The aim of the present paper is to describe a new control strategy for a three level NPC converter connected to the grid, characterized by its high efficiency. This efficiency is achieved using a square wave modulation or SWM, which has the advantage of a quasi absence of switching losses. The produced active and reactive power can be controlled by voltage magnitude adaptation in the input of the inverter, together with a phase shift between the grid and output inverter voltages. Therefore, the phase-locked loop (PLL) can be considered to be an important part of the power generation system connected to the grid. The contributions of the paper include an approach to achieve a robust PLL, a discussion of the properties and operation performance of the new control strategy, as well as the corresponding characteristic curves. The performance will be illustrated through simulation examples as well as experimental results.