A new topology for battery systems: Reconfigurable Cascaded Multilevel Converter

Electric vehicles cover a fundamental role for the sustainability transition of the transport sector. In the past ten years, the investments and efforts dedicated to the developments and improvements of electric powertrain have been increasingly significant from almost all automotive companies. Battery electric vehicles are the most promising solution, guaranteeing zero local emissions and high efficiency in energy conversion. However, the electrical architecture adopted until now, consisting in a stiff connection between a battery pack and a two-level inverter, presents two main drawbacks related to the converter topology and the energy system configuration. On one hand, the two-level inverter can guarantee high efficiency only at nominal loads, showing drastic drops for partial loads operations. On the other hand, the battery pack is characterized by a fixed serial and parallel connection of battery cells, limiting the energy delivery of one string to the weakest cell. Both drawbacks are extremely critical in electric vehicles applications, since the motor is required to often work at partial loads - i.e. in city traffic - and the driving range may significantly decrease with the low efficiency operations of the battery pack. Therefore, some research works have suggested a conceptual revision of the converter and battery pack used in electrical powertrain structures. In this framework, multilevel converters started to gain attention as valid candidates to replace conventional converters in powertrain architectures. Splitting the battery pack in several modules allows to limit the serial connection of cells and consequently decrease the probability of having multiple weak cells. Moreover, the presence of more submodules reduce the stress on the devices, permitting a theoretical indefinite increase of the DC-link voltage. Since an optimal battery system management relies on on the possibility to access the single battery cells, other works in the scientific literature suggested the utilization of Reconfigurable Battery Systems. These systems allow to perform active management of the battery system by controlling the connection between the battery cells through a coherent placement of electronic switches. The role of Multilevel converters in electrical powertrain is considered central in this work. This research work presents a new topology, called Reconfigurable Cascaded Multilevel converter able to simultaneously implement the power conversion and the battery management. In each submodule, battery cells are serially connected in groups of three through a pattern of switches, forming a single unit called Reconfigurable Battery Module. In this way, each battery cell can be controlled, enhancing sorting algorithms during both charging and discharging processes and fault tolerant strategies. The new topology is explained in details and then used in different case scenarios to prove its validity.