MODULAR MULTILEVEL CONVERTER WITH HARMONICS COMPENSATION FOR STATE-FEEDBACK CONTROLLER

Abstract

The voltage variations among the sub-module (SM) capacitors result in harmonics
superimposed unto the circulating current. The conventional control of the SM voltages uses
a closed-loop proportional Integral (PI) controller based on carrier shifted pulse width
modulation (CSPWM). The PI-controller provides a dc reference circulating current for the
control of the modular multilevel converter (MMC) measured circulating current. To
decrease the influence of the voltage variation and improve the voltage control of the SM
capacitors with harmonics elimination, this paper proposes a simple but an improved
circulating current control within the MMC using a state-feedback controller. The proposed
method employs state-feedback controller shaped by a linear quadratic regulator for a better
reference tracking. The voltage disturbance in the PI-control loop is considered and
disturbance rejection by feed forward compensation concepts is employed in the proposed
scheme to produce an opposing compensation current to the circulating current reference.
The feed forward current produced in the proposed method opposes and cancels the ac-
harmonics due to the voltage disturbance/variations with simplicity and without the use of
any additional filters or harmonic controllers such as resonant controllers. The MMC
performance in the conventional method and proposed method is presented in a simulation.
Finally, results from experiment are given to verify the simulation.