Investigation of a Multilevel Inverter for Electric Vehicle Applications

Electrified vehicles on the market today all use the classical two-level inverter as the propulsion inverter. This thesis analyse the potential of using a cascaded H-bridge multilevel inverter as the propulsion inverter. With a multilevel inverter, the battery is divided into several parts and the inverter can now create voltages in smaller voltage levels than the two-level inverter. This, among other benefits, reduces the EMI spectrum in the phase cables to the electric machine. It is also shown that these H-bridges can be placed into the battery casing with a marginal size increase, and some addition of the cooling circuit performance. The benefit is that the separate inverter can be omitted. In this thesis, measurements and parameterisations of the battery cells are performed at the current and frequency levels that are present in a multilevel inverter drive system. The derived model shows a great match to the measurements for different operating points and frequencies. Further, full drive cycle simulations are performed for the two analysed systems. It is shown that the inverter loss is greatly reduced with the multilevel inverter topology, mainly due to the possibility to use MOSFETs instead of IGBTs. However, the battery packs in a multilevel inverter experience a current far from DC when creating the AC-voltages to the electric machine. This leads to an increase of the battery loss but looking at the total inverter-battery losses, the system shows an efficiency improvement over the classical two-level system for all but one drive cycle. In the NEDC drive cycle the losses are reduced by 30 % but in the demanding US06 drive cycle the losses are increased by 11 % due to the high reactive power demand at high speed driving. These figures are valid for a plug-in hybrid with a 50 km electrical range where no filter capacitors are used. In a pure electric vehicle, there is always an energy benefit of using a multilevel converter since a larger battery will have lower losses. By placing capacitors over the inputs of the H-bridges, the battery current is filtered. Two different capacitor chemistries are analysed and experimentally verified and an improvement is shown, even for a small amount of capacitors and especially at cold operating conditions.

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