In short. A microgrid is a local system of sources, storage and loads with a controller and a defined boundary, able to run connected to the grid or on its own. For critical power it offers local resilience and flexibility, but it adds control and protection complexity that must be engineered and tested.
Building blocks
| Block | Typical content |
|---|---|
| Sources | Diesel or gas generators, solar, grid connection |
| Storage | BESS for fast response and energy shifting |
| Loads | Classified into critical, essential and deferrable |
| Interface | Point of common coupling with protection and breaker |
| Controller | Dispatch, mode control, shedding, resynchronisation |
| Protection | Settings for grid-connected and islanded states |
See microgrids and microgrid controller architecture.
Operating modes
- Grid-connected: import or export under the utility’s rules; storage and generators may reduce cost or support the grid.
- Islanded: runs independently. See islanded operation.
- Transition: planned and unplanned moves between the two.
- Black start: restart from nothing. See black start.
Design decisions that matter
- Boundary: which loads are inside the microgrid and which are shed or separated.
- Sizing: generation and storage for critical load with redundancy and the longest outage to survive.
- Grid-forming source: which unit sets frequency in the island.
- Controller architecture: centralised, distributed or hierarchical, with a fallback if the controller fails.
- Protection: settings adapting to grid and island fault levels.
- Interconnection approvals: from the utility.
Resilience considerations
- Controller failure should leave sources in a safe, autonomous state.
- Communications need redundancy.
- Fuel and storage reserves define autonomy; solar alone will not carry night loads.
- Test seamless transitions and failures, not only normal operation.
Economics
Microgrids may earn value from demand-charge reduction, energy cost optimisation and avoided outage costs. Treat resilience value separately, because it is real but hard to monetise. Do not allow economic dispatch to deplete the reserve needed for an outage.
Worked illustration
A hospital campus has 1.5 MW critical load, two 1 MW generators and a 500 kW BESS. The microgrid controller keeps the BESS at a minimum state of charge for outage bridging, dispatches the generators during outages, and sheds the non-critical 800 kW of the campus. In normal operation it uses the BESS for peak shaving only above the reserve line. The design is judged by the outage test, not by the savings.
A microgrid for critical power
A critical-power microgrid combines sources (utility, generators, BESS, sometimes solar) with controllers so the site can operate connected or islanded and keep critical loads running through grid events.
| Element | Function |
|---|---|
| Point of common coupling breaker | Connects or isolates the site from the grid |
| Generators | Long-duration energy and grid-forming capability |
| BESS | Instant response, peak support |
| Solar PV (optional) | Fuel saving, needs a forming source in island |
| Microgrid controller | Mode control, dispatch, protection settings changes |
| Critical bus | Section with priority loads |
Design principles
- Define the critical loads and the required island duration first.
- Choose which source forms the grid in island mode and its backup.
- Use adaptive protection or settings groups for grid and island modes.
- Plan for solar variability if present: do not count solar as firm capacity.
- Design resynchronisation and return procedures.
Value stack
The microgrid can support peak shaving, reduce diesel run hours and participate in utility programmes where available, which helps justify the investment beyond resilience alone.
Cautions
Complexity increases the number of failure modes. Keep logic simple and test every mode. See microgrid controller and grid-forming BESS.
Common mistakes
- Treating a microgrid controller as a single point of failure.
- Using the resilience reserve to pursue savings.
- Protection that works only in grid-connected mode.
What to send us
Send the loads, existing sources and the resilience goal. Send your application details. See also islandable microgrids.
Questions engineers ask
What makes a microgrid different from a generator backup system?
It has an integrated controller, storage and often renewables, and can operate in both grid-connected and islanded modes with defined transitions.
Does a microgrid need a BESS?
Not always, but storage helps frequency control, seamless transitions and efficient generator loading.
How is a microgrid tested?
By testing grid loss, islanded steps, source failures, controller failure and resynchronisation.
Do I need utility approval?
Usually for grid-connected systems. Requirements vary by location.
This guide gives general principles. Microgrid design requires site-specific engineering studies.