A model-based controller for a single-phase n-level CHB multilevel converter

Abstract

This work introduces a generalized methodology for mathematical modelling and controller design of a single-phase cascaded H-bridge multilevel-converter comprising the connection of N H-bridges, which results in 2N+1 voltage levels. Towards this end, the average model of the converter is first transformed into a decoupled mathematical structure based on a new definition of state variables. Out of this, the control objectives, current tracking and voltage regulation, are decoupled from the voltage balance problem for such a general case of N H-bridges. To show the benefits of the proposed methodology, a shunt active power filter application has been considered in this paper, where the control objectives are the compensation of the power factor and harmonic distortion due to nonlinear loads, plus regulation and balance of all DC-bus voltages. In particular, this work focus on the balance issue, which leads to the design of N-1 balance loops. A tuning strategy is also introduced to design the gains of the proposed control scheme, which are based on time scale separation and a desired closed-loop transient performance. The proposed generalized methodology is evaluated in a 7-level cascaded H-bridge converter both through simulations and experimentally.