Calculation
uk = Uk/UN; Ik = IN/uk = IN·UN/Uk
The smaller short-circuit voltage is relative to rated voltage, the greater the unconstrained short-circuit current.
Busch 2006, Abschnitt 8.3.3.2 Kurzschlussversuch
Transformer short-circuit current
The smaller short-circuit voltage is relative to rated voltage, the greater the unconstrained short-circuit current.
MINTSI
02
Result
Select a target and calculate.
Understand the inputs
- Steady short-circuit current Ik — Symmetrical steady value from the simplified equivalent circuit at rated voltage.
- Rated voltage UN — Rated voltage on the same transformer side to which rated and short-circuit current refer.
- Short-circuit voltage Uk — Reduced voltage required in the short-circuit test to make exactly rated current flow.
- Rated current IN — Rated current on the same winding side as UN and Uk.
Example
UN=230 V, Uk=18.3 V and IN=4 A give uk≈7.96% and Ik≈50.3 A.
Assumptions and limits
Rated voltage from an ideal stiff source and a linear transformer equivalent circuit; supply impedance, cables, protection clearing and transient peak current are excluded.
Understand Transformer short-circuit current
This calculator uses short-circuit voltage as a compact measure of transformer impedance and estimates steady fault current at rated applied voltage.
What does this quantity describe?
Uk is the small test voltage that drives IN with the secondary shorted. The ratio uk=Uk/UN scales prospective steady current: Ik=IN/uk.
Formula and variables
uk = Uk/UN; Ik = IN/uk = IN·UN/Uk
uk = Uk/UNIk = IN/ukIk = IN·UN/Uk
| Symbol / input | Meaning |
|---|---|
| Steady short-circuit current Ik | Symmetrical steady value from the simplified equivalent circuit at rated voltage. |
| Rated voltage UN | Rated voltage on the same transformer side to which rated and short-circuit current refer. |
| Short-circuit voltage Uk | Reduced voltage required in the short-circuit test to make exactly rated current flow. |
| Rated current IN | Rated current on the same winding side as UN and Uk. |
Choose the inputs correctly
UN, Uk and IN must refer to the same winding side and tap. Uk is a voltage from the short-circuit test; if nameplate uk is given in percent, Uk=uk·UN/100.
How to use the calculator
Take rated voltage/current from the nameplate and matching short-circuit voltage from test or manufacturer data. Use the result only as transformer contribution and add supply/cable impedances for protection studies.
Worked example
With 230 V rated voltage, 18.3 V short-circuit voltage and 4 A rated current, uk=7.96% and steady short-circuit current is about 50.3 A.
Understand the result and units
Lower uk means a stiffer source but greater fault current. This result is neither inrush current nor the first asymmetrical peak.
UN and Uk may use different selectable voltage units because their ratio is formed in SI. IN and Ik refer to the same side.
Typical applications
Preliminary protection estimates, laboratory tests and comparison of transformer impedances.
Assumptions, limits and common mistakes
Simplified steady transformer value; upstream supply, cables, arc impedance, saturation, DC offset and clearing time are excluded.
Common mistake: Do not confuse Uk with UN or mix winding sides. Do not treat the steady value as peak short-circuit current.
Frequently asked questions
What is “Transformer short-circuit current” used for?
Preliminary protection estimates, laboratory tests and comparison of transformer impedances.
Where do the input values come from?
UN, Uk and IN must refer to the same winding side and tap. Uk is a voltage from the short-circuit test; if nameplate uk is given in percent, Uk=uk·UN/100.
What does the result not cover?
Simplified steady transformer value; upstream supply, cables, arc impedance, saturation, DC offset and clearing time are excluded.