Active/reactive power control of photovoltaic grid-tied inverters with peak current limitation and zero active power oscillation during unbalanced voltage sags

This paper proposes an analytical expression for the calculation of active and reactive power references of a grid-tied inverter, which limits the peak current of the inverter during voltage sags. The key novelty is that the active/reactive power references are analytically calculated based on the dc-link voltage and grid codes, while they do not depend on the implemented current reference calculation algorithm and, as a general formulation, can be implemented in combination with various current reference calculation algorithms. Furthermore, based on the inverter nominal current and the injected reactive power to the grid during voltage sags, an analytical algorithm is introduced for the calculation of the active power reference, which can be extracted from PV strings. The proposed algorithm ensures that the maximum current capability of the inverter is used for the enhancement of the grid voltages during voltage sags, while it always complies with the reactive power injection requirement of grid codes and avoids increasing the dc-link voltage excessively. An unbalanced current injection algorithm is also applied for the grid-tied inverter which results in zero active power oscillation. Experimental results of a grid-connected 3.3- kVA, three-level, neutral-point-clamped inverter laboratory prototype are presented to demonstrate the effectiveness of the proposed controller.