Patent ID: 7920393

Claim:
A three-level neutral point clamped converter comprising: a first pole; a second pole; a third pole; a fourth pole, each of the first, second, and third poles having a first switch, a second switch, a third switch, and a fourth switch coupled in series between a positive DC bus and a negative DC bus, the fourth pole having a first switch and a second switch coupled in series between the positive DC bus and the negative DC bus, an AC bus being connected between the second switch and the third switch of the first, second, and third poles, a neutral line being connected through a diode between the first switch and the second switch and between the third switch and the fourth switch of the first, second, and third poles, the first and second switches defining an upper portion, the third and fourth switches defining a lower portion; and a controller that switches states of the respective first, second, third, and fourth switches of the first, second, and third poles to produce zero common mode voltage, the fourth pole enabling all neutral point clamped switch states of the first, second, and third poles to be used for producing zero common mode voltage, and modulates the first and second switches of the fourth pole to balance the positive DC bus and the negative DC bus.