Mathematical Description of 3L NPC Back-to-Back Converter

Abstract

This paper presents a mathematical description of a three-level neutral point clamped (3L NPC) back-to-back converter based on active front-end rectifier (AFE) and voltage source inverter (VSI). This description
allows investigation of electromagnetic processes and analysis of the electromagnetic compatibility (EMC) of the grid – 3L NPC back-to-back converter – load system taking into account the switching functions of AFE and VSI and voltage unbalance in the DC link. The known mathematical models of the 3L NPC back-to-back converter do not provide a block diagram for development of a control system because they use the principle of electrical system separation into subsystems. The developed mathematical model provides a convenient block diagram for the synthesis of a control system neglecting the voltage unbalance in the DC link, a perfect synchronization and switching function linearization. The model is identical to the well-known two-level block
diagram of the frequency converter. Linearization at the rated value of the DC voltage was considered.