Limits of the Neutral-Point Balance in Back-to-Back-Connected Three-Level Converters

Abstract

This paper explores the limits of neutral-point current control in back-to-back connected three-level converters. The theoretical analysis used is based on a mathematical neutral-point current model, which can be extended to apply to converters with higher numbers of levels. The low-frequency ripple, which appears in the neutral-point voltage for some operation conditions, can be removed for an extended operating area when two converters are connected back to back to the same dc bus. As a consequence, a lower voltage is applied to the devices, and the value of the capacitors can also be significantly reduced. Some practical graphics are given to represent the design according to the specifications of the application.