Source - Vattenfall Arholma Microgrid (2025)

Two web clippings from Vattenfall about the Arholma microgrid project in the northern Stockholm archipelago.

Documents

1. “Ö-drift Arholma” — Vattenfall Eldistribution operational status page with live dashboard. URL: https://www.vattenfalleldistribution.se/var-verksamhet/vart-arbete/innovation/arholma/. Captured 2026-04-26.
2. “Mikronät gör Arholma mindre beroende av fastlandet” — Vattenfall Group corporate news article. URL: https://group.vattenfall.com/se/nyheter-och-press/nyheter/2025/mikronat-gor-arholma-mindre-beroende-av-fastlandet. Published: 2025-01-28.

The Arholma project

Arholma is an island in the northern Stockholm archipelago with approximately 250 permanent and seasonal residents. Vattenfall Eldistribution operates the island’s electricity grid via a submarine cable connection to the mainland.

System specification

• Two 160 kW lithium-ion battery systems (total 320 kW)
• Two-hour island capacity: the batteries are sized to supply the entire island for 2 hours during a mainland cable outage — “more than sufficient for a normal interruption”
• Solar panels on at least one of the battery container roofs
• Real-time control software: scans and controls the microgrid continuously; detects a fault on the mainland submarine cable in milliseconds and automatically activates island mode (ö-drift) by opening the relevant breakers
• Commissioned: August 2023

Institutional structure

The project is organized as a Power-as-a-Service arrangement:

• Vattenfall Eldistribution is the grid operator and project client (beställare)
• Vattenfall Elanläggningar (part of Vattenfall Network Solutions business unit) owns and operates the BESS hardware, delivering it as a service to Vattenfall Eldistribution
• Vattenfall R&D participates in research and evaluation

This is an internally commercialized model where the DSO (Vattenfall Eldistribution) does not own the storage asset — consistent with the Art. 36 DSO storage ownership restrictions — even within the same corporate group. The storage is delivered as a capacity service.

Operation

The live dashboard shows the island’s current status — battery SoC levels, solar output (% of installed capacity), and whether the mainland connection is active. As of capture (2026-04-26): batteries at ~80% SoC; solar at ~88.85%; mainland connection active.

When a mainland cable fault occurs, the software triggers ö-drift automatically. The island then operates as a self-contained microgrid powered by its batteries (and solar when available). When reconnecting to the mainland, the system must prevent cold-load pickup — the risk that too many appliances switching on simultaneously causes a new fault.

Next phase: customer demand-side management

The January 2025 news article reports a significant development: load on Arholma has increased substantially since the system was designed in 2019, particularly in winter, such that battery capacity is no longer sufficient to cover demand peaks.

Vattenfall’s solution is to extend the microgrid control system to include customer assets:

• Remote-controlled switches installed at customer premises to disconnect loads during islanding
• Sequential reconnection: loads are reconnected one by one after mainland power returns, preventing simultaneous cold-load pickup that could re-trip the system
• Assets targeted: heat pumps, electric floor heating, heat storage systems
• Customer compensation: discount on the grid tariff (not a flexibility market payment)

Niklas Sjöberg (Senior Innovation & Business Strategist, Vattenfall Eldistribution): “Kundernas efterfrågan är den största potentiella flexibla resursen i våra elnät, och vi måste börja utnyttja denna möjlighet för att överbrygga de kapacitetsrelaterade utmaningarna.” (“Customer demand is the largest potential flexible resource in our grids, and we must start using this opportunity to bridge capacity-related challenges.”)

Customer meetings were beginning in January 2025 to map which appliances can be controlled. The project for optimizing customer demand was planned to run during 2025.

Key observations

Compensation model: Tariff discount rather than a flexibility market price. This is implicit demand response — customers are rewarded through reduced network costs, not through a competitive market mechanism. This contrasts directly with E.ON’s SWITCH explicit flexibility market approach.

Contradiction with “no flex markets” stance: Vattenfall has stated in its DNDP that it sees no conditions for market-based flexibility at any constrained location. Yet Arholma demonstrates active DSO-controlled demand-side management — just through a rules-based/implicit mechanism rather than a competitive market. The distinction is: Vattenfall is skeptical of market-based explicit DR, not of load control itself.

International interest: A large Japanese delegation visited Vattenfall R&D and Vattenfall Eldistribution in autumn 2024 to exchange experiences from similar island projects in Japan.

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