Neutral Point Potential Control Based on Second Order Harmonic Current Injection of Hysteresis Controlled 3-Level AC/DC Converter

The neutral point potential (NPP) drifting problem of 3-level neutral point clamped (NPC) converter is usually solved by adding extra feedback loop into the control system. Methods based on zero sequence DC current or modulation wave injection are simple and effective solutions. But in this paper, it was found that these commonly used methods are hard to apply in 3 phase 3 wire (3P3W) case when hysteresis control strategy is adopted. So this paper proposes a novel method, which achieves NPP control by dynamic second order harmonic current injection. Computing power requirement of this method is low, and it is compatible with 3P3W and 3P4W NPC converters under both linear modulation and hysteresis control strategies. Principle, characteristics and parameter synthesis of the proposed NPP control method are introduced in this paper. Experiment results verifies effectiveness of it.