VOLTAGE BALANCING SYSTEM AND METHOD FOR MULTILEVEL CONVERTERS

• US 20140293667A1
• Filed: 03/29/2013
• Published: 10/02/2014
• Est. Priority Date: 03/29/2013
• Status: Active Grant

• Current Assignee: GE Energy Power Conversion Technology Limited (GE Vernova, Inc.)
• Original Assignee: General Electric Company
• Inventors: Schroeder, Stefan, Chen, Quingyun
• Granted Patent: US 9,246,407 B2
• Time in Patent Office: Days
• US Class Current: 363/60
• CPC Class Codes: H02M 1/0009 Devices or circuits for detecting current in a converter
• CPC Class Codes: H02M 1/0012 Control circuits using digital or numerical techniques in DC/DC converters H02M3/157, H02M3/33515; in DC-AC converters H02M7/53873
• CPC Class Codes: H02M 1/0095 Hybrid converter topologies, e.g. NPC mixed with flying capacitor, thyristor converter mixed with MMC or charge pump mixed with buck
• CPC Class Codes: H02M 7/483 Converters with outputs that each can have more than two voltages levels
• CPC Class Codes: H02M 7/4833 Capacitor voltage balancing
• CPC Class Codes: H02M 7/4837 Flying capacitor converters
• CPC Class Codes: H02M 7/487 Neutral point clamped inverters

Abstract

A method of operating a flying capacitor multilevel converter having a direct current link and a plurality of phase legs each having a plurality of flying capacitors includes employing redundant states to balance flying capacitor voltages by charging or discharging flying capacitors. The redundant states are employed by obtaining a load current of the flying capacitor multilevel converter. If a load current value is lower than a threshold value then a capacitor current of a phase terminal capacitor is utilized to determine redundant states else a load current direction is utilized to determine the redundant states.

Claims

1. A method of operating a flying capacitor multilevel converter including a direct current link and a plurality of phase legs each having a plurality of flying capacitors, the method comprising:

employing redundant states to balance flying capacitor voltages by charging or discharging flying capacitors, wherein employing redundant states include

obtaining a load current of the flying capacitor multilevel converter;

utilizing a capacitor current of a phase terminal capacitor to determine redundant states when a load current value is lower than a threshold value; and

utilizing a load current direction to determine redundant states when the load current value is higher than the threshold value.

2. The method of claim 1, wherein the phase terminal capacitor comprises a filter capacitor or a phase terminal stray capacitance.

3. The method of claim 1, wherein the threshold value is determined based on parameters including a capacitance value of the phase terminal capacitor, a voltage change at a phase terminal of the flying capacitor multilevel converter, and switching frequency and duty cycle of the flying capacitor multilevel converter.

4. The method of claim 1, wherein the threshold value is determined by equating a charge of the phase terminal capacitor with a charge generated by a load current.

5. The method of claim 1, wherein utilizing the capacitor current of the phase terminal capacitor comprises determining a phase terminal capacitor current direction based on an expected voltage transition at a phase terminal of the flying capacitor multilevel converter.

6. The method of claim 5, wherein determining the phase terminal capacitor current direction includes marking the phase terminal capacitor current direction as flowing into the phase terminal if the expected voltage transition is from high to low.

7. The method of claim 5, wherein determining the phase terminal capacitor current direction includes marking the phase terminal capacitor current direction as flowing out of the phase terminal if the expected voltage transition is from low to high.

8. The method of claim 5, wherein the expected voltage transition at a phase terminal of the flying capacitor multilevel converter is determined based on a reference voltage value.

9. The method of claim 1, wherein redundant switching states include at least two switching states which generate equal voltages.

10. The method of claim 9, wherein determination of the redundant state is decided based on charging or discharging requirement of the flying capacitor.

11. The method of claim 1, wherein a time constant of the phase terminal capacitor is smaller than a carrier period.

12. The method of claim 1, wherein the phase terminal capacitor charges or discharges within one switching cycle.

13. A multi-level converter system comprising:

a converter including a direct current (DC) link and a plurality of phase legs each having a plurality of flying capacitors; and

a converter controller configured to

employ redundant states to balance flying capacitor voltages by charging or discharging the plurality of flying capacitors, wherein the controller employs redundant states by

utilizing a capacitor current of a phase terminal capacitor to determine redundant states when a load current value is lower than a threshold value; and

utilizing a load current direction to determine redundant states when the load current value is higher than the threshold value.

14. The multi-level converter system of claim 13, wherein the phase terminal capacitor comprises a filter capacitor or a phase terminal stray capacitance.

15. The multi-level converter system of claim 13, wherein the threshold value is determined based on parameters including a capacitance value of the phase terminal capacitor, a voltage change at a phase terminal of the flying capacitor multilevel converter, and switching frequency and duty cycle of the flying capacitor multilevel converter.

16. The multi-level converter system of claim 13, wherein the threshold value is determined by equating a charge of the phase terminal capacitor with a charge generated by a load current.

17. The multi-level converter system of claim 13, wherein the controller is configured to utilize the capacitor current of the phase terminal capacitor by determining a phase terminal capacitor current direction based on an expected voltage transition at a phase terminal of the flying capacitor multilevel converter.

18. The multi-level converter system of claim 13, wherein the controller is configured to determine the redundant state based on charging or discharging requirement of the flying capacitor.

19. The multi-level converter system of claim 13, wherein a time constant of the phase terminal capacitor is smaller than a carrier period.

20. The multi-level converter system of claim 13, wherein the phase terminal capacitor charges or discharges within one switching cycle.