PWM Carrier Effects on the Harmonic Distortion of Output Voltage in Single-Delta Bridge Cell MMC

A modular multilevel converter (MMC) based on single-delta bridge-cells is a promising multilevel converter for various high-voltage application. When the cluster voltage synthesis method based on the conventional triangular carrier wave of phase-shifted pulse width modulation (PSPWM) is applied, the quality of the cluster output voltage differs depending on the parity (which is the property of an integer of whether it is even or odd) of the number of cells per cluster. These differences of the output voltage quality are caused by the different effective level of cluster output voltage under the conventional triangular carrier based PSPWM. In other words, the total harmonic distortion (THD) of the cluster output voltage is indirectly related to the number of levels of the cluster output voltage. In this paper, the THD of the cluster output voltage is analyzed according to the relationship between the number of cells per cluster and the type of the carrier. In addition, the recommended carrier type for PSPWM is determined by whether the number of cells per cluster is odd or even. To demonstrate the analysis and comparison in this paper, a full-scale real-time simulation has been conducted.