What is the difference between grid-forming and grid-following inverters?

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The key idea is who sets the grid’s voltage and frequency. A grid-forming inverter acts as a voltage source that can create and regulate the grid’s voltage and frequency on its own, which is essential when the microgrid is islanded or temporarily disconnected from the main grid. It can start the system and provide inertia-like support through control methods such as droop control or virtual synchronous machine concepts. In contrast, a grid-following inverter is designed to connect to an existing grid, measure its voltage and frequency, and synchronize to that grid using a phase-locked loop, injecting current in step with the grid. It relies on the grid as the reference and cannot establish voltage or frequency if the grid isn’t there. Therefore, grid-forming’s ability to establish voltage and frequency in islanded mode, versus grid-following’s role of synchronizing to the grid’s existing voltage and frequency, captures the essential difference.