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4th International Conference on Renewable Energy Research and Applications Palermo, Italy, 22-25 Nov 2015 Energy Management of Auxiliary Battery Substation Supporting High-Speed Train on 3 kV DC Systems Vito Calderaro1, Vincenzo Galdi1, Giuseppe Graber1, Alfonso Capasso2, Regina Lamedica2, Alessandro Ruvio2 Antonio Piccolo1 1 DIIn - University of Salerno 2 DAEEE - University of Rome "La Sapienza" Fisciano (SA), Italy Rome, Italy [email protected], [email protected] [email protected], [email protected] [email protected], [email protected] [email protected] Abstract —The paper propose an energy management strategy for high efficiency battery-based substation (Auxiliary Battery Substation - ABS) able to power weak railways in areas without energy supply from the grid. The proposed control algorithm makes the ABS able to sustain part of the peak current absorbed during traction by high performance trains operating on traditional 3 kV DC rail networks. The proposed solution also, according to the state of charge and of the line voltage, allows the ABS system to recover the train's braking energy making it available to the next train departure. Several simulations are Fig. 1. ABS basic diagram. performed on a real Italian 3 kV railway system feeding a new Another benefit introduced by the ABS is related to the generation high speed train, where the ABS supports existing increase of the energy efficiency of the overall railway system. supply system. The simulation results show that the ABS and its In fact, the ABS storage units in a not regenerative railway control allow the use of high performance trains even on 3 kV plant can host the energy during the braking phase of a train traditional lines not properly powerful. located near the ABS, charging its battery, and it feeds the line Keywords—battery storage system, energy management, railway during the peak of the energy and power demand (for instance system simulation, voltage drop. due to a HS/HC train in starting phase) by using the recovered energy, minimizing losses due to the energy flow coming from I. INTRODUCTION the main substation. Fig. 1 shows a basic electric diagram of The transport sector in the third millennium are definitely the auxiliary battery substation [3]. The ABS can be directly moving towards three directions and in each of them the connected to the traction power supply system or to the electric energy carrier is the protagonist: in the city, mass substation bus-bar by means of the connection unit, which transit is becoming increasingly widespread in metro and tram consists in the disconnector, the high-speed DC circuit-breaker systems; at the level of individual mobility, there is great and the pre-charging unit. The connection unit and the storage excitement towards the increasingly popular hybrid-electric unit are joined by the DC/DC converter, which functions as a and pure electric vehicles. In medium-distance train more and step-up/step-down converter [4]. Research activities about more replaces the plane. The current technological trend energy storage in the railway systems are mainly focused on suggests trains characterized by higher and higher metro and light rail networks [3-5]. Several works propose the performance, such as the high-speed/high-capacity (HS/HC) use of stationary or onboard storage systems in DC metro one [1]. Thanks to the multi-voltage technology characterizing networks, [6]-[7]. In particular, optimization algorithms for such HS/HC trains, more and more often they are also used on the siting and sizing of stationary supercapacitors are proposed traditional 3 kV DC lines as well as on high-voltage AC lines. in [6] and [8]. Some electric energy storage systems were The widespread penetration of HS/HC trains on existing 3 kV recently installed in Japan and USA, in 1.5 kV DC feeding DC feeding lines has a considerable impact on the voltage lines, mainly to compensate voltage drops and to recover drop and power losses, as well as on the train performance of braking energy. The East Japan Railway Company, installed a which is forced to limit the power absorption. The natural li-ion battery system at HAIJIMA substation on the Ome line solution that would an enhancement of feeding line very often in 2013, [9], whereas the test results of a Ni-MH based storage is not very convenient, because of the railway lines distance system for DC railway at New York City Transit in 2010, are the from the high voltage transmission network. Auxiliary reported in [10]. However, with reference to the 3 kV railway battery substation (ABS) in stand-alone configuration systems, in technical and scientific literature only a few works represents an innovative solution in railway 3 kV DC systems dealing with the energy storage systems, [11] [12]. In [13], the [2]: the ABS sustains the absorbed peak current during the authors propose a control technique to improve battery starting phase by high performance trains having a peak power lifetime in power compensator for DC railway system, higher than line and substation capacity, such as HS/HC train whereas the design procedures for battery energy storage operating on traditional 3 kV railway networks. In particular, systems for railway application are described in [14] and [15]. an ABS reduce the effect of peak current and voltage drops on Finally, other works are focused on specifications and design weak feeding line due to the high performances trains. criteria of bilateral DC/DC converter for battery energy storage systems supporting railway DC feeder systems [4]. ICRERA 2015 4th International Conference on Renewable Energy Research and Applications Palermo, Italy, 22-25 Nov 2015 This paper evaluates the performance of an ABS control vmR 2 )( algorithm able to efficiently manage stored energy supporting BASE 21 a HS/HC train operating on traditional 3 kV railway network. RLINE mg sin x mg The model pointed out and implemented in a software bxr simulator takes into account: i) track topology - slopes and In (2), and depend on the train characteristics and curves – ii) the electrical features of the feeding line, iii) the 1 2 mechanical characteristics of the train and its timetable. The the train speed, and can be calculated by the train data or simulation tests are performed on a real Italian railway system. obtained by literature; g is the gravitational acceleration and The paper is organised as follow: Section II describes the γ(x) is the slope grade. Second term of RLINE is the curve kinematic of the train and, the electrical models used for the resistance given by empirical formulas, as the Von Röckl’s implementation of the railway simulator. In Section III, the formula, where r(x) is curvature radius, and a, b are proposed ABS control algorithm is presented and its coefficients which depend on the track gauge, tabled in [16]. characteristics are described, whereas case study and results of Trains are modelled as ideal current sources absorbing power several simulations are presented and discussed in Section IV. at the accelerating time and generating power at the Finally, conclusions are listed in Section V. regenerative breaking time. The power at the wheels required to overcome the vehicle inertia, slopes and curves, II. MODELLING OF THE RAILWAY SYSTEM aerodynamic and rolling frictions, is calculated starting from a The model pointed out is obtained by the integration of given speed cycle. Going upstream the vehicle components three different sub-models: railway vehicle kinematics, ABS, and their related efficiencies, the power requested from the and the conventional feeding system. electrical substations and the absorbed current are determined by the following equation: A. Vehicle The longitudinal dynamic of vehicles evolves according to dv m r vF the force balance equation described by the model expressed dt PVEHICLE PAUX _ SERVICES by: img dv P m BASE LINE xRvRF I VEHICLE dt VEHICLE VLINE . dx v dt In (3), PAUX_SERVICES is the power for on board auxiliary where m is the mass of the vehicle, ρ is a correction factor services (lighting, cooling or heating), m is the total mass of taking into account the rotating mass, v and x are the train the train - including the passengers -, v is the vehicle speed, ηg, speed and position respectively, F is the traction (if positive) ηm, and ηi represent, respectively, the gear box efficiency, the or braking (if negative) force, which is lower and upper motor efficiency and the inverter efficiency. Fr is total bounded, [6]. resistive forces, computed as sum of two terms: the basic resistance RBASE, and the line resistance RLINE, defined in (2). RBASE (v) is the basic resistance including roll resistance and air To bring into account that the voltage along the track is not resistance, and RLINE (x) is the line resistance caused by track constant, the railway vehicle is modelled as an ideal current slopes and curves, and they are expressed by: generator IVEHICLE, whose value is calculated as the ratio between vehicle power and line voltage VLINE, (3). B. Auxiliary battery substation The ABS electrical model includes the battery modules, the DC/DC converter and the power flow controller (Fig. 2a). During the charging period, ABS receives the regenerative power from the vehicles and during the starting time, delivering power to the trains: therefore, the ABS is modelled as ideal current sources, whereas a simple constant resistor models the power converter. The DC/DC converter charges or discharges the battery modules, using an energy management strategy, according to the line voltage and the batteries state of Fig. 2a. ABS electric model. charge (SoC). In Fig. 2b is shown the first-order equivalent circuit of battery modules consisting in four elements [17].