Low-voltage ride-thorough capability of photovoltaic grid-connected neutral-pointclamped inverters with active/reactive power injection

Low-voltage ride-thorough capability is among the challenges in the operation of medium- and large-scale gridconnected photovoltaic power plants (PVPPs). In addition, reactive power injection during voltage sags is required by power system operators in order to enhance the voltage of the point of common coupling. The performance of medium- and largescale grid-connected PVPPs during these events is studied. An algorithm for the calculation of current references, in the dqframe, during voltage sags is introduced, which considers the inverter current limitation, grid code requirements and the amount of extracted power from photovoltaic strings. The proposed algorithm uses the full current capacity of the inverter in injecting active or reactive powers to the grid during voltage sags, which leads in a better grid voltage enhancement. The performance of proposed control strategies is investigated on a 150-kVA PVPP connected to the 12.47-kV medium-voltage test-case system simulation model during different fault conditions. An experimental setup of the 3.3-kVA grid-connected three-level neutral-pointclamped inverter with a dc/dc converter illustrates and validates the performance of the controller in injecting required active/ reactive power and supporting the network voltage.