An autotransformer’s terminal power passes through a combination of conductive connection and magnetic transformation. The terminal MVA can therefore be larger than the winding-section MVA. Those quantities should not be used interchangeably in procurement or a system model.
Define an ideal single-phase step-down connection. The common section runs from neutral to the low-voltage tap, and the series section runs from that tap to the high terminal. Let r = VL/VH, with aligned voltage phasors and 0 < r < 1. Ignore losses, excitation current and any tertiary load.
Derive the current and power shares
Take IH entering the high terminal and IL leaving the low terminal. Ideal complex-power balance gives VH IH* = VL IL*, so IH = rIL. The common-section current, referenced downward from the tap to neutral, is IC = IH − IL = −(1 − r)IL.
The series voltage is VS = VH − VL. Its apparent duty is |VS IH*| = (1 − r)|Sthrough|. The common section has the same apparent duty magnitude: |VL IC*| = (1 − r)|Sthrough|. These are two winding-section duties associated with the same magnetic transfer; adding them does not double the transformed power.
| Illustrative single-phase quantity | Value | Interpretation |
|---|---|---|
| High and low voltage | 100 and 80 kV | Ratio r = 0.8 |
| High and low current | 100 and 125 A | Same 10 MVA terminal duty |
| Series voltage/current | 20 kV and 100 A | 2 MVA section duty |
| Common voltage/current | 80 kV and 25 A magnitude | 2 MVA section duty |
| Conductive share | 8 MVA in this ideal partition | Remaining portion of terminal transfer |
The values are an algebraic example, not equipment ratings. With consistent complex phasors, the transformed complex-power share is (1 − r)Sthrough and the conductive share is rSthrough under the stated ideal connection. Reactive power must follow the same phasor relationship; multiplying unrelated RMS values and power factors would break the balance. All numerical inputs and limits in the illustrative cases are assumptions unless a catalogue or standard source is explicitly identified.
Keep the basis with every number
A quoted “transformed capacity” may describe this ideal partition, a winding thermal rating or a manufacturer’s design basis. Ask which definition is intended. The simple fraction does not automatically describe a loaded tertiary or an auxiliary regulating arrangement.
Likewise, a through-power rating does not independently specify every internal current limit. Their relationship depends on ratio and connection. Reconfiguration can change it even if the core and conductor remain unchanged. Keep connection, tapping and network assumptions explicit in the application assessment [1].
Use the derivation to check a rating statement: terminal complex powers should balance, section currents should satisfy the tap-node equation, and each winding duty should use its own voltage. That three-part check prevents a terminal MVA from being silently substituted for the magnetic or conductor duty it was never meant to describe. Record rated quantities and test conditions on the general power-transformer basis of IEC 60076-1 [2].
Check the winding-duty limit before quoting through-MVA
All numerical values above are assumptions for an ideal single-phase transfer at 50 Hz. They give r = 80/100 = 0.80 and transformed duty (1 − r)×10 = 2.0 MVA. If the assumed common conductor permits only 20 A while the series conductor permits 100 A, then IL ≤ 20/(1 − 0.8) = 100 A and through-duty is limited to 80 kV×100 A = 8.0 MVA. Its associated series current is 80 A. Select the lower internal constraint rather than retain a 10 MVA transfer solely because one conductor can carry 100 A [2]. Assess the relevant measured quantities with a defined measurement model and propagated uncertainty [3].
References
- IEC, IEC 60076-8:1997, Power transformers – Part 8: Application guide, 1997, official public scope.
- IEC, IEC 60076-1:2011, Power transformers – Part 1: General.
- JCGM, JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement, 2008.