A neutral point balancing controller for three-level inverter with full power-factor range and low distortion

This paper proposes a dynamic neutral-point (NP) potential balancing algorithm for Neutral-Point-Clamped three-level inverters. It is known that the original NP current without NP control cannot contribute to the dc balancing because it is dc-free and dominated by the 3rd harmonic. In our proposal, this online pre-calculated original NP current pattern is modified by different gains dynamically according to the sign, if a dc balancing current is desired. This modified pattern is sent as an instantaneous reference NP current to output the command common mode (CM) voltage using a lookup table. If a strong dc balancing current is desired, the CM voltage is so effectively injected that the maximum NP current equal to the physical limit is obtained at any instant. At low unbalance conditions, the original NP current pattern is only slightly modified so that the amplitude of the resultant CM voltage is quite low, i.e. our NP controller has low impact on the original design (i.e. without NP control). With the help of the proposed novel control loop, the CM voltage is intrinsically changing from quasi-DC injection at high power factors (PF) to a 6th harmonic dominated injection at low PFs automatically and smoothly. This algorithm has superior THD performance and the maximum balancing ability at any power factors, which is proven via a comparison between different NP control algorithms in this paper.