Comparison Between PLL-Based and PLL-Less Grid-Forming Converters

Grid-forming (GFM) converters are growingly treated as solutions to address the challenges posed by the phase-out of synchronous generators and the growing share of grid-following (GFL) renewables in power systems. In this paper, a comparative study of PLL-based and PLL-less GFM converters is provided. PLL-based GFM converters still rely on the PLL to passively synchronize with the grid, while PLL-less GFM converters use the current-synchronization loop (CSL) to generate an internal voltage reference. The similarities in the structure of both converters enable them to have similar grid-supporting functionalities and current-limiting capability. This comparison helps to reveal the impact of synchronization loops on their dynamic response, synchronization stability and small-signal stability and provides insights about GFM design fulfilling the needs of evolving power systems. All the cases are tested in an experimental hardware setup.