A novel hybrid modulation scheme for an isolated high-frequency-link fuel cell inverter

This paper describes a novel hybrid modulation scheme for the operation of an isolated high-frequency and high-power direct-power-conversion (DPC) inverter. It enables: (a) removal of the dc-link filter evident in conventional inverters where the diode rectifier stage is followed by low-pass filter and a voltage-source inverter (VSI); (b) significant reduction in switching loss of the inverter by modulating the switches of 2 of the 3 legs of the 6-switch ac/ac converter at 120 Hz while modulating the 3rd leg at high frequency. Unlike the VSI approach, in the DPC approach hybrid modulation enables the retention of the sine-wave modulated switching information at the output of the diode rectifier without rather than filtering it. Overall, the following is demonstrated: i) modulation scheme and uniqueness, and analytical derivation of the Fourier transformation of the output voltage; ii) derivation of a reachability condition for the DPC topology that outlines its global stability and performance; iii) demonstration of the switching and conduction losses and harmonics; and iv) scaled experimental validation. It is noted that, the term hybrid modulation has no similarity with the modulation scheme for a hybrid converter (which are conjugation of two types of converters based on a slow and fast device) reported in literature. The term hybrid modulation scheme is simply chosen because at any given time, only one leg of the inverter output stage carries out forced switching while the other two legs are not switching at all. The outlined hybrid modulation scheme is unlike all reported discontinuous modulation schemes where the input is a dc and not a pulsating modulated dc and at most only one leg stays on or off permanently in a 60° or 120° cycle.