How DC-DC converters are used in railway systems

Electric mobility systems such as high-speed trains rely on a constant current supply both to power the motors that move the vehicle and for the electrification systems related to control circuits. It is worth remembering that these electric mobility solutions are extremely more efficient from an energy point of view than any other propulsion system deriving from fossil fuels such as coal, diesel or gas, reducing at the same time CO2 emissions.

DC-DC converters play a crucial role in improving the energy efficiency and power quality of electrified railways. The electronic components used within these applications will be subject to extreme temperatures, humidity, vibration and shock mechanical conditions. For this reason, they require a high degree of structural integrity to withstand these conditions and function reliably.

So what are the requirements for DC-DC converters in railway systems and how are they used? Finally, what are the international standards that guarantee efficiency and reliability?

A look at electric mobility trends and systems

The first railway electrification systems were based on low voltage direct current from which power was supplied through diode rectifiers in traction substations along the track, distributing the power to the train engines via overhead lines and rails. The most common voltages for DC traction systems are 600 V for older trams, 750 V for newer metros and 1500 V for more complex suburban line systems. Finally, some projects of considerable geographical size use 3000 V systems in order to reach even long distances.

The first trend that can be noticed is therefore the growing and constant growth in terms of tension. For the same power, increasing the voltage allows a decrease in the current; lower voltage systems will therefore require a higher level of current, with a consequent increase in the cross-section of the cables to be used.

Another trend can be identified in the constant increase in energy efficiency to reduce CO2 emissions. Among the most mature systems we find energy reuse systems through regenerative braking, where the kinetic energy of the rotor is converted into electrical energy to be reused by the main engine.

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How DC/DC converters are used in railway systems

The use of DC/DC converters is central within this particular application category, bringing with it some very important characteristics. In order to guarantee correct energy distribution inside vehicles such as trains, trams and subways, multiple series of DC/DC converters are required to provide voltage from primary systems dedicated to movement and secondary systems dedicated to all the services on board. As seen above, nominal input voltages can range from as low as 24V to as high as 110V, with a regulated output of 3.3, 5, 12, 15, or 24V. It is necessary to remember that the European regulation EN 50155 for railway electrical equipment requires that the nominal input voltage can oscillate between 0.7 and 1.25 times the nominal voltage , with short-term deviations between 0.6 and 1.4 times the rated input voltage.

From these numbers, we can deduce that a 110V system would require a continuous voltage range of 67V to 120V, and a float voltage range of 66V to 154V. The converters offered by Digimax support wide input ranges to cover more than one nominal input voltage. The RCD series made by P-Duke for example, was designed and built to guarantee ultra-wide coverage from the point of view of the input voltage range.

Focusing attention on actual applications, regenerative braking represents an emblematic example of the use of DC/DC converters: these solutions are in fact extremely more compact compared to bulky converters bi-directional and are capable of returning power to the battery pack or energy storage system from the main power line, or reliably and continuously distributing power to all electronic devices and subsystems within the vehicle. The reliability of the auxiliary systems present inside electric vehicles is also a central element and guaranteeing the functionality of critical systems such as engine drive controls, braking systems, indicators and warning lights, information displays and door opening and closing systems, is truly central.

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