Stability margin definition for a converter-grid system based on diagonal dominance property in the sequence-frame

The increased presence of power converters in power networks has set new power system stability challenges. New methodologies have been proposed to analyse these systems and one of the most promising is the impedance-based stability criterion. To date using this method the grid interfaced converter is represented in terms of impedances and it is assumed that the converter impedances within the sequence-frame are decoupled. However, coupling between the components of the sequence-frame converter impedances have been observed, and different studies have shown how relevant this might become to the stability performance. Hence, a stability margin that takes such coupling into account is presented in this paper. This results in a more conservative measure of the stability robustness of the system, compared to the gain and phase margin figures employed in the impedance-based stability criterion. The presented study has been used to assess how the relative stability of a VSC system connected to a weak grid varies with its operating point, and experimental data have been used to support the analytical study.