Analysis of a three-phase flying converter cell rectifier operating in light/no-load condition

The “Flying” Converter Cell (FCC) rectifier allows the extension of an existing passive diode bridge rectifier to a low-harmonic unity power factor input stage by adding a combination of additional converter topologies to the DC-side of the passive circuit. In general, light-load condition of such an active rectifier, however, may lead to undesired effects as, e.g., impaired total harmonic distortion of input currents (THDi). In this paper the operation of the active rectifier circuit under no-load or light-load conditions and corresponding effects are therefore analyzed in detail also considering the control of the two FCC DC voltages. Three different operating modes are proposed and different parameters and characteristics of these modes are discussed accompanied by simulation results. It is shown that the inductance value of the coupled three-phase current injection choke directly influences the operating behavior and design guidelines for this coupled inductor are therefore derived. This work further comprises a closer look at current control and voltage balancing of FCC DC voltages during light- and no-load condition and resulting optimization issues evoked due to appropriate control of dedicate control structures. Discussed side effects are finally verified by experimental results taken from a laboratory prototype of 10kW output power and 10 kHz switching frequency.