Contribution to the Ultra-Fast Charging of Electric Vehicles The Configurable Modular Multilevel Converter (CMMC)

It is generally agreed that one of the drawbacks of a conventional vehicle is the inability to always reach the best operating point of the engine and the inability to regenerate the kinetic energy. Meaning, even with the help of the gearbox or automatic transmission, there is no way to reach more efficient operating points without increasing the power and then there is nowhere to store the excess of the energy converted. Electric vehicles (EVs) offer an opportunity for intelligent motion by working at the best operating point during driving mode, as well as a smart energy management during charging mode. However, it is well-known that the EV has been criticised due to its range anxiety, its limited compatibility with various charging points, its high cost, as well as its limited capability to ride-through a thermal runaway, etc. Today, the range anxiety seems to be resolved by the fact that a significate development is foreseen in the field of the battery cells market for portable equipment such as cell phones, pads, laptops, etc; the same battery technology can be deployed for EVs. Actually, battery packs in EVs tend to have high energy and power densities which will exert a strong thermal stress upon them. This means that individual battery cells are more prone to decline away and eventually fail early because of an excessive loss of capacity and an increase of the internal resistance caused by accelerated aging. Moreover, for electromechanical reasons, each cell is totally unbalanced in operating mode. Therefore, a suitable battery management system should be installed and the capability of the EV to ride-through thermal runaways must be guaranteed. Furthermore, in order to overcome the difference between AC and DC charging station standards, the EV on-board charger has to be flexible for various levels of charging modes (from AC household basic supply to AC or DC ultrafast charging). In order to meet the EV requirements, an advanced power converter which driven efficiency, power density is required. Therefore, the configurable modular multilevel converter (CMMC) has been introduced. It is part of a family of modular multilevel converters, also known as MMC, M2C or DSCC. Hence, its heritage allows bringing the whole advantages to EV applications. In addition it carries the surname of configurable because the converter plays a double role: in fact, it merge the traction converter and the bidirectional charger as well as the battery balancer converter, which not only increases the modularity and the scalability of the power modules but it reduces the cost, condenses the volume. In detail, the concept is linked to the idea of replacing the branch inductance of the standard MMC by the tapped stator windings of the traction motor itself. This allows operating the stator windings as current filter during charging mode. In addition, the famous EV battery pack is split into the MMC submodule. This will increase the fault ride-through capabilities of the Flex-EV battery against thermal runaways, by increasing the capability of the system to work under fault condition. In addition, states of charge (SoC) balancing are implemented at the level of modulation in order to balance that submodule battery state of charge. This is based on a selecting and sorting algorithm, which connect and disconnect the batteries submodules actively according to the sign of the generated AC current wave form and the SoC of the submodules batteries.

Rufer, Alfred
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis6934-6

 Record created 2016-02-29, last modified 2018-03-17

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