Grid-Tied vs Off-Grid vs Grid-Forming (GFL/GFM): What Actually Differs
Summary: The real difference is one question: who provides the voltage and frequency reference? A clear explainer of GFL, off-grid and grid-forming for microgrids.
The difference comes down to one question
The three most confused terms differ in who establishes the voltage and frequency reference.
Grid-tied / Grid-Following (GFL)
When the grid exists, the PCS follows grid voltage and frequency and charges/discharges per EMS — used for peak-shaving and self-consumption. It cannot supply loads alone.
Off-grid
When the grid drops, the PCS powers local loads and must stabilise voltage/frequency itself — used for backup and weak-grid sites. Note: "off-grid" is a system state, while "grid-forming" is a control capability.
Grid-Forming (GFM)
The PCS actively builds the grid reference, supports multi-unit paralleling and works with diesel and PV — ideal for no-grid, island and mining sites. Stable off-grid operation usually requires grid-forming capability.
VGRID grid-forming products
SP125HCPS is a grid-forming microgrid PCS paralleling up to 15 units off-grid; Leona-H135K is grid-forming with 20+ units off-grid; SP250HAG2T all-in-one supports diesel-hybrid and grid-forming. See grid-forming / weak-grid microgrid.
FAQ
Are off-grid and grid-forming the same?
No. Off-grid is a state (no grid); grid-forming is a capability (actively building the reference). Off-grid operation usually requires grid-forming capability.
Why does a weak grid need grid-forming PCS?
Weak-grid voltage/frequency is unstable; a grid-following device struggles, while a grid-forming PCS actively supports the reference and improves continuity.
Is there a paralleling limit?
Limits vary by model and architecture. SP125HCPS parallels up to 15 units off-grid; beyond that, a system-level PCC-controller + contactor scheme is used (e.g. the 2.5MW mine project).
Must an island project be grid-forming?
Islands without a stable utility grid typically rely on grid-forming PCS to build the reference with redundant paralleling and diesel support — e.g. the Maldives island project.
