Energy Efficiency Comparison Between Two-level and Multilevel Inverters for Electric Vehicle Applications

Licentiate thesis, 2013

In order to contribute to the development of energy and cost efficient electric vehicles, a thorough analysis of two different electric converter topologies is performed, considering different drive cycles and different control strategies. In the electric vehicles available on the market today, a high voltage (200−400 V ) battery pack composed of several battery modules is connected to one or more inverters which create AC voltages to the electric propulsion machines. This thesis analyses the use of a modular battery-inverter concept, the cascaded multilevel inverter. Here, the battery modules have one inverter for each battery module, the inverters are then connected in series and controlled in order to create the AC voltages for the electric machines. The simulation results shows that this concept lowers the inverter losses, mainly due to the possibility to use MOSFETs instead of IGBTs. The losses in the battery are on the other hand increased. If there was a possibility to filter out reactive power and harmonics from the battery (using input capacitors for each inverter), the drive train with the multilevel inverter will have lower losses than the two-level inverter for all the analysed driving cycles. If such filter capacitors can not be used, it is not beneficial to use the multilevel inverter for high speed driving cycles, such as the US06. For the more moderate new European driving cycle, the accumulated drive cycle losses in the battery and inverter are reduced by 25 % when using the multilevel inverter, even without filter capacitors, compared to the two-level inverter. Furthermore, when using infinitely large supporting capacitors in parallel to the battery modules, the loss reduction for the inverter and the battery becomes 72 % compared to the two-level inverter. The multilevel inverter is able to control from which battery modules the energy is taken from, therefore the battery capacity is no longer limited by the weakest battery module. If one of the modules is damaged and the available energy is lower, the inverter can be controlled to utilize the remaining energy in the battery pack so the effect of the faulty module is not so severe. When using the propulsion inverter as a charger, the multilevel inverter also shows an increase in efficiency compared to the two-level inverter.