Converts per-unit and percentage impedance from original equipment manufacturer (OEM) nameplate ratings to common system-wide study bases. Verifies physical ohmic impedance invariance across transformations.
Governing Formulas & Standards
Standards Basis: IEEE Std 399 / Stevenson Elements of Power System Analysis
Z_{\text{pu}}^{\text{new}} = Z_{\text{pu}}^{\text{old}} \cdot \left(\frac{V_{\text{base}}^{\text{old}}}{V_{\text{base}}^{\text{new}}}\right)^2 \cdot \left(\frac{S_{\text{base}}^{\text{new}}}{S_{\text{base}}^{\text{old}}}\right)
Scales per-unit impedance proportionally with MVA base ratio and inversely with the square of the voltage base ratio.
Worked Engineering Example: Rebasing 25 MVA, 132 kV, 10% Impedance Transformer to 100 MVA Base
- Compute Voltage Ratio Squared: (132 / 132)² = 1.0 → 1.0
- Compute MVA Ratio: 100 MVA / 25 MVA → 4.0
- Compute New Per-Unit Impedance: Z_pu_new = 0.10 × 1.0 × 4.0 → 0.40 pu (40.0%)
Final Solution: New Impedance: 0.4000 pu (40.0%) | Ohmic Invariant: 69.696 Ω
Frequently Asked Questions
- Does the actual physical impedance in Ohms change when rebasing?
- No. The physical impedance Z (in Ohms) is an intrinsic property of the copper windings and core geometry. Only the dimensionless per-unit representation changes.
Interactive calculation engine and real-time CAD solver available online at https://amithvijayan.in/tools/per-unit-base-conversion.