Control of MMC-based Grid-Forming STATCOM with DC supercapacitors for energy storage

Due to the increase in renewable energy resources, the characteristics of the power system are changing rapidly, thus introducing different challenges. Among many others, three challenges are particularly significant, namely a reduced power system inertia, dynamic reactive power support, and operation under weak grid scenarios. To bring these challenges under control, some system operators are installing classical devices such as synchronous condensers. A device that may outdo the synchronous condensers is the grid-forming STATCOM, given its capability of having an advanced and case-tailored control structure. Out of the different alternatives, in particular, MMC-based grid-forming STATCOMs with supercapacitors on the DC side are getting much attention from various stakeholders. Prior research on STATCOM for grid-forming applications has focused predominantly on two-level topologies, and even the few studies investigating MMCs have primarily considered constant DC voltage. This paper presents a detailed control structure for this device, emphasising inertia provision considerations and the DC side energy management. Additionally, a DC side control design method is introduced so that the overall control can provide the inertia required by the asset. The grid-forming performance demonstrated in simulation results verifies that the proposed control structure and the proposed design method can successfully provide inertia and effectively manage the energy on the DC side.