1. Introduction
In the European Union (EU), four railway electrification systems are mainly used: DC, at 1.5 kV or 3 kV, Medium Voltage AC (MVAC) of 15 kV at the special frequency of 16.7 Hz and 25 kV at the standard line frequency of 50 Hz [
1]. The first two systems were introduced at the beginning of the 20th century. The last one was developed after World War II. From that moment, the countries that had not yet electrified their railway lines chose the 25 kV/50 Hz system. The other electrification systems, already in place, were kept and even extended during the second half of the 20th century. Today, in the EU, 43% of electrified railway routes are powered by DC, 30% at 15 kV/16.7 Hz, and 27% at 25 kV/50 Hz [
2].
Faced with the collective need to favor modes of transport that are more respectful of the environment, train traffic, for freight and passengers, is expected to increase in the years to come [
3]. From the point of view of the railway network operator, an increase in traffic is only possible if a minimum contact-line voltage can be ensured to allow the locomotives to draw the power necessary to keep to their timetables [
4]. Due to its relatively low voltage level, the DC electrification system is penalized by the level of current absorbed by the trains which can reach several kAs. Conventional solutions to reduce the voltage drop consist of increasing the cross-section of overhead lines or reducing the length of sectors by installing new substations [
5]. Additionally, paralleling stations (PS) at the middle of the sector can be added to reduce the contact-line resistance. Previous studies have demonstrated that the middle of the sector between two substations is the critical point in terms of voltage drop. Excluding the paralleling stations (since the majority of the DC railway lines already have a PS installed), the other solutions are expensive and not always feasible. As of 2014, several papers proposing a Medium Voltage DC (MVDC) electrification system were published [
6,
7,
8,
9]. In [
10], it was demonstrated that a nominal contact-line voltage of 9 kV made it possible to obtain performances similar to those of 25 kV/50 Hz electrification system in terms of railroad traffic, substation spacing, and contact-line cross-section.
Figure 1 illustrates a DC-electrified railway system. When the line is electrified at 1.5 kV, the distance between substations, referred to as
D, is in the order of 10–20 km. Nevertheless, for a line electrified at 9 kV, the substations could be spaced approximately 50 km apart.
However, changing the voltage level to MVDC is challenging cost-wise. On the one hand, new rectifier groups have to be installed in substations and MVDC circuit breakers have to be developed. On the other hand, updating all the rolling stock traction converters is not a simple task even considering advances in high blocking-voltage semiconductors [
10]. Therefore, the MVDC electrification system is considered only as a long-term solution. In the short term, several reinforcement solutions can be deployed. The principle of a three-wire DC power supply is illustrated in
Figure 2. This solution can be considered as an intermediate stage in converting a classical DC power supply, e.g., 1.5 kV or 3 kV, to MVDC. MVDC rectifier groups are installed in existing substations and an additional feed-wire is deployed on the support posts of the contact line. Then DC/DC converters are implemented at the PS to reinforce the power supply and boost the voltage on the contact lines [
11].
Other reinforcement solutions, also shown in
Figure 2, rely on energy storage systems and renewable energy sources, e.g., photovoltaic (PV) systems, integrated with the DC power electrification lines [
12,
13,
14]. Solutions involving battery energy storage systems (BESS) have gained attention in the last few years. Due to the environmental pressure, the road transportation sector is shifting from fossil fuels to more sustainable electric power. This demand drives advances and developments in battery technologies, creating more reliable, efficient, and less expensive solutions [
15,
16].
Several works in the literature have proposed incorporating BESS in railway applications [
17,
18,
19,
20,
21]. Japan railways are the pioneers in this field and have installed around 20 BESS rated at 100 kWh [
22,
23]. The authors in [
24] propose a sizing and control strategy optimization of a hybrid energy storage system based on batteries and ultracapacitors in urban rail transit to reduce the substation energy cost and achieve the peak shaving function. Additionally, the authors in [
25] propose a battery and ultracapacitor hybrid storage system for high-speed railway applications. In [
26], the impact assessment of energy storage systems supporting DC railways on AC power grids is investigated, considering both wayside and on-board solutions. A techno-economic sizing of auxiliary-battery-based substations in 3 kV Italian DC railway systems is established in [
27]. Also, recent research progress and applications of energy storage systems in China’s high-speed railways are developed in [
28]. However, until now, no BESS has been implemented on the French rail network and there is still no dedicated industrial product.
Therefore, this paper focuses on the sizing of a Modular Battery Energy Storage System (MBESS) based on the association of 300 kW elementary converters, including an isolated DC-DC converter and battery racks. The case studies considered in this paper are for two critical sectors of the French Rail Network where, during peak hour traffic, the contact-line voltage drops below 1.3 kV, limiting the power that the locomotives can draw. Simulations are performed considering real railroad traffic to determine the number of elementary blocks of the MBESS.
This paper is structured as follows:
Section 1 introduces the subject of study;
Section 2 describes the topology of the MBESS and its control strategy;
Section 3 models the railway sector as an equivalent electrical circuit and designs the MBESS for the proposed application. In
Section 4, the sizing method is applied for two critical sectors of the French rail network electrified at 1.5 kV. Then, simulation results are presented and discussed. Finally,
Section 5 presents the conclusions of the paper and perspectives for future works.
5. Conclusions
This paper proposed a modular BESS solution to reinforce DC railway lines. By reducing the contact-line voltage drop, the MBESS prevents the onboard locomotive control system from limiting the traction power. Therefore, trains can respect their timetables. Furthermore, the power losses in the contact line and the rails are reduced and the energy efficiency of the electrification system is improved. Thanks to the galvanic isolation included in the DC/DC converters, the solution is suitable for different contact-line voltage levels, including for future 6 kV or 9 kV electrified lines still under consideration.
The sizing method of the MBESS was presented in
Section 3. The number of elementary blocks depends on the electrical parameters of the sector considered and the rail traffic. To run the simulations, traffic information, and power consumed by trains must be provided by the rail infrastructure manager. The two case studies covered in this paper were provided by SNCF-Réseau, the French rail infrastructure manager. The first one concerns a sector of a main line where there is a high traffic density with high-power trains in circulation. The second one concerns a sector of a regional line with large substation spacing and a low contact-line cross-section. Respectively, the power ratings of the MBESS are 3 MW and 2.4 MW which represents about half of the nominal power of a substation used on lines electrified in 1.5 kV DC.
Taking into account the characteristics of the batteries initially selected, the stored energies are 2.58 MWh and 2.06 MWh. These values are oversized regarding the evolution of the battery SOC during the peaks of traffic. A way of optimizing the storage energy and the power capability of the MBESS could be simply increasing the battery’s nominal voltage. However, this is problematic given the battery management system currently available.
Finally, it is important to say that the work presented here is the design of a planned prototype to be installed on a line of the French rail network. Further studies are expected to achieve the integration of the MBESS in the railway line, design of input filters, different energy management strategies, lifetime evaluations and so on.