In short. When the grid disappears, local generation must either shut down quickly or form a controlled island. Detection must be fast enough to be safe and reliable enough not to trip on grid disturbances. The island must then match its load to its generation.
Detection methods
| Method | Principle | Note |
|---|---|---|
| Voltage and frequency windows | Trip when outside limits | Simple; may miss balanced islands |
| Rate of change of frequency (ROCOF) | Fast change signals islanding | Sensitive to settings and grid events |
| Vector shift | Sudden phase jump | Can nuisance trip |
| Breaker status of the utility tie | Direct signal | Most reliable where available |
| Communication-based transfer trip | Utility sends a trip signal | Needs a reliable channel |
Planned versus unplanned
- Planned: adjust generation to match load, set the leader, open the tie, no disturbance.
- Unplanned: detect loss, open the tie, assign leader, shed load to fit capacity, stabilise. See load shedding.
Sequence for unplanned islanding
Grid loss detected → tie opens → leader source set → frequency and voltage stabilised → shed to capacity → critical loads supplied → restore in stages.
Resynchronisation
When the utility returns, the island must match voltage, frequency and angle before closing the tie. A sync check and utility permission apply. See sync-check logic.
Utility rules
Utilities require that generators not energise a dead network. Where a generator can parallel or export, anti-islanding protection with agreed settings and test is mandatory in most places. See interconnection studies.
Illustrative example
A factory with a 1 MW generator and solar detects grid loss in about 100 ms through the tie breaker status and ROCOF. The tie opens, solar inverters trip as required, the generator becomes the leader and a shed table sheds enough load to fit its capacity. On return of the grid, the controller synchronises before closing the tie.
What islanding means
An island forms when a part of the network continues to be energised by local generation after the utility connection has opened. Unplanned islands are hazardous to line crews and equipment, because out-of-phase reclosing by the utility can damage generators.
Detection methods
| Method | Principle | Notes |
|---|---|---|
| Under/over voltage and frequency | Island drifts outside limits | Fails if generation matches load closely |
| Rate of change of frequency (ROCOF) | Fast frequency change on imbalance | Setting must avoid nuisance trips |
| Vector shift | Phase jump when grid is lost | Fast, can false-trip on grid events |
| Transfer trip | Utility sends a signal when its breaker opens | Most certain, needs communications |
Planned island transition
- Detect loss of grid or receive an intentional command.
- Open the point-of-common-coupling breaker.
- Check that the generation can carry the load; shed if not.
- Establish voltage and frequency control.
- Switch protection settings to island mode.
Reconnection
Do not reclose until the utility is stable, then synchronise with sync-check and reconnect. Respect any utility waiting time.
Settings caution
Take values from the utility’s requirement and the interconnection study, and test with injection. See interconnection study and microgrid controller.
The master engineering question
Within what time does the site know the grid is gone, and what stops local generation from feeding a dead network or holding an unstable island?
What to send us
Send utility connection details and relay data. Send your application details.
Questions engineers ask
Why can islanding be dangerous?
Unplanned energisation of a network endangers line workers and can damage equipment.
Can ROCOF cause nuisance trips?
Yes, if set too sensitive. Tune with the utility.
Is islanding allowed?
Only as permitted by the utility and local rules.
How is it tested?
By witnessed simulation of grid loss and protection tests.
This guide gives general principles. Follow the utility’s requirements and local regulation.