Method and apparatus for reducing common mode voltage in multi-phase power converters

A multi-phase power conversion system for providing power to a multi-phase load having a plurality of inputs and a neutral node and for reducing common mode voltage, comprising:

• (a) a DC-to-AC power inverter having an even number of inverter legs connected between positive and negative DC bus lines, wherein each inverter leg includes inverter switching devices responsive to switching device control signals to connect the positive or negative DC bus lines to an inverter leg output;
• (b) means for symmetrizing a DC source voltage provided to the positive and negative DC bus lines such that the DC source voltages applied to the positive and negative DC bus lines are symmetric about ground;
• (c) a multi-phase filter connected between the inverter leg outputs and the inputs but not the neutral node of the load such that the multi-phase filter presents a balanced impedance to the inverter;
• and
• (d) switching device controller means for providing the switching device control signals to the inverter switching devices in the inverter legs to switch the inverter switching devices in a pattern which implements a modulation strategy such that AC voltage waveforms are synthesized on at least some of the inverter leg outputs, and wherein the modulation strategy is subject to a modulation constraint such that at all times an equal number of the inverter leg outputs are connected to the positive DC bus line by the inverter switching devices as are connected to the negative DC bus line by the inverter switching devices.

A method of converting power provided to a multi-phase load having a plurality of inputs and a neutral node and reducing common mode voltage, comprising the steps of:

• (a) applying a DC source voltage to positive and negative DC bus lines of a DC-to-AC power inverter having an even number of at least four inverter legs connected between the positive and negative DC bus lines, wherein each inverter leg has an output;
• (b) balancing the DC source voltage applied to the positive and negative DC bus lines such that the DC source voltage applied to the positive and negative DC bus lines is symmetric about ground;
• (c) synthesizing a multi-phase AC voltage waveform on at least three of the inverter leg outputs by connecting the inverter leg outputs to the positive and negative DC bus lines in a pattern defined by a modulation strategy;
• (d) applying a modulation constraint to the modulation strategy such that at all times an equal number of the inverter leg outputs are connected to the positive DC bus line as are connected to the negative DC bus line;
• and
• (e) connecting the inverter leg outputs to the inputs but not the neutral node of the load.

A power conversion system for reducing common mode voltage, comprising:

• (a) a two terminal DC-to-AC inverter having two inverter legs wherein each inverter leg includes two inverter switching devices responsive to switching device control signals connected together in series at an inverter leg output terminal between positive and negative DC bus lines and two inverter switching devices responsive to switching device control signals connected together in series between the two inverter leg output terminals to conduct in opposite directions when turned on;
• (b) means for symmetrizing a DC source voltage provided to the positive and negative DC bus lines such that the DC source voltages applied to the positive and negative DC bus lines are symmetric about ground;
• and
• (c) switching device controller means for providing the switching device central signals to the inverter switching devices in the inverter to switch the inverter switching devices in a pattern which implements a modulation strategy such that an AC voltage waveform is synthesized on the inverter leg output terminals, and wherein the modulation strategy is subject to a modulation constraint such that at all times either switching devices in the inverter legs are turned on to connect the positive DC bus line to a first leg output terminal and to connect the negative DC bus line to a second leg output terminal, switching devices in the inverter legs are turned on to connect the negative DC bus line to the first leg output terminal and to connect the positive DC bus line to the second leg output terminal, or the two switching devices connected between the two inverter leg output terminals are turned on to thereby short the terminals.