Neutral point clamped converter control systems and methods

A control method for balancing a neutral point voltage between a source-side converter and a line-side converter of a power conversion system, the control method comprising:

• generating a first fundamental command to implement fundamental power conversion operation for the source-side converter;
• generating a second fundamental command to implement fundamental power conversion operation for the line-side converter;
• obtaining a first modulation index of the source-side converter and a second modulation index of the line-side converter;
• determining whether the first modulation index is less than the second modulation index;
• when the first modulation index is less than the second modulation index, generating a first source-side compensation command for injection into the first fundamental command and, when the first source-side compensation command is not sufficient to balance the neutral point voltage, further generating a first line-side compensating command for injection into the second fundamental command;
• and
• when the first modulation index is greater than the second modulation index, generating a second line-side compensation command for injection into the second fundamental command and, when the second line-side compensation command is not sufficient to balance the neutral point voltage, further generating a second source-side compensating command for injection into the first fundamental command.

A power conversion system;

• a source-side converter;
• a line-side converter;
• a DC link coupling the source-side and line-side converters;
• and
• a control system configured to provide commands to the source-side and line-side converters for balancing a neutral point voltage at the DC link, the control system comprising:
• a first fundamental control unit configured to generate a first fundamental command to implement fundamental power conversion operation for the source-side converter;
• a second fundamental control unit configured to generate a second fundamental command to implement fundamental power conversion operation for the line-side converter;
• and
• a compensation control unit comprising:
• a switch element configured to receive a fundamental compensation command used to balance the neutral point voltage;
• and
• a comparator configured to compare a first modulation index of the source-side converter and a second modulation index of the line-side converter and control the switch element to provide the fundamental compensation command for injection into at least the first fundamental command to balance the neutral point voltage if the first modulation index is less than the second modulation index and control the switch element to provide the fundamental compensation command for injection into at least the second fundamental command to balance the neutral point voltage if the first modulation index is greater than the second modulation index.