Abstract
Unbalanced voltage sags are common phenomenon in power systems which affects the performance of grid-connected photovoltaic (PV) systems. The unbalanced voltage conditions results in double-line frequency power oscillations. These oscillations increase the dc-link voltage ripple leading to higher stresses in dc-link capacitors. In this case, the literature presents many power control strategies in order to improve the performance of PV systems during unbalanced conditions. Nevertheless, a deep comparison of these strategies has been missing. This work presents a complete analysis of the main power control strategies proposed in the literature. The energy storage requirements of the converter are analytically deduced and some guidelines for the dc-link design are presented. Furthermore, a case study based on a 11 kW PV system is used to compare the performance of the most interesting strategies in terms of dynamic behaviour, power oscillations and dc-link voltage ripple. Furthermore, the dc-link capacitors stress is discussed, considering the characteristics of reactive power injection, which is required by the modern grid codes. The power losses and the thermal stresses in dc-link capacitors are evaluated and the worst operational scenario is obtained for each power control strategy.
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