Dynamic Performance Limitation and Enhancement of Grid-Forming Converters

Grid-forming converters (GFMCs) are emerging as promising candidates in more-electronics power systems for their capabilities of forming voltage and frequency. Regarding the control of GFMCs, lots of research have been done on the active and reactive power control loop and the voltage control loop of standalone voltage source converters. However, little literature elaborates the design of the voltage and current controllers when GFMCs are connected to the grid. This paper analyzes the stability of standard dual-inner-loop GFMCs. The requirements for the stability are analyzed and the facts that limiting the control bandwidth are identified. Specifically, it is revealed that in the synchronous dq frame, the coupling terms introduced by the grid impedance have a non-negligible impact on system dynamics. More importantly, the conventional dual-loop control scheme inherently limits the control bandwidth of GFMCs to be less than 50 Hz. To improve the speed of tracking the reference, a novel control scheme with a band-pass filter and a band-stop filter is proposed. With the proposed control scheme, the bandwidth of the GFMC is increased without causing instability.