Multilevel converter systems and methods with reduced common mode voltage

A power conversion system, comprising:

• a DC bus circuit comprising first and second DC bus terminals;
• a rectifier including a plurality of multilevel rectifier converter stages, individual ones of the multilevel rectifier converter stages comprising an AC input terminal adapted to receive AC input power from a corresponding one of a plurality of AC input phases, a pair of DC output terminals individually coupled to a corresponding one of the first and second DC bus terminals of the DC bus circuit, and a multilevel rectifier circuit operable according to a corresponding set of rectifier switching control signals to convert AC input power to provide a DC output voltage at the DC output terminals;
• an inverter including a plurality of multilevel inverter stages, individual ones of the multilevel inverter stages comprising a pair of DC input terminals individually coupled to a corresponding one of the first and second DC bus terminals of the DC bus circuit, an AC output terminal adapted to provide AC output power directly or indirectly to a corresponding one of a plurality of AC output phases to drive a load, and a multilevel inverter circuit operable according to a corresponding set of inverter switching control signals to convert DC input power from the DC bus circuit to provide an AC output voltage at the AC output terminal;
• and
• a controller providing the rectifier and inverter switching control signals to facilitate offsetting of common mode voltage contributions of the rectifier and the inverter;
• wherein the individual multilevel converter stages of the rectifier and of the inverter are configured to provide an integer number N AC voltage levels, where N is an even number greater than or equal to 4;
• wherein the controller provides the rectifier and inverter switching control signals via space vector modulation according to corresponding rectifier and inverter reference vectors using a corresponding subset of possible switching states with minimal absolute common mode voltages;
• and
• wherein the controller provides the switching control signals to a first one of the rectifier and the inverter alternately using switching states of a first subset of possible switching states for which a common mode voltage contribution of the corresponding multilevel stage is a minimal positive value and a different second subset of possible switching states for which the common mode voltage contribution of the corresponding multilevel stage is a minimal negative value when a modulation index associated with the reference vector associated with the first one of the rectifier and the inverter is greater than a predetermined value and a modulation index associated with the reference vector associated with the other one of the rectifier and the inverter is less than or equal to the same predetermined value, and wherein the controller provides the switching control signals to the other one of the rectifier and the inverter using the switching states of the same subset currently used for the first one of the rectifier and the inverter.

A method for operating a multiphase AC-AC conversion system having a rectifier including a plurality of multilevel rectifier converter stages, an inverter including a plurality of multilevel inverter stages, and an intermediate DC bus circuit, the method comprising:

• providing rectifier and inverter switching control signals to the multilevel stages of the rectifier and the inverter via space vector modulation according to corresponding rectifier and inverter reference vectors using a corresponding subset of possible switching states with minimal absolute common mode voltages to facilitate offsetting of common mode voltage contributions of the rectifier and the inverter;
• and
• providing the rectifier and inverter switching control signals using the same subset of possible switching states when a modulation index associated with the rectifier reference vector is less than or equal to a predetermined value and a modulation index associated with the inverter reference vector is less than or equal to the predetermined value;
• wherein the individual multilevel converter stages of the rectifier and of the inverter are configured to provide an integer number N AC voltage levels, where N is an even number greater than or equal to 4.

A method for operating a multiphase AC-AC conversion system having a rectifier including a plurality of multilevel rectifier converter stages, an inverter including a plurality of multilevel inverter stages, and an intermediate DC bus circuit, the method comprising:

• providing rectifier and inverter switching control signals to the multilevel stages of the rectifier and the inverter via space vector modulation according to corresponding rectifier and inverter reference vectors using a corresponding subset of possible switching states with minimal absolute common mode voltages to facilitate offsetting of common mode voltage contributions of the rectifier and the inverter;
• wherein the individual multilevel converter stages of the rectifier and of the inverter are configured to provide an integer number N AC voltage levels, where N is an even number greater than or equal to 4;
• and
• providing the switching control signals for a first one of the rectifier and the inverter alternately using switching states of a first subset of possible switching states for which a common mode voltage contribution of the corresponding multilevel stage is a minimal positive value and a different second subset of possible switching states for which the common mode voltage contribution of the corresponding multilevel stage is a minimal negative value when a modulation index associated with the reference vector associated with the first one of the rectifier and the inverter is greater than a predetermined value and a modulation index associated with the reference vector associated with the other one of the rectifier and the inverter is less than or equal to the predetermined value, and providing the switching control signals to the second one of the rectifier and the inverter using the switching states of the same subset currently used for the first one of the rectifier and the inverter.