In short. A grid-forming inverter creates its own voltage and frequency reference, so it can energise a dead bus and hold an island, while a grid-following inverter needs another source to follow. In a hybrid with generators it can take fast steps and keep the system stable, but its current limit, energy and protection impact must be designed in.
Grid-forming and grid-following
| Property | Grid-forming | Grid-following |
|---|---|---|
| Behaviour | Acts like a voltage source | Acts like a current source |
| Needs a reference | No | Yes; a live grid or other source |
| Black start | Possible | Not possible |
| Island holding | Yes | Only with another forming source |
| Fault current | Limited by inverter rating | Limited by inverter rating |
See grid-forming and grid-following coordination.
What it enables
- Energising a dead bus and restoring the system without a generator running.
- Seamless transition to island with little or no break.
- Fast support for load steps and motor starts.
- Letting generators start and synchronise to a stable reference.
- Running at times on storage and renewable energy alone, saving fuel.
Limits to understand
- Current limit: typically around 1.1 to 1.5 times rated for a short time, which constrains motor starting and fault clearing.
- Energy: finite; state of charge governs how long it can hold the island.
- Thermal: overload times depend on the inverter’s cooling.
- Interaction: multiple forming sources need coordination to avoid circulating power and oscillation.
Working with generators
A common approach is for the BESS to form the island, generators to synchronise to it and then share load. After the generators take the base load, the BESS can switch to a following or support role. Another approach keeps the generator as the primary former and uses the BESS for steps. The chosen scheme defines who sets frequency and how the handover works. See hybrid dispatch control.
Protection effects
Because inverter fault current is limited, overcurrent protection may not detect faults in an island supplied only by the BESS. Use voltage-based, differential or directional protection, and verify with fault studies. See inverter fault behaviour.
Worked illustration
A site has a 1 MW grid-forming BESS and a 1 MW generator. A 400 kW motor starts DOL with a starting current of six times full load. At roughly 2,400 kW-equivalent apparent demand during the start, the BESS alone cannot supply it within its 1.2 times current limit. The plan: the generator starts first, the BESS supports it, or the motor uses a soft starter so the demand falls within the BESS’s limit. This kind of check, using actual start currents, decides whether the island holds.
Verification
- Black-start a dead bus from the BESS.
- Island transition at various loads.
- Largest motor start and largest load step.
- Generator synchronise to the BESS and handover.
- Fault tests to confirm protection.
Why grid-forming matters for resilience
A grid-forming BESS inverter creates its own voltage and frequency reference. It can hold the critical bus the instant the utility fails, with no gap, and can host the generator start without a break.
| Capability | Benefit |
|---|---|
| Seamless island transition | Load never sees the outage |
| Fast frequency and voltage support | Cushions generator load steps |
| Black start | Can re-energise a dead bus (if designed for it) |
| Works with weak or no grid | Stable island |
Limits to respect
- Current limit. Inverters supply roughly 1.1 to 2 times rated current, so they cannot start large motors the way a generator can.
- Energy limit. A battery has finite energy; the generator or grid must take over.
- Sharing. Two forming sources on one bus need coordinated droop control.
- Protection. Low fault current may not trip overcurrent devices.
Hybrid operation sequence
BESS forms; the generator starts and synchronises to the BESS-formed bus; load shifts to the generator; the BESS becomes a follower or stays as a forming source in droop; recharge happens within generator capacity.
Verify with study and test
Dynamic simulation of transitions, a motor start and a generator trip. Factory and site tests with real equipment.
See grid-forming coordination and BESS for critical power.
Common mistakes
- Sizing power on running load and ignoring starting currents.
- Two sources both trying to set frequency.
- Protection not verified at low fault current.
What to send us
Send the loads, motor list and the sources you plan. Send your application details. See also BESS for critical power.
Questions engineers ask
Can any BESS do grid-forming?
No. It needs inverters with grid-forming control capability. Confirm with the manufacturer.
Is grid-forming always better?
It is needed for islanding and black start; otherwise grid-following can suffice. Choose by the use case.
How does a BESS handle a motor start?
Within its current limit. Large DOL starts may exceed it, so soft starters or generator support may be needed.
What about two grid-forming sources?
They need a coordination scheme, such as droop, so they share load without conflict.
This guide gives general principles. Inverter capabilities vary by manufacturer; confirm with data sheets.