Cascade multilevel inverters for large hybrid-electric vehicle applications with variant DC sources

This thesis investigates using the cascade multilevel inverter as an alternative to conventional pulse width modulated inverters for large hybrid-electric vehicle (HEV) drivetrain applications. Previous research considered constant and equal dc sources with invariant behavior; however, this research extends earlier work to include variant dc sources, which are typical of lead-acid batteries when used in HEVs. This thesis also investigates methods to minimize the total harmonic distortion of the synthesized multilevel waveform and to help balance the battery voltage. The harmonic elimination method was used to eliminate selected lower dominant harmonics resulting from the inverter switching action. Switching points (angles) were determined using an iterative technique to solve the system of nonlinear transcendental equations. The total harmonic distortion was investigated over a wide range of possible output control voltages and number of voltage levels used to synthesize the output waveform. As expected, the line-to-line voltage of the three-phase multilevel inverter's voltage was observed to be zero when used with an ideal low-pass filter; however, the total harmonic distortion increased significantly for both the phase- and line-voltages as the number of synthesis voltage levels decreased. Also, a switching pattern that would help balance and equalize the individual battery voltages within an HEV battery pack was developed. The individual batteries with the higher voltages would be assigned the longer duty cycle, and the batteries with the lower voltages would be skipped or assigned the lower duty cycles.