Grid Impedance Characterization To Provide a Robust Phase-Locked Loop Design for PV Systems

The operation of rooftop photovoltaic (PV) systems can be challenged by the electric characteristics of the low voltage (LV) feeders. On this subject, high-impedance LV feeders can result in PVs facing the loss of synchronization due to instability in the phase-locked loop (PLL) unit of their inverter, mainly because PLLs are commonly designed to operate under nominal grid conditions and their performance can be highly affected when the loading of the feeder deviates from the nominal one. One way to avoid such a problem is by characterizing the network impedance at the PV connection point, and use this characterization to provide a robust PLL design. This paper presents a methodology to construct a stochastic representation of the network impedance seen at the PV point of connection. To do so, the distribution of the grid resistance and inductance, under various operational scenarios, is extracted using a Monte Carlo simulation framework and then modeled via a Gaussian distribution. This distribution is employed to obtain an ellipse that embraces the values of the aforementioned resistance and inductance. This ellipse determines the range of the variations of the network impedance to be regarded in the robust design of the PLL.