A bipolar two-stage photovoltaic system based on three-level neutral-point clamped converter

This paper proposes a two-stage photovoltaic (PV) system for large-scale grid-connected applications. The proposed PV system consists of multiple dc-dc boost converters and one large inverter. The inverter is based on the neutral-point clamped (NPC) technology with a grounded dc-link midpoint. This enables a bipolar structure and doubles the net dc voltage, while the 600-V North American standard is respected. The boost converters independently control the dc voltages of their corresponding PV arrays, while their output voltages are regulated by the inverter. Further, they limit the dc-link voltage of the inverter if the power cannot be dispatched to the grid, for example, due to network faults or failure of the inverter. The proposed PV system also offers an enhanced MPPT performance and energy yield, due to its multi-MPPT capability. The NPC technology permits the employment of low-voltage switches for the inverter, despite the doubled net dc voltage. Principles of operation, mathematical model, and control schemes of the proposed PV system are described in the paper. Further, the performance of the proposed PV system is demonstrated through time-domain simulations and study cases that include system startup, normal operation, shading and partial shading scenarios, and various faults.