Research on neutral-point potential balancing for three-level NPC voltage source inverter

A significant problem with neutral-point-clamped three-level inverters is the fluctuation in the neutral-point voltage. When three-level inverter is loaded, the neutral-point voltage leads to the distortion on output voltage space-vector waveform of the inverter. A novel and simple judge rule based on the virtual space-vector is introduced. The mathematics models of neutral-point circuit and neutral-point potential are developed while the neutral-point potential is unbalanced. And the region of neutral-point potential can be balanced in a sample period is described.