H∞-Based Double Grid Forming Controller of Multi-Modular Converters in a Hybrid AC/DC Grid

The power system is witnessing an increasing penetration of power converters due to the integration of renewables, HVDC, DC and AC microgrids, etc. In line with this increasing penetration, the system needs units with grid-forming capabilities on the AC and DC sides. We propose a dual grid forming controller for power converters, particularly for modular multilevel converters (MMC), that forms the voltage in the DC grid and voltage and frequency on the AC grid. Nowadays, the parameter tuning of MMC controllers heavily relies on manual effort and rich engineering experience, which usually leads to suboptimal power system operation. Instead, we propose an optimal $H_{\infty}$ controller, which considers AC and DC grid dynamics. We impose grid-forming conditions, among other control objectives, by using weighting functions. Finally, using a simple AC/DC grid we demonstrate that the proposed double grid forming algorithm is suitable for hybrid grid applications.