In short. Islanded operation means the site runs on its own sources, separated from the utility. It needs a way to detect the loss of grid and disconnect, a source that sets frequency and voltage, a load that matches generation, enough reserve for steps and a safe path back to the grid.
What changes in an island
On the grid, frequency and voltage are held by the utility. In an island, the site’s own sources hold them. Anything the grid used to absorb, such as a load step, now falls on the island’s sources, and the system is much weaker. Fault levels drop, which affects protection.
Core functions
| Function | Requirement |
|---|---|
| Detection | Reliable loss-of-mains detection without nuisance trips |
| Separation | A breaker that opens quickly and verifiably at the interface |
| Frequency and voltage control | At least one source in a controlling (grid-forming or isochronous) mode |
| Load balance | Generation matches load, shedding if short |
| Reserve | Spare capacity and energy for steps and faults |
| Protection | Settings work at low fault levels |
| Resynchronisation | Controlled reconnection to the grid |
Transition styles
- Intentional planned island: the site is moved to island mode ahead of a known event.
- Unplanned island: the grid fails and the site separates and carries on. May involve a brief interruption unless storage bridges the transition.
- Seamless transition: possible with grid-forming BESS or fast controls.
See islanding detection and transition control.
Protection in islands
Inverter-based sources supply limited fault current and generators supply less than the grid. Overcurrent relays that worked on the grid may not trip in the island. Review settings or use directional, voltage-restrained or differential schemes. See inverter fault-current behaviour.
Reserve and stability
- Keep spinning reserve or storage power for the largest credible step.
- Avoid running one generator near its limit with no fast backup.
- Use frequency-based load shedding as a last defence.
Return to the grid
Reconnect only when the grid is stable and the island is synchronised to it, using sync-check and the utility’s permission. Alternatively, open-transition back with a short break. See islandable microgrids.
Worked illustration
A campus with 2 MW of load has 2.5 MW of generators and a 1 MW BESS. On a grid fault, the interface breaker opens in a few cycles and the BESS forms the island while generators start and synchronise. During this time the BESS carries the load; the generators pick up and the BESS returns to reserve. The design checks that the BESS can cover the 20 to 30 seconds needed and that protection operates at the lower island fault level. Without these checks the island may collapse.
Testing
- Controlled separation tests at several load levels.
- Generator trip and load step tests while islanded.
- Resynchronisation tests with the grid.
What changes when the site runs alone
In islanded operation, generation must follow load in real time with no grid to absorb mismatches. Voltage, frequency, protection and reserve all behave differently.
| Issue | On grid | Islanded |
|---|---|---|
| Frequency | Held by grid | Held by the generators |
| Fault current | High | Lower, may not trip protection |
| Load step | Absorbed by grid | Absorbed by generators, so frequency dips |
| Spinning reserve | From grid | Must be provided on site |
| Voltage control | Grid stiff | AVR and reactive sharing matter |
Island design checklist
- Total capacity at least the island load, with N+1 where required.
- Largest motor start inside voltage and frequency limits.
- Protection settings for low fault current, checked by study.
- Load shedding to prevent overload.
- Fuel and cooling for the full island duration.
- Voltage and frequency windows suitable for UPS and other sensitive loads.
Duration planning
For a 72 hour island at 600 kW, fuel is roughly 600 kW × 0.27 L/kWh × 72 h ≈ 11,600 litres as an order of magnitude, before reserves. Confirm with engine data and arrange delivery for longer events.
Transition
Island start and return transitions are covered in islanding transition control. See microgrid architecture.
Common mistakes
- No source in a frequency-setting mode.
- Protection settings that only work with grid fault levels.
- Insufficient reserve for load steps.
What to send us
Send the single-line diagram, sources and load profile. Send your application details. See also islanding readiness.
Questions engineers ask
Is islanding the same as backup power?
Backup power may supply a separated load only. Islanding continues to run a connected system, with its generation and loads, independently.
Why is protection harder in an island?
Low fault current from generators and inverters may not trip standard overcurrent relays.
Does the utility need to approve?
Usually yes for grid-connected installations that can island or reconnect. Check local rules.
Can renewable sources join the island?
Yes with suitable controls, and usually with storage or a grid-forming source to hold the frequency.
This guide gives general principles. Grid-interconnection rules and utility approvals must be checked for your site.