Example Data Table
| Case |
Base MVA |
Base kV |
Actual Value |
Expected Result |
| Voltage check |
100 |
132 |
125 kV |
0.94697 pu |
| Impedance check |
100 |
132 |
42 ohm |
0.24105 pu |
| Base conversion |
50 to 100 |
11 to 11 |
0.12 pu |
0.24 pu |
Formula Used
Per unit value = actual value / base value.
Three phase base current = Sbase / (sqrt(3) × Vbase).
Single phase base current = Sbase / Vbase.
Base impedance = Vbase² / Sbase.
Base admittance = 1 / base impedance.
Impedance base change = old pu × (new MVA / old MVA) × (old kV / new kV)².
Admittance base change = old pu × (old MVA / new MVA) × (new kV / old kV)².
Estimated fault current = base current / impedance pu.
How to Use This Calculator
Enter the system type first. Use three phase for most power network studies. Enter the base MVA and base kV. Add any actual voltage, current, power, impedance, resistance, reactance, or admittance values you want to normalize. For base conversion, enter the old value and both base sets. Press the submit button. Review the results shown above the form. Use CSV or PDF buttons to save the calculated table.
Power System Per Unit Analysis Guide
Per unit analysis turns electrical quantities into normalized values. It is widely used in power engineering. The method compares each actual value with a selected base value. This keeps transformer, line, and fault studies easier to read. It also reduces large voltage and current numbers.
Why Per Unit Values Matter
Power networks often include several voltage levels. A generator may feed a transformer, a transmission line, and another transformer. Direct ohm and ampere calculations can become confusing. Per unit values place those items on a common reference. Engineers can compare equipment ratings faster. They can also find unusual results before detailed studies begin.
Choosing Bases
A study normally starts with base power and base voltage. Three phase systems usually use line to line voltage. Once these two bases are known, base current and base impedance follow automatically. The calculator uses the selected system type to choose the correct current formula. Impedance base remains based on voltage squared divided by power.
Changing Bases
Many studies receive equipment data on different bases. A transformer nameplate may give impedance on its own MVA rating. A system model may need the same impedance on a network base. The conversion formula moves the per unit value from the old base to the new base. This prevents manual scaling mistakes.
Practical Study Uses
This calculator supports voltage, current, power, resistance, reactance, impedance, and admittance checks. It also estimates fault current from a selected impedance. That result assumes a one per unit prefault voltage. It is useful for screening. Final protection settings should still use complete standards and verified network data.
Reading the Results
A value near one per unit means the actual value is close to base. A low value shows a smaller quantity. A high value shows a larger quantity. Review the base section first. Then compare each normalized result. Export the table when you need a record for reports, assignments, or design notes.
Accuracy Notes
Use consistent units before entering values. Enter kV, kA, MVA, ohms, and siemens as labeled. Round only after calculation. Keep transformer voltage bases matched across connected windings. Recheck zero or negative inputs. This supports safer early review. Use engineering judgment for final approval.
FAQs
What is a per unit value?
A per unit value is an actual electrical value divided by its selected base value. It shows the size of a quantity relative to the study base.
Which voltage should I use for three phase systems?
Use the line to line voltage base for three phase power studies. This matches common equipment ratings and standard base impedance calculations.
Can I convert transformer impedance to another base?
Yes. Enter the old per unit impedance, old MVA, old kV, new MVA, and new kV. The calculator applies the base change formula.
Does the calculator handle admittance?
Yes. You can enter admittance in siemens. You can also choose admittance for old to new base conversion.
What does estimated fault current mean?
It is a screening value based on one per unit prefault voltage and the selected impedance. Detailed protection studies need verified network data.
Why is base current different for single phase?
Three phase current uses the square root of three factor. Single phase current does not use that factor.
Can negative reactance be entered?
Yes. Negative reactance can represent capacitive behavior. The impedance angle will reflect the entered resistance and reactance signs.
Why use CSV or PDF export?
Exports help keep a record of inputs and results. CSV supports spreadsheet review. PDF is useful for reports and submissions.