Autotransformer equivalent rating

Equivalent (self) rating and the series and common winding currents for a three-phase autotransformer. The co-ratio is 1 − VLV/VHV. Percent impedance is not split, and this page does not show a turns ratio.

Co-ratio α = 1 − VLV/VHV
Equivalent (self) rating
LV line current
Series winding current
Common winding current
Validation example. Inputs: throughput 100 MVA, HV line voltage 132 kV, LV line voltage 66 kV. Expected: co-ratio 0.5, equivalent rating 50 MVA, LV line current 874.8 A, series winding current 437.4 A, common winding current 437.4 A. References: IEEE C57.12.10 uses the co-ratio α = 1 − VLV/VHV for the equivalent two-winding rating, Seq = Sthrough × α. IEC 60076-1 rated current is I = S / (√3 · U). Series current is the HV line current. Common current is ILV − IHV.

Method

The co-ratio α = (VHV − VLV) / VHV is the fraction of throughput that is transformed. IEEE C57.12.10 uses that co-ratio for the equivalent two-winding (self) rating: Seq = Sthrough × α. The rest of the throughput is conducted, which is why the autotransformer is smaller than a two-winding unit of the same throughput.

Three-phase line current is I = S / (√3 · U), the rated-current relationship in IEC 60076-1. Series-winding current equals the HV line current. Common-winding current is ILV − IHV, which is ILV × α. Voltages are line-to-line. This sheet is three-phase only.

Percent impedance referred to the same throughput is not divided by the square of the voltage ratio. A single uk% is not split into a common-winding impedance on this page.

Questions about a specific unit: hello@transformerpath.com. The 3D series and common winding model is on the autotransformer page.

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