Abstract
With the neutral-point-clamped (NPC) converter widely applied to direct-drive wind energy generation system, wheather to guarantee the dc-link neutral-point potential (NPP) balance has a crucial influence on the system. Research on gridside NPC three-level converter, firstly, this paper established NPP fluctuation mathematical model, analyzed the mechanism of NPP AC-DC fluctuations, deduced the relationship between NPP error calculation and system operation mode (rectifier-mode or inverter-mode), and established NPP control model. Secondly, the inherent law of NPP balancing control and the controller parameter tuning method were given by theoretical analysis. In order to suppress NPP AC fluctuations at the extreme, this paper proposes a method designing controller parameters according to the largest open-loop cutoff frequency by system frequency characteristic analysis. The NPP control method has the following advantages: effectively suppressing the NPP AC-DC fluctuations at the same time, fast dynamic response of positive and negative dc-link voltage and high utilization rate of DC voltage. Finally, the correctness of the theoretical analysis was verified by MATLAB simulation. Keywords-three-level converter; neutral-point potential; mathematical model; cutoff frequency
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