The compensation strategy on the drift voltage problem of dual modulation waves based neutral point potential control for three level converter

Even if the dual modulation waves based neutral-point-potential (NPP) control for three-level converters is able to eliminate the low frequency NPP fluctuation, it just has the natural balancing characters for the NPP. Thus, a compensation strategy is necessary to solve the drift voltage problem of the NPP. The traditional compensators overcome the drift voltage problem by adjusting an offset voltage. Since there is no theoretical basis to determine the gain of the offset voltage, it may cause the over-adjustment. It is also difficult to master the law of the output waveform adjusted by the offset voltage. Given these defects of the traditional compensators, a new compensation factor (CF) is raised and the relationship of the CF, the NPP, the phase current and the Dc-link capacitors is analyzed in this paper. By adjusting the CF in a limit region analyzed in this paper, the drift voltage problem is well solved without over-adjustment. Since it is easy to know the law of the output waveform adjusted by the new CF, it can also decrease the switching frequency and avoid the three level modulation degrading to the two level modulation in some operation conditions. Simulation results verify the correctness and effectiveness of the proposed method.