Calculate network impedance from short-circuit power

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Calculate network impedance from short-circuit power

Inputs

Known quantity

Nominal system voltage

V

Short-circuit power S_k″

MVA

Short-circuit current I_k″

kA

Voltage factor c

R/X ratio of the source

Result

Network impedance

Ω

Short-circuit current I_k″

kA

Short-circuit power S_k″

MVA

Resistance R

Ω

Reactance X

Ω

Formula

• From power: Z = c · U² / S_k″
• From current: Z = c · U / (√3 · I_k″)
• Reverse: I_k″ = c · U / (√3 · Z)
• Split: X = Z / √(1 + (R/X)²), R = (R/X) · X

Assumptions and standards

• The voltage factor c and the R/X ratio are entered here. Both depend on the voltage level and the case considered; the standard you apply governs.
• The calculation gives the symmetrical initial short-circuit current, not the peak short-circuit current.
• The impedance applies at the point for which the network operator gave the figure. Everything downstream adds to it.

A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.

FISEKON GmbH – Fischer Elektrokonstruktion · https://fisekon.com