Calculator Inputs
Enter generator nameplate data and reactances to estimate symmetrical and peak fault duty.
Example Data Table
These sample cases illustrate how reactance, voltage factor, and external impedance affect available fault duty.
| Case | Rating | Voltage | Xd"" | Xd' | Xd | Zext | I""k | I'k | Ik | ip |
|---|---|---|---|---|---|---|---|---|---|---|
| Diesel Generator A | 5 MVA | 11 kV | 18% | 28% | 160% | 0% | 1.458 kA | 0.937 kA | 0.164 kA | 3.757 kA |
| Hydro Unit B | 12 MVA | 6.6 kV | 16% | 24% | 140% | 4% | 5.511 kA | 3.936 kA | 0.765 kA | 13.898 kA |
| Gas Turbine C | 20 MVA | 13.8 kV | 14% | 22% | 130% | 6% | 4.602 kA | 3.287 kA | 0.677 kA | 11.364 kA |
Formula Used
The calculator uses a generator base-current method for a balanced three-phase bolted fault. All reactances are converted from percent to per-unit values on the machine base.
| Item | Equation | Meaning |
|---|---|---|
| Base Current | Ibase = S / (√3 × V) | Rated current from apparent power and line voltage. |
| Initial Symmetrical Current | I""k = c × Ibase / Z""pu | First-cycle RMS current using subtransient reactance plus external impedance. |
| Transient Current | I'k = c × Ibase / Z'pu | Intermediate RMS current using transient reactance plus external impedance. |
| Steady Current | Ik = c × Ibase / Zpu | Longer duration RMS current using synchronous reactance plus external impedance. |
| Time-Domain Estimate | I(t) = Ik + (I'k − Ik)e−t/Td' + (I""k − I'k)e−t/Td"" | Decay estimate between initial and longer-duration current values. |
| Peak Asymmetrical Current | ip = κ × √2 × I""k | Approximate first-peak current using the entered X/R ratio. |
| Peak Factor | κ = 1.02 + 0.98e−3/(X/R) | Approximation used for the DC-offset contribution. |
| Fault Level | Fault MVA = Sbase / Zpu | Short-circuit level referenced to the selected reactance stage. |
This page is intended for balanced three-phase generator faults. Unbalanced faults require sequence impedances and grounding details.
How to Use This Calculator
- Enter the generator apparent power and select kVA or MVA.
- Enter the rated line voltage and choose volts or kilovolts.
- Provide power factor for rated real-power context.
- Set the voltage factor if you want prefault voltage adjustment.
- Enter subtransient, transient, and synchronous reactance values in percent.
- Add any external impedance to represent transformer, cable, or reactor effects.
- Enter X/R ratio to estimate the first peak asymmetrical current.
- Enter Td"" and Td' plus an evaluation time for current decay analysis.
- Press Calculate Fault Current to show the result above the form, then export it as CSV or PDF.
Frequently Asked Questions
1) What current values does this calculator report?
It reports symmetrical RMS subtransient, transient, and steady fault current. It also estimates current at a selected time and the first peak asymmetrical current.
2) Why are three generator reactances needed?
Xd"" represents the earliest fault period, Xd' represents the following decay period, and Xd represents the longer steady condition. Different protection checks use different stages.
3) What does external impedance represent?
It models additional impedance between the generator and the fault, such as transformers, cables, reactors, or buswork. Higher external impedance lowers calculated fault current.
4) Why does the page ask for X/R ratio?
X/R ratio is used to estimate the DC offset effect in the first current peak. Higher X/R usually produces a larger asymmetrical making duty.
5) Does power factor directly change the fault current?
Not in this simplified balanced-fault model. Power factor is included mainly to show approximate rated real power from the entered generator apparent power.
6) Can I use this page for remote faults?
Yes, approximately. Add equivalent external impedance on the generator base to represent transformers, cables, or reactors between the machine and the fault point.
7) Is this valid for single-line-to-ground faults?
No. Single-line-to-ground, line-to-line, and double-line-to-ground faults require positive, negative, and zero-sequence networks plus grounding information.
8) Why is steady-state current much smaller than initial current?
Synchronous reactance is usually much larger than subtransient reactance. That increased impedance causes the current to decay after the earliest fault cycles.