Abstract
This paper proposes a novel control strategy for the two-stage three-phase Photovoltaic (PV) systems to improve the operation under unbalanced grid faults. In case that a three-phase power converter operates under unbalanced grid voltage faults, power related issues, including injected currents, active and reactive power can be negatively affected. In grid-connected systems, the unbalanced faults may lead to Double Grid Frequency (DGF) oscillations of active and/or reactive power or injection of non-sinusoidal three-phase currents. The DGF oscillations of the injected active power can cause similar variations in the dc-link voltage of conventional two-stage PV inverters. In such systems with an electrolytic capacitor in the dc-link, the oscillations of the dc-link voltage with DGF can deteriorate the capacitor lifetime, and thus the entire system. The proposed Low-Voltage Ride-Through (LVRT) control strategy benefits from a reference current generation method, which can eliminate the oscillations at the dc-link and in the active power during unbalanced voltage dips. In addition, a reliable current limiting method is introduced, which can efficiently prevent overcurrent failure under grid faults. Performance of the proposed LVRT strategy is verified on a 2-kW system by simulations under unbalanced voltage dips.
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