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    Abstract: Based on three-level neutral point clamped (NPC) converter, a photovoltaic grid-connected inverter system is designed in this paper. This system allows as many PV strings as possible to be connected to two DC busbars within the power limit of the NPC, and the output voltage shape can operate at a lower switching frequency which could improve efficiency and power quality but with low loss. Firstly, the LLC half-bridge resonant converter is selected to track the maximum power point in DC-DC conversion circuit. Secondly, a balance factor is introduced to control the neutral point voltage, and the dq phase-locked loop is utilized to test the voltage of symmetrical power grid. Thirdly,second-order generalized integrator (SOGI) is to test the voltage dip when the voltage on line side is unbalanced by the method of positive and negative sequence separation control and finally lock the phase. In conclusion, the proposed system could provide an effective networking when the voltage on line-side is tested both rising and dropping.

    Key words: photovoltaic grid-connected; NPC; LLC half-bridge resonant converter; midpoint voltage balance method