Progressive Natural Balance of Neutral-Point Voltage of Three-Level NPC Inverter With a Modified SVM Scheme

It is well known that the neutral point voltage of three-level NPC (neutral point clamped) inverter tends to deviate during operation. The voltage deviation can cause uneven voltage distribution among switching devices and additional harmonic distortion. The problem is normally mitigated by a closed loop control of the neutral point voltage. In this paper, the problem is fully analyzed from mathematical point of view. It is proven mathematically that the neutral point voltage can be naturally balanced from initial unbalanced neutral-point voltage when the inverter output PWM (pulse width modulation) sequences do not contain even-order harmonics, non-zero load resistance and different three phase switching states. The experimental results are provided to verify the theoretical analysis.