Island Operation

Island operation (Swedish: ö-drift) is the mode in which a grid section operates as a self-contained electrical “island” — with local generation maintaining balance and frequency, without connection to the wider grid. In Sweden, ö-drift functions both as a normal operational mode for isolated communities and as a preparedness instrument of last resort for sustaining critical societal functions during prolonged transmission failures.

Framing and context

Svenska kraftnät‘s public position is unambiguous: “A national, interconnected electricity supply is the best outcome for Sweden.” Ö-drift at the local/regional level is a fallback — relevant only in “genuinely severe situations” where national restoration plans cannot quickly reconnect areas after transmission failure. It is not a normal operating mode. (Source - Svk Om Ö-drift (2025))

ö-drift as a localization principle: Svk’s February 2025 localization report explicitly cites civil defense and ö-drift capability as one rationale for siting plannable (dispatchable) production near cities and areas with inflexible consumption. The argument is that co-location improves local access to generation during system restoration scenarios, reducing dependence on north-south transmission. Large industrial establishments with limited flexibility also benefit from nearby plannable production for the same reason. This is notably a transmission-level planning argument, not an operational grid argument — ö-drift readiness is embedded in Svk’s geographic guidance for new connection applications. (Source - Svk Planering för ökad elanvändning (2025))

This framing matters because island operation is sometimes discussed as a proactive resilience strategy for remote communities. In the Swedish regulatory context, the primary driver is emergency preparedness (elberedskap), not normal grid economics.

Four levels of island operation in Sweden

Svenska kraftnät distinguishes four scales, with different responsible actors:

The Arholma and Simris microgrids are level 2–3 cases: DSO-led projects serving a defined island community. (Source - Vattenfall Arholma Microgrid (2025), Source - InterFlex Simris Microgrid (2018))

Technical requirements

Black start (dödnätsstart) — the binding constraint

The single most essential prerequisite. The island must have local generation capable of starting from a completely dead grid — without an external voltage reference to synchronize to. Without black start capability, ö-drift is impossible. In practice, most potential ö-drift areas rely on small hydro, diesel, or (increasingly) BESS-based black start.

Frequency and voltage regulation

Once the island is energized, load and generation must be continuously balanced to maintain 50 Hz and acceptable voltage. This requires dispatchable, controllable generation — not just variable renewables (solar and wind can participate but cannot alone sustain stable frequency). BESS with grid-forming inverter control can fulfill this role.

Weak-grid operation

An isolated microgrid is a svagt nät (weak grid) — fundamentally different from normal grid-connected operation:

• Low fault currents: inverter-based sources (batteries, solar, wind) deliver ≤2 p.u. fault current vs 6+ p.u. from synchronous generators. Standard protection relay settings designed for strong-grid operation may fail to detect faults in island mode.
• No voltage stiffness: frequency and voltage are supported only by local resources; any sudden large load change or generation trip creates larger transients than in the mainland-connected case.
• Coordination complexity: DSO must manage protection, switching, and dispatch under abnormal conditions with reduced margins.

The protection engineering challenge for Swedish microgrids is documented in detail in Source - Energiforsk 2023-957 Felbortkoppling i Mikronät (2023) (Simris and Arholma cases, evaluating differential/distance protection over phase-overcurrent, Type A/BESS fault ride-through, and communication requirements generally) and Source - Lund Arholma Microgrid Fault Detection (2025) (Arholma PowerFactory simulations, which centers specifically on the low-voltage circuit breaker (LVCB) as a practical enabling component for fault detection and isolation in inverter-dominated island grids — the LVCB-centrality finding traces to the Lund thesis, not equally to the Energiforsk report).

Cold-load pickup

When reconnecting to the mainland after an islanding event, thermostatically controlled loads (heating, refrigeration) that have been deprived of power all switch on simultaneously — potentially exceeding grid capacity and causing a new fault. Mitigation: sequential reconnection, where loads are re-energized one by one. The Arholma next-phase plan includes remote-controlled switches at customer premises specifically to manage sequential reconnection. (Source - Vattenfall Arholma Microgrid (2025))

Organizational requirements

Technical capability alone is insufficient. Functioning ö-drift requires:

• A written ö-driftsplan covering all foreseeable contingencies
• Trained personnel who can operate a weak, isolated grid — considerably more demanding than normal operations
• Clear ö-driftsledare (island operation leader) with defined authority and communication protocols
• Coordination with emergency services, hospitals, water utilities — actors outside the electricity supply chain that Svk cannot directly mandate

Limitations

Cannot fully test: a genuine live drill replicating complete isolation is not possible without inducing real outages. Svk points to testing (provningar) and simulations. The consequence: function cannot be guaranteed with 100% certainty. (Source - Svk Om Ö-drift (2025))

Hard load prioritization: in most ö-drifts, installed generation is sufficient only for the most critical societal functions. The DSO must decide, in advance, which loads receive power and which do not. Non-critical consumers are expected to arrange their own backup (diesel generators etc.).

Reliability analysis (Arholma, 2015–2019 data): Source - Energiforsk 2023-948 Reliability Analysis Microgrid (2023) modelled Arholma’s reliability with the microgrid system. Results: SAIDI improvement −45% in full island mode, −18% in hybrid mode; but residual capacity shortfall LOLP of 1.5% / ~130 hours per year — the 2×160 kW BESS was not sized to guarantee full coverage of demand peaks.

Swedish case studies

Arholma — Vattenfall Eldistribution

Vattenfall Eldistribution‘s Arholma microgrid in the northern Stockholm archipelago is Sweden’s most documented operational example. Key specifications:

• 2 × 160 kW lithium-ion BESS (total 320 kW; Vattenfall’s operational communication describes a “two-hour system,” but the pre-commissioning design thesis stated a 1-hour-at-99%-probability target — the two figures are unreconciled, a data gap the thesis itself flags)
• Solar panels on one battery container
• Real-time control system: detects mainland cable fault in a fraction of a second, automatically opens breakers and activates island mode
• Commissioned August 2023; ~250 permanent and seasonal residents
• Power-as-a-Service model: Vattenfall Elanläggningar owns and operates the BESS hardware; Vattenfall Eldistribution (the DSO) purchases capacity as a service — consistent with Art. 36 DSO storage ownership restrictions even within the same corporate group

Since commissioning, winter load growth on the island has outpaced the original 2019 design assumptions. Vattenfall’s next phase extends control to customer assets (heat pumps, floor heating) via remote switches, with sequential reconnection and tariff-discount compensation. This is implicit demand response rather than a market mechanism. (Source - Vattenfall Arholma Microgrid (2025))

Simris — E.ON

E.ON’s Simris Local Energy System in Skåne was a 2015–2018 H2020 demonstration (InterFlex project). 333 kWh / 800 kW BESS, 500 kW wind, 442 kWp PV, ~150 customers. The project included a 12-hour islanding test, demonstrating sustained island operation with inverter-based resources. Cost comparison: BESS + power conversion system up to 4× cheaper than a conventional grid upgrade for mitigating voltage deviations caused by newly-connected renewable generation on the 24 km MV line (the source’s own framing — a capacity/voltage-quality driver, not specifically a “reliability requirement” comparison). (Source - InterFlex Simris Microgrid (2018))

Loading strategies (pålastningsstrategier) — a simulated case

During black-start and island operation startup, load must be connected gradually — connecting all loads simultaneously can cause frequency and rotor angle instability. A simulation study of a real anonymous municipality in central Sweden (Uppsala thesis, 2025) compared four strategies using a 30.6 MVA CHP plant and a hydropower plant, supplying 15 prioritized buildings. (Source - Från dödnätsstart till ödrift Uppsala Thesis (2025))

Key results:

• Maximum frequency dip with “Alla samtidigt” (max load scenario, CHP plant): 49.87 Hz — within RfG limits but near the lower bound
• Frequency, rotor angle, and voltage stabilize ~20 seconds after the last load connection, across all scenarios
• Stabilized voltage within ±5% p.u. in all scenarios; transients recover in ~0.6 ms (hydro) to ~1 ms (CHP)
• CHP was more robust than hydro in this model due to higher available capacity relative to the 15 priority buildings’ demand — the one exception: hydro’s “Alla samtidigt” max-load scenario did not stay above 49 Hz, the sole case in the study where frequency regulation could not handle the load step

Practical implication: loading strategy design is as important as generator sizing. Strategies with time-separated connections are significantly more stable than simultaneous connection. Kluster offers a practical middle ground between operational simplicity and stability.

Feasibility planning — Skåne läns förstudie (2023)

A 2023 feasibility study by Energikontor Syd (funded by Region Skåne) mapped Skåne’s potential for crisis island operation. (Source - Förstudie Krisberedskap och ö-drift Skåne (2023))

Key finding: No suitable object could be identified for further detailed investigation among existing CHP plants or waterpower stations. Main barriers:

Financing: Svenska kraftnät’s elberedskapsanslag is the primary funding source for achieving ö-driftförmåga. CHP plants can additionally benefit from revenue from stödtjänst markets (FCR-N and mFRR where ramp capability and budget volume allow) — investment in control capability serves both preparedness and commercial purposes.

June 2023 development: Svk decided to preserve Öresundsverket (Malmö) for island operation by 2025 — its output nearly covers the full power need of the Malmö-Burlöv distribution area.

Future options: gasturbiner at bio- or vätgas production sites; new CHP converted from heat-only värmeverk; decentralized battery + gas turbine combinations for smaller load clusters; V2G at island network access points for supplying remote priority loads.

Additional Swedish examples

Ludvika: hydropower (3.5 MW) + 400 kWh battery (300 kWh reserved for crisis) + 218 kW solar. Battery enables dödnätsstart of the hydro plant; hydro takes over voltage/frequency regulation. Investment ~6 MSEK excl. solar (just under half from Energimyndigheten). Study conducted 2006–2007; capability realized subsequently (exact completion date not stated in source).

Jönköping: biomass CHP + waterpower backup. Modifications needed: dödnätsstart capability, cooling at bio boiler, communications. Planned completion end-2024. Municipal plan: 7-day capability.

Skälleryd (Mönsterås): run-of-river hydro (1200 kVA, no reservoir). Proposed modifications include dump load (100 kW) and battery storage (400 kWh) for frequency smoothing. Neither implemented as of 2023.

Gotland — total defence island operation planning

Gotland is the most strategically significant island operation planning case in Sweden, both technically and from a total defence perspective: a three-month island-operation planning basis drawn from MCF’s total-defence Typsituation 4 (not a funded capability), a 2030 transmission-level AC connection alongside the existing HVDC link, and the Försvarsmakten FRaM project addressing wind/military land-use coexistence. Full detail on Gotland Island Operation — Total Defence Planning.

Relationship to flexibility and DER

Island operation creates a direct value case for local generation and storage that is separate from the flexibility market arguments:

• A BESS that provides ö-drift capability is earning a reliability service (grid resilience) for the DSO, not just a balancing market revenue
• Heat pumps, EV chargers, and other controllable loads can extend island duration by reducing peak demand during islanding events
• Solar panels provide daytime generation but cannot alone sustain frequency — they require either storage or dispatchable backup

This points toward a value-stacking architecture: local DERs that participate in Balancing Markets and Flexibility Markets during normal operation can simultaneously provide ö-drift capability as a third revenue/service stream. No Swedish market currently prices ö-drift capability explicitly — Svk’s elberedskap funding covers capability costs, not market payments.

CHP plants are another possible source of ö-drift or black-start capability. Energimyndigheten’s October 2026 report on district heating and CHP says some systems can provide these functions where technical and organisational conditions exist, but not all can, and recommends assessing them function by function (Source - Energimyndigheten ER 2026-23 Fjärr- och Kraftvärmens Roll (2026)).

Svk’s elberedskap funding mechanism

For level-2 local/regional ö-drift, Svenska kraftnät in its elberedskapsmyndighet role can compensate the costs of maintaining ö-drift capability, planning, and testing. This is a grants/compensation mechanism, not a market payment. Svk can decide on preparedness measures only towards actors within the electricity supply — not municipalities or other actors outside it. (Source - Svk Om Ö-drift (2025))

Unintentional islanding (oavsiktlig ö-drift)

Unintentional islanding — a grid section unexpectedly disconnecting while local generators continue to supply it, without the preparation and organizational readiness that intentional ö-drift requires — is the mirror problem of the planned island operation covered on this page, and is detailed on its own page given the size and distinct protection-engineering scope: Unintentional Islanding and Anti-Islanding Protection (Non-Detection Zone, passive/active/hybrid detection methods, RfG/IEC 62116 regulatory requirements, protection challenges, and two real E.ON case studies).

Cybersecurity threats to island-sustaining DERs

RISE (2023) simulated a coordinated cyberattack on connected heat pumps using the Nordic32 grid test model. Sweden has ~300,000 connectable liquid-based heat pumps — approaching a critical mass where a synchronized activation or deactivation attack could create grid frequency disturbances. (Source - RISE Cyberhot mot Elsystemet (2023))

The mechanism is relevant to island operation: a synchronized step change in consumption from a DER botnet (heat pumps, EV chargers, BESS inverters) in an isolated island network — which has no external frequency support — could drive frequency outside acceptable limits and trigger automatic protection disconnection. The same attack vectors that threaten the main grid are amplified in the low-inertia island context.

Attack vectors for DER botnet recruitment: product vulnerabilities (hard-coded passwords, unpatched firmware), cloud service compromise, user credential theft. Devices with long lifespans (heat pumps: 15–20 years) are particularly vulnerable to outdated firmware.

See Source - RISE Cyberhot mot Elsystemet (2023) for recommended mitigations across myndigheter, energibolag, leverantörer, installatörer, and användare. For how islanding (decentralized resilience) trades off against aggregation (concentrated control) in the overall security picture, see Security and Resilience of the Digitalized Flexible Grid.

Regulatory framework: Energimyndigheten (the Swedish Energy Agency — distinct from Ei/Energimarknadsinspektionen) publishes cybersecurity guidance for the energy sector. Energy sector actors (including DSOs with island operation assets) are expected to maintain systematic risk management, incident handling, continuity planning, supply chain security, security effectiveness measurement, and threat reporting. Suspected intrusions should first be contained/remediated, then reported to Myndigheten för civilt försvar (MCF), which appears to be the successor to MSB for civil defence functions; CERT-SE provides active incident response support. (Note: Energimyndigheten’s own guidance does not name a specific statute, NIS2, or an in-force date for a supervisory mandate — an earlier version of this page’s cited source asserted a “Cybersecurity Act SFS 2025:1506” citation that could not be traced to the source and has been removed; treat any NIS2/statutory claim as unverified until independently sourced.) (Source - Energimyndigheten Cybersäkerhet Energisektorn (web, 2026))

Related pages

• Gotland Island Operation — Total Defence Planning — Gotland’s three-month total-defence ö-drift planning basis, 2030 AC transmission upgrade, and FRaM wind/military coexistence project
• Unintentional Islanding and Anti-Islanding Protection — the accidental/protection-engineering counterpart: NDZ, detection methods, RfG/IEC 62116, real E.ON incidents
• Svenska kraftnät — elberedskapsmyndighet role; transmission-based restoration
• Vattenfall Eldistribution — Arholma project; implicit DR in island context
• Energy Storage — BESS as black start and frequency regulation resource; grid-forming vs grid-following
• Demand Response — load management during islanding; cold-load pickup mitigation; V2G for remote priority loads
• Electric Power Distribution — distribution grid as ö-drift network; protection challenges
• Generator Connection Requirements — RFG type A/B/C/D; black start capability; islanding detection requirements; EN 50549; IEC 62116

Data gaps

• How many Swedish distribution networks with significant DER penetration have implemented network protection (nätvärn) analogous to the E.ON Case 2 topology?
• Outcome of Jönköping island operation implementation — was the end-2024 target met?
• Status of Öresundsverket preparedness readiness — was the 2025 island operation target achieved?