Stability-Oriented Design of Frequency Drift Anti-Islanding and Phase-Locked Loop Under Weak Grid

The stability of a grid-tied, power conditioning system (PCS)-equipped with frequency drift anti-islanding-is analyzed in this paper. To reduce the nondetection zone (NDZ), the positive feedback gain (PFG) of the frequency drift method should be increased. However, it was reported that PFG is restricted due to stability issues of the system under a weak grid. Careful analysis revealed that such a restriction is also related to the parameters of the phase-locked loop (PLL). As the parameters of PLL are represented by the damping coefficient and natural frequency, which demonstrate the second-order filter characteristics, the investigation shows that the damping coefficient has a close relation to the maximum PFG for a stable operation of the system in the weak grid. To enhance the stability margin and increase PFG, an alternative method is proposed to estimate grid frequency. Since the proposed method reduces the negative resistance region in PCS, it can be stabilized more than the conventional estimation method. Furthermore, the PFG can be increased to reduce NDZ.