Traction transformers
A traction transformer is the power conversion unit that steps down high-voltage supply — whether from the catenary on an AC-fed train or from the utility grid at a substation — to the voltage levels required by traction and auxiliary systems.
The transformer sits at both ends of the traction power chain: onboard the vehicle, and in the fixed power supply and electrification infrastructure that feeds it. The two roles impose very different design constraints.
Onboard transformers
On AC-fed electric multiple units and locomotives, the traction transformer is mounted on the vehicle — typically under the floor or in a dedicated equipment bay — and steps the catenary voltage down to the levels required by the four-quadrant converter and traction inverter chain.
A multi-system transformer for a vehicle operating under both 15 kV/16.7 Hz and 25 kV/50 Hz typically carries four secondary traction windings plus an auxiliary winding for onboard passenger and comfort systems (hotel power). Voltage changeover between the two AC systems occurs on the secondary side.
DC operation on 1.5 kV or 3 kV networks uses the traction secondary windings as input filter reactors rather than for voltage transformation.
Operating conditions and standards
Onboard traction transformers must meet dimensional and weight constraints that stationary transformers do not face. They operate under continuous mechanical vibration, shock from track irregularities, rapid thermal cycling from the traction load profile, and ambient conditions spanning −40 °C to +70 °C.
The governing standard is EN 60310 (IEC 60310), which defines terminology, service conditions, and test requirements for onboard traction transformers. EN 61373 specifies the shock and vibration profiles the transformer must withstand as a complete assembly.
Substation transformers
Fixed traction transformers at substations step down grid voltage to the catenary supply voltage. A 25 kV AC substation transformer takes a primary feed from the 110–400 kV transmission grid and delivers 25 kV single-phase to the overhead contact system.
The fixed-installation standard is EN 50329, which notes that these units are subject to frequent short circuits and current shocks from the traction load. Testing requirements reference IEC 60076-5 for short-circuit withstand capability.
Substation transformers may be oil-cooled or dry-type; oil-cooled units dominate at high power ratings due to better heat dissipation.
Insulation and cooling
Traction transformer insulation is built on one of two base media: mineral oil, thermally rated Class A, or dry-type synthetic resin, thermally rated Class H. A hybrid Class F rating, using upgraded oil-based materials rated for higher continuous temperatures, is also used where a higher thermal margin is needed without moving to a fully dry design. Dry-type cast-resin transformers are used in underground and urban installations where oil presents a fire risk.
Cooling is a critical design parameter: onboard units are constrained by the vehicle’s aerodynamic envelope and weight limits, while substation units can use forced oil cooling with external radiators.
Traction transformers — both onboard and catenary-connected units — are explicitly excluded from the EU’s Ecodesign efficiency regulation for power transformers. Loss measurement and declaration instead follow EN 50329 and EN 60310, while actual efficiency levels are set in individual procurement specifications.
Higher-efficiency core materials, including amorphous alloy steel, reduce no-load losses compared with conventional grain-oriented electrical steel, and are increasingly specified on that procurement basis.