Improved SVPWM strategy based on neutral-point charge balance for three-level neutral-point-clamped converter

The three-level neutral-point-clamped (NPC) voltage-source inverter has been widely used in many areas. However the problem of neutral-point (NP) voltage imbalance has not been solved well. An improved modulation strategy based on the nearest three-vector (NTV) modulation method is proposed in this paper. In the regions where two pairs of redundant short vectors are contained, the neutral-point charge caused by short vectors in a sampling period is calculated to select appropriate switch sequence. In the regions where the medium vectors is used, the adjacent large vectors are adopted to reduce the action time of the medium vectors when neutral-point voltage cannot be balanced,. The proposed method in this paper can effectively reduce the neutral-point voltage ripple under any modulation indexes and power factors. The simulation results verify the validity of the proposed strategy.