Current limiting algorithms and transient stability analysis of grid-forming VSCs

Abstract

This paper consists first in comparing the well-known virtual impedance and current saturation algorithms in case of a three-phase short circuit. During the fault, the virtual impedance has an indirect action on the AC current since it depends mainly on the AC voltage controllers dynamics, whereas, the current saturation algorithm has a direct action on the AC current. Thus, a first study consists in comparing the dynamic behavior of the AC current for both methods separately. Once the fault is cleared, the grid-forming converter should be able to resynchronize to the main grid. In this paper, the impact of each current limiting algorithm on the transient stability is highlighted. Based on this comparative approach between the virtual impedance and the current saturation algorithms, a hybrid current limiting control is proposed to benefit the advantages of each algorithm in terms of current limitation and transient stability. To show the effectiveness of the proposed control, time-domain simulations and experimental test cases have been performed.