Converter and method for the control thereof

1. A method for controlling a converter having a plurality of switching modules, each of the plurality of switching modules having controllably interruptible semiconductor switches and an energy store, wherein at least a number of the plurality of switching modules being switching modules of a first type, wherein the first type being full-bridge switching modules, and at least a further number of the plurality of switching modules being switching modules of a second type, wherein the second type being half-bridge switching modules, the method comprises the steps of:

regulating a switching frequency of the plurality of switching modules based on at least one integral limiting value, wherein the at least one integral limiting value is variably determined over time based on energy store voltages of the plurality of switching modules;

executing, by means of additional switching operations, by reference to an evaluation of the energy store voltages, a symmetrization of the energy store voltages, the symmetrization of the energy store voltages being executed based on a voltage difference in the energy store voltages of the switching modules of the first type and a voltage difference in the energy store voltages of the switching modules of the second type, wherein the voltage difference of the switching modules of either of the first or second types is a difference between an energy store voltage of a respective switching module and an average value for all of the energy store voltages of the switching modules of the same type;

comparing the voltage difference of either of the first or second types with a differential limiting value and, if the voltage difference exceeds the differential limiting value, reducing the at least one integral limiting value for switching frequency control, wherein for switching the frequency control, determining an average switching frequency of the plurality of switching modules, and filtering the average switching frequency by means of a moving average filter; and

generating, during an operation of the converter assembly, a positive switching module voltage, a negative switching module voltage or a zero voltage on connection terminals of the full-bridge switching modules, and generating a positive switching module voltage or a zero voltage on connection terminals of the half-bridge switching modules.

2. The method according to claim 1 , which further comprises executing the switching frequency control by means of a frequency droop.

3. The method according to claim 1 , which further comprises executing the switching frequency based on the number of the switching modules of the first and second types.

4. The method according to claim 1 , which further comprises controlling the plurality of switching modules of a converter arm with a modulation factor greater than one.

5. A converter for medium-voltage applications and high-voltage applications, the converter comprising:

a plurality of switching modules each having controllably interruptible semiconductor switches and an energy store, wherein at least a number of said plurality of switching modules being switching modules of a first type, wherein said first type being full-bridge switching modules, and at least a further number of said plurality of switching modules being switching modules of a second type, wherein said second type being half-bridge switching modules; and

a controller for controlling the converter, said controller configured to:

regulate a switching frequency of said plurality of switching modules based on at least one integral limiting value, wherein the at least one integral limiting value is variably determined over time based on energy store voltages of said plurality of switching modules;

execute, by means of additional switching operations, by reference to an evaluation of the energy store voltages, a symmetrization of the energy store voltages, the symmetrization of the energy store voltages being executed based on a voltage difference in the energy store voltages of said switching modules of said first type and a voltage difference in the energy store voltages of said switching modules of said second type, wherein the voltage difference of said switching modules of either said first or second types is a difference between an energy store voltage of a respective switching module and an average value for all of the energy store voltages of said switching modules of the same type;

compare the voltage difference of either of the first or second types with a differential limiting value and, if the voltage difference exceeds the differential limiting value, reducing the at least one integral limiting value for switching frequency control, wherein for switching the frequency control, determining an average switching frequency of said plurality of switching modules, and filtering the average switching frequency by means of a moving average filter; and

generate, during an operation of the converter, a positive switching module voltage, a negative switching module voltage or a zero voltage on connection terminals of said full-bridge switching modules, and generating a positive switching module voltage or a zero voltage on connection terminals of said half-bridge switching modules.