Grid-following Converter Dynamics under Large Sub-synchronous Voltage Fluctuations

The growing integration of converter-interfaced power sources into distribution grids leads to novel dynamic stability concerns. This paper investigates the stability of a grid-following converter under large sub-synchronous square-wave modulations of the grid voltage amplitude. By modelling only the critical dynamical processes of the converter control architecture, specifically direct voltage control and the phase-locked loop, we arrive at a non-linear fourth-order differential-algebraic system. Under these voltage disturbances, we study the influence of grid strength and the damping of the phase-locked loop on system behaviour. Our results show that the dynamic response is much stronger than predicted by small-signal analysis. For extreme grid conditions, we identify an instability associated with the converter’s loss of synchronisation. This study provides insights into the non-linear dynamic behaviour of grid-following converters under abnormal voltage profiles, plausibly induced by equipment malfunction or load altering attacks.