Increasing DC system stability thanks to energy control in MMC based DC-DC converters

DC voltage regulation of DC networks is essential to the operation of the system. Recently a new control method called virtual capacitor control has been proposed for the Modular Multilevel Converter (MMC) to increase the inertia of HVDC systems. In the development of future DC grids, DC-DC converters will be needed. Most of DC-DC converters proposed in the literature for HVDC are based on the MMC, thus some of the features of this converter can be extended for DC-DC converters. In this paper, the virtual capacitor control is proposed for the Front-to-Front MMC. The proposed control approach is validated in transient simulations, analysing the impact on the interconnection of two DC systems. The simulations results show that the variation of the voltage of a DC network after a power disturbance can be decreased with the proposed control and at the same time the DC-DC converter offers a decoupling to the second DC network which is not affected by the perturbation. Moreover, the results show how the virtual capacitor concept can be extended to MMCs that are not directly connected to the DC network where the service is provided.