Let me answer this question from the principle point of view. In the grid connected inverter you have to synchronize the output voltage of the inverter with that of the grid to satisfy the coupling condition with the grid.This means that the inverter voltage must have the same frequency and phase of the grid voltage. The synchronization can accomplished by the PLL not by FLL because the pll minimizes the phase between two periodic wave forms while the FLL minimizes the frequency difference between two periodic wave forms. So the condition of the phase lock in a pll is exactly equal frequencies of the reference and that of the voltage controlled oscillator of the phase locked loop. The residual phase is made very small by increasing the dc loop gain.
On the other side the FLL can not force equal frequencies between the reference and the voltage controlled oscillator. However it can be used in conjunction with the pll for lock acquisition where at larger frequency difference the loop will act as FLL. Once the frequency difference is decreased to the lock in frequency of the pll, the pll automatically goes into lock.
wish you the best
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