In short. A grid-forming source sets voltage and frequency. A grid-following source reads them and injects power. An island needs at least one grid-forming source, and two sources fighting for the lead cause oscillation. Coordination defines the leader, the followers and what happens when the leader changes.
Definitions
| Type | Behaviour | Examples |
|---|---|---|
| Grid-forming | Acts like a voltage source behind impedance; sets frequency and voltage | Synchronous generators, grid-forming battery inverters |
| Grid-following | Acts like a current source; needs a stable voltage reference | Typical solar inverters, some battery modes |
Why coordination matters
- A solar-heavy island with no grid-forming source cannot start or stay stable.
- Two grid-forming sources without droop or sharing control fight each other.
- Following inverters can trip if frequency or voltage moves faster than they accept.
- A weak island (low short-circuit strength) causes following inverters to hunt.
Coordination approaches
- Single leader: one generator or battery is isochronous; all others follow.
- Droop sharing: several grid-forming sources share by droop settings.
- Virtual inertia: inverters emulate inertia to slow frequency changes.
- Staged leadership: battery leads for the first seconds, generator takes over, battery follows.
See microgrid controller.
Mode transitions
Changing a source from following to forming, or the reverse, must be controlled so that voltage and frequency do not jump. Use ramps, hold states and handshakes with the PMS. See island transitions.
Protection implications
Inverters give limited fault current. In islands with mainly inverters, overcurrent relays may not see enough current. See inverter fault behaviour.
Illustrative example
A site with a diesel generator, solar and a battery in island mode sets the generator as leader when running. When the generator is off, the battery leads and solar follows, limited to the battery’s capacity. A test of the handover from battery to generator shows a brief frequency dip, and ramp settings are tuned to remove it.
Two inverter personalities
| Mode | Behaviour | Needs |
|---|---|---|
| Grid-following | Follows voltage and frequency set by another source, injects current accordingly | A stable voltage source, such as the utility or a generator |
| Grid-forming | Sets voltage and frequency itself, behaves like a voltage source | Proper control design, current limit, sometimes headroom |
Why it matters on a generator site
- With the utility present, the BESS can follow.
- When the utility fails and a generator starts, the generator forms the bus; the BESS follows it, or forms the bus first to give a bumpless ride-through.
- If the BESS forms and the generator then synchronises to it, the generator closes onto an inverter-formed bus.
- If two grid-forming sources share a bus, they need coordinated droop or they will fight.
Transition scenarios to design
| Scenario | Control action |
|---|---|
| Mains loss, BESS grid-forming | BESS holds bus while generator starts; generator syncs; BESS steps back to following |
| Generator trips while BESS follows | BESS must switch to forming or trip; define which |
| Mains return | Resynchronise bus to mains before closing |
Limits
Inverters have a current limit of about 1.1 to 2 times rated, so they cannot start large motors as a generator can. Keep this in the study. See inverter fault current behaviour.
See microgrid controller.
The master engineering question
In every operating mode, exactly which source holds voltage and frequency, and what happens during the second when that responsibility moves?
What to send us
Send source types and inverter models. Send your application details.
Questions engineers ask
Can solar run alone in an island?
Only with a grid-forming source or capable inverter.
Do generators count as grid-forming?
Yes, synchronous generators naturally set voltage and frequency.
Why do inverters trip in weak islands?
Voltage and frequency move more, and control loops can interact.
Is virtual inertia needed?
For inverter-dominated islands it helps; study the case.
This guide gives general principles. Confirm with inverter manufacturer data.