Analysis and Control of Neutral-Point Voltage for Transformerless Three-Level PV Inverter in LVRT Operation

The modulation strategy LMZVM (large, middle, and zero vector modulation) can be widely adopted in the three-level 7 photovoltaic (PV) inverter, because it reduces the ground current and produces little neutral-point (NP) voltage in normal operation. However, in low-voltage ride-through (LVRT) operation, the NP voltage fluctuation becomes large and has not been researched nowadays. In this paper, according to this problem, four critical weighing factors about the NP voltage based on symmetrical component method are proposed. We can see that the maximum peak-to-peak value among all dip types happens when the single-phase grid voltage dips to 0. Then, an original balancing strategy, which replaces the zero vector with two opposite P-type or N-type small vectors of equal duty cycle, is put forward. Experimental results are provided to validate that the proposed strategy can reduce the NP voltage fluctuation in LVRT operation.