Source - Svk Driftsäkerhet Augusti 2025

Svenska kraftnät’s report on grid operational security (driftsäkerhet) in the Swedish and Nordic power system, published August 2025. Covers the technical challenges arising from the transition to a high-inverter-based resource (IBR) system and Svk’s planned requirements development.

Bibliographic details

• Title: Driftsäkerhet i kraftsystemet — rapport augusti 2025
• Publisher: Svenska kraftnät
• Type: Technical report
• Raw file: raw/svk/rapport_driftsakerhet_i_kraftsystemet_augusti_2025.pdf

Core framing: six system needs

The report organizes operational security requirements around six fundamental system needs that any power system must satisfy:

These six needs map onto five stability categories used in academic and regulatory literature: frequency stability, voltage stability, angular stability, resonance stability, and inverter stability (the last being the newly emphasized category).

Grid-forming vs grid-following inverters

The central technical distinction in the report:

Grid-following (nätföljande omriktare)

• The dominant mode for all current wind, solar, and battery inverters
• Synchronizes by measuring the grid voltage and matching it via a phase-locked loop (PLL)
• Cannot function in isolation — requires an external voltage reference to lock onto
• At high IBR penetration, if grid-following devices dominate, the grid loses the stable voltage reference that all devices depend on (circular dependency)
• Cannot contribute to the synchronization system need in a meaningful way; is a “consumer” of grid stability

Grid-forming (nätformande omriktare)

• Creates its own internal voltage reference and imposes it on the network
• Synchronizes via an internal algorithm (e.g., virtual synchronous machine, droop control) rather than by measuring the grid
• Can form a voltage reference in a weak or de-energized grid segment
• Supports island operation and black start
• Can contribute active damping of oscillations
• Not the current norm: essentially no large-scale grid-forming inverters are operational in the Nordic system as of 2025

Why this matters now

As synchronous generation (hydro, nuclear, thermal) is displaced by IBR, the system loses:

• Rotational inertia (which naturally resists frequency changes)
• Built-in voltage regulation from synchronous machines
• Automatic damping from physical machine behavior

Grid-forming inverters can substitute for these properties if properly designed and controlled — but require explicit technical requirements from Svk.

Svk’s grid-forming development roadmap

Svk is developing mandatory grid-forming requirements in phases:

1. HVDC connections first — new HVDC interconnectors and large HVDC projects (including NordSyd’s HVDC segments) will be required to meet grid-forming specifications
2. Large batteries — grid-scale battery storage connected at transmission level
3. Offshore wind — large offshore wind parks (which connect via dedicated cable and converter stations)
4. Onshore wind and solar — the final phase; this represents the bulk of IBR capacity

The rationale for sequencing HVDC first: HVDC converters are already custom-engineered projects where design changes are feasible; the technical capability exists; and HVDC makes the highest per-unit contribution to system strength.

Existing stability support mechanisms

Frequency regulation

• FCR-D (Frequency Containment Reserve for Disturbances): activated when frequency deviates >0.1 Hz from 50 Hz
• FCR-N (Frequency Containment Reserve for Normal operation): continuous regulation within ±0.1 Hz
• Both FCR products were revised in 2023 with updated symmetry and response requirements
• FFR (Fast Frequency Response): ultra-fast reserve (< 2 seconds); battery-based FFR has been procured since 2020; growing share of FFR provision from batteries vs synthetic inertia from wind/hydro

Reactive power and voltage

• Three STATCOMs installed in recent years at key transmission nodes (specific locations named in report)
• Large batch of shunt reactors contracted in 2024, covering needs through approximately 2032
• From 2027: Svk will introduce a reactive power tariff incentive — network users who provide reactive power compensation receive a tariff credit; users who fail to meet reactive power obligations face fines (tariff additions)
• This tariff mechanism addresses the growing need for distributed reactive compensation as power flows intensify under new load patterns

N-1 criterion

The Swedish transmission system is planned and operated to the N-1 standard: loss of any single component (generator, line, transformer) must not cause cascading failure or unacceptable voltage/frequency deviations. NordSyd investments are partly driven by the need to maintain N-1 compliance as flows increase.

Relevance to wiki

• Grid-Forming Inverters — primary source for the concept page; defines the grid-forming/grid-following distinction and Svk’s requirements roadmap
• Svenska kraftnät — operational security program; reactive power tariff from 2027; STATCOM investments
• Balancing Markets — FFR procurement history; FCR requirement revisions 2023; battery participation in frequency reserves
• Island Operation — ö-drift capability as one of the six system needs; grid-forming inverters as enablers
• NordSyd — N-1 criterion context; HVDC grid-forming requirements apply to NordSyd HVDC segments
• Electric Power Transmission — stability categories; six system needs framework

Data gaps

• Specific STATCOM installation locations and MVAr ratings
• Quantified inertia threshold at which Svk considers mandatory grid-forming requirements triggered
• Timeline for Svk publishing formal grid-forming technical requirements (HVDC phase)
• Whether FFR volumes in KMA2025 reflect grid-forming or grid-following battery assumptions