An LVRT control strategy for reducing DC-link voltage fluctuation of a two-stage photovoltaic multilevel inverter

This paper proposes a control method for reducing the dc-link voltage of a two-stage photovoltaic (PV) inverter under low voltage ride through (LVRT) by injecting reactive power to meet the standard requirement and real power to mitigate the fluctuation in the dc-link voltage without exceeding the current limit of the inverter. The PV generation system investigated in this paper is a two-stage topology consists of a three-level boost converter and a grid-connected three-level neutral point clamped (NPC) inverter. Most grid-connected guidelines inquire distributed renewable power generations (DRPGs) to inject reactive current only under LVRT without restriction on the real power. Thus, during LVRT period, this paper proposes a method which provides minimum-required reactive current and when there is any residual current capacity, the proposed method delivers real power to the grid as much as possible, of course without excess of the inverter current. Consequently, the dc-link voltage increase rapidly at the beginning of the voltage drop and then returns to normal value faster than only providing reactive current. A simulation was conducted to verify the validity of the proposed method.