Standby and charging of modular multilevel converters
View Patent ↗The present invention proposes a hybrid converter branch operating mode for a Modular Multilevel power Converter MMC with MMC cells in distinct subsets operating according to a “pulse blocked” cell operation mode with DC cell voltage increase or according to a “bypass” cell operation mode without DC cell voltage increase. Repeated cell subset assignment and corresponding alternation of cell operating mode allows to reduce or at least manage a mean deviation of the cell capacitor DC voltages of the converter cells. The invention also reduces no-load losses of the MMC in standby mode and a charging voltage in an MMC charging mode.
1. A method of operating a Modular Multilevel Converter MMC having a converter branch with a plurality of converter cells with a nominal cell voltage, comprising
applying a branch voltage to the converter branch, the branch voltage being inferior to a nominal branch voltage cumulative of the nominal cell voltages of the cells of the branch;
selecting a first subset of the plurality of converter cells of the branch, and selecting a second subset of the plurality of converter cells which is distinct from the first subset; and
operating successively the first and second subset in a pulse blocked cell operation mode with cell voltage increase, and operating a respective complementary subset of the plurality of converter cells of the branch in a bypass cell operation mode without cell voltage increase;
wherein firing pulses or gate pulses to the plurality of converter cells being blocked or suppressed in the pulse blocked cell operation mode.
2. The method of claim 1 , wherein at subset selection a highest cell voltage of the first and second subset does not exceed a lowest cell voltage of the respective complementary subset.
3. The method of claim 1 , wherein the MMC is operated in standby mode without nominal power exchange, including
connecting the converter branch to an electric power.
4. The method of claim 1 , wherein the MMC is operated in charging mode, including
connecting the converter branch to an auxiliary power supply.
5. The method of claim 3 , wherein the MMC is a Statcom for producing and/or absorbing reactive power.
6. The method of claim 3 , wherein the MMC is a frequency converter for converting power grid line frequency current to single phase traction supply current in railway applications.
7. An MMC controller for operating a Modular Multilevel Converter MMC having a converter branch with a plurality of converter cells with a nominal cell voltage, the MMC controller being adapted to, following application to the converter branch of a branch voltage inferior to a nominal branch voltage that is cumulative of the nominal cell voltages of the cells of the branch;
select a first subset of the plurality of converter cells of the branch, and select a second subset of the plurality of converter cells which is distinct from the first subset;
operate successively the first and second subset in pulse blocked cell operation mode with cell voltage increase, and operate a respective complementary subset of the plurality of converter cells of the branch in bypass cell operation mode without cell voltage increase;
wherein firing pulses or gate pulses to the plurality of converter cells being blocked or suppressed in the pulse blocked cell operation mode.
8. The MMC controller of claim 7 , which is further adapted to select the first and second subset such that a highest cell voltage of the first and second subset does not exceed a lowest cell voltage of the respective complementary subset.
9. The method of claim 2 , wherein the MMC is operated in standby mode without nominal power exchange, including
connecting the converter branch to an electric power grid.
10. The method of claim 2 , wherein the MMC is operated in charging mode, including
connecting the converter branch to an auxiliary power supply.
11. The method of claim 4 , wherein the MMC is a Statcom for producing and/or absorbing reactive power.
12. The method of claim 4 , wherein the MMC is a frequency converter for converting power grid line frequency current to single phase traction supply current in railway applications.
13. The method of claim 3 , wherein the MMC is a frequency converter for converting variable frequency current to power grid line frequency in hydro power applications.
14. The method of claim 4 , wherein the MMC is a frequency converter for converting variable frequency current to power grid line frequency in hydro power applications.