Auto-transformer — savings, tertiary, applications

Reference notes

The auto-transformer uses ONE physical winding shared by primary and secondary circuits, achieving substantial copper and iron savings over a conventional 2-winding transformer when the voltage ratio is close to 1. Use Next → to walk through the kVA-savings formula, the role of the delta tertiary winding, application guidelines, and the disadvantages that limit auto-transformer deployment.

Construction

• Single winding with a tap somewhere along its length.
• Series winding = portion between supply terminal and tap; carries only primary input current I_1.
• Common winding = portion between tap and ground/neutral; carries primary + secondary current = I_2 − I_1.
• Voltage ratio a = V_1 / V_2 = turns ratio (same as conventional transformer).
• NO electrical isolation between primary and secondary circuits.

Copper / iron savings

Sweet spot: 1.05 ≤ a ≤ 3. Beyond a ≈ 3, the no-isolation drawback dominates.

Tertiary winding (in large grid auto-transformers)

Nearly all utility-scale auto-transformers (above ~50 MVA) include a third winding, typically delta-connected at 13.8–33 kV, rated 1/3 of auto kVA. Three roles:

1. Stabilize neutral — provides a path for zero-sequence current circulation in wye-wye autos, preventing neutral drift during unbalanced loading.
2. Sink triplen harmonics — 3rd, 9th, 15th harmonics are all zero-sequence and cannot flow in wye lines. The delta tertiary circulates them, suppressing magnetizing distortion.
3. Supply local loads — substation auxiliary, shunt capacitor banks, shunt reactors, local generation.

Disadvantages

• No electrical isolation — primary and secondary share the common winding. Faults propagate. Codes prohibit auto-transformers between systems with incompatible grounding philosophies (e.g., effectively-grounded transmission and ungrounded industrial).
• Low per-unit impedance — typically 5–12 % vs 8–15 % for equivalent 2-winding. Result: higher through-fault current downstream → larger interrupting ratings required for switchgear.
• Grounding sensitivity — wye-wye auto with grounded neutrals on both sides is standard, but requires the delta tertiary for triplen suppression.

Re-connecting a 2-winding as an auto

A small 2-winding transformer rated S_2w (with windings V_1 and V_2) can be re-connected as an auto-transformer with a much higher kVA rating S_auto at a new output voltage V_auto. The auto rating scales by (1 + a) where a = V_2/V_1 (or similar, depending on direction). A classic PE-exam scenario: a 5 kVA 240/120 V 2-winding reconnected as a 240/360 V step-up auto becomes a 15 kVA device (S_auto = 15 kVA at V_auto = 360 V). The increased rating is what makes auto-reconnection an economical option for certain field retrofits.

Applications

• Grid intertie auto-transformers — 138/230, 230/345, 345/500, 500/765 kV interconnections. The dominant application.
• Motor starting auto-transformers — reduced-voltage start of large induction motors. Standard taps at 50 / 65 / 80 % of full voltage. Korndorfer connection.
• Distribution voltage regulators — pole-mounted auto-transformers with motor-driven tap changers. ±10 % in 16 steps. ANSI 90 control.
• Variac — variable auto-transformer with continuous brush-tap movement. Standard lab equipment for adjustable AC.
• Audio impedance matching — tube-amplifier output transformers often use auto-transformer construction for output impedance matching.

NOT used for

• Generator step-up (GSU) — ratio 13.8 → 138-500 kV (a = 10-36) is too large; no savings, isolation needed for fault containment.
• Distribution service transformer — 13.8 / 4.16 / 0.480 kV ratios with public-facing secondaries require isolation for safety.
• Between grounding-incompatible systems — prohibited by codes regardless of ratio.