A Comprehensive Study of Neutral-Point Self-Balancing Effect in Neutral-Point-Clamped Three-Level Inverters

Neutral point (NP) balancing problems in neutral-point-clamped (NPC) three-level inverters have been widely discussed in the past. Most of the previous researches have focused on the active NP control algorithms based on the common-mode duty cycle injection or the differential-mode harmonic current injection. Various active control methods have been proven to work well in real applications. On the other hand, it is well known that sometimes the NP is self-balanced, even without any active NP control. However, due to the uncertainty and lack of the theoretical explanations, active NP controllers are typically preferred for real applications, and the self-balancing characteristic of the NPC topology did not draw much attention in the past. Our paper covers this gap: the mechanisms of self-balancing characteristic are explored systematically. With the help of a precise mathematical model, readers can easily evaluate the self-balancing capability of their own design. In addition, this paper discusses some methods to enhance the self-balancing capability of an existing system. It is shown here that the self-balancing effect depends strongly on the switching frequency. Especially, for high-power insulated gate bipolar transistor/integrated gate-commutated thyristor/gate turn-OFF thyristors (IGBT/IGCT/GTO) applications, the effect is so strong at all power factors that this "do-nothing" control is sufficient and no other active NP controls are needed.