MODULAR MULTILEVEL CONVERTER WITH STATE- FEEDBACK CONTROLLER FOR HARMONICS SUPPRESSION

Abstract

Control of submodule capacitor voltages and the circulating current within modular
multilevel converter (MMC) is essential to MMC performance. The circulating current has
an influence on the losses, rating of the power devices and voltage ripple. The circulating
current tends to have csurrent harmonics due to the voltage disturbance on the submodule
capacitor voltages. To decrease the influence of the harmonics for an improved control of
the submodule capacitor voltages, a voltage control approach by suppressing the current
produced by the voltage disturbance is introduced. The approach is implemented using a
virtual-impedance compensation that increases the voltage reference of the averaging
voltage control of the circulating current reference in order to decrease the impact of the
disturbance. The improved capacitor voltage control produces a new circulating current
reference to a state-feedback controller provided to control the measured circulating
current using a linear quadratic regulator to shape the gain of the controller. The main
advantage of the approach is that it allows wideband ripple suppression without having
knowledge of the harmonics frequencies; this is unlike the usual multiresonant controllers
which 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 conventional
proportional integral controllers. The results from the proposed method and the
conventional method are shown in a simulation. Finally, the experiment results are
presented to verify the simulation.