Small signal study of grid-forming converters and impact of different control structures and parameters

Towards transitioning to a carbon-free power system, new dynamic phenomena and interactions involving grid-forming converters (GFMs) will become important as their proliferation occurs to support this transition. The multi-loop control incorporating inner cascaded voltage and current controllers (VC and CC) often utilised within GFMs are generally expected to cause interactions in higher frequency ranges than the (well-studied) dynamics of interest associated with a purely synchronous machine-based system. However, the restricted control bandwidth associated with low switching frequency of large power rated VSCs results in the need for much slower control time constants, causing potentially destabilizing, lower-frequency (or even non-oscillatory) interactions. This paper offers an extensive insight into the small-signal, multi-machine interactions involving large power rated GFMs in a transmission network: the IEEE 39-bus New-England test system (NETS). Furthering the contribution of this paper, multi-loop controllers are employed within the GFMs, offering an insight into their interactions with other power system elements to help aid the ongoing discussions on model appropriateness and direct AC voltage control versus multi-loop control. Finally, parametric sweep sensitivity analyses are performed for the GFMs which are implemented as virtual synchronous machines (VSMs).