Discrete-Time Sliding-Mode Observer for Capacitor Voltage Control in Modular Multilevel Converters

This paper proposes a discrete-time sliding-mode observer (SMO) for equivalent capacitor voltages control in modular multilevel converters, aiming to reduce the communication complexity in applications with several series-connected submodules. The observed voltages of the equivalent capacitors are used in the voltage control loops, resulting in a simpler implementation, which demands low communication effort, since the individual capacitor voltages are not used in the central processor. The proposed SMO was obtained from discrete-time analysis with a stability improvement, differently from discretized observers previously proposed in the literature. A discrete-time methodology is presented to design the observer and ensure the proper operation of the control system. The proposed SMO design methodology in the discrete-time domain enables a simple design of the observer, ensuring transient and steady-state observer performances. Simulation results are presented to demonstrate an example of the proposed methodology, where the observer operates with the control system under distinct conditions. Additionally, experimental results are included to demonstrate the effectiveness of the proposed control system with discrete-time SMO for the equivalent capacitor voltages.