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The principle of a diode neutral point clamped three-phase three-level inverter with neutral line was analyzed, and the mathematical model of the main circuit is established on the basis of the operation principle. The oversample and multi-time calculation control strategy was proposed to reduce the typical delay in digital control and improve the system calculation accuracy, lower the inverter output voltage THD. In order to further improve the inverter output voltage waveform, the logic external execution was also proposed. The proposed control method was verified on a 20kVA inverter controlled by microchip processor IC30F5015 DSP. The experiment results indicated that the THD of the inverter output voltage is decreased considerably.

With the NPC converter widely applied to distributed generation system, the ability to ensure the DC bus neutral-point potential (NPP) balance has a crucial impact on the system. Research on grid-connected NPC three-level converter, in this paper, mathematical model of NPP fluctuation is established firstly, the mechanism of NPP ac-dc fluctuations is analyzed, the relationship between NPP error calculation and system operation mode (rectifier-mode or inverter-mode) is deduced, NPP control model based on equalizing loop is established. Secondly, the inherent relationship for NPP balancing control and controller parameter tuning method are given by theoretical analysis. In order to maximize suppress NPP AC fluctuations, the paper proposes a method that controller parameters is designed according to the largest open-loop shear frequency by system frequency characteristic analysis. The NPP Control method has the following advantages: effectively suppressing of NPP ac/dc fluctuations at the same time, positive and negative DC bus voltage fast dynamic response, high utilization rate of dc voltage. Finally, the correctness of the theoretical analysis is verified by MATLAB simulation.