A control apparatus for use with a voltage source converter, the voltage source converter comprising: a transformer connected to an AC grid for receiving a system voltage (VGR, VGS, VGT); and legs each of which is connected to one of the phases of the transformer and configured by a plurality of converter modules each having a capacitor and a voltage sensor for detecting the capacitor voltage (VCjk), wherein: each of said legs has a first DC terminal on a positive side and a second DC terminal on a negative side between which a high-side switching device and a low-side switching device of the converter modules are connected; said control apparatus is operative to create command pulses for operating the converter modules; the command pulses are calculated by a PWM, pulse-width modulation, method using a PWM carrier wave (Vcar*) made of a triangular wave whose phase is shifted for each converter module and a modulating wave (Vmjk*) which is based on a moving average of the capacitor voltages (Vcjk) of the converter module; the command pulses have a frequency that is a non-integral multiple of a frequency of the system voltage (VGR, VGS, VGT); and the time increment of the moving average is determined by the reciprocal of the greatest common divisor of said system frequency and the product of the system frequency with said non-integral multiple. a transformer connected to an AC grid for receiving a system voltage (VGR, VGS, VGT); and legs each of which is connected to one of the phases of the transformer and configured by a plurality of converter modules each having a capacitor and a voltage sensor for detecting the capacitor voltage (VCjk), wherein: each of said legs has a first DC terminal on a positive side and a second DC terminal on a negative side between which a high-side switching device and a low-side switching device of the converter modules are connected; said control apparatus is operative to create command pulses for operating the converter modules; the command pulses are calculated by a PWM, pulse-width modulation, method using a PWM carrier wave (Vcar*) made of a triangular wave whose phase is shifted for each converter module and a modulating wave (Vmjk*) which is based on a moving average of the capacitor voltages (Vcjk) of the converter module; the command pulses have a frequency that is a non-integral multiple of a frequency of the system voltage (VGR, VGS, VGT); and the time increment of the moving average is determined by the reciprocal of the greatest common divisor of said system frequency and the product of the system frequency with said non-integral multiple. each of said legs has a first DC terminal on a positive side and a second DC terminal on a negative side between which a high-side switching device and a low-side switching device of the converter modules are connected; said control apparatus is operative to create command pulses for operating the converter modules; the command pulses are calculated by a PWM, pulse-width modulation, method using a PWM carrier wave (Vcar*) made of a triangular wave whose phase is shifted for each converter module and a modulating wave (Vmjk*) which is based on a moving average of the capacitor voltages (Vcjk) of the converter module; the command pulses have a frequency that is a non-integral multiple of a frequency of the system voltage (VGR, VGS, VGT); and the time increment of the moving average is determined by the reciprocal of the greatest common divisor of said system frequency and the product of the system frequency with said non-integral multiple.