Damping higher order harmonics in average dc link voltages

A method for controlling a converter system, the converter system comprising at least two branches, wherein each branch comprises an alternating current to direct current (AC-to-DC) converter and a DC link cascade connected with each other, and wherein the method comprises:

• determining at least two AC-side currents, each of which is input into one of the AC-to-DC converters;
• determining from the AC-side currents, a voltage reference for each of the AC-to-DC converters, such that the AC-side current follows a current reference for the respective AC-to-DC converter;
• determining a DC link ripple indicator, wherein the DC link ripple indicator is an average DC link voltage of the DC links, and/or an average DC link current of the DC links and/or an average active power input into the at least two branches;
• determining a converter reference correction from the DC link ripple indicator, such that a higher order harmonic in the average DC link voltage is damped;
• determining corrected voltage references for the AC-to-DC converter by adding the converter reference correction to the voltage references of the AC-to-DC converters or by adding the converter reference correction to an average current reference for the AC-to-DC converters;
• and
• controlling the AC-to-DC converters with the respective corrected voltage references.