Grid-forming control VSC-based including current limitation and re-synchronization functions to deal with symmetrical and asymmetrical faults

Abstract

Grid-forming converters operate as voltage sources behind impedance. This property makes them robust against Short-Circuit Ratio (SCR) variation, but vulnerable against large grid disturbances. As a precaution, grid-forming converters have to embed adequate control algorithms to ensure a stable system operation under various grid conditions, to deal with excessive overloadings mainly caused by faults, and to guarantee a stable re-synchronization after fault clearance. These expectations have been met in previous works considering balanced conditions. Nevertheless, the extension of the grid-forming control to deal with unbalanced grid conditions considering current limitation and angular stability is a point rarely discussed in the literature. To fill this research gap, this paper proposes an Extended Power Synchronization Method (EPSM) that allows the system to operate under balanced and unbalanced grid conditions while meeting the Fault Ride-Through requirements (FRT). The proposed method is a direct voltage control-based, which embeds a threshold current control loop, which is enabled only when a fault is detected. Additionally, the control is equipped with an algorithm that modifies the active power control during faults to aid the power converter to remain synchronized after fault clearance. The effectiveness of the proposed control has been demonstrated through time-domain simulations.