Three paradigm shifts in microgrid inverters: off-grid, seamless transfer and always grid-forming
Summary: The VGRID POWER technical library divides microgrid inverter evolution into three stages: a simple off-grid inverter, a dual-mode inverter with seamless on/off-grid transfer, and a grid-forming inverter that always behaves as a voltage source. This is not merely a reduction of transfer time from hundreds of milliseconds to 10 ms; the dividing line is whether the control structure changes abruptly. In always grid-forming operation, grid-connected and islanded are only connection states, not operating modes.
The VGRID POWER technical library divides microgrid inverter evolution into three stages: a simple off-grid inverter, a dual-mode inverter with seamless on/off-grid transfer, and a grid-forming inverter that always behaves as a voltage source. This is not merely a reduction of transfer time from hundreds of milliseconds to 10 ms; the dividing line is whether the control structure changes abruptly. In always grid-forming operation, grid-connected and islanded are only connection states, not operating modes.
Stage one: the simple off-grid inverter
Early units were off-grid inverters that converted DC to AC, operated as a voltage source with stable voltage and frequency, and never synchronised with a utility grid. Remote standalone power was the typical application. The limitations were equally clear: no grid backup, low PV utilisation and reliability resting entirely on one device, so a single failure blacked out the whole site.
Stage two: seamless on/off-grid transfer
As distributed PV spread, dual-mode hybrids appeared: a current source following the grid when connected, a voltage source establishing the island when the grid failed. Transfer time fell from hundreds of milliseconds to tens and then to the widely quoted sub-10 ms. Yet grid-following and grid-forming remain two different control laws. At the switching instant the control structure jumps, so a transient is unavoidable: however fast, it is still a switching event rather than a continuous process.
Stage three: always grid-forming
A grid-forming inverter is permanently a voltage source. With the utility present it runs in parallel with other voltage sources; when the utility leaves it continues to establish voltage and frequency, with no change of role. Three qualitative gains follow: no transfer transient, flexible aggregation and separation of microgrids at equal or small phase difference, and natural paralleling with diesel gensets and gas turbines, which are voltage sources too. The white paper summarises seven dimensions - grid operation, island operation, mode switching, genset paralleling, multi-unit paralleling, microgrid interconnection and underlying topology - and only stage three supports paralleling in all conditions and interconnection by design.
Frequently asked questions
Does a sub-10 ms transfer make an inverter grid-forming?
No. The test is whether it remains a voltage source while connected and whether it depends on a PLL to follow the grid. A dual-mode unit still changes control structure, however fast the switch.
Why do grid-forming units pair naturally with diesel gensets?
Both are voltage sources and can be paralleled directly. A grid-following inverter usually needs extra control strategy to operate in island mode alongside a genset.
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FAQ
Does a sub-10 ms transfer make an inverter grid-forming?
No. The test is whether it remains a voltage source while connected and whether it depends on a PLL to follow the grid. A dual-mode unit still changes control structure, however fast the switch.
Why do grid-forming units pair naturally with diesel gensets?
Both are voltage sources and can be paralleled directly. A grid-following inverter usually needs extra control strategy to operate in island mode alongside a genset.
