Circulating current injection control
In one example, a power converter includes a modular multilevel converter (MMC) electrically coupled between a first power system and a second power system. The MMC includes an arrangement of switching submodules, and the switching submodules include an arrangement of switching power transistors and capacitors. The MMC also includes a controller configured to inject a common mode frequency signal into a circulating current control loop. The circulating current control loop is relied upon to reduce at least one low frequency component in power used for charging the capacitors in the switching submodules. By injecting the common mode frequency signal into the circulating current control loop, the switching submodules can be switched at higher frequencies, the capacitances of the capacitors in the MMC can be reduced, and the power density of the MMC can be increased.
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:
an arrangement of switching submodules, at least one of the switching submodules comprising an arrangement of switching power transistors and capacitors, the switching power transistors comprising high-current, high-frequency medium-voltage semiconductor switches; and
a controller comprising a proportional-integral control loop and a proportional-resonant control loop, the controller being configured to inject a high frequency common mode signal into a circulating current control loop of the MMC, wherein:
based on the high frequency common mode signal injected into the circulating current control loop, the controller is configured to switch the switching power transistors at a higher frequency to reduce at least one low frequency component in power used for charging the capacitors and increase a power density of the MMC; and
the controller is configured to inject the high frequency common mode signal into the circulating current control loop of the controller to reduce a magnitude of voltage ripple in the capacitors.
2. The power converter according to claim 1 , wherein a size of the capacitors can be reduced based on a reduction of the magnitude of voltage ripple in the capacitors.
3. The power converter according to claim 1 , wherein the high frequency common mode signal is injected into a circulating current reference used by the proportional-resonant control loop.
4. The power converter according to claim 1 , wherein the proportional-resonant control loop is configured to increase a gain associated with at least one of a direct current (DC) component, a harmonic line frequency component, or a common mode frequency component of a circulating current in the MMC.
5. The power converter according to claim 1 , wherein:
the first power system comprises a direct current (DC) power system; and
the second power system comprises an alternating current (AC) power system.
6. The power converter according to claim 1 , wherein the switching power transistors comprise silicon carbide (SiC) metal oxide semiconductor field effect transistors (MOSFETs).
7. 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 leg comprising a cascade arrangement of switching submodules, at least one of the switching submodules comprising an arrangement of switching power transistors and capacitors, the switching power transistors comprising high-current, high-frequency medium-voltage semiconductor switches; and
a controller comprising a proportional-integral control loop and a proportional-resonant control loop, the controller being configured to inject a high frequency common mode signal into a circulating current control loop of the MMC, wherein:
based on the high frequency common mode signal injected into the circulating current control loop, the controller is configured to reduce at least one low frequency component in power used for charging the capacitors; and
the controller is configured to inject the high frequency common mode signal into the circulating current control loop of the controller to reduce a magnitude of voltage ripple in the capacitors.
8. The power converter according to claim 7 , wherein a size of the capacitors can be reduced based on a reduction of the magnitude of voltage ripple in the capacitors.
9. The power converter according to claim 7 , wherein, based on the high frequency common mode signal injected into the circulating current control loop, the controller is configured to switch the switching power transistors at a higher frequency to reduce the at least one low frequency component in power used for charging the capacitors.
10. The power converter according to claim 7 , wherein the high frequency common mode signal is injected into a circulating current reference used by the proportional-resonant control loop.
11. The power converter according to claim 7 , wherein the proportional-resonant control loop is configured to increase a gain associated with at least one of a direct current (DC) component, a harmonic line frequency component, or a common mode frequency component of a circulating current in the MMC.
12. The power converter according to claim 7 , wherein:
the first power system comprises a direct current (DC) power system; and
the second power system comprises an alternating current (AC) power system.
13. The power converter according to claim 7 , wherein the switching power transistors comprise silicon carbide (SiC) metal oxide semiconductor field effect transistors (MOSFETs).