A new control strategy for megawatt scale multilevel photovoltaic inverters under partial shading

Abstract

In this paper, a new control strategy for megawatt scale multilevel photovoltaic (PV) inverters under partial shading is proposed. In the proposed system, the photovoltaic arrays are divided into zones and each zone is connected to DC to AC inverter. The DC to AC inverters of multiple zones are connected in series to form the required medium voltage and transfer power to the grid. In such a system where all zones are uniformly illuminated, the output voltage vectors of all the inverters are of equal magnitude and in phase with each other. However, under partial shading, the output voltage magnitude and phase angles of individual DC to AC inverters need to be adjusted in such a way that the available real power is transferred to the grid and the power factor of the overall system is as close to unity as possible. A control strategy is developed to operate the series connected multilevel inverter configuration under partial shading, by continuously monitoring the real and reactive powers supplied by each inverter. The developed strategy does not require a central controller and/or communications between the various DC to AC inverter blocks. The proposed control strategy is simulated for a 6.6 kV, three phase utility scale PV system rated at 5 MW.