The Case Against Phase-Locked Loops in Weak AC Grids

With increasing deployment of inverter-based sources in microgrids, inverter control methods are constantly being modified and improved. What remains constant, however, is the phase-locked loop (PLL). Regardless of the control technique adopted and the type of microgrid, a PLL is used to synchronize the output PWM signal of the inverter with the main grid or other inverters. But as penetration of inverter-based sources increases and grids become weaker, the impact of PLLs on controller behavior becomes more pronounced. The issues caused by these influences are described in this paper to make a case for finding a solution for inverter synchronization that better fits the needs of inverter-dominated microgrids than PLLs. Simulation results from a study of a low-voltage microgrid supported by parallel inverters are also presented to demonstrate some of these characteristics.