AC/DC/AC Converter Modulation Strategy With Natural Zero sequence Rejection Using Only One Six-Switch Inverter Module

This paper reports an innovative power converter modulation strategy that converts energy from a poly-phase variable frequency source to a single-phase fixed frequency load using only one of the customary two six-switch bridge circuits characteristic of a dc link based power converter. To achieve this, a six-switch dc link CPRWM inverter is creatively modulated with two voltage components. The application is single- phase, fixed frequency power generation from a variable frequency wind energy conversion system, which uses an induction generator, a converter, and a zero sequence filter. Modulating the converter for a three-phase balanced positive sequence voltage component at the induction generator's optimum operating frequency transfers energy from the generator to the converter. At the same time, modulating the converter for a three-phase fixed frequency zero sequence voltage component transfers the energy from the converter to the load through a zero sequence filter. The two voltage components are separated by the ungrounded induction motor's natural rejection of zero sequence components and the zero sequence filter's natural rejection of positive sequence components. Unity power factor output is readily achieved. This paper presents operation, control, theoretical calculations, simulations, and experimental results that reveal important limitations of this energy conversion scheme with a single converter.