This manuscript presents results of study and research of synchronous adjustment in the overmodulation control zone of three neutral-clamped inverters (NCIs) of the specific configuration of three-phase grid-connected photovoltaic system. It has been proved, that the corresponding modification of techniques and algorithms of pulsewidth modulation (PWM) for control of NCIs, assure symmetry and advanced harmonic composition of inverter-side winding voltage of multi-winding power transformer, thereby helping to reduce losses in windings of the transformer, and to improve the efficiency of photovoltaic installations.