Railway Power Design: EN 50155-Compliant 230 VAC Power Systems

This article was originally published on: Digikey_230 VAC in Train Applications

Contents

• Why AC Power Is Entering Train Applications
  • Train Power System Architecture
  • Why AC Has Advantages Over DC
• Case Study: A Redundant Power Supply for Traction Motor Cooling
• The TBF500 & XTBF500 Solution
  • Mechanical Design for Harsh Environments
  • Specifications and EN 50155 Compliance
  • Redundant Configuration with Load Sharing
• Summary 

Why AC Power Is Entering Train Applications

For many years, 115/230 VAC power systems have already been used in trains to supply air conditioning units, fans, lighting, power outlets or small AC/DC power supplies for USB C sockets at passenger seats. However, most power supply inquiries for rolling stock applications were for units with DC inputs that comply with the supply voltages specified in EN 50155 (Figure 1).

Figure 1: Worldwide train DC supply voltages.

P-DUKE has a broad range of products compliant with these voltages and qualified for environmental conditions and EMI requirements in trains. A customer asked P-DUKE whether an existing AC/DC product could be qualified for compliance with EN 50155. The application was the liquid cooling system of an electric train propulsion system. Why are AC systems considered such applications, after decades of dominance by DC grids?

Train Power System Architecture

First, let’s look at the simplified schematic of a modern electric train’s power system shown in Figure 2.

Figure 2: Simplified block diagram of a traction transformer with propulsion and auxiliary converters and battery charger.

Energy is supplied either via overhead wires and pantographs or via sliding contacts (“shoes”) and third rails along the tracks. Typical supply voltages are 25 kVAC/50 Hz and 15 kVAC/16.7 Hz for long distance trains or 600 – 3000 VDC for subways. Traction transformers on the train step down the high voltages to the voltages needed in the train propulsion and auxiliary systems.   

Auxiliary inverters generate AC bus systems used to supply downstream loads and converters including the battery charger.

Why AC Has Advantages Over DC

P-DUKE has received information from customers that the increasing energy demand resulting from growing electrification could overload the existing DC power grid on trains. Manufacturers therefore consider using the existing 230 VAC power grid not only to supply simple loads but also for more critical applications.

Expanding a 230 VAC grid is often easier than adding wires, connectors, fuses and switches to a DC grid.   

Currents in a 230 VAC system are over 50% lower compared to a 110 VAC system and thinner wires can be used for the same power. Handling failures like short circuits or switching off loads is easier when using AC as the periodic zero-crossing of AC currents extinguishes arcs between wires and contacts. This is also why AC fuses are simpler and smaller than their DC counterparts.

Additionally, components for 230 VAC mains are manufactured in large quantities and therefore significantly cheaper than the specialized equipment designed for DC supply voltages in trains.

Case Study: A Redundant Power Supply for Traction Motor Cooling

However, everything installed on a train must also comply with the relevant standards, such as EN 50155. This was why the customer asked P-DUKE whether the TBF500, a 500W AC/DC power supply in a Full Brick package, or the XTBF500, a 500 W plug-and-play AC/DC power supply, could be qualified for use in trains. The customer was looking for a redundant 24 V/370 W power supply solution for pumps in the liquid cooling of traction motor drives.

The TBF500 & XTBF500 (Figure 3) are AC/DC power supplies with an input voltage range of 85 – 264 VAC which is compliant to the auxiliary AC voltages specified for train applications. Unlike the DC power grid (Figure 4), EN 50155 does not yet specify interruptions for the AC auxiliary power grid but the TBF500 & XTBF500 has a hold-up time of at least 16 ms, which enables operation at full load even during short interruptions of the AC grid.

Figure 3: TBF500 & XTBF500 — 500 W Full Brick & Plug-and-Play AC/DC Power Solutions. 

The TBF500 & XTBF500 Solution

The following are brief summaries of the TBF500 & XTBF500 power supplies:

• TBF500: Full-brick AC/DC module combines 500 W output, 93% efficiency, and a wide input range with EN 50155 and IEC/UL/EN 62368-1 certification. Its robust baseplate cooling and comprehensive protection suite make it a trusted power solution for railway rolling stock and harsh industrial environments.
• XTBF500: The XTBF500 delivers 500 W with full system integration — EMC filters, bulk capacitors, and inrush limiters included. Certified to EN 50155, with EN 55032 Class B EMC performance and OVC III rating, it is engineered for reliable operation in railway and mission-critical environments.

Figure 4: DC supply interruption.

Mechanical Design for Harsh Environments

This product was designed for harsh environments, is potted and approved for the environmental conditions of MIL-STD-810F. With baseplate cooling it is ideal for rolling stock applications.

Specifications and EN 50155 Compliance

The rugged design of the TBF500 series enabled P-DUKE to pass the approvals for EN50155, and with a few external components, the customer was able to design a complete solution including fuse, EMI filter, and thermal protection (Figure 5).

Figure 5: TBF500 with a few external components.

Instead of using heatsinks like in Figure 5, cooling was realized by mounting the module on the liquid cooling system of the motor drive. With an operating baseplate temperature of -40°C to +105°C, the module also meets OT4 (-40°C to +70°C) as well as the ST1 requirements for 10 minutes operation at extended operating temperatures.

Redundant Configuration with Load Sharing

As shown in Figure 2, most train traction transformer configurations offer two separate AC lines. If one line fails, the other one is still available. This enables the dual redundant configuration with two power supplies in parallel shown in Figure 6.

Figure 6: Redundant configuration with two TBF500 & XTBF500 modules, outputs are connected in parallel with O-Ring diodes. 

If one AC line or one power supply fails, the remaining TBF500 & XTBF500 will deliver the 370 W needed by the pump.

However, such a configuration only works reliably if appropriate load sharing is implemented. The TBF500 & XTBF500 modules are available with a load sharing option to make sure that both modules deliver the same power.

The droop slope characteristics (Figure 7) slightly reduces the output voltage (Vset - ∆Vset) with increasing output current. This passive load sharing method is easy to implement and does not require any external components. More details can be found in the TBF500 & XTBF500 Application Manual.

Figure 7: Output voltage V-I characteristic of the droop mode current sharing. 

Summary

By using this modular approach, the customer was able to design a EN50155 compliant AC/DC power solution with worldwide safety approvals in a very short time. It is just one example for a P-DUKE AC/DC power supply used in a rolling stock application. The increasing electrification of trains with growing power demands combined with the request for lower cost solutions will open more opportunities for the TBF500 & XTBF500 modules and other AC/DC power supplies from P-DUKE, designed for demanding applications in harsh environments with operating temperatures from -40°C up to +85°C.