In the railway sector, service continuity is a system requirement: a shutdown, an anomaly or non-compliant behaviour affects not only productivity, but also operational reliability and safety.
In this context, Ortea was selected by an Italian company belonging to a large multinational that delivers complete, integrated infrastructure solutions for the global railway industry, for an annual supply programme with quarterly deliveries of multiple transformers dedicated to UPS systems.
This article explains what it takes to build a UPS transformer for railway applications and how Ortea approaches the design challenge when strict technical specifications, tight dimensional constraints and railway standards come into play.
HOW DO YOU DESIGN AN ISOLATION TRANSFORMER FOR A RAILWAY UPS?
A railway UPS transformer is not a standard isolation transformer. It often has to work with 12-pulse rectifiers, manage harmonics, meet short-circuit impedance/inductance values required by the technical specification, limit inrush and no-load currents, fit within prescribed dimensions and comply with railway standards (in addition to general standards).
Ortea designs it using multi-physics calculation – magnetic + thermal + mechanical – and industrialises it with production planning and repeat deliveries.
WHY IS UPS TRANSFORMER DESIGN MORE DEMANDING IN THE RAILWAY SECTOR?
A railway UPS must ensure continuity even under non-ideal conditions: mains disturbances, sensitive loads, installation constraints and, above all, requirements that are rarely fully aligned. In these applications, the transformer is not a secondary component: it directly influences power quality, compatibility with converters (rectifier/inverter), thermal performance and compliance with specifications.
In the reference case, the UPS chain requires two transformer families: an input transformer feeding the rectifier (12-pulse architecture) and a downstream inverter transformer for voltage adaptation and harmonic control.
Complexity increases due to current and voltage harmonics affecting losses and heating, restrictive thermal limits (insulation class and temperature rise), tight footprint and mechanical/connection requirements, and dedicated railway standards.
TYPICAL ARCHITECTURE: RECTIFIER TRANSFORMER + INVERTER TRANSFORMER
TRANSFORMER FOR A 12-PULSE RECTIFIER (SIX-PHASE SYSTEM)
To build a 12-pulse rectifier – useful to contain ripple and reduce disturbance back to the mains – you need a transformer with two secondary windings that generate two three-phase sets.
Typical requirements include high impedance between primary and secondaries (in the described case: a significant Vcc%), often achieved via an integrated reactor (shunt) to reduce footprint and cost without compromising linearity, with careful attention to saturation. Equally critical is the impedance between the two secondaries to limit overcurrents in the converter; this depends on geometry and winding spacing, where calculation accuracy makes the difference.
A further frequent constraint is low no-load current, often specified in the technical specification and influenced by steel quality, selected flux density and manufacturing execution, not only by the design “on paper”.
TRANSFORMER ON THE INVERTER OUTPUT (STEP-UP + HARMONIC CONTROL)
Downstream of the inverter, a transformer is needed to adapt voltage (for example to 400 V) and, at the same time, limit current harmonics through leakage reactance. A typical solution is air leakage with a dedicated Vcc%. In addition, the core must handle the thermal effect of voltage harmonics injected by the inverter.
MULTI-PHYSICS CALCULATION: A CONTROLLED DESIGN TRADE-OFF
In railway projects, strict and partly conflicting targets are common: minimising footprint, limiting losses and heating, containing inrush current, meeting the Vcc%/Xcc% values required by the technical specification and maintaining low no-load current. This calls for a controlled design compromise.
Designing a railway UPS transformer means precisely controlling how core and windings behave at rated conditions and under harmonics, intended (or imposed) leakage, geometric constraints and thermal limits. Ortea addresses this complexity with a multi-physics approach integrating magnetic design (flux, induction, core losses, leakage, inter-winding impedance), electrical design (Vcc/Xcc, short-circuit currents, harmonic impact and compatibility with rectifiers/inverters), thermal design (copper/core losses, allowable temperature rise, heat dissipation vs footprint and ventilation) and mechanical/manufacturing design (internal layout, clearances, terminals, wiring and production repeatability).
In practice, it is not about adapting a standard transformer but designing around the railway specification to find the optimal balance between performance, compliance and industrialisation, ensuring consistent repeat supplies over time.
SHORT-CIRCUIT INDUCTANCE/IMPEDANCE (XCC/VCC%) AND INTEGRATED REACTORS
When high values are required between primary and secondaries, an integrated reactor is often the most effective way to limit footprint. It must be sized to avoid saturation and ensure linearity in the expected conditions.
CORE LOSSES AND STEEL SELECTION (GO)
Harmonics can significantly increase core losses. Selecting grain-oriented steel (GO) and its thickness is a decisive design choice because it directly affects temperature, cost and transformer reliability. In railway applications, where thermal limits and continuity requirements are stringent, this choice must be governed by accurate calculations and specialist know-how.
NO-LOAD CURRENT AND INRUSH CURRENT
Reducing inrush current can work against no-load current and core losses (for example by increasing the air gap). The best outcome comes from a combination of steel selection, air-gap sizing and manufacturing quality.
TERMINALS AND MECHANICAL CONSTRAINTS
Railway projects often require a specific terminal arrangement and dedicated connections. This translates into tight mechanical accuracy and a non-trivial internal layout.
TEMPERATURE: MAXIMUM TEMPERATURE RISE AND INSULATION CLASS
The design aims to contain losses and optimise heat-dissipating surfaces to meet thermal limits. In the cited case: insulation class B with limited temperature rise, in line with the technical specification.
COMPLIANCE: DESIGNING TO THE SPECIFICATION, NOT ADAPTING AFTERWARDS
In the railway sector, compliance is not an end attachment: it is a design driver. A correct approach starts from analysing the technical specification and requirements (electrical, thermal, dimensional, connections), continues with magnetic design aligned to the specification, and is completed by verification and industrialisation focused on repeatability, so each batch delivers the same performance and installation constraints.
In the reference case, the transformers are declared compliant with dedicated railway standards as well as general standards.
BEYOND “DESIGN”: CONTINUOUS SCHEDULED SUPPLIES
The project includes a supply programme with annual planning and quarterly deliveries. This requires standardisation where possible and customisation where needed, control of the bill of materials and critical raw materials, variant management and component traceability, repeatable production and testing processes, and logistics coordination for timed deliveries aligned with the customer’s needs.
For the customer this means supply continuity, with confidence that each quarter the delivered product matches the technical specification, expected performance and installation constraints, reducing the risk of downtime, rework or site delays.
WHY CHOOSE ORTEA AS A PARTNER FOR RAILWAY UPS TRANSFORMERS
Ortea combines multi-physics know-how (magnetic + thermal + mechanical) to handle complex and often conflicting requirements, experience with UPS architectures (12-pulse rectifiers and inverter transformers) with attention to harmonics, losses and real-world magnetic dynamics, and a truly specification-driven approach where footprint, terminals, Vcc/Xcc, no-load and inrush currents, thermal limits and installation constraints become controlled design parameters.
This is supported by proven supply reliability: the ability to plan production and repeat deliveries, particularly valuable when customers run progressive projects, sites or multi-site programmes and require batch-to-batch consistency.
FAQ
IS A RAILWAY UPS TRANSFORMER THE SAME AS A STANDARD ISOLATION TRANSFORMER?
No. It often has to meet additional requirements linked to rectifiers/inverters (harmonics, inter-winding impedance, no-load and inrush currents), footprint constraints and compliance with railway standards.
WHY USE A 12-PULSE RECTIFIER, AND WHAT DOES IT CHANGE FOR THE TRANSFORMER?
A 12-pulse solution helps contain ripple and reduce disturbance back to the mains, but it requires a six-phase system achieved with a dual-secondary transformer (two three-phase sets) and precisely designed inter-winding impedance.
WHAT DOES “INTEGRATED REACTOR” MEAN IN A TRANSFORMER?
It is a solution that integrates reactance effects to reach the required impedance while limiting footprint. It must be designed to avoid saturation and ensure linearity under the expected conditions.
WHY ARE HARMONICS SO IMPORTANT FOR THE CORE AND TEMPERATURE?
Because they increase losses (including core losses) and therefore heating. Material selection and core optimisation are essential to meet the thermal limits set by the technical specification.
