Modular multilevel converters (MMCs) have been recognized as the most attractive technology for high voltage direct current (HVDC) applications during the last years. Their advantages include the ability to freely control the internally stored energy in the distributed submodules (SMs) and to independently control AC and DC power. Recent studies showed that an MMC can take advantage of this degree of freedom to supply ancillary services with low energy requirements such as frequency response or power oscillation damping by connecting energy storage systems (ESSs) to their SM capacitors. This paper proposes a modular DC-DC converter as an interface between the energy storage element and the SM capacitor. As it allows to share voltage and to increase switching frequency, it is shown that the volume of the passive components can be reduced. The proposed control strategy enables to not transfer the harmonic of a SM to the ESS that avoid to drastically oversize the system. The proposed control method also provides an appropriate energy exchange without affecting the control philosophy of an MMC and address balancing issues. Simulations and a downscaled prototype confirm the effectiveness of the solution.
Florien Errigo, Florent Morel, Cedric Mathieu de Vienne, Laurent Chedot, Ali Sari, Pascal Venet
Published in IEEE Transactions on Power Delivery