Capacitor voltage balancing strategies for flying capacitor converters.

摘要

The capacitor voltage balancing is the main technical challenge of a multi-level Flying Capacitor Converter (FCC). The capacitor voltages of an FCC diverge from their nominal values, if not properly controlled by a closed-loop capacitor voltage balancing method. This research develops two closed-loop voltage balancing methods for an n-level FCC: (i) A Space Vector Modulation (SVM)-based strategy that takes the advantage of the redundant switching states of an FCC to counteract capacitor voltage drift. In this method, the internal control of the FCC is performed by a SVM switching strategy and conventional PI controllers are used for current control purposes. (ii) A Model Predictive Current Control (MPCC) strategy that is based on a discrete mathematical model of the FCC system. The MPCC strategy does not involve any modulator and PI controllers and perform the FCC internal and external control all in one algorithm. Based on simulation studies, satisfactory performance of a grid-connected four-level FCC under the proposed control strategies, for various operating conditions, is demonstrated.