Minimizing the battery losses in MMC via Zero-Sequence Voltage injection: an open challenge

This work focuses on battery-powered Modular Multilevel Converters (MMC) and explores several methods aimed at improving overall efficiency by reducing the energy losses of electrochemical sources. MMCs represent a valid alternative to more traditional structures such as Voltage Source Inverters (VSI) because they possess several advantages. MMCs are less problematic from the standpoint of electromagnetic compatibility, and they exhibit significantly reduced switching losses. They can be used when the total voltage level is too high to be applied to a single transistor and must be distributed across multiple power electronic components. Furthermore, they ensure considerably reduced voltage distortion. Another advantage is their ability to balance individual batteries powering a single module without resorting to additional electronics. On the downside, MMCs are associated with more complex control electronics, especially when talking about a high number of levels. Finally, MMCs expose the batteries to greater energy losses than VSIs, significantly worsening overall efficiency. This gives rise to the need to find methods and control techniques aimed at reducing or minimizing energy losses in batteries. This work discusses how techniques recently proposed in the literature manage to affect battery losses by injecting a proper zero-sequence voltage. Nevertheless, as this work demonstrates the goal of minimizing losses has not yet been achieved. This paper then analyzes the complexity of the unresolved problem and investigates possible alternative approaches.