About the Short Circuit Analysis (PSS)
Short-circuit level calculation determines the maximum fault current available at solar plant HT busbars for both three-phase symmetrical faults (highest magnitude) and single-line-to-ground faults — the two design-governing fault types that set equipment interrupting ratings, busbar ratings, and protection settings.
Formula
I_sc3 = V / (√3 × Z_total)
- V
- Nominal line voltage (V)
- Z_total
- Total positive-sequence impedance from the source to the fault point (Ω)
- I_sc3
- Symmetrical three-phase fault current (A rms)
How to calculate it
- 1Obtain the grid short-circuit MVA from the DISCOM at the point of common coupling (PCC). Convert to impedance: Z_grid = kV² / SC_MVA.
- 2Add the transformer impedance: Z_tr = u_k% × kV² / MVA_rated.
- 3Add cable/line impedance from the transformer HV terminals to the fault point.
- 4Total Z_total = Z_grid + Z_tr + Z_line. Calculate I_sc3 = V / (√3 × Z_total).
- 5Multiply I_sc3 by the asymmetry factor (1.5–1.8 for near-source faults per IEC 60909) to obtain peak fault current for mechanical withstand checks.
Worked example
33 kV bus: grid SC level = 500 MVA, Z_grid = 33² / 500 = 2.18 Ω. Transformer 12.5 MVA, 6% impedance: Z_tr = 0.06 × 33² / 12.5 = 5.24 Ω. No significant cable: Z_total = 7.42 Ω. I_sc3 = 33,000 / (1.732 × 7.42) = 2,568 A ≈ 2.57 kA. Equipment must be rated ≥ 2.57 kA symmetrical. Peak current = 2,568 × 1.7 = 4,366 A.
What you can calculate
- 3Ø fault kA
- IEC 60909
- SLG fault
- Asymmetrical factor
Standards & references
Frequently asked questions
Does solar plant generation increase or decrease the short-circuit level?
Modern grid-following inverters contribute very little fault current (typically 1–1.5 pu for 100–150 ms, then current-limited). Grid-forming inverters can sustain higher fault current for longer. In practice, the solar plant generation is usually ignored in the short-circuit calculation because its contribution is small and conservative to omit. However, if the plant uses multiple synchronous condensers or large BESS with grid-forming inverters, their contribution must be included.
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