This free short-circuit ratio calculator helps engineers check grid strength before connecting a solar, wind, or battery storage plant. Enter the short-circuit MVA at each point of interconnection (POI) and the plant size to see the short-circuit ratio (SCR), weak-grid flags, and the largest plant each site can host. It also screens nearby plants together using the weighted short-circuit ratio (WSCR) and estimates synchronous condenser cost if a site is too weak.
What is the Short-Circuit Ratio?
The short-circuit ratio (SCR) compares the grid’s short-circuit strength at a bus with the size of the plant connecting there: SCR = short-circuit MVA ÷ plant MW. Inverter-based resources supply limited fault current, so in weak parts of the grid they can face control instability and protection challenges. Screening values below about 3 are commonly treated as weak, and below about 2 as very weak, although there is no universal threshold and the inverter manufacturer’s minimum SCR always applies.
How to Use the Short Circuit Ratio Calculator
Click Load example to see how it works, or enter your own plants. Open Cost screening to set a target SCR and your own costs. Everything runs in your browser, and nothing you enter is sent anywhere.
SCR-Screen Lite
NOTE: Runs entirely in your browser. Nothing you enter is sent anywhere.
Your Power Plants
Short-circuit MVA should exclude inverter-based resources (as NERC defines SCR). Power plants with the same group label are also screened together (WSCR).
Flag thresholds (SCR)
Cost screening (optional)
Set the SCR you want each plant to have. Plants below it are marked in the results with two ways to get there: a smaller plant, or a synchronous condenser at the point of interconnection. Flags stay on the standard screening bands above. Enter your own costs; this calculator includes no cost data.
Results
Each plant on its own (SCR)
| Plant | SCR | Flag (screening bands) | Max size, no flag (MW) | Target check | Condenser (MVA) | SCR after condenser | Total cost |
|---|
* Incomplete: a needed cost rate is blank.
Plants screened together (WSCR)
How it is calculated. SCR = short-circuit MVA ÷ plant MW. WSCR = Σ(SCMVAᵢ × Pᵢ) ÷ (ΣPᵢ)², assuming the plants in a group are fully interacting (ERCOT/NERC). Max size (largest plant before a weak flag) = SCMVA ÷ weak threshold. Condenser MVA = max(0, MW × target SCR − SCMVA) × (X″d + Xt), assuming the condenser is at the point of interconnection. Default flag bands follow HVDC planning practice (IEEE Std 1204-1997); there is no universal weak-grid threshold.
Screening only. Results do not replace interconnection studies, detailed positive-sequence or EMT analysis, or the transmission provider's requirements. SCR-based metrics assume grid-following inverters and do not replace the inverter manufacturer's minimum SCR. Note that cost screening excludes network upgrades, which are often the largest interconnection cost.
Need a Full Network Scan?
The full SCR-Screen tool calculates short-circuit MVA directly from a network model, scans every bus, and compares the classical and IEC 60909 methods. It is free and open-source: install it with pip install scr-screen, or see the documentation on GitHub.
