A new capacitor voltage balancing control for hybrid modular multilevel converter with cascaded full bridge

Abstract

Recently, a hybrid modular multilevel converter with cascaded full bridges (HMMC-CFB) is proposed. This converter has mainly two parts: a director part (DP) and a wave shaping part (WSP). The DP consists of a conventional modular multilevel converter (MMC) with half bridge submodules (HBS). The WSP is a series connection of full bridge submodules (FBS) and attenuates the voltage harmonics produced by DP. The energy exchanged by WSP changes with modulation index and causes improper capacitor voltage regulation. For satisfactory operation of HMMC-CFB, the total capacitor voltage of WSP should remain constant. This paper presents an analysis of energy exchanged by WSP and proposes a new control technique to regulate the total dc voltage of WSP. In the proposed control, the energy exchanged by WSP is controlled by changing the slope of the voltage reference signal of DP. In addition, analysis of circulating current of HMMC-CFB is presented. A circulating current suppressing controller (CCSC) is used to control the second harmonic component. The performance of the proposed control technique for the standalone model of HMMC-CFB is evaluated by using time domain simulation studies in the PSCAD software environment. To test fault blocking capability of converter, a grid connected model is built in PSCAD and analyzed for dc fault tolerant capability.