| Abstract |
Ensuring robust control stability to varying grid conditions is a key requirement when installing a newHVDC station to the AC grid. With its very accessible criteria for stability margins, the Nyquist theoremfor single input/single output systems is commonly used to determine the robustness of the control system of HVDC converter stations and tune it accordingly. However, three-phase AC system are not single input/single output systems and the validity as well as the underlying assumptions of this investigation has rarely been under scrutiny. In this paper, the commonly used impedance based approach to assess stability and robustness of HVDC converter stations is reviewed from a control theoretical point of view. Starting with a clarification on the properties of commonly used reference system transformations, the repercussions for robustness investigations in a multivariable control environment are discussed. Based on that, possible shortcuts to allow classical single input/single output investigations as well as limitations to that approach are derived and explained in detail on a generic control model. |
