About the PSS Busbar Sizing
Substation busbar sizing selects the cross-section and conductor material (copper or aluminium) for HV and LV busbars in a solar plant's outdoor switchyard or indoor switchgear — ensuring the busbar can carry continuous load current and withstand the peak fault current without thermal or mechanical failure.
Formula
I_thermal = J × A
- J
- Current density (A/mm²) — typically 1.5–2 A/mm² for copper
- A
- Busbar cross-sectional area (mm²)
- I_thermal
- Thermally permissible continuous current (A)
A_fault = I_fault × √t / K
- I_fault
- Prospective short-circuit current (A rms)
- t
- Fault clearance time (seconds)
- K
- Material constant — 143 for copper, 94 for aluminium
- A_fault
- Minimum cross-section for thermal withstand (mm²)
How to calculate it
- 1Determine continuous rated current for each busbar section from the connected transformer or feeder rating.
- 2Select busbar cross-section so I_thermal ≥ rated current, using 1.5–2 A/mm² for copper (indoor) or 2–2.5 A/mm² for copper (outdoor).
- 3Calculate the fault withstand cross-section A_fault using the available short-circuit level and protection clearance time.
- 4Take the larger of the two cross-sections as the minimum required size.
- 5Check the selected standard bar size (e.g. 50×6, 80×8, 100×10 mm copper) against the mechanical span requirements for rigid busbars.
Worked example
A 10 MW solar plant with a 33 kV, 12.5 MVA transformer: rated secondary current ≈ 219 A. Using J = 2 A/mm², required area = 110 mm² → select 50×6 mm copper (300 mm²). Fault level at 33 kV = 12.5 kA, clearance time = 0.5 s: A_fault = 12,500 × √0.5 ÷ 143 = 62 mm². Thermal sizing governs; use 50×6 mm copper.
What you can calculate
- Current rating
- IS 5082
- SC withstand
- Temperature rise
Standards & references
Frequently asked questions
When should I choose aluminium over copper for solar substation busbars?
Aluminium is preferred for large outdoor busbars (>200 mm² cross-section) in utility-scale solar plants because it is lighter and cheaper per ampere at large sizes. Copper is preferred for indoor switchgear busbars, connections near terminal lugs, and situations where the busbar must fit in a compact panel. Aluminium has ~60% of copper's conductivity, so a larger cross-section is needed for the same current rating.
What fault clearance time should I use for busbar thermal withstand sizing?
Use the main protection clearance time for the fault level at the busbar: typically 0.1–0.3 s for a numerical differential relay on the transformer, 0.3–0.5 s for a directional overcurrent relay at 33 kV, and 0.5–1.0 s for IDMT overcurrent protection at 11 kV. Add the breaker interrupting time (1–3 cycles, ~0.02–0.06 s) to get total fault duration.
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