Powering High Speed: The Auto-Transformer (2x25kV) System Explained

The Auto-Transformer (AT) System, often referred to as the 2x25kV System, is the preferred power supply configuration for High-Speed Lines (HSL) and heavy-haul freight railways. Unlike standard electrification, it utilizes a dual-voltage transmission method to minimize energy loss and allow trains to draw higher power loads over longer distances.

How the 2x25kV System Works

In a conventional 1x25kV system, power is supplied via the contact wire and returns through the rails. The AT system introduces a third conductor: the Negative Feeder Wire.

• Contact Wire: Carries +25kV relative to the rails.
• Feeder Wire: Carries -25kV relative to the rails.
• Rails: Maintained at 0V (Earth potential).
• Total Transmission: The voltage between the Contact Wire and Feeder Wire is 50kV.

Auto-Transformers are placed at regular intervals (typically every 10–15 km) along the track. They connect the Contact Wire, the Feeder Wire, and the Rails, effectively balancing the voltage and forcing the return current to flow through the Feeder Wire rather than the rails/earth, except in the immediate section where the train is located.

Comparison: Classic (1x25kV) vs. Auto-Transformer (2x25kV)

The choice between a simple system (often with Booster Transformers) and an AT system depends on the operational requirements of the line.

Why is it Essential for High-Speed Rail?

High-speed trains (like the TGV or Shinkansen) consume massive amounts of power. Using a standard system would result in unacceptable voltage drops, forcing the train to slow down. The AT system ensures stable voltage at the Pantograph even when the train is far from the Traction Substation. Furthermore, because the currents in the Contact Wire and Feeder Wire flow in opposite directions, their magnetic fields largely cancel each other out, protecting nearby signaling and telecommunication cables from interference.