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]. The ideal voltage source represents the open circuit voltage (OCV), which is affected by battery SoC; the series resistor Rint represents the internal resistance, whereas, rd and Cd are the RC parallel circuit describing the charge transfer and double layer capacity, respectively. In (4) the four main equations describing the electrical model of the battery are represented. Specifically, the first equation represents the Fig. 2b. First-order equivalent circuit of battery modules. Kirchhoff's voltage law, while the second one is the n-
ICRERA 2015 4th International Conference on Renewable Energy Research and Applications Palermo, Italy, 22-25 Nov 2015 polynomial relation between OCV and SoC. The third III. ABS CONTROL ALGORITHM equation models the SoC update law, according to the current The ABS controller ensures the auxiliary substation drawn from the battery module, and finally the differential supplying or recovering energy keeping the line voltage within equation describing the RC parallel circuit. an interval defined by a lower and upper threshold value that are lower enough than protections line. The flow chart of the batt tV dT int batt d tutIRtOCV dT )()()()( n n1 proposed ABS control algorithm is shown in Fig. 4, where it is SoCOCV )( nSoC n1SoC ... 0 possible to note an external control action on the maximum current supplied from the ABS and an inner control action for batt tV dT batt tI )()( dT (4) tSoC dT tSoC )()( the line voltage regulation. In particular, if the line voltage 3600 Cbatt exceeds the upper threshold value, Vmax, (usually if one or tu dT tu )()( battd tIr )( more vehicles are in braking phase), then the ABS, according tu dT )( Cr d dd to the battery modules SoC, stores the regenerative braking dT 0 energy, reducing the line voltage. If, instead, the line voltage where ud(t) is the rdCd parallel circuit voltage, dT is the time is below the lower threshold value, Vmin, (as when a train step, the … are interpolation coefficients and accelerates or starts) the ABS supports the railway system by 0 n using the previously stored energy to the feeding the contact Cbatt [kWh] is the ABS capacity. C. 3 kV DC feeding system Conventional substations are represented by ideal DC voltage sources, series resistance and series diode, if the substations are not reversible [6], [8]. The contact wire is modelled as a set of electric resistances that change their value according to the vehicle position. If x(kdT) is the train position at the time kdT, the value of the resistance upstream Ra and downstream Rb to the vehicle towards a generic node of the railway feeding system (conventional substation, ABS or another train) are calculated by:
a kdTxrR b kdTxdrR where, Ra and Rb are expressed in [Ω], r [Ω/km] represents the resistive coefficient, d [km] is the distance between the two nodes - upstream and downstream the train -, and x(kdT) [km] is the distance between the train and the upstream node at the each time step kdT. Finally, the electric model of the overall railway system, one side supplied contact line, is shown in Fig. 3. Furthermore, it is necessary to improve the train electric model with some small capacitances in parallel to the vehicles in order to describe the receptivity of the network under regenerative braking conditions [8]. They models the voltage rise along the contact wire during the first phase of the Fig. 4. ABS control algorithm flow-chart. regenerative breaking that is used by the ABS control to detect the availability of breaking energy along the track.
Fig. 3. Electric model of the overall one side DC supplied contact line. Fig. 5. ABS current control function.
ICRERA 2015 4th International Conference on Renewable Energy Research and Applications Palermo, Italy, 22-25 Nov 2015
line. The auxiliary substation can also charge itself absorbing charge itself using the constant current ICH; since ICH>0 then energy from the line, in constant current mode, until reaching the entire CF is shifted upward by an amount equal to ICH. an intermediate state of charge, SoClim when it does not detect a power absorption in the train supply line. The intermediate IV. SIMULATION FRAMEWORK SoC value ensures a good trade-off between the ability to The railway system model was implemented in a railway support the DC feeding line during the starting phases and to simulator based on the ‘quasi static’ backwards looking recover most of the braking energy, improving the energy method, due to its short simulation times for estimating energy efficiency of the railway system. The constant charging consumption of vehicles following an imposed speed cycle current ICH and the intermediate SoC value SoClim are control [6], [12]. The power needed to satisfy the speed cycle is parameters affecting significantly the auxiliary substation determined at the wheel level. Then, the power provided by performances, in terms of losses and HS/HC trains scheduling. the feeding line is estimated by means of efficiency of each Fig. 5 illustrates the operational characteristic of the ABS electrical and mechanical components of the powertrain. control function (CF) linking the ABS reference current IRef Several simulations are carried out in which one HS/HC train and the line voltage variation related to its nominal value ΔV. moves in the two opposite directions following the same If the line voltage is increased by regenerative breaking, the driving cycle. battery is charged (IRef>0), and it is discharged (IRef<0) when the line voltage is decreased by powering trains. The A. Case study maximum charging current IMAX_charge is set to the battery The effectiveness of the proposed ABS is verified on the maximum current, depend on the battery technology and railway line linking Roma Termini railway station to the characteristics, for absorbing the regenerative energy as large Rome international airport Leonardo da Vinci. At present, two as possible. The maximum discharge current IMAX_discharge is different types of trains provide the service: a direct train chosen in a similar way. The voltage variation values within (Leonardo Express – peak power 2800 kW) leaving from the ±ΔV_sb interval, is a standby region of the ABS where no Roma Termini station every 30 minutes, and a high traffic action are required. When |ΔV| is greater than |ΔV_sb|, the ABS train (peak power 3500 kW) leaving from Roma-Tiburtina is ready to supply/recover energy and is more or less station every 15 minutes. In particular, the tests are performed responsive according to the slope of the CF and the IMAX_charge on the route section MAGLIANA–FIUMICINO (15.8 km and IMAX_discharge values. In the ΔV_sb interval, the ABS also long and shown in Fig. 6a) of the ROMA TERMINI- FIUMICINO railway line. The DC line consists in a 3 kV one side supplied contact line fed by one conventional (not reversible) electric substation, located in Magliana, and by a Li-ion ABS assumed by the authors, at Fiumicino station. It is considered only one vehicle moving along the track having ETR 1000 electrical and mechanical characteristics. Driving cycle consist in a starting phase, followed by a stretch of line path at constant speed depending on the speed limit along the track, and finally ending with the braking phase. Driving cycle and altimetric profile are shown in Fig. 6b. The maximum
TABLE I. RAILWAY SYSTEM PARAMETERS ETR 1000 Net weight 447 t Loaded weight 500 t Fig. 6a. Railway line MAGLIANA-FIUMICINO - Google Earth view. Maximum traction power 9800 kW Maximum train speed 400 km/h Accessories power 1000 kW Coefficient of auxiliary use 0.75 - Powertrain overall efficiency 0.86 - Track Track’s length 15.8 km Rail electric resistance 0.062 Ω/km Substation maximum power 5400 kW Substation DC voltage 3000 V Substation internal resistance 0.013 Ω Maximum line voltage (+20% - CEI EN 50163) 3600 V Minimum line voltage (-33% - CEI EN 50163) 2000 V ABS Battery technology Li-ion - Maximum power 2000 kW Nominal capacity 500 kWh ABS nominal voltage 2000 V
Maximum SoC value 90 % Fig. 6b. ETR 1000 drive cycle and and altimetric profile on the line Minimum SoC value 30 % MAGLIANA-FIUMICINO.
ICRERA 2015 4th International Conference on Renewable Energy Research and Applications Palermo, Italy, 22-25 Nov 2015
3,7 3,2 2,7 2,2 Voltage [kV] Voltage 1,7 25200 26200 27200 28200 29200 30200 31200 32200 Time [s]
Fig. 7a. Minimum line voltage in normal traffic conditions.
3,7 3,2 2,7 2,2 Voltage [kV] Voltage 1,7 25200 26200 27200 28200 29200 30200 31200 32200
Time [s] Fig. 9. Train and electrical substation current trends on the line Fig. 7b. Minimum line voltage by adding one ETR 1000 every 30 MAGLIANA-FIUMICINO. minutes. are show the line voltage trends by using ABS and two speed reached by the train on a stretch of the path is different control functions compared with the line voltage highlighted in dotted black line and its maximum value is 125 trend by not using ABS. In particular, the CF 1 allows to use km/h. The ETR 1000 train, ABS, track and DC feeding line the 50% of the ABS maximum discharge current, whereas the are characterized by the parameters listed in Table I. second one the 80%. On the line MAGLIANA-FIUMICINO, both the ABS control functions allow to obtain a limited B. Numerical results variation of the line voltage avoiding the use of the braking chopper due to the overvoltage. In detail, the CF 2 shows a Several simulation are carried out both to evaluate the greater reduction in voltage drop (about 15%) and in losses impact on 3 kV DC supply system of a HS/HC train as an (3.75%). On the line FIUMICINO-MAGLIANA, without ETR 1000, and to evaluate the benefits introduced by the ABS the voltage drops exceed the allowed limit, making the proposed ABS control algorithm. The actual conditions and existing feeding line not able to circulate a HS/HC train. The the impact in terms of voltage drop, evaluated with the obtained results show that a 2 MW ABS is not enough to calculation code RECUPERA [18], on the feeding line due to support the feeding line and is necessary at least a 4 MW ABS an ETR 1000 train is shown in Fig 7a and Fig. 7b, to obtain the line voltage variation within the limits. respectively. The average useful voltage on the DC line, Specifically, the CF 2 compared with the CF 1 presents a adding to the existing service one ETR 1000 train is better reduction in voltage drop (5.16%) and in losses significantly reduced compared to the voltage value without (1.14%). Fig. 9 shows main substation and ABS current trends high performances train and is not enough to reliably manage on the line MAGLIANA-FIUMICINO comparing the the railway line. In particular, the line voltage reaches almost performance of the two CFs. The current trend of the main 1700 V, a value well below the minimum allowed. In Fig. 8
Fig. 8. Comparison on line voltage: no ABS and ABS on the track. Fig. 10. Comparison between control 1 and control 2: ABS current trends on the line FIUMICINO-MAGLIANA.
ICRERA 2015 4th International Conference on Renewable Energy Research and Applications Palermo, Italy, 22-25 Nov 2015
Finally, the effect of the ABS constant current charging allows to increase the number of simultaneous active trains in line.
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Effect of the constant charging current: ABS current and SoC DC Electrified Railway (Traction Power Substation)” in Electrical trends on the line MAGLIANA-FIUMICINO. Systems for Aircraft, Railway and Ship Propulsion, IEEE 2012, pp.1-6. [6] V. Calderaro, V. Galdi, G. Graber, A. Piccolo, “Siting and sizing of substation without ABS corresponds to the train current in the stationary SuperCapacitors in a Metro Network”, in AEIT Annual same condition, whereas by using an ABS we obtain a slightly Conference, IEEE 2013, pp. 1-5. reduction in the train peak current and a better line voltage [7] M.C. Falvo, R. Lamedica, R. Bartoni, G. Maranzano, “Energy Saving in profile. The effect of ABS on line current corresponds also to Metro-Transit Systems: Impact of Braking Energy Management”, in a reduction of the main substation peak current: CF 1 International Symposium on Power Electronics, Electrical Drives, Automation and Motion, IEEE 2010, pp.1374-1380. supplying about 500 A reduces the substation current by 30%, [8] V. Calderaro, V. Galdi, G. Graber, A. Piccolo, “Optimal Siting and whereas the CF 2 allows to obtain a reduction of 40%. Fig. 10 Sizing of Stationary Supercapacitors in a Metro Network using PSO”, in shows main substation and ABS current trends and the International Conference on Industrial technologies, IEEE 2015, pp. 1-6. performance of the two CFs are compared. The substation [9] H. Hayashiya, D. Hara, M. Tojo, K. Watanabe, M. Hino, T. Suzuki, H. current trend without ABS is not proposed because the Okamoto, H. Takahashi, T. Kato, M. Teshima “Lithium-ion battery existing supply system is not by itself able to move the train. installation in traction power supply system for regenerative energy The 4 MW ABS supplies 1500 A and 2400 A peak current utilization”, in 16th International Power Electronics and Motion Control Conference and Exposition, 2014, pp. 119-124. values by using the CF 1 and CF 2, respectively. The main [10] K. Ogura, K. Nishimura, T. Matsumura, C. Tonda, E. Yoshiyama, M. substation peak currents are about 1300 A and 1700 A, Andriani, W. Francis, R.A. Schmitt, A. Visgotis, N. Gianfrancesco, respectively. Finally, In Fig. 11 the ABS current and SoC "Test Results of a High Capacity Wayside Energy Storage System Using trend on the line MAGLIANA-FIUMICINO by using ICH=0 A Ni-MH Batteries for DC Electric Railway at New York City Transit" in and ICH=50 A are shown. The ABS current trend with constant Green Technologies Conference, IEEE 2011, pp. 1-6. charge current only differs at times when the line voltage is [11] R. Lamedica, A.Ruvio, V.Galdi, G. Graber, P. Sforza, G. G. Buffarini, within the stand-by zone and it shows a constant absorption C. Spalvieri, “Application of Battery Auxiliary Substations in 3kV from the feeding line. The final SoC value is also increased by Railway Systems” in AEIT Annual Conference, IEEE 2015, pp. 1-6. about 5% compared to the case with no I current. [12] M. Sadakiyo, N. Nagaoka, A. Ametani, S. Umeda, Y. Nakamura, J. CH Ishii, “An optimal operating point control of lithium-ion battery in a power compensator for DC railway system”, in 42nd International V. CONCLUSIONS Universities Power Engineering Conference, 2007, pp. 681-686. The paper proposed a control algorithm to manage the [13] Taku Niwa, N. Nagaoka, N. Mori, A. Ametani, S. Umeda, "A control energy stored in the ABS installed on weak railway systems. method of charging and discharging lithium-ion battery to prolong its The overall railway network is modelled and a simulation tool lifetime in power compensator for DC railway system", in 43rd International Universities Power Engineering Conference, 2008, pp. 1-5. to quantify the power flow on the feeding line is implemented. [14] M. Conte, F. Vellucci, M. Pasquali, “Design Procedures of Lithium-ion The proposed ABS control algorithm is tested on a real Italian Battery Systems: the Application to a Cable Railway”, in International 3 kV DC railway system in which circulates an ETR 1000, the Conference on Clean Electrical Power, 2011, pp. 257-264. new HS/HC train of Ferrovie dello Stato Italiane. The [15] I. Masatsuki, "Development of the battery charging system for the new obtained results show that a properly control algorithm makes hybrid train that combines feeder line and the storage battery" in the ABS able to support the conventional railway supply International Power Electronics Conference, 2010, pp. 3128-3135. system that would not be able alone to move a HS/HC train on [16] F. Perticaroli, “Sistemi elettrici per i trasporti”, 2nd Edition, 2001, CEA. the track. ABS and its control strategy guarantees compliance [17] Yao Low Wen, J.A. Aziz, "Modeling of Lithium Ion battery with with the limits defined by the CEI EN 50163 standards with nonlinear transfer resistance", Applied Power Electronics Colloquium reduced voltage drop. The greater stabilization of the line (IAPEC), IEEE 2011, pp. 104-109. voltage leads to a slight reduction of the train peak current and [18] A. Capasso, M.C. Falvo, U. Grasselli, R. Lamedica, R. Bartoni, M. Francisi, G. Maranzano, “A plannyng study on power systems of metro- to a significant reduction of the substation peak current. transit transportation system” in Int. Sym. on Power Electronics, Electrical Drive, Automation and Motion, 2008, pp. 1027-1032.
ICRERA 2015