In short. A microgrid controller manages a local network that can run with or without the utility. It sets which source leads, keeps frequency and voltage stable, shares load, and moves between grid-connected and island operation. Its hardest job is the transition, not the steady state.
Control layers
| Layer | Role |
|---|---|
| Primary | Local droop or grid-forming control in each source, fast and independent |
| Secondary | Restores frequency and voltage to nominal, shares reactive power |
| Tertiary | Economic dispatch and schedule, links to EMS. See EMS |
Modes
- Grid-connected: utility sets frequency; sources follow and optimise cost.
- Planned island: controlled disconnect, a leader source set before opening the tie.
- Unplanned island: utility lost; fast detection and response. See islanding detection.
- Black start: restoring from no power. See black start.
- Resynchronisation: reconnect to the utility. See sync-check logic.
Who leads in the island
One source, or a defined group, must set frequency and voltage. Generators with droop can share the lead; inverter sources must be designated grid-forming. Unclear leadership leads to oscillation. See grid-forming and grid-following.
Design checks
- Minimum generation and spinning reserve in the island.
- Fault current available in island mode and protection settings. See inverter fault behaviour.
- Load priorities and shedding to match island capacity.
- Time of transition and which loads ride through.
- Communication failure behaviour.
Illustrative example
An industrial site with a 2 MW generator plant, 1 MW solar and a 1 MWh battery loses the utility. The controller declares an island, makes the battery inverter the grid-forming source for the first seconds while a generator starts, sheds non-priority loads to fit available capacity, then hands the lead to the generator and returns the battery to a supporting role. On utility return, it matches voltage, frequency and angle before closing the tie.
What a microgrid controller must do
A microgrid combines generation, storage and load that can operate connected to the utility or islanded. The controller manages both states and the transitions.
| Mode | Controller role |
|---|---|
| Grid-connected | Optimise import and export, support peaks |
| Islanding transition | Detect loss of grid, open the point of common coupling, establish voltage and frequency |
| Islanded | Keep supply and demand balanced, share load, control frequency |
| Resynchronisation | Match to utility and reconnect |
| Black start | Restore from zero |
Control hierarchy
- Primary: local droop control in each source for fast sharing.
- Secondary: restores frequency and voltage to nominal.
- Tertiary: economic dispatch.
Key design decisions
- Which source forms voltage and frequency in island mode.
- Minimum spinning reserve before islanding.
- Load priority and shedding in island mode.
- Protection settings that remain effective with low fault current.
Cautions
Inverter-heavy islands have low fault current and little inertia, so frequency can move quickly. Study before committing. See islanding and grid-forming coordination.
The master engineering question
When the utility disappears, which source holds the island in the first second, and which source holds it after one hour?
What to send us
Send source list, ratings and load priorities. Send your application details.
Questions engineers ask
Is a microgrid just a generator with solar?
No. The controller, protection and transitions make it a microgrid.
Can it run without a battery?
Yes, but solar-heavy islands need fast sources or limits.
Does utility approval apply?
Usually, for parallel operation and export. Check local rules.
What if the controller fails?
Sources should fall back to safe local control, as defined.
This guide gives general principles. Microgrids need a project-specific study.