Project snapshot
| Sector | Urban rail transit — new metro line, trackside traction substations |
| Region | [Latin America ] |
| Application | Auxiliary power for signalling, station services and tunnel loads |
| Product | Cast resin dry-type auxiliary transformer |
| Rating | [ ] kVA |
| Voltage ratio | [ ] / 0.4 kV |
| Vector group | [Dyn11] |
| Cooling | AN with forced-air (AF) headroom |
| Enclosure | [IP23 / IP54] |
| Standard | IEC 60076-11 (dry-type), routine tests per IEC 60076 |
| Delivered | [Q · 20 ] |
The challenge
A traction substation on an urban metro line does two jobs at once. The rectifier transformers feed the DC traction network; a separate set of auxiliary transformers steps the same medium-voltage bus down to 400/230 V for everything that keeps the railway running — signalling and interlocking, telecoms and SCADA, station lighting and ventilation, platform screen doors, fire pumps and escalators.
That second job is where the design constraints bite. Four of them shaped this project.
Space. Trackside equipment rooms on an underground line are fixed by the civil works. The units had to fit the room as built, with cable bending radius and escape clearance intact — not the other way round.
Vibration and shock. Train passages produce continuous mechanical vibration through the slab, and it never stops during service hours. Winding clamping, core bracing and every bolted connection have to hold for the design life of the line, not just survive FAT.
Fire safety in an enclosed space. A trackside room below street level with limited egress is the worst possible location for a flammable insulating liquid. An oil-filled design would have dragged in bundled containment, fire-rated separation and oil-condition monitoring — none of which the room could accommodate.
Continuity of the signalling load. Auxiliary power is not a convenience load. If it drops, signalling drops, and the line stops. Availability expectations here are closer to a critical-care facility than to a commercial distribution board.
On top of that, the units sit electrically alongside rectifier groups, so the winding and insulation system has to tolerate the harmonic spectrum those rectifiers inject into the bus.
Our solution
Cast resin construction — no flammable liquid
We supplied cast resin dry-type auxiliary transformers. Windings are vacuum-cast in epoxy resin, giving a self-extinguishing, non-hygroscopic insulation system with no oil to contain, monitor or dispose of. For an enclosed, personnel-adjacent room this removes the fire-load question from the design review entirely and lets the room stay exactly as built. Where a project genuinely needs oil, the trade-offs are set out in our comparison of oil-immersed vs dry-type transformers.
Mechanical design for continuous vibration
Vibration resistance was engineered into the assembly rather than bolted on afterwards. Core and coil assemblies are pre-stressed and clamped to keep winding pre-load stable over the service life; the core is stepped and braced to limit magnetostriction-induced movement; and all terminal and busbar connections use locking arrangements suited to a vibrating environment. The enclosure is stiffened so that panel resonance does not amplify the input.
Design was aligned to the railway supply and interface requirements the EPC specified for the line, with documentation packaged for the project’s approvals process.
Harmonic tolerance next to the rectifier units
Auxiliary transformers on a traction bus do not see a clean sinusoidal supply. Rectifier groups generate characteristic harmonics, and the auxiliary units must not overheat or resonate under that spectrum. Winding and shield arrangements were selected to keep eddy-current and stray losses in check under harmonic load, with thermal margin preserved so that rated output does not depend on an ideal supply.
Compact footprint and maintainability
Cast resin construction removes oil containment from the layout, which is usually what frees the most floor area. Units were arranged to preserve withdrawal routes and inspection access in a room that could not be enlarged, with forced-air (AF) cooling available as headroom for peak and future load rather than as a permanent dependency.
Technical specification
| Parameter | Value |
|---|---|
| Rated power | [ ] kVA |
| Primary / secondary | [ ] / 0.4 kV |
| Frequency | [50 / 60 Hz] |
| Vector group | [Dyn11] |
| Impedance voltage | [ ] % |
| Insulation class | [F / H] [temperature rise … K] |
| Cooling | AN / AF |
| Enclosure | [IP23 / IP54], [RAL …] |
| Vibration / shock | |
| Standards | IEC 60076-11, IEC 60076, [EN/IEC ] |
Testing and verification
Every unit passed routine tests in accordance with IEC 60076 before release: winding resistance measurement, voltage ratio and vector group verification, no-load loss and current, load loss and impedance voltage, applied and induced voltage withstand tests, and insulation resistance checks.
If you need a reference for what a unit should pass before shipment, see our guide to transformer routine testing.
Result
Auxiliary power for signalling, station and tunnel loads across the new line, delivered in a footprint the civil works had already fixed, with no flammable insulating liquid anywhere in the installation. The units entered service with the line and have operated under continuous traffic vibration since commissioning.
Planning a metro, light rail or mainline project? Review our rail transit transformer solutions, or send us your duty cycle and room dimensions for a proposal.
Frequently asked questions
What is the difference between an auxiliary transformer and a traction rectifier transformer?
A rectifier transformer feeds the DC traction network via rectifier groups — its output is DC at 750 V or 1500 V (or other values defined by the railway supply standard). An auxiliary transformer is a conventional distribution transformer: it steps the medium-voltage bus down to 400/230 V AC to power signalling, telecoms, lighting, ventilation, platform screen doors and fire systems. Both sit in the same substation, but they serve entirely different loads and are specified differently.
Why use dry-type transformers in a metro traction substation?
Trackside and underground equipment rooms have limited egress and, in many cases, no practical way to provide oil containment and fire-rated separation. Cast resin dry-type units carry no flammable insulating liquid, which removes bundled containment, oil monitoring and disposal from the design — and typically frees floor area as well. They also tolerate the damp, dusty tunnel environment better than ventilated oil-filled designs.
How is vibration resistance achieved and verified?
It is a mechanical design task rather than a test you pass at the end: pre-stressed winding clamping, braced core construction, stiffened enclosure and locking arrangements on all connections. Where a project specifies it, verification is by type or special tests agreed with the EPC against the railway standard in force .
How do harmonics from the rectifiers affect the auxiliary transformer?
Rectifier groups inject characteristic harmonics into the medium-voltage bus. The auxiliary transformer must not overheat under that spectrum, so winding arrangement, shielding and loss distribution are selected for harmonic load conditions, with thermal margin kept so rated output does not depend on an ideal sinusoidal supply.