Neutral-point voltage balancing principle of NPC inverter modulated by SPWM injected zero-sequence voltage

The neutral point voltage balancing control of three-level neutral-point-clamped inverter is still a hot researching point, the solutions of which can be classified into two categories: adding accessory hardware circuit and improving modulating means. In respect of size, loss and stability of the system, the latter is more preferred. SVM has a good performance on three-level NPC inverter voltage balancing while the complexity increases largely as a result of the increasing levels, by contrast SPWM injected zero-sequence voltage is much simpler. This paper makes a study on the control mechanism of SPWM injected zero-sequence voltage. It is theoretically proved that only 6n times zero-sequence voltage injected into reference voltages could draw a dc part which can be adjusted by amplitude and initial phase angular of zero-sequence in the neutral point over a fundamental period to do voltage balancing control, neither do the 6n-3 times zero-sequence voltage. Numerical simulations are identical to the conclusion.