The Control Algorithm of Three-Level NPC Inverter under Unbalanced Input Voltage Conditions

This paper performs the control algorithm of three-level Neutral-point-clamped (3L-NPC) inverter system including the neutral point voltage balance control under unbalanced voltage conditions because the 3L-NPC inverter as compared with a conventional two-level inverter has an inherent problem of neutral point potential variation. The control of 3L-NPC inverter system under unbalanced input supply conditions is as follows. First, the reference input currents are selected to eliminate the constant component of reactive power and the second-order harmonics of active power, and these are induced by the dc-link voltage control. Second, to operate the inverter system under unbalanced conditions, both positive-and negative sequence component of grid currents should be controlled simultaneously. Finally, to neutral point voltage control, the simplified neutral point voltage balancing method is used to control the neutral point voltage. This method is achieved by the calculated off-set voltage including the neutral point balancing factor. So, the overall control algorithm is composed with a dc-link voltage control, a current control, and neutral point voltage control. The validity of overall control algorithm with the neutral point voltage balancing control method has been proven through the simulation results.