Unintentional Islanding and Anti-Islanding Protection
IEC 62116, IEEE 1547, and UL 1741 SA look like interchangeable islanding-detection standards but aren't interoperable — Sweden specifically requires IEC 62116, so equipment sourced or certified against the US standards doesn't automatically qualify here.
A real 2022 E.ON event shows this isn't just a theoretical inverter-era risk — a conventional synchronous hydro generator, not any inverter-based DER, sustained an unintentional island for about six minutes after a relay mis-trip, precisely because its own frequency-regulation capability kept voltage and frequency inside the safe range long enough to go undetected.
Unintentional islanding (Swedish: oavsiktlig ö-drift) is the mirror problem of planned Island Operation: a grid section unexpectedly disconnects from the upstream network while local generators continue to supply the isolated section, without the deliberate preparation, black-start planning, or organizational readiness that intentional ö-drift requires. This creates risks of personal injury (workers assume disconnected lines are dead), fire, equipment damage, and impeded fault clearing. (Source - Energiforsk 2025-1128 Oavsiktlig ö-drift med Distribuerad Generering (2025))
Where Island Operation covers the planned, prepared case (Svk’s ö-drift levels, Swedish microgrid case studies, Gotland total-defence planning), this page covers the protection-engineering problem of detecting and preventing islands that form by accident — a distinct technical discipline (anti-islanding protection) rather than a planning/preparedness one, split out as its own page given its size and self-contained scope.
The Non-Detection Zone (NDZ)
The fundamental challenge is detection. The frequency range in which generators must remain connected — 47.5–52 Hz and 0.9–1.1 Un per RfG — defines an unavoidable Non-Detection Zone (NDZ): if the isolated section achieves near-zero power exchange imbalance at the moment of disconnection, frequency and voltage remain within these bounds indefinitely. Passive monitoring cannot detect the island.
Inverter-based resources face a wider NDZ than synchronous generators: they produce only ~1–1.5× rated fault current (vs 4–7 p.u. for synchronous generators) (Source - Energiforsk 2023-957 Felbortkoppling i Mikronät (2023)), so perturbations are smaller and harder to distinguish from normal fluctuations. Grid-forming BESS makes detection harder still — by actively sustaining voltage and frequency, it suppresses the very signals passive detection relies on.
Detection methods
Passive methods: monitor frequency, voltage, ROCOF (df/dt), phase angle jump, impedance change, or voltage THD. Effective only when power imbalance at disconnection is large enough to drive values outside the NDZ. Insufficient as sole detection method.
Active methods — inject controlled perturbations; the grid’s response differs depending on whether it is connected to an upstream source (which stiffens the response) or isolated:
| Method | Principle |
|---|---|
| AFD (Active Frequency Drift) | Shifts inverter current zero-crossing to induce frequency drift |
| SFS (Sandia Frequency Shift) | Positive feedback on frequency — accelerates drift when islanded |
| SMS (Slip Mode Shift) | Phase-shift positive feedback |
| SVS (Sandia Voltage Shift) | Reactive power variation to detect voltage response |
| RPV (Reactive Power Variation) | Systematic reactive power injection variation |
| Negative sequence current injection | Creates voltage imbalance between phases; effective but problematic in multi-DG systems |
| Modern positive feedback strategies | Combined frequency perturbation + positive feedback; reduces NDZ |
| JEM algorithms (Japanese) | Step reactive power injection; fast detection |
Active methods are more reliable than passive but can be overwhelmed in large islands where perturbations are diluted.
Hybrid methods: passive trigger → active confirmation. Less power quality impact than pure active; reduced NDZ vs pure passive. Tradeoff: longer detection time and higher complexity. Examples: SFS-based hybrid, ROCOFOP/ROCOVOP variants.
Communication-based methods:
- Phase angle measurement at both upstream and island reference points — angle diverges on disconnection; requires communication link
- Intertrip: direct trip signal to generators when specific breakers open. Permitted under RfG Art. 15(5)(b)(iii) as one component of the detection method but cannot be the sole method — must be supplemented by local detection
IEC 62116 is the test procedure standard required in Sweden: 2-second detection time limit; test setup uses DC source + RLC load resonant at 50 Hz + simulated grid that can be disconnected. Must detect disconnect and shut down within specified time under multiple balanced and unbalanced load conditions.
Regulatory requirements
Under RfG Articles 15–16 (EU 2016/631, implemented in Sweden as EIFS 2018:2):
- Type C and D generators: must have an islanding detection method agreed with the TSO/DSO; the method cannot rely solely on switchgear position signals
- EN 50549 (type A/B generator connection to distribution networks): islanding detection required; must not conflict with fault ride-through requirements
Standards comparison:
| Standard | Origin | Key feature |
|---|---|---|
| IEC 62116 | IEC (global) | Required in Sweden; 2-second detection limit |
| IEEE 1547 | IEEE (US) | Broader DER coverage; not interoperable with IEC 62116 |
| UL 1741 SA | UL (North America) | Meets or exceeds IEEE 1547 |
Compliance with one standard does not imply compliance with another. Swedish market requires IEC 62116.
Protection challenges in islanded distribution networks
Inverter-based resources drastically change fault behavior:
| Source type | Fault current | Protection implication |
|---|---|---|
| Synchronous generator | 4–7 p.u. rated | Standard overcurrent protection works |
| Inverter (grid-following) | 1–1.5 p.u. rated | Phase overcurrent may fail |
| Grid-forming inverter | Configurable, limited | Design choice; protection must account for limit |
In high-impedance earthed MV networks, the neutral resistance is typically located outside the island. When unintentional islanding occurs, the earth fault detection system becomes non-functional. Solution: zero-sequence voltage protection (öppet-delta VT) at each generation connection point to the MV network.
Real events — E.ON case studies
Case 1: 2022-09-26 unintentional island (Sweden) 130 kV earth fault → distance protection trips → auto-reclose 1 second later → re-energization transients trigger a wrong-zone relay trip at the 40 kV busbar → Station B (40/20/10 kV switchboards) isolated with 1.6 MW hydro on 10 kV (plant type not specified in the report). Control room sees Station B still energized at elevated voltage. Operator manually opens the 20 kV transformer breaker → voltage/frequency increase → frequency protection trips the hydro. Island duration: ~6 minutes. Lesson: conventional synchronous generators (not only inverter-based DER) create unintentional islands; the hydro’s frequency regulation capability sustained the island. (Source - Energiforsk 2025-1128 Oavsiktlig ö-drift med Distribuerad Generering (2025))
Case 2: Nätvärn (preventive network protection) A regionnät topology with three wind farms, one BESS, and two distribution stations on a radial. If the transmission connection is lost, power balance between wind, BESS, and consumption could sustain an island. Preventive measure: nätvärn (network protection) monitors specific breaker positions. If any monitored breaker opens, a trip signal is sent to the BESS (the only unit with frequency regulation capability). Without the BESS, wind farms cannot sustain frequency and trip within their passive protection limits. Lesson: complex topologies require engineering analysis of which combinations can sustain power balance, and specific preventive protection for each. (Source - Energiforsk 2025-1128 Oavsiktlig ö-drift med Distribuerad Generering (2025))
Related pages
- Island Operation — the planned/prepared counterpart; Svk’s ö-drift levels, Swedish case studies, Gotland total-defence planning
- Generator Connection Requirements — RfG type A/B/C/D islanding detection requirements; EN 50549
- Grid-Forming Inverters — grid-forming control widens the NDZ, making detection harder
- Energy Storage — BESS role in nätvärn preventive protection and as the frequency-regulating asset in mixed-topology islands