Modular multilevel converter capacitor voltage ripple reduction
Aspects of capacitor voltage ripple reduction in modular multilevel converters are described herein. In one embodiment, a power converter system includes a modular multilevel converter (MMC) electrically coupled and configured to convert power between two different power systems. The MMC includes one or more phase legs having a cascade arrangement of switching submodules, where the switching submodules include an arrangement of switching power transistors and capacitors. The MMC further includes a control loop including a differential mode control loop and a common mode control loop. The differential control loop is configured to generate a differential control signal based on a target modulation index to reduce fundamental components of voltage ripple on the capacitors, and the common mode control loop is configured to inject 2 nd order harmonic current into a common mode control signal to reduce 2 nd order harmonic components of the voltage ripple on the capacitors.
1. A power converter, comprising:
a modular multilevel converter (MMC) electrically coupled between a first power system and a second power system, the MMC comprising:
a phase leg comprising a cascade arrangement of switching submodules, the switching submodules comprising an arrangement of switching power transistors and capacitors; and
a controller to reduce voltage ripple on the capacitors in the switching submodules, the controller comprising:
a differential control loop configured to generate a differential control signal for switching the switching power transistors based on a target modulation index; and
a common mode control loop configured to generate a common mode control signal for switching the switching power transistors based on an injection of a 2 nd order harmonic current as an input to the controller according to a power factor of the MMC.
2. The power converter according to claim 1 , wherein the control loop is configured to reduce both fundamental and 2 nd order harmonic components of voltage ripple on the capacitors in the switching submodules.
3. The power converter according to claim 1 , wherein:
the first power system comprises a direct current (DC) power system having an output voltage of V dc ;
the second power system comprises an alternating current (AC) power system having an envelope output voltage of V a ; and
the target modulation index, M, is defined as M=2V a /V dc .
4. The power converter according to claim 3 , wherein the switching submodules comprise at least one of a full bridge arrangement or a semi full bridge arrangement of a subset of the switching power transistors, and M is greater than about 1.
5. The power converter according to claim 3 , wherein the switching submodules comprise at least one of a full bridge arrangement or a semi full bridge arrangement of a subset of the switching power transistors, and M is about 1.15.
6. The power converter according to claim 1 , wherein the control loop further comprises an upper switching submodule balance controller and a lower switching submodule balance controller.
7. The power converter according to claim 1 , wherein the 2 nd order harmonic current comprises an angle component at a non-unity power factor of the MMC.
8. The power converter according to claim 1 wherein:
a first of the switching submodules comprises a full bridge arrangement of a first subset of the switching power transistors;
a second of the switching submodules comprises a half bridge arrangement of a second subset of the switching power transistors; and
the control loop is further configured to inject a 3 rd order harmonic voltage into a combination of the differential control signal and the common mode control signal.
9. The power converter according to claim 1 , wherein the switching submodules comprise a first switching submodule having a full bridge arrangement of a first subset of the switching power transistors and a half bridge arrangement of a second subset of the switching power transistors.
10. A power converter system, comprising:
a direct current (DC) power system;
an alternating current (AC) power system; and
a modular multilevel converter (MMC) electrically coupled and configured to convert power between the DC power system and the AC power system, the MMC comprising:
a phase leg comprising a cascade arrangement of switching submodules, the switching submodules comprising an arrangement of switching power transistors and capacitors; and
a controller to reduce voltage ripple on the capacitors in the switching submodules, the controller comprising:
a differential control loop configured to generate a differential control signal for switching the switching power transistors based on a target modulation index; and
a common mode control loop configured to generate a common mode control signal for switching the switching power transistors based on an injection of a 2 nd order harmonic current as an input to the controller according to a power factor of the MMC.
11. The power converter system according to claim 10 , wherein the target modulation index, M, is defined as M=2V a /V dc .
12. The power converter system according to claim 11 , wherein the switching submodules comprise at least one of a full bridge arrangement or a semi full bridge arrangement of a subset of the switching power transistors, and M is about 1.15.
13. The power converter system according to claim 10 , wherein the control loop further comprises an upper switching submodule balance controller and a lower switching submodule balance controller.
14. The power converter system according to claim 10 , wherein the 2 nd order harmonic current comprises an angle component at a non-unity power factor of the MMC.
15. The power converter system according to claim 10 , wherein:
a first of the switching submodules comprises a full bridge arrangement of a first subset of the switching power transistors;
a second of the switching submodules comprises a half full bridge arrangement of a second subset of the switching power transistors; and
the control loop is further configured to inject a 3 rd order harmonic voltage into a combination of the differential control signal and the common mode control signal.
16. The power converter system according to claim 10 , wherein the switching submodules comprise a first switching submodule having a full bridge arrangement of a first subset of the switching power transistors and a half bridge arrangement of a second subset of the switching power transistors.
17. A modular multilevel converter (MMC), comprising:
a phase leg comprising a cascade arrangement of switching submodules, the switching submodules comprising an arrangement of switching power transistors and capacitors; and
a control loop to reduce 2 nd order harmonic components of voltage ripple on the capacitors in the switching submodules, the controler comprising:
a differential mode control loop configured to generate a differential control signal for switching the switching power transistors based on a target modulation index to reduce fundamental components of voltage ripple on the capacitors in the switching submodules; and
a common mode control loop configured to generate a common mode control signal for switching the switching power transistors based on an injection of a 2 nd order harmonic current as an input to the control loop according to a power factor of the MMC.
18. The MMC according to claim 17 , wherein:
the MMC is electrically coupled between a first power system and a second power system
the first power system comprises a direct current (DC) power system having an output voltage of V dc ;
the second power system comprises an alternating current (AC) power system having an envelope output voltage of V a ; and
the target modulation index, M, is defined as M=2V a /V dc .
19. The MMC according to claim 18 , wherein the switching submodules comprise at least one of a full bridge arrangement or a semi full bridge arrangement of a subset of the switching power transistors, and M is greater than about 1.
20. The MIVIC according to claim 17 , wherein:
a first of the switching submodules comprises a full bridge arrangement of a first subset of the switching power transistors;
a second of the switching submodules comprises a half full bridge arrangement of a second subset of the switching power transistors; and
the control loop is further configured to inject a 3 rd order harmonic voltage into a combination of the differential control signal and the common mode control signal.