# What counts as genuine grid-forming: four verifiable criteria Being able to run in island mode does not make an inverter grid-forming. The VGRID POWER technical library gives four criteria: permanent voltage-source behaviour, virtual inertia and damping, communication-free paralleling, and flexible interconnection of microgrids. Each can be turned into a tender clause and a site test, preventing a dual-mode inverter from being purchased as a grid-forming unit. Being able to run in island mode does not make an inverter grid-forming. The VGRID POWER technical library gives four criteria: permanent voltage-source behaviour, virtual inertia and damping, communication-free paralleling, and flexible interconnection of microgrids. Each can be turned into a tender clause and a site test, preventing a dual-mode inverter from being purchased as a grid-forming unit. Criterion one: always a voltage source A grid-forming inverter establishes and maintains its own output voltage amplitude and frequency instead of tracking grid voltage through a PLL. A grid-following unit is like a boat drifting with the waves and loses its reference when the grid disappears; a grid-forming unit generates its own reference and stays stable with or without the utility. Ask suppliers for the grid-connected control block diagram, not just an island-mode demonstration. Criterion two: virtual inertia and damping A synchronous machine uses rotor kinetic energy to resist a fast frequency drop, then its governor and damper windings restore steady state. A grid-forming inverter reproduces the rotor swing equation in software and delivers an inertial response to load steps or frequency disturbances. The PQ, VF, droop and VSG modes listed for the LEONA-B750K and H135K are exactly the selectable external characteristics for different operating conditions. Criteria three and four: communication-free parallel and flexible interconnection With droop or VSG control, several inverters share active and reactive power automatically from local voltage amplitude and frequency deviations, with no paralleling communication cable; capacity grows plug-and-play like adding batteries. The documented 2.25 MW multi-unit scheme and the VSG scheme with no stated limit on unit count rely on this. Because two grid-forming microgrids are both voltage sources with inertia and damping, connecting them through a tie line lets them pull their phases into alignment through power exchange, like two coupled pendulums, without a strict synchroniser; after separation each continues to run stably. Frequently asked questions How should a tender specification avoid wordplay? Make the four criteria measurable: grid-connected control block diagram, frequency rate-of-change and recovery time under a load step, measured load sharing of two units without a communication cable, and waveform records of closing and opening two microgrids with a phase difference. Are droop control and VSG mutually exclusive? No. Both achieve communication-free sharing; VSG additionally models rotor inertia and damping explicitly. Product documentation lists them as selectable modes for different conditions. Related equipment and guides [SP1350H54SW](/en/product/sp1350h54sw) · [SP250HAG2T](/en/product/sp250hag2t-2) · [STS200](/en/product/sts200-400-600) · [SP125HCPS](/en/product/sp125hcps) · [All technical guides](/en/blog/category/blog) · [Product catalogue](/en/products)。 ## How should a tender specification avoid wordplay? Make the four criteria measurable: grid-connected control block diagram, frequency rate-of-change and recovery time under a load step, measured load sharing of two units without a communication cable, and waveform records of closing and opening two microgrids with a phase difference. ## Are droop control and VSG mutually exclusive? No. Both achieve communication-free sharing; VSG additionally models rotor inertia and damping explicitly. Product documentation lists them as selectable modes for different conditions.