from a classic linear control point of view. The analysis that is presented in this article begins with the extraction of two different models for the DC bus which permits the deduction of ranges of nominal values for the DC bus capacitor (C_DC) and for the load resistance (R_L) which ensures system stability. A stability analysis of these models as a function of the operating mode of the VSC (rectifier, inverter), of the DC-bus capacity and of the value and sign of the current across the DC-bus is the objective of the second part of this article. Finally, a control scheme for both models is proposed. Special interest is dedicated to the effects of DC-bus parameters (R_L and C_DC) over the controller operation and to a correct chose for that values: an analytic method for constant design is presented. Control system correct operation is verified under a non-linear back-to-back simulation environment.