A solar power conversion system comprising: a photovoltaic array having photovoltaic modules for generating direct current power; a power converter for converting the DC power to alternating current power; a transformer coupled between the power converter and a power grid for transmitting the AC power to the power grid, wherein the transformer is connected to the power grid at the point of common coupling and to the power converter at output terminals; a reactance estimation module configured to estimate 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; a maximum reactive power estimation module to estimate a maximum reactive power based on the estimated reactance, the measured voltage at output terminals, and the measured reactive power at the output terminals, wherein the maximum reactive power estimation module estimates the maximum reactive power based on following equation: Q MAX t ≈ Q t + V MAX − V t ⋅ V t X ⋅ 1 − X ⋅ Q t V 2 t where Q MAX (t) is maximum reactive power at time t, Q(t) is measured reactive power at time t, V MAX is the maximum allowable voltage by the power converter at time t, V(t) is the measured voltage V at time t and X is the short circuit reactance; and a controller for generating switching command signals for the power converter based on the measured voltage at output terminals and the estimated maximum reactive power. a photovoltaic array having photovoltaic modules for generating direct current power; a power converter for converting the DC power to alternating current power; a transformer coupled between the power converter and a power grid for transmitting the AC power to the power grid, wherein the transformer is connected to the power grid at the point of common coupling and to the power converter at output terminals; a reactance estimation module configured to estimate 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; a maximum reactive power estimation module to estimate a maximum reactive power based on the estimated reactance, the measured voltage at output terminals, and the measured reactive power at the output terminals, wherein the maximum reactive power estimation module estimates the maximum reactive power based on following equation: Q MAX t ≈ Q t + V MAX − V t ⋅ V t X ⋅ 1 − X ⋅ Q t V 2 t where Q MAX (t) is maximum reactive power at time t, Q(t) is measured reactive power at time t, V MAX is the maximum allowable voltage by the power converter at time t, V(t) is the measured voltage V at time t and X is the short circuit reactance; and a controller for generating switching command signals for the power converter based on the measured voltage at output terminals and the estimated maximum reactive power. where Q MAX (t) is maximum reactive power at time t, Q(t) is measured reactive power at time t, V MAX is the maximum allowable voltage by the power converter at time t, V(t) is the measured voltage V at time t and X is the short circuit reactance; and a controller for generating switching command signals for the power converter based on the measured voltage at output terminals and the estimated maximum reactive power.