Finnish transmission system operator Fingrid has published technical requirements obliging new large wind and solar plants in converter-dominated parts of its network to provide grid-forming control, either through their own inverters or through separate equipment built at or near the site.

Fingrid announced the measure in late Setpember and published the detailed requirements earlier this month, under its VJV2024 grid code for power plants. They apply to type D plants connecting where the equivalent short-circuit ratio (ESCR) at the connection point is below 1.0. Existing plants and projects that already hold a connection agreement are exempt.

Wind generation in Finland is concentrated in such areas, notably along the west coast, and grid security can be disrupted where there is too little synchronous generation, Fingrid said. Its requirements document says grid-following converters in these areas can cause poorly damped voltage oscillations, weak fault ride-through and overvoltages during three-phase faults, risking the disconnection of plants at scale, and that tuning control parameters alone can’t solve the problem.

Developers can choose between batteries, energy-storage-equipped static synchronous compensators (ES-STATCOMs) and synchronous machines such as synchronous condensers. Minimum sizing rises as the grid weakens. A battery must be rated at ±10% of the plant’s capacity where the ESCR is between 0.9 and 1.0, ±20% between 0.7 and 0.9, and ±30% below 0.7. The equivalent thresholds are 7%, 13% and 20% for an ES-STATCOM, and short-circuit power of 33%, 66% and 100% of plant capacity for a synchronous machine.

The equipment must be online whenever the plant runs at 50% of its rated output or more, and in service before the plant first exports power.

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