The unbalanced neutral point voltage of three-level inverter can not only cause increased voltage stress on the switching device

but also increase the total harmonic distortion(THD)of the output current. So

the control of the unbalanced neutral point voltage of three-level inverter is always the research focus for scholars. A control strategy that balances the neutral point voltage for three-level neutral-clamped inverter systems is proposed. The proposed control strategy makes continual virtual voltage vectors with finite discrete voltage vectors of three-level inverter. Then

the cost function

which can flexibly control the comprehensive performance of the inverter

is defined according to the control goals including reference voltage and balance of neutral point voltage. The virtual voltage vectors

which track reference voltage and balance neutral point voltage

as well as minimize cost function

can be gotten with the optimization method. This method can not only guarantee good tracking effect of output voltage

but also has strong neutral point voltage balancing and anti-interference ability. Effective verification has been made by simulation and experiment.