Operating a multilevel converter

A method of operating a multilevel converter, the method comprising:

• providing the multilevel converter with a plurality of phase modules that are connected in parallel with one another between a first DC voltage connection and a second DC voltage connection of the multilevel converter,
• each of the phase modules having a plurality of modules each with at least two electronic switching elements and an electrical energy storage device;
• each of the phase modules having a first phase module branch, which is connected to the first DC voltage connection, a second phase module branch, which is connected to the second DC voltage connection, a third phase module branch, which connects the first phase module branch with the second phase module branch, and a switching device having a first switching position that connects an AC voltage connection of the phase module to a first connecting point between the first phase module branch and the third phase module branch and a second switching position that connects the AC voltage connection to a second connecting point between the third phase module branch and the second phase module branch;
• and the method further comprising:
• determining for each of the phase modules an energy summation variable, which is a measure of a sum of the energies of the first phase module branch and the second phase module branch;
• determining for each of the phase modules an energy difference variable, which, in the first switching position, is a measure of a difference between the energies of the second phase module branch and the third phase module branch and which, in the second switching position, is a measure of a difference between the energies of the first phase module branch and the third phase module branch, depending on the switching position of the switching device of the respective phase module;
• and
• controlling circulating currents of the multilevel converter in dependence on the energy summation variables and the energy difference variables of the phase modules in order to compensate for energy imbalances between the phase module branches.