Three-level circuit and control method for balancing neutral point voltage of the same

A control method for balancing neutral point voltage of a neutral point clamped three-level circuit, wherein the three-level circuit is for converting a DC voltage into a three-phase AC voltage, wherein the three-level circuit comprises a capacitance bridge arm including upper and lower capacitors connected in series and three switch bridge arms electrically connected to the capacitance bridge arm and being a, b and c phase switch bridge arms, respectively, each of the a, b and c phase switch bridge arms includes at least four switch devices to output a positive level, a zero level, or a negative level, wherein the method comprises:

• step 11, obtaining a first neutral point voltage to represent a voltage difference between the upper and lower capacitors;
• step 12, acquiring a first value range of a duty ratio of a zero level of x phase switch bridge arm through a modulating reference voltage of the x phase switch bridge arm and the DC voltage, wherein x represents a, b and c phase switch bridge arms included in the three-level circuit, and presetting a first set value of the duty ratio of the zero level of the x phase switch bridge arm within the first value range;
• step 13, when an absolute value of the first neutral point voltage is larger than a first threshold value, adjusting the first set value to be within the first value range, distributing a second set value for a duty ratio of the positive level of the x phase switch bridge arm and a third set value for a duty ratio of the negative level of the x phase switch bridge arm on the basis of the adjusted first set value, and modulating the switch devices of the x phase switch bridge arm according to the adjusted first, second and third set values, such that the absolute value of the first neutral point voltage is smaller than the first threshold value.