Modeling and Control of a Power Electronic Cascade for the Multi DC Bus Supply of a Seven-Level NPC Voltage Source Inverter

The stability problem of the input DC voltages in a seven-level Neutral Point Clamping (NPC) Voltage Source Inverter (VSI) is recalled and illustrated. A first solution involving six controlled-voltage (CV) rectifiers is presented. A knowledge model of this converter using switching functions and a PWM strategy is detailed. For an economic point of view, a second power structure is proposed. It consists in using only 3 CV rectifiers but the instability of intermediary DC voltages is observed. To solve this problem the power structure is modified by adding clamping bridges across capacitors in order to limit their deviations. Additional control functions are then introduced. The obtained results are full of promise to be used to stabilise the input DC voltages of these converters.