Validation of the Quasi-Two-Level Operation for a Flying Capacitor Converter in Medium-Voltage Applications

Abstract

Standard medium-voltage converters are operated at low switching frequencies using bulky passive components. One concept to change this involves the quasi-two-level operation (Q2O) of multilevel converters that use fast-switching semiconductors to minimize the need for passive components. The flying capacitor converter (FCC) uses SiC semiconductors and operates with Q2O to minimize passive components. In this paper, two different quasi-two-level algorithms are analyzed. A medium-voltage prototype was built and low-voltage and medium-voltage measurements were used to validate the concept. A particular focus is on the overvoltage, the dv/dt behavior of the converter, as well as the dynamic behavior.

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“…With the Q2L operation of the ac front-end, the converter cells' capacitors are only subject to a notable current flow during the brief staggered switching transitions (but not during the majority of the switching period where the switch node a is either connected to the phase terminal a or to the star point n). Thus, the required cell capacitance can be calculated according to [26], using a peak-to-peak voltage ripple criterion U C,pp , as…”

“…The Q2L operating mode has initially been proposed for diodeclamped converters [21,22] and then also been applied to flyingcapacitor bridge legs [23][24][25][26] and to MMC bridge legs [27][28][29][30][31][32][33] (note that a similar concept is known as "integrated capacitor-blocked transistor cells" [34][35][36]). Essentially, Q2L operation employs the available intermediate voltage levels of the multilevel converter structure only very briefly during the transitions between a high and a low output voltage level; for example, the output node is connected to either of the Q2L-MMC bridge leg's two input terminals for most of the time; see also Figure 1.…”