Controller Interaction and Stability Margins in Mixed SCR MMC-Based HVDC Grid

Using HVDC grids for interconnection of ac grids across countries posses questions regarding interactions and stability. Important aspects that may be considered are the different short circuit ratio (SCR) values of the corresponding ac grids and outages. The analysis of such mixed SCR will be time-consuming since; detailed models and multiple simulations of the entire system will be required in order to develop a complete understanding of the modes of interactions and existing stability margins. Use of small signal model will allow for easy parametric variation after the initial barrier of model development is overcome. The article presents an analysis technique that uses the mode identification and participation factor augmented with eigen-sensitivity to analyze modes of interactions and stability margins of the HVDC grids when system parameters and controller gains are varied. The modes are classified as local and inter-area modes based on the participation of controller states for HVDC grid. The method does not require detailed switched models to be developed. The paper also proposes an adaptive control technique which results in a reduction of interactions and prevents instability when system operating conditions are changed. The results obtained using the proposed analysis technique are verified with a detailed time-domain simulation of the HVDC grid. The interaction modes and participation of states are dependent on the converter operation modes, loading characteristics, SCR values, PLL gains and loading conditions.