In this paper, it is proposed a robust Second-Order Super-Twisting Sliding Mode Control for a Grid-connected inverter of a PV power plant. The proposed control strategy is able to maximize the energy extracted from the PV while regulating the DC-link voltage and achieving a power factor compensation and the reduction of currents harmonic distortion. Both in balanced or unbalanced condition, the grid is supplied by the PV plant with the desired active power and with the developed strategy, it is possible to use the inverter as a reactive power compensator. In order to prove the goodness of the proposed algorithm in terms of high efficiency and performance, some simulated and experimental results are carried out.