Offset-Free Optimal Control of Cascaded H-Bridge Converters Based on a Kalman Filter Harmonic Compensator

Cascaded H-bridge (CHB) converters are suitable candidates for numerous applications, including electrical drives, static synchronous compensators, and battery energy storage inverters. Optimal control strategies for CHB converters have attracted significant interest in recent decades due to their flexibility in including multiple control objectives and their simple design process. However, the steady-state performance of these control strategies deteriorates if the CHB converter model has parameter mismatches and/or the sub-module (SM) capacitor voltage ripples are not measured. This work proposes a Kalman Filter (KF) based strategy to eliminate the steady-state error and undesired low-frequency harmonic components in the CHB converter output currents. The proposed KF strategy estimates the instantaneous arm voltage harmonics representing the converter modelling errors and unaccounted disturbances. Then, these estimated voltage harmonics are used to improve the arm current predictions and obtain a compensation term for the steady-state arm voltage references to be used by the optimal control strategy. Experimental results for three different optimal control schemes are provided for a three-phase CHB converter with nine SMs to confirm the effectiveness of the proposed KF strategy.