A solar power conversion system includes a photovoltaic array having photovoltaic modules for generating direct current (DC) power. A power converter is provided in the system for converting the DC power to alternating current (AC) power. A transformer is coupled between the power converter and a power grid for transmitting the AC power to the power grid. The transformer is connected to the power grid at the point of common coupling (PCC) and to the power converter at output terminals. A reactance estimation module is provided in the system for estimating a short circuit reactance at PCC based on a small change in a measured voltage at output terminals with respect to a small change in a measured reactive power at the output terminals. Further, a maximum reactive power estimation module estimates a maximum reactive power based on the estimated reactance, the measured voltage at output terminals, and the measured reactive power at the output terminals. A controller in the system generates switching command signals for the power converter based on the measured voltage at output terminals and the estimated maximum reactive power.