Modulation technique for series-connected h-bridge multilevel converters with equal load sharing

An improved duty-cycle modulation scheme for multilevel inverters is introduced. The scheme exploits two-to-three axis coordinate transformation to achieve duty-cycles of each phase directly from the location of the reference vector in the Cartesian coordinate system. There is no need for trigonometric functions, which makes the applicability of the algorithm very good in a digital signal processor or a field-programmable gate array. The method also guarantees a low common-mode voltage output, because a zero-component is added only when operating at a voltage amplitude larger than 75% of the maximum amplitude of the reference vector. The selection of zero-component is also simple and mathematically easy. The pulse-formation is formulated in such a way that the modules of the inverter are equally loaded, and the balance of the individual DC link voltages is maintained throughout the voltage region. The methods are verified with simulations and experimental tests on a seven-level 100 kVA, 3 kV prototype induction motor drive.