Grid-Forming Inverters

Grid-forming inverters (Swedish: nätformande omriktare) are power electronic converters that establish their own internal voltage and frequency reference, allowing them to create a stable grid signal rather than merely following an existing one. They are a critical enabling technology for power systems with high shares of inverter-based resources (IBR) — wind, solar, and battery storage.

The core distinction

All large-scale wind, solar, and battery connections to the grid use power electronic inverters to convert between DC and AC (or between AC frequencies). These inverters currently come in two fundamental types:

Grid-following (nätföljande)

The current industry norm. A grid-following inverter:

• Measures the existing grid voltage and frequency using a phase-locked loop (PLL)
• Synchronizes its output to match what it measures
• Is fast and efficient under normal conditions
• Cannot operate in isolation — it requires an existing voltage reference to lock onto
• At very high IBR penetration, creates a circular dependency: all devices are following each other, with nothing providing a stable reference

Grid-forming (nätformande)

A grid-forming inverter:

• Contains an internal voltage and frequency model (e.g., a virtual synchronous machine algorithm or droop control)
• Synthesizes a voltage waveform from its internal reference and imposes it on the network
• Can energize a de-energized grid segment and create the voltage reference that other devices follow
• Provides properties analogous to a synchronous generator: inertia response, voltage support, damping
• Can function in weak grids and during disturbances where grid-following inverters would lose synchronization

Why this matters — the transition challenge

Traditional power systems are anchored by synchronous generators (hydro, nuclear, thermal): large rotating machines physically coupled to the grid, whose rotational inertia naturally resists frequency changes and whose electromagnetic properties provide built-in voltage regulation and damping.

As synchronous generators are displaced by IBR, the system loses:

• Inertia: frequency deviates faster after a generation/load imbalance (higher rate-of-change-of-frequency, RoCoF)
• Voltage regulation: synchronous machines automatically absorb or inject reactive power; IBR must be explicitly programmed to do so
• Synchronization strength: the “short-circuit ratio” at any node declines, making the grid weaker and more susceptible to inverter instability
• Oscillation damping: synchronous machines provide inherent inter-area oscillation damping; IBR does not unless designed to

Grid-forming inverters can substitute for these properties if designed and controlled appropriately. They do not replicate the physics — but they can replicate the functional behavior that the system needs.

Five stability categories

Source - Svk Driftsäkerhet Augusti 2025 organizes stability challenges into five categories:

Inverter stability is the newest and least-well-understood category, arising specifically from the interaction of many grid-following IBR devices in a low-synchronous-generation environment.

Six system needs

Related to the stability framework, Svk structures operational security requirements around six system needs:

1. Active power balance — continuous match of generation and load; managed via frequency reserves (FCR, aFRR, mFRR, FFR)
2. Reactive power balance — voltage control; managed via synchronous generators, STATCOMs, shunt reactors, and reactive tariff incentives
3. Synchronization — common frequency/angle reference across all connected devices; historically provided by synchronous machines; increasingly dependent on grid-forming inverters
4. Damping — suppression of electromechanical oscillations; provided by power system stabilizers (PSS) on synchronous machines and potentially by active damping algorithms on grid-forming inverters
5. Fault tolerance — N-1 criterion; survival of single-component failure without cascade
6. Restoration — black start and island reconnection capability; requires grid-forming capable sources to energize segments without an external reference

Nordic TSO coordination — the CONDON position

In November 2025, the four Nordic TSOs (Fingrid, Energinet, Statnett, and Svenska kraftnät) published a joint position through CONDON — their coordinating body — establishing a common definition and implementation framework for grid-forming requirements across the Nordic synchronous area. (Source - CONDON Nordic Position on Grid-Forming (2025))

Urgency: In 2022, for the first time in Nordic history, more than half of power during peak renewable penetration came through converters. By 2030, converter-connected capacity is expected to more than double. CONDON frames this as a present-tense vulnerability, not a future one.

CONDON’s GFM definition: A control strategy enabling power-electronic interfaced devices (PEIDs) to function as a controlled voltage source behind an impedance with self-synchronization capability. This distinguishes GFM from grid-following (PLL-based current source, cannot operate without an existing reference) and island mode (generates a waveform but cannot self-synchronize with other voltage sources).

The four key behaviors a GFM converter must exhibit:

International alignment: The CONDON definition aligns with ACER/ENTSO-E (voltage source behind impedance), the InterOPERA HVDC/PPM specification, the German 4-TSO paper, NERC BESS white paper, AEMO voluntary specification, and GB grid code GC0137.

Testing framework: CONDON is developing standardized test protocols using envelope curve methodologies, harmonized across all four Nordic countries — strengthening the common negotiating position with manufacturers.

Svk’s requirements development roadmap

Svenska kraftnät’s requirements development is phased by technology type, now coordinated across all four Nordic TSOs under the CONDON framework:

Svk’s HVDC specification, already in use, is quoted in the CONDON report: “The GFM control mode shall enable dynamic voltage magnitude and phase control similar to a controllable voltage source behind an impedance.”

For wind and solar, GFM requirements will follow the evolution of EU network codes (NC RfG, NC HVDC). CONDON commits that test scenarios and parameters will be “fully developed” by the time future network codes are introduced, so requirements can be implemented immediately on code entry into force.

(Source - Svk Driftsäkerhet Augusti 2025, Source - CONDON Nordic Position on Grid-Forming (2025))

Relationship to existing stability support

Grid-forming inverters do not replace all existing stability mechanisms but complement them:

• FFR (Fast Frequency Response): battery-based FFR has been procured by Svk since 2020 and addresses the high-RoCoF problem; grid-forming batteries would provide both FFR and synchronization support simultaneously
• STATCOMs: three installed in the Swedish transmission system as of 2025; address reactive power / voltage support; grid-forming inverters could partially substitute for STATCOMs at generation/storage connection points
• Shunt reactors: passive reactive compensation; addresses steady-state reactive needs; not a substitute for grid-forming
• FCR-D/N: frequency containment reserves revised 2023; grid-forming devices could qualify as FCR providers while also providing synchronization support

Relevance to other wiki topics

• Island Operation — grid-forming inverters are essential for ö-drift capability; a grid-following inverter cannot black-start a segment
• Svenska kraftnät — Svk’s requirements development program is an active regulatory process
• Balancing Markets — grid-forming batteries may qualify for multiple reserve products simultaneously, changing the economics of battery investment
• NordSyd — NordSyd’s HVDC segments will be subject to the first wave of grid-forming requirements
• Electric Power Transmission — grid strength, short-circuit ratio, and stability framework context

Data gaps

• Svk’s published timeline for HVDC grid-forming specifications — CONDON (November 2025) confirms requirements already being introduced for HVDC, STATCOM, and BESS (Source - CONDON Nordic Position on Grid-Forming (2025))
• Whether any Nordic TSO has already mandated grid-forming for specific projects — CONDON confirms requirements already being introduced; Svk HVDC spec quoted in the report
• EU-level harmonization — CONDON confirms alignment with ACER/ENTSO-E and InterOPERA; wind/solar requirements will follow NC RfG/NC HVDC evolution
• Technical performance metrics — CONDON envelope curve testing methodology; testing frameworks nearing completion for HVDC/FACTS/BESS