Shuvangkar Das, PhD

2 minute read

If you’re working with EMT (Electromagnetic Transient) simulation in distribution or transmission networks and planning to connect an inverter at a particular bus or node, one key metric you must calculate is the Short-Circuit Ratio (SCR). It’s essential for understanding how strong or weak the grid is at that connection point.

When I first encountered SCR, I was honestly confused. There were so many formulas, so many explanations—it was overwhelming. That’s why I wrote this blog: to give you a simple, practical guide to calculating SCR using your existing EMT simulation setup.

What is Short-Circuit Ratio (SCR)?

SCR is a measure of grid strength at the point where an Inverter-Based Resource (IBR) is connected. It helps determine how well the grid can support that inverter.

\[\text{SCR} = \frac{S_{\text{SC}}}{P_{\text{IBR}}}\]

Where:

  • $S_{\text{SC}}$: Available short-circuit power at the bus (in MVA)
  • $P_{\text{IBR}}$: Rated power of the inverter (in MW or MVA)

A higher SCR indicates a stronger grid, meaning the voltage remains more stable when the inverter injects or absorbs power.

Step-by-Step: How to Calculate SCR in EMT Simulation

You likely already know the inverter rating ($P_{\text{IBR}}$). The next step is to calculate the short-circuit power $S_{\text{SC}}$ at the bus. Here’s how you can do that:

🔧 Steps:

  1. Disable the IBR at the point of interconnection (POI).
  2. Run a three-phase bolted fault at that bus.
  3. Measure the fault current, $I_{\text{SC}}$.
  4. Use this equation to compute short-circuit power:
\[S_{\text{SC}} = \sqrt{3} \cdot V_{\text{LL}} \cdot I_{\text{SC}}\]

Where:

  • $V_{\text{LL}}$ is the rated line-to-line RMS voltage (e.g., 13.2 kV)
  • $I_{\text{SC}}$ is the measured RMS fault current in kA

✅ Final SCR Calculation:

\[\text{SCR} = \frac{S_{\text{SC}}}{P_{\text{IBR}}}\]

That’s it. Run the fault, get the current, plug into the equation, and you’re done.

How to Interpret SCR

SCR is more than just a number—it tells you how likely you are to face stability and control challenges. Here’s a widely used classification:

SCR Grid Strength Description
> 3 Strong Grid can support IBRs with minimal stability issues
2 – 3 Weak Stability issues likely; detailed studies are required
< 2 Very Weak High risk of instability; needs advanced controls or mitigation strategies

This classification is consistent with NERC and industry practices for planning and stability studies.

Final Thoughts

When I was first learning this, I struggled with multiple definitions, conflicting formulas, and too much theory. But once I started running EMT simulations and applying these steps, calculating SCR became second nature.

Whether you’re designing microgrids, studying renewables, or tuning inverter controllers, knowing how to compute and interpret SCR will help you make smarter engineering decisions.

Shuvangkar Das, PhD Knoxville, Tennessee, USA

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