RAILWAY INDUSTRY The solid state transformer - an essential device in the evolution of DC railway electrification systems

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Currently, electrical network, research laboratories operators and electrical equipment manufacturers are considering the use of direct current in electrical power distribution. In fact, renewable energy sources and associated storage devices are easier to integrate into a DC grid. Thus, new microgrid concepts are emerging with medium voltage direct current (MVDC).

In the European Union, three railway electrification systems are commonly used: DC (1.5 kV or 3 kV), AC at a special frequency (15 kV/16.7 Hz) and AC at the standard utility frequency (25 kV/50 Hz). The first two systems were introduced at the beginning of the 20^th century. The last one (25 kV/50 Hz) was developed by the French National Railways after the 2^nd World War. In the EU, around 110,000 km of railway routes are presently electrified. 43.5 % of them are electrified with DC, while AC systems, at the special and utility frequencies, represent respectively 30.5 % and 26 %.

The current DC electrification system suffers from a relatively low voltage level that limits locomotive power and traffic density due to the high currents in the overhead-lines, whereas, thanks to their medium voltage level, AC electrification systems allow the use of overhead lines with small cross-sections. In AC systems, the voltage drop is mainly related to the reactance of the overhead line and reactive power compensation is sometimes used to boost the line voltage. In this respect, the 25 kV/50 Hz system is particularly penalized. Although the 15 kV/16.7Hz system requires a specific power supply network, it allows, as in the DC system, a parallel connection of the substations. In the 25 kV/50 Hz system, single-phase substations require high short-circuit power at the connection point to avoid voltage unbalances on the three-phase grid. Modern AC locomotives include a single-phase step-down transformer, a rectifier, a low-frequency shunt filter and a three-phase voltage source inverter that supplies the AC traction motor whereas, the on-board traction converter of a DC locomotive is much simpler and is reduced to an input filter and a three-phase voltage-source inverter.

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Currently, research laboratories, electrical network operators and manufacturers of electrical equipment are seriously considering the use of DC in electrical power distribution. Indeed, renewable energy sources and associated storage devices are simpler to integrate into a DC grid. Thus, new micro-grids concepts at medium-voltage direct current (MVDC), are emerging. The same concept can be envisaged for railway electrification and it is obvious that a DC electrification with a higher voltage combines the favourable aspects of existing electrification systems discussed previously. DC benefits from the absence of inductive voltage drop, the absence of reactive power, the absence of voltage unbalance at the grid connections and simplification of the traction chain for the rolling stock. A higher voltage level allows the use of light overhead-lines and a reduced number of substations.

In 2016, a study of an MVDC electrification system was jointly initiated by the LAPLACE Laboratory of the University of Toulouse and SNCF-Réseau (the Railway Infrastructure Administration in France). The results showed that an MVDC electrification system with a nominal voltage of 9 kV has similar performances to the 25 kV AC electrification system. This represents a major technological breakthrough in the world of railways and opens up new perspectives for the use of DC electrified lines, which fits perfectly into a sustainable development scenario. In the long term, an MVDC electrification system could therefore constitute, on a large scale, the backbone of an electrical energy hub and distribution network integrating renewable sources and storage elements. Reversible AC/DC converters could be installed and offer ancillary services to the grid operator and Solid-State Transformers (SSTs) would play a major role since they allow the interconnection of different energy sources and storage elements.

Currently, the European Project “Future Unified DC Railway Electrification System”¹⁾ supported by Shift2Rail Joint Undertaking, presents a deep forward-thinking scenario and proposes related work to define the future of DC railway electrification systems in order to increase transport capacity while improving energy consumption, limiting environmental impacts and decreasing investment and operating costs.

The implementation of the new MVDC power supply seems conceivable within a few years in countries where the electrification of railways is still to be fully developed. In European countries which already use DC electrification systems, it cannot be envisaged in the short term. It is indeed impossible to simultaneously modify the rolling-stock and the infrastructure but SSTs do offer solutions which are valuable for both rolling stock and power supply during a transition period: stepdown SSTs could be installed on-board locomotives to allow an operation both on classical DC lines and new MVDC lines and three-wire systems can be implemented for the power supply of the contact line.

Thus, the initial power system is preserved, an additional MVDC feed-wire is installed and stepdown SSTs are used to reinforce the power supply of the trains. This solution limits the voltage drop and enhances the capacity of a railway line by allowing the circulation of more powerful locomotives or the increase of the traffic. In this scenario, the feed-wire is supplied by AC/DC converters which are installed in some of the existing substations. SSTs are connected to the 1.5 kV or 3 kV contact line at the sector mid-points. This makes it possible to take advantage of switches and circuit breakers already installed in the Paralleling Stations (PS).

One solution for realising the SST is based on an association of elementary isolated DC/DC converters. The Input Seriesed and Output Paralleled (ISOP) configuration, allows natural voltage and current balancing. The power ratings will differ according to the application: around 2 MW for rolling stock and 6 MW for the reinforcement of the track-side power supply of existing lines. For the on-board SST, water cooling will be used in order to obtain maximum compactness whereas, for fixed installations, an air-cooled solution will be possible. A container layout will be preferred so that the SSTs can be reused on another line pending migration to MVDC.

In any case, the efficiency is an essential criterion and it is recommended to have a sufficiently high switching frequency in order to reduce filter elements, noise pollution, volume and mass of the transformers. At present, the first 3.3 kV SIC-MOSFET power modules are commercially available and medium-frequency transformer technology is state-of-the-art for some transformer manufacturers. That is why, in 2019 SNCF-Réseau and LAPLACE decided to demonstrate the feasibility of an elementary isolated DC/DC converter at a power level of a few hundred kW and with an operating frequency of more than ten kilohertz.

In power electronics, as soon as the power level reaches several hundred kW, experimentation becomes essential. Therefore, a test bench, rated for full power, is fundamental to obtain results far closer to reality than those that theoretical calculations and simulations could provide. The use of an opposition method is preferable in order to limit the costs of the tests (low power supply, no load). This makes it possible to accurately measure the efficiency at all operating points. For the considered application, the test bench built at LAPLACE includes a transformer with a turns-ratio of 1 so that a direct connection between input and output is possible. The voltage source (VDC) imposes the voltage (Vin) on the input DC-Bus while the current-source regulates the output current (Iout) flowing in the converter. Thus, the two power supplies provide only converter losses.

The Medium Frequency Transformer (MFT) was rated to operate with the 3.3 kV MOSFET power modules which are available now from several semiconductor manufacturers.

This test bench allows converter losses to be measured both electrically and thermally. Three water-cooling circuits are used in parallel to measure the losses by calorimetry. This method consists of measuring the cooling-water flow and the inlet and outlet temperatures of each of these three circuits and in order to limit heat-loss by convection, exposed parts are foam-insulated.

The efficiency was measured for a switching frequency of 15 kHz, an input voltage of 1.8 kV and for three different 3.3 kV SiC power modules. The test results showed outstanding converter efficiency but also revealed that a proper understanding of switching waveforms and choice of semiconductors devices are necessary to achieve this.

The results obtained with this prototype are very promising and they pave the way for the commercialisation of SSTs. As for the other components of the new MVDC electrification system, the technologies are mature. Modular Multilevel Converters are already used to supply the 15 kV/16.7 Hz AC lines. They can also be applied to realise MVDC sub-stations. Solid State Circuit Breakers have been optimized for multi-terminal HVDC systems and it seems therefore realistic to envision versions for much lower voltage levels.

From the rolling stock point of view, the DC/DC power electronic traction transformer requires a much simpler topology than the solutions which were investigated in recent years for AC power systems. Moreover, engineering samples of SiC-dies up to 15 kV are becoming available and three-phase voltage-source inverters operating at 10 kV DC input-voltage have been tested. Thus, within a few years, it will be possible to design a medium-voltage traction converter based on a single voltage-source inverter. In addition, the development of medium-voltage traction-motors will lead to very simple on-board power conversion chain for the rolling stock.

All these considerations should encourage Railway Infrastructure Administrations to move to MVDC electrification systems. In recent years, Power Electronics has allowed considerable advances in the development of electric vehicles and more-electric aircraft; the world of railways should also experience such technological breakthroughs.

_{1) FUNDRES project involves 4 partners, “Laboratoire Plasma et Conversion d’Energie” (University of Toulouse - France), “Ecole Polytechnique Federale de Lausanne” (Switzerland), “Politecnico di Milano” (Italy) and “Union Internationale des Chemins de Fer” (France). This project has received funding from the Shift2Rail Joint Undertaking (JU) under grant agreement No 881772. The JU receives support from the European Union’s Horizon 2020 research and innovation program.}

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