Source - Energiforsk 2025-1128 Oavsiktlig ö-drift med Distribuerad Generering (2025)

Energiforsk Report 2025:1128 — Oavsiktlig ö-drift med distribuerad generering (Unintentional island operation with distributed generation). Published September 2025 by DNV on behalf of Energiforsk. Project leader Henrik Hemark (DNV); co-authors Daniel Karlsson, Lars Messing, Florin Stelea, Johan Stelin, Alexander Svensson, Lucas Finati Thomée. Reference group includes Vattenfall, E.ON, Ellevio, Skellefteå Kraft, Jönköping Energi, Öresundskraft and others.

Structured in two phases (A and B) covering detection methods, protection engineering, case studies, and standards. ~69 pages.

Summary

The central problem is detecting unintentional island operation (oavsiktlig ö-drift) in grids with increasing distributed generation. When a grid section isolates from the upstream network, local generators may sustain the island — creating risks of personal injury, fire, damage to equipment, and impeded fault clearing. Passive detection methods (frequency/voltage monitoring) fail when the isolated segment has near-zero power exchange imbalance at the moment of disconnection. Active methods that inject perturbations into the network are necessary.

Detection methods

Passive methods

Based on monitoring electrical variables (frequency, voltage, rate of change) and tripping when values exceed thresholds:

• Over/under frequency, over/under voltage
• ROCOF (Rate of Change of Frequency, df/dt)
• Phase angle jump
• Change in voltage THD (total harmonic distortion)
• Monitoring of impedance change at point of connection

Fundamental limitation — Non-Detection Zone (NDZ): the frequency range required by RfG (47.5–52 Hz, 0.9–1.1 Un) where generators must not trip defines an unavoidable dead band. If the isolated network achieves power balance within these limits, passive methods cannot detect the island transition. This is the NDZ problem.

Inverter-based generators face a larger NDZ than synchronous generators — they produce lower fault current (~1.0–1.2× rated vs. 6+ p.u. for synchronous generators) and respond faster to perturbations, making small frequency/voltage deviations harder to distinguish.

Active methods

Inject controlled perturbations; the response differs depending on whether the system is connected to the upstream grid (which stiffens it) or isolated:

1. AFD (Active Frequency Drift) — shifts the zero-crossing of the inverter’s output current slightly, creating a frequency drift when islanded
2. SFS (Sandia Frequency Shift) — positive feedback on frequency; accelerates drift when islanded
3. SMS (Slip Mode Shift) — phase-shift positive feedback
4. SVS (Sandia Voltage Shift) — reactive power variation to detect voltage change
5. RPV (Reactive Power Variation) — systematic variation of reactive power injection
6. AFD with reactive power — combined
7. Negative sequence current injection — creates voltage imbalance between phases; effective but problematic in multi-DG systems
8. Modern positive feedback strategies — combines frequency perturbation with positive feedback; reduces NDZ
9. JEM algorithms — Japanese inverter techniques using step reactive power injection; fast detection time

Active methods are considered more reliable than passive but carry risks in large islands: if the island has substantial local generation capacity, the injected perturbation may be “drowned out” and fail to propagate detectably.

Hybrid methods

Two-stage approach: passive monitoring first; active perturbation only when passive thresholds suggest possible islanding. Benefits: less impact on power quality than pure active; reduced NDZ compared to pure passive. Drawbacks: longer detection time; higher complexity.

Examples: SFS-based hybrid, SMS-based hybrid, voltage unbalance methods, ROCOF-based hybrid, ROCOP/ROCORP/ROCOFOP/ROCOVOP variants (monitoring rate of change of power and voltage in addition to frequency).

Communication-based methods

• Phase angle measurement: measure phase angles at both the upstream network and within the potential island; angle difference grows when isolated. Requires communication link.
• Intertrip (remote trip): identify circuit breakers whose opening would create an island; send a direct trip signal to generators in the island via communication link. Permitted under RfG Article 15(5)(b)(iii) as one component of the detection method, but cannot be the sole method — it must be supplemented by local detection.

Regulatory framework for islanding detection

RfG requirements

EU Regulation 2016/631 (RfG, implemented in Sweden as EIFS 2018:2):

• Type C and D generators (Article 13 and 14): must have island detection agreed with the TSO/DSO; the method cannot rely solely on switchgear position signals
• Type A and B: basic frequency/voltage protection contributes but no specific islanding detection mandate in RfG

Frequency limits requiring generators to remain connected: 47.5 Hz < f < 52 Hz, voltage: 0.9 Un < U < 1.1 Un — these define the unavoidable NDZ.

EN 50549

European standard for type A and B generator connection to distribution networks. Requires islanding detection protection functions; specifies the protection must not conflict with fault ride-through (FRT) requirements.

International standards comparison

Standards are not interoperable: compliance with IEEE 1547 does not imply IEC 62116 compliance. Swedish market requires IEC 62116. Equipment must be tested and certified per the relevant regional standard.

IEC 62116 test procedure: DC source simulating PV + RLC load tuned to resonance at 50 Hz + simulated grid connection that can be disconnected. Inverter must detect the disconnect and shut down within the specified time under multiple balanced and unbalanced test conditions; must recover normally when grid is restored.

Power electronics and grid control

Rectifiers (AC→DC), inverters (DC→AC), and converters (AC↔DC↔DC) are controlled by microprocessors/DSPs running algorithms for voltage and frequency regulation, protection, and optimization. Key capability distinction:

• Grid-following: inverter synchronizes to existing grid voltage/frequency; used in most PV and battery installations; cannot sustain frequency without an external reference
• Grid-forming: inverter establishes and maintains its own voltage/frequency reference; enables black-start and island operation; creates “synthetic inertia” (syntetisk svängmassa)

Grid-forming is the technical prerequisite for BESS and V2G participation in intentional island operation. In unintentional islanding, grid-forming capability makes detection harder — the inverter actively sustains the island.

BESS in islanding context

BESS can both cause and mitigate islanding:

• Grid-forming BESS can sustain island operation, increasing dwell time before detection
• BESS with grid-following control will disconnect faster under passive detection
• BESS is the primary enabling technology for intentional island operation (ö-drift)

V2G considerations

Two topologies:

• AC-coupled (inverter in vehicle): vehicle is mobile; registration as a generation source at each connection point would be required; hardware protection in the EV charger still needed; V2G is a mobile BESS
• DC-coupled (inverter in chargepoint): vehicle as DC battery; all regulation requirements on the chargepoint; cleaner regulatory interface

No EU network codes specifically address V2G as of 2025 — BESS is included in EN 50549 and national implementations; V2G regulatory gap identified by ACER and ENTSO-E as needing resolution in future RfG revision. From islanding perspective, no technical difference between AC and DC V2G topologies.

Protection challenges in islanded distribution networks

Inverter-based vs synchronous generator fault currents

Protection methods for generation in island networks

• Phase overcurrent: detects faults within the generation unit; cannot detect external faults (fault current from inverter ≈ rated current)
• Undervoltage: alternative to overcurrent; requires voltage transformers; long delay for selectivity
• Impedance (distance) protection: requires both CTs and VTs; inverter must provide negative sequence current for asymmetric faults
• Differential (line): absolute selectivity, high sensitivity, fast — preferred when topology allows
• Transformer differential: recommended for all transformers serving the island
• Earth fault: in high-impedance earthed MV networks, neutral resistance is outside the island — the earth fault detection system is non-functional in islanding. Solution: zero-sequence voltage protection (öppet-delta VT) at each generation connection point to MV network

NDZ for inverter-based systems

Inverter-based resources face a larger NDZ problem than synchronous generators because:

1. Lower fault current makes frequency/voltage perturbations smaller
2. Faster dynamic response can mask or dampen frequency drift
3. Grid-forming BESS actively suppresses voltage and frequency deviations

Case studies (E.ON)

Case 1: Unintentional island — 2022-09-26

A real Swedish event demonstrating that unintentional islanding occurs not only with renewables but with conventional hydro.

• 130 kV radial: earth fault → distance protection trips
• 1 second later: successful auto-reclosure, but re-energization introduces DC and harmonic transients (due to transformer winding coupling without equalizing winding, direct earthed 130 kV neutral connected to MV neutral)
• Transient triggers a faulty relay trip at the 40 kV busbar, disconnecting the 40 kV radial to Station B
• Station B (40 kV + 20 kV + 10 kV switchboards) is isolated with a 1.6 MW run-of-river hydro connected at 10 kV
• Control room notices Station B is still energized with elevated 40 kV voltage
• Operator manually opens the 20 kV transformer breaker → voltage/frequency increase → frequency protection trips the hydro unit
• Island duration: ~6 minutes
• Hypothesized cause of high voltage: 40/10 kV transformer tap changer responding to high 10 kV voltage

Lesson: conventional synchronous generators (not only inverter-based DER) create unintentional islands; frequency regulation capability of hydro is precisely what sustains the island.

Case 2: Preventive network protection (nätvärn)

A regionnät topology with three wind farms, one BESS installation, and two distribution stations on a radial network. If the transmission connection is lost, power balance between wind production, BESS storage, and consumption could be achieved — creating an unintentional island.

Preventive measure: nätvärn (network protection) monitors a set of circuit breaker positions. If any of the monitored breakers opens, a trip signal is sent to the BESS (the only unit with frequency regulation capability). Rationale: if the BESS is disconnected, the wind farms cannot sustain frequency and will also disconnect within their passive protection limits.

Lesson: in complex topologies, an engineering analysis of which topologies can sustain power balance must be performed, and specific preventive protection must be designed.

Conclusions from the report

1. Passive methods based on frequency/voltage/ROCOF are insufficient in the general case — they fail when the isolated network has near-zero power exchange imbalance at disconnection.

2. Active methods are necessary, either via primary apparatus switching or injection of perturbing signals from inverter-based generation.

3. ROCOF (df/dt) and ROCOU (dU/dt) are the common triggers for activating active detection; these should be configured conservatively to avoid false trips from large generator disconnections on the mainland grid.

4. SCADA-based manual detection is a valid complement: complete real-time breaker position monitoring allows operators to identify unintended island situations — but requires comprehensive SCADA coverage and trained personnel.

5. Recommendations: live testing at commissioning of large new generation installations (both inverter-based and synchronous); detection method agreed between generator owner and DSO/TSO per RfG Article 13.

Relevance to other wiki pages

• Island Operation — technical depth on detection methods, NDZ, case studies
• Generator Connection Requirements — RfG type C/D requirements for island detection; EN 50549; standards mapping
• Energy Storage — BESS in islanding context; grid-forming vs grid-following
• Demand Response — V2G regulatory gap; EVs as mobile BESS
• Electric Power Distribution — protection engineering in distribution networks with DER

Data gaps

See Island Operation › Data gaps and Generator Connection Requirements › Data gaps for canonical tracking of EIFS 2018:2 islanding detection guidance and nätvärn deployment questions.