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on01/08/2022
[White Paper] What to Look for in a DC/DC Converter for Railway Applications

Railway electrification systems rely on a steady DC supply to both power their DC motors and
the control/energy circuits. These DC-traction systems are more energy efficient than their coal, gas, or
diesel-based counterparts while also reducing CO2 emissions. DC-to-DC converters play a crucial role
in improving the energy efficiency and power quality of electrified railways. Electronic components used
in these applications will be subjected to extremes in temperature, humidity, vibration, and mechanical
shock. As such, they require a high degree of electrical and structural integrity to be able to withstand
these conditions and reliably operate.
This article explores the requirements for DC-to-DC converters in railway systems, including a general
description of DC railways, how DC-to-DC converters are used in these systems, and the standards that
allow these devices to function regardless of operating conditions.


A Look at DC Electric Railway Systems
Early railway electrification systems were based on low-voltage DC where the supply was provided by
diode rectifiers in traction substations (TSs) along the track, with power distributed to the train’s motors
by relying on a power supply given from the overhead lines and rails. The most common voltages for
DC-traction systems are 600V for older tramways and underground systems, 750V systems in newer
metro, and 1500V used by more suburban lines. Large 3000V systems are also used in suburban
networks.


Trends for DC Electric Railway Systems
There is a general trend for railway systems to steadily increase in voltage and efficiency.
The increase in voltage allows for a decrease in current where higher gauge (thinner) cables are
sufficient. The lower voltage systems will require a high current level, which lead to thicker overheadlines and short distances between substations. The increase in energy efficiency reduces CO2
emissions by reducing the energy consumption of the installation. Energy reuse can also be employed
through regenerative braking where the kinetic energy of the rotor is converted into electricity and sent
back to the power source.


While many of these parameters ensure the device does not meet premature failure due to undesired
electrical, mechanical and environmental conditions, the mean time between failures (MTBF) is
a critical parameter that represents the long term wearout of while the device is operating in steady-state conditions.


How DC-to-DC Converters Used in Railway Systems
Regenerative braking relies on the use of a large bidirectional DC-to-DC converter to send power back
to the battery bank or energy storage system (ESS) from the DC feeder line. Smaller DC conversion is
necessary in modern railways to distribute a continuous supply of power to all electronic devices and
subsystems within the coach. These auxiliary systems include higher voltage critical systems such as
engine controls, motor drive controls, and braking systems, as well as lower voltage secondary systems
including lighting, battery charging, indication lamp, pump, and information displays, and electric door
openers. The conventional solution often involves the use of an input filter, a 3-phase inverter, and a low
frequency transformer in order to provide electrical isolation between the overhead line voltage and
the auxiliary power supply equipment.

 

Typically, this transformer is large and bulky, adding extra mass to an already space- and
weight-constrained application. Instead, isolated DC-to-DC converters can be used to meet the size,
weight, and power requirements of modern railway applications. The goal is to convert the high 750V,
1500V, and/or 3000V input voltage to a regulated 24, 28, 36, 48, 72, 96, or 110 volt output.
The power distribution within the train from the main supply also requires a chain of DC-to-DC
conversion to the secondary systems. As stated earlier, the nominal input voltages can vary from as low
as 24 volts to as high as 110 volts; from there, a regulated 3.3, 5, 12, 15,or 24 volt output is necessary.
The EN 50155 European standard for railway electrical equipment requires that the nominal input
voltage can fluctuate between 0.7 and 1.25 times the rated voltage. The standard also allows short-term
deviations between 0.6 and 1.4 times the nominal input voltage.
 

In other words, a 110V system would require a continuous voltage range between 67.2 volts and
120 volts as was a fluctuation voltage range from 66 volts to 154 volts. DC-to-DC converters can support
wide input ranges to cover more than one nominal input voltage. For example, a converter for
the common 72V and 110V railway input voltages would need an operating input voltage range from
43.2 volts to 154 volts (see Table 1). P-DUKE offers the RHKW, RHMW, and RHDW series of
mountable DC-to-DC converters rated for railway and industrial applications. These devices all offer
3,000 VAC of reinforced isolation as well as a wide input range from 36 volts up to 160 volts,
allowing them to be used in 72V, 96V and 110V railway applications.

input specifications
Table 1: Standard voltage range specifications from EN 50155 for railway electrical equipment


Transformer Considerations in Isolated Converters
Isolated DC-to-DC converters will leverage a transformer to provide galvanic isolation between the input
and the output of the device. Typically, the DC-to-DC conversion is accomplished by first translating
the signal into AC with an inverter, sending the signal through a transformer, and then through a rectifier
for a regulated DC output (so there is no metallic or conductive path between the two parts of the circuit).
The amount of isolation is dependent on the clearance and creepage distances, often defined rigorously
by the relevant standards.

 

Clearance is the distance through air between two conductors, while creepage is the distance along
the surface between two conductors. The clearance distance will prevent arcing, and the creepage
distance mitigates the chance a short will occur in the event the surface becomes contaminated and
conductive.


The transformer and the number of transformations from DC to AC (and back) will limit the level of
efficiency the converter can achieve. The isolation barrier stops the output from being directly controlled
for better output regulation. The efficiency of the transformer also has to be taken into account which can
limit the efficiency of the converter itself. P-DUKE’s patented transformer design enables operation at
efficiencies up to 90.5% even with their 4.5 mm clearance and creepage distances (Figure 1).

 

Figure 1: Patented tranformer design enables efficiencies up to 90.5% despite the 4.5mm clearance and creepage distance

 

Meeting the Harsh Conditions of Railway Applications
Outside of the EN 50155 standard, the EN61373 standard lists out the test requirements for equipment
used in railway vehicles. These devices will be subjected to near-constant vibration and, in some
instances, mechanical shock. Tests for vibration will include the simulated long-life testing where
the device under test is subjected to at least 15 hours of vibrations at a specific amplitude. For shock
testing, the device undergoes a sequence of half sine impulses to reproduce the effects of the transport.
The certifications and safety approvals for electronic equipment used in railway systems span between
EN 50155, EN 61373, and EN 44545-2 to ensure these systems performance regardless of electrical,
environmental, or fire hazards respectively. Additional standards include the IEC/UL/EN 62368-1
standard for the safety requirements of electrical and electronic equipment in the field of audio, video,
information, and communication equipment -- systems that are invariably used in railway auxiliary
systems.


There are no direct EMI standards for railway applications outside of the EN 50121-2 standard for
maximum emission levels measured at the railway boundary fence; however, the electromagnetic
environment of these systems are demanding with potential sources from the substations, mains supply,
overhead lines, and rolling stock. For both railway and industrial applications, it is beneficial to have
a converter that has a level of electromagnetic immunity.

 

P-DUKE’s Railway DC-to-DC Converter Offering
P-DUKE’s RHKW (3W/6/10W), RHMW (20W), and RHDW (40W) series DC-to-DC converters are
ideal for 72V, 96V, and 110V railway inputs and meet EN 50155, EN 61373, and EN 445545-2 railway
standards as well as the IEC/UL/EN 62368-1 standard (with reinforced insulation) for electrical
equipment (Figure 2). These converters have built-in EMI circuits that are designed according to
IEC/EN 55032 class A as well as IEC/EN 50121-3-2. This massively simplifies PCB design and saves
board real-estate. The transformer design allows for up to a 90.5% efficiency while also meeting
the safety and isolation requirements of both harsh industrial and railway applications. They are tested
for vibration and shock with a wide operating temperature from -40°C to +105°C and are operational at
5000 meters for rolling stock that may run through mountainous regions.

 

RHD40W
Figure 2: RHD40W series of railway DC/DC converters

 

Conclusion
DC-to-DC converters used in railway applications have a number of considerations from basic voltage
requirements to isolation and construction. This allows these devices to operate regardless of the harsh
electrical, mechanical and environmental conditions. Not only do these devices need to meet common
railway standards, but should also have a degree of immunity against electromagnetic interference.
P-Duke offers such DC-to-DC converters from 3W to 300W which meet all above criterias.
https://www.pduke.com/jp/products.php?type=2&CNo=7

 

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