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Transformer short circuit fault current calculator with equations

Calculates the short circuit fault current level of a 3-phase, core type transformer with a Dyn winding connection.

See Also

Transformer fault current calculator parameters

  • Transformer rating (S): Rating in kVA.
  • Voltage rating (VLL): Line-to-line voltage rating of the secondary windings in volts.
  • Impedance (\(Z_{\%}\)): Per-unit impedance of the transformer in percentage. Can be found on the nameplate. Typically between 4% and 10%.

Infinite bus assumption

The transformer fault level calculator assumes that the transformer is supplied from an infinite bus.

This means, the fault level on the primary side of the transformer is considered to be infinite.

This assumption will give the worse case fault level on the secondary side of the transformer, which is sufficient for sizing of equipment according to fault rating.

However, this assumption is not reliable for the folllowing:

  • Earth loop impedance calculations.
  • Protection grading calculations.
  • Motor start-up calculations.

How to calculate transformer short circuit current?

Three-phase fault \(I_{3p}= \dfrac{S}{\sqrt{3} \times V_{LL} \times Z_{1}}\)
Phase-to-phase fault \(I_{pp}= \dfrac{S}{ V_{LL} \cdot \left( Z_{1} + Z_{2} \right) }\)
Phase-to-neutral fault \(I_{pn}= \dfrac{3 \cdot S}{\sqrt{3} \cdot V_{LL} \cdot \left( Z_{0} + Z_{1} + Z_{2} \right) }\)
Phase-to-earth fault \(I_{pe}= \dfrac{3 \cdot S}{\sqrt{3} \cdot V_{LL} \cdot \left( Z_{0} + Z_{1} + Z_{2} \right) }\)
Where, Z0, Z1, and Z2 are the zero, positive and negative sequence impedances.

How to calculate transformer zero, positive and negative sequence impedances?

Positive sequence impedance \(Z_{1} = \dfrac { Z_{\%} } { 100 } \)
Negative sequence impedance \(Z_{2} = Z_{1} \)
Zero sequence impedance \(Z_{0} = 0.85 \cdot Z_{1} \)
Core type, with Dyn (delta-star-earth) connections.