Ui-Min Choi, Frede Blaabjerg, and Kyo-Beum Lee, "Independent Control Strategy of Two DC-link Voltages for Separate MPPTs in Transformerless Photovoltaic Systems using Neutral-Point-Clamped Inverters," in Proc. APEC Conf., Mar. 2014, pp. 1718–1724. > Paper

To improve the efficiency of the photovoltaic (PV) system, the centralized topology using three-level inverters are widely used. In this

system, PV modules are separately connected to the split DC-links. This causes a decrease of maximum power point tracking (MPPT)

efficiency under the partial shading condition. This paper proposes an independent control of two DC-link voltages for separate

MPPT of each PV module in three-level inverters. The proposed method is simply implemented by adding or subtracting the time-

offset to the three-phase turn-on times and modifying the reference voltages without complex calculations or any additional

hardware. Simulations and experimental results verify the validity and feasibility of the proposed methods.