MODULAR MULTILEVEL CONVERTER WITH STATEFEEDBACK CONTROLLER FOR HARMONICS SUPPRESSION

Control of submodule capacitor voltages and the circulating current within modularmultilevel converter (MMC) is essential to MMC performance. The circulating current hasan influence on the losses, rating of the power devices and voltage ripple. The circulatingcurrent tends to have csurrent harmonics due to the voltage disturbance on the submodulecapacitor voltages. To decrease the influence of the harmonics for an improved control ofthe submodule capacitor voltages, a voltage control approach by suppressing the currentproduced by the voltage disturbance is introduced. The approach is implemented using avirtual-impedance compensation that increases the voltage reference of the averagingvoltage control of the circulating current reference in order to decrease the impact of thedisturbance. The improved capacitor voltage control produces a new circulating currentreference to a state-feedback controller provided to control the measured circulatingcurrent using a linear quadratic regulator to shape the gain of the controller. The mainadvantage of the approach is that it allows wideband ripple suppression without havingknowledge of the harmonics frequencies; this is unlike the usual multiresonant controllerswhich are restricted to the particular knowledge of frequencies. From the results obtained,the control using the proposed method has the advantages of an improved voltage control,reduced capacitor voltage ripple and harmonics compared to the conventionalproportional integral controllers. The results from the proposed method and theconventional method are shown in a simulation. Finally, the experiment results arepresented to verify the simulation.