A weak grid is not a grid that runs a few volts low. It is a grid with high source impedance, a low short-circuit ratio and very little rotating inertia. Those three physical properties cause three specific failure modes in a battery system, and whether your equipment survives them is decided by four numbers on the datasheet — numbers that are usually not printed there.
What “Weak” Actually Means
Engineers describe grid strength with the short-circuit ratio: the short-circuit power available at a point divided by the rated power of the equipment connecting there. A ratio above 10 is a strong grid. Below 3 is weak. Below 2, most grid-following inverters cannot maintain stable control at all.
| Strong grid | Weak grid | What it breaks | |
|---|---|---|---|
| Short-circuit ratio | > 10 | < 3 | Phase-locked loop stability |
| Source impedance | Low | High | Voltage moves when you push current |
| Inertia | High (large generators) | Low (inverter-dominated) | Frequency moves fast, RoCoF is steep |
| Voltage THD | < 3% | 5–10% or worse | PLL locks onto a distorted angle |
| Frequency variation | ±0.05 Hz | ±0.5 Hz or more | Anti-islanding trips on legitimate events |
The practical consequence: on a weak grid, the storage system’s own current injection changes the voltage it is measuring. A 1 MW system on a 3 MVA short-circuit point will shift the local voltage by several percent just by exporting. Grid-following control assumes the grid is a stiff voltage source; on a weak grid that assumption is false.
Failure Mode 1 — The Inverter Trips on a Voltage Sag
A fault 30 km away on a weak feeder pulls the local voltage down to 70% for 200 ms. A strong-grid inverter rides through. A grid-following inverter on a weak grid often trips, because its phase-locked loop loses lock when the voltage phasor jumps.
The rating that decides this is the voltage ride-through profile — how low the voltage can go, for how long, and whether the inverter must stay connected and inject reactive current during the dip. European grid codes require this explicitly: EN 50549-1 defines the profile, and it is not optional for anything that exports.
- Ask for: the ride-through curve, plotted, with the test standard named.
- Common gap: a datasheet that says “wide voltage range 85–110%” is describing the operating window, not the ride-through requirement. These are different specifications and only one of them is a grid-code function.
Failure Mode 2 — Frequency Drift Trips the Anti-Islanding
On a weak, inverter-dominated grid, frequency moves further and faster than on a generator-backed grid. A 0.5 Hz excursion is normal on some island and mini-grid systems. Standard anti-islanding protection, set for a stiff grid, sees that excursion as an islanding event and disconnects.
The relevant parameter is rate of change of frequency (RoCoF) withstand. Older relays tripped at 0.5 Hz/s; modern settings go to 1–2 Hz/s and some markets have removed RoCoF tripping entirely because it was causing more outages than it prevented.
If your system is on an island grid or a feeder with high solar penetration, ask what RoCoF the inverter can withstand without disconnecting, and whether the protection settings are adjustable in the field. Fixed settings are a problem, because the grid operator will change their requirement at least once during the project’s life.
Failure Mode 3 — Harmonics and Voltage Unbalance
Weak grids are also usually distorted grids. High source impedance means harmonic currents from other customers produce large harmonic voltages. A phase-locked loop trying to extract a clean angle from a 8% THD waveform will jitter, and a jittering PLL produces more harmonics — a feedback loop that ends in a trip.
Unbalance is the same story with a different mechanism. On a single-phase-heavy rural feeder, negative-sequence voltage of 2–3% is common. Grid-following inverters inject negative-sequence current in response, which heats the DC bus capacitors and, on some designs, causes DC-link voltage ripple that trips the overvoltage protection.
On a weak grid, the inverter is not connected to a voltage source. It is connected to a neighbour, and the neighbour reacts to everything the inverter does.
The Four Ratings to Ask For
These are the four numbers that predict whether a system will survive a weak grid. Most datasheets do not print them, which is itself informative.
| # | Rating | What good looks like | Why it matters |
|---|---|---|---|
| 1 | Voltage ride-through profile | Zero-voltage ride-through for 150 ms, full recovery by 1.2 s, per EN 50549-1 or the local code | Decides whether a distant fault takes your site down |
| 2 | RoCoF withstand | ≥ 2 Hz/s without disconnecting, field-adjustable | Prevents nuisance tripping on inverter-dominated grids |
| 3 | Overload capability | 1.5–3× rated for 10 s to 1 min | Carries motor starts and rides through voltage recovery |
| 4 | Grid-forming capability | Voltage source behaviour, black-start capable, droop control | The only way to run a weak grid or an island stably |
The fourth one is the decision point. A grid-following inverter can operate on a weak grid if the short-circuit ratio is above roughly 3. Below that, or on a true island, you need grid-forming control — an inverter that establishes voltage and frequency rather than following them.
When You Must Specify Grid-Forming
Grid-forming is more expensive and harder to parallel. It is also the only correct answer in four situations:
- Island operation, even temporarily. A grid-following inverter cannot black-start an island.
- Short-circuit ratio below 3. Above that, grid-following is usually stable and cheaper.
- Very high renewable penetration on the same feeder, where there is no synchronous machine left to hold the angle.
- Sites where the utility requires it. A growing number of island and mini-grid codes now mandate grid-forming for new storage.
Below those thresholds, grid-following with a good ride-through profile and adequate overload is the cheaper and perfectly adequate answer. Specifying grid-forming everywhere is as much a mistake as never specifying it.
Frequently Asked Questions
What short-circuit ratio makes a grid “weak”?
Below 10 is generally considered weak, below 3 is very weak, and below 2 most grid-following inverters cannot maintain stable control. The ratio is calculated at your point of connection, using the utility’s short-circuit MVA at that point divided by your system’s rated MVA.
Can a standard grid-tied inverter run on a weak grid?
Often yes, if the short-circuit ratio is above about 3 and the ride-through profile matches the local code — and what “the local code” means changes at every border, as set out in the EU grid connection guide. Below that, or where frequency excursions are large, grid-forming control is required. The failure mode with grid-following on a very weak grid is not gradual degradation — it is a trip, usually at the worst moment.
How do I find my grid’s short-circuit ratio?
Ask your utility for the short-circuit MVA at the point of connection, or the fault level in kA at the connection voltage. Divide by your system’s rated MVA. If the utility cannot provide it, that is itself a signal that the interconnection study has not been done yet.
Does grid-forming cost more?
Yes, typically 10–25% more for the PCS, and more again if you need multiple units operating in parallel, because grid-forming units must share load without a communication link or with a very fast one. It is still usually cheaper than the alternative on a true island, which is a diesel plant plus the fuel logistics to keep it running.
What is the difference between grid-following and grid-forming?
A grid-following inverter measures the grid voltage angle and injects current in phase with it — it follows. A grid-forming inverter sets the voltage and frequency itself, behaving like a synchronous generator with controlled impedance. Grid-following needs an existing grid to follow; grid-forming can create one.
Send Us Your Grid Data
Short-circuit MVA at the point of connection, typical voltage and frequency excursion, and whether the site ever runs islanded. With those three numbers we can tell you whether grid-following will hold or whether you need grid-forming.