Impedance-Based Modeling of a Three-level Converter Under Balanced and Unbalanced Condition for the Stability Analysis of Bipolar LVDC Grids

Abstract

This paper presents an impedance-based assessment to identify potential instabilities in bipolar LVDC grids where the voltage is controlled by a three-level DC-DC converter. As an essential step to perform the assessment, the small-signal admittance matrix of the three-level DC-DC converter is analytically derived and verified by a non-linear switched model implemented in the EMT-type software PSCAD. It is shown that the converter admittance matrix exhibits non-passive behavior at frequencies above 500 Hz. Furthermore, the non-passivity is found to be dependent on the operating point of the converter. This behavior can negatively interact with resonances of bipolar LVDC systems, e.g. LC-filter resonances, leading to unstable high-frequency oscillations and system instability.