PLL (Phase-Locked Loop)
A control algorithm inside a grid-following inverter that continuously tracks the grid voltage angle and frequency, allowing the PCS to synchronise its output with the AC network. PLL performance degrades on weak grids with low short-circuit ratios, which is one reason grid-forming control is gaining adoption.
In energy storage
On a strong grid with a high short-circuit ratio, the PLL locks accurately and the inverter operates stably. On weak grids — common at remote BESS sites with long cable runs to the nearest substation — the PLL can lose synchronisation or oscillate, which is a key technical driver behind the transition to grid-forming inverter control that does not rely on a PLL.