Why Midpoint Balancing of an NPC Is Still a Challenge in Medium-Voltage Applications

In medium-voltage applications, neutral-point clamped converters (NPCs) are often utilized with optimized pulse patterns. While their utilization improves the current quality significantly without impacting the switching losses, the midpoint balancing becomes a challenge. This paper analyzes the known midpoint balancing methods for NPCs which utilize optimized pulse patterns. Furthermore, it derives a novel model capable of predicting the usability of these balancing methods in various operation points. The model demonstrates that currently, none of the investigated publicly-available methods is capable to reliably balance the dc-link capacitors in operation points with purely inductive power factor, which is also validated by a time-domain simulation.