Grid-forming vs grid-following, and when each one is right.

Both modes hit the same wall

A grid-following loop is already commanding current, so the limit clips a command it was issuing anyway. A grid-forming controller is commanding a voltage, so how it limits is a design choice — saturating the current reference hands voltage control away while the limit binds; a virtual impedance holds it as a voltage source through the fault.

All three codes below legislate that choice, and disagree about how far the guarantee reaches. Finland writes it flat: restricting the current must not interrupt the grid-forming controller, and the limits must match the plant's real capability and use its short-term overload. Switzerland writes nearly the same sentence and scopes it — once the converter reaches the limit, it says, no grid-forming control can be guaranteed, so the requirement holds only below that point. Chile writes it as two thresholds: a phase jump of at least 30 degrees ridden with the limiting functions off, then stable operation with them on. In none of the three is it the vendor's call.

Every number above is about a simulation until someone names the model class that produced it.

Where is grid-forming actually mandatory?

Three jurisdictions, and all three mandates are recent. Chile requires every storage installation interconnecting to the national system to be grid-forming, with the operator able to instruct a change back, and transitional carve-outs that turn on the date the plant energises. Switzerland requires converter-based transmission-connected storage to use grid-forming-capable converters, active whenever connected. Finland requires storage to operate constantly in grid-forming mode, Type C and, by inheritance, Type D. Those are 3 of the 36 codes whose parameter tables are published here — a fact about these tables, not a finding that the other 33 permit both modes. The grid-forming entry names what Great Britain and North America specify instead, and how binding each one is.

For most projects the other direction matters more: 65 of the 66 codes in the ride-through atlas set their envelopes without reference to mode — Chile, which draws a figure for each, is the exception.

How a buyer decides

1. Is black start or islanded operation in scope? Then it is decided: a grid-following converter cannot energize a dead network or hold an island alone. Black start is the one binary here, and it must be specified and tested, never inferred.
2. Otherwise start at the connection point. Short-circuit ratio is fault MVA at the node over the plant's own MW rating, so resizing changes it — double the rating and you halve the ratio. Get it for your point of interconnection from the operator, and ask which definition.
3. Read the code before the datasheet: most write duties both modes must meet, and a few write the mode into the requirement.

That is enough to decide. Code names the mode, or the ratio is at or below the vendor's stated minimum: grid-forming, with the project-specific stability study budgeted. Code silent and the ratio still clear of that minimum in the weakest credible case — after the outage that thins the network, and with the neighbouring converters counted against you rather than ignored — grid-following. Chile writes that second test into law for a grid-following plant, which must verify a short-circuit ratio of at least 3 and an equivalent ratio, counting neighbouring converters against you, of at least 1.5. Between the two it is a study question before a mode question: the weak-grid one. Which model class answers it, and what the answer has to say, is not a detail. A service you intend to sell that prices the capability overrides the ratio.

Then the question that outlives it. Grid-forming is often — not always — the same hardware running different control software; Chile assumes as much, limiting an instructed mode change to software and communications. So ask each bidder whether the unit is grid-forming capable or field-upgradeable, what it costs, what headroom backs the rated response at your ratio, and what type test and model delivery stand behind the answer: a capability answer is worth what its evidence is, and the grid-forming and grid-following entries carry what to demand. A plant studied in one mode must be re-studied for the other.

Whether that headroom must be bought as oversizing belongs to the code and the contract, and the three are not answering about one quantity. Chile makes at least 1.3 pu of rated current for 5 seconds a design requirement, with reactive and active overcurrent margins held against it. Finland and Switzerland say only that the storage capacity need not be overdimensioned for grid-forming control — a different quantity, and no waiver of the current headroom. A reserved state-of-charge band is a market-and-contract matter too, and belongs in the capacity warranty.