Modelling and Control of an MMC-HVDC Submodule with Energy Storage for Fast Frequency Response

Recent works have shown that energy storage systems (ESSs) can be distributed in a modular multilevel converter (MMC) for the enhancement of high voltage direct current (HVDC) converter stations in order to provide ancillary services. In this case, DC-DC converters are compulsory to interface energy storage elements to submodule (SM) capacitors. However, the choice of the converter topology and its control is not straightforward due to the complex working principle of an MMC. This paper proposes a suitable interface converter with a control strategy to address these issues. Special emphasis is put on the modelling of the converter to highlight all the interactions inside a SM and ease the design of the controllers. To conclude, a downscaled prototype validates the effectiveness of the proposed solution.