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Model-based investigation of an uncontrolled LTO wayside energy storage system in a 750 V tram grid
Abstract
Wayside energy recovery systems (WERS) can increase energy efficiency in DC railway grids. Almost all commercial systems connect energy storage system and grid via power electronics, and most studies investigate this approach. Power electronics increase complexity, space demand, and investment costs.
Our work presents a novel concept for an application in 750 V grids: a direct grid-coupled, uncontrolled WERS based on a commercial lithium-ion-titanate-oxide (LTO) cell.
We present a detailed simulation model to investigate the application of the novel concept on tramline 112 in Oberhausen/Mulheim (Germany). Field measurements on two vehicles support the investigations. We consider the auxiliaries’ specific behaviour and use measured traction power profiles as simulation input. As a result, the application of the proposed WERS could save around 182,500 kWh per year. Economic operation is thus possible with a service life of two years at investment costs of less than 53,000 €.
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The paper proposes an optimal siting and sizing methodology to design an energy storage system (ESS) for railway lines. The scope is to maximize the economic benefits. The problem of the optimal siting and sizing of an ESS is addressed and solved by a software developed by the authors using the particle swarm algorithm, whose objective function is based on the net present value (NPV). The railway line, using a standard working day timetable, has been simulated in order to estimate the power flow between the trains finding the siting and sizing of electrical substations and storage systems suitable for the railway network. Numerical simulations have been performed to test the methodology by assuming a new-generation of high-performance trains on a 3 kV direct current (d.c.) railway line. The solution found represents the best choice from an economic point of view and which allows less energy to be taken from the primary network.
This paper investigates the benefits of using the on-board energy storage devices (OESD) and wayside energy storage devices (WESD) in light rail transportation (metro and tram) systems. The analysed benefits are the use of OESD and WESD as a source of supply in an emergency metro scenario to safely evacuate the passengers blocked in a metro train between stations; the use of OESD for catenary free sections, the benefits of using the WESD as an energy source for electrical car charging points and tram traction power supply; the benefits of using a central communication system between trams, cars, WESD and electrical car charging points. The authors investigated the use of: OESD with batteries for a catenary free section for different scenarios (full route or a catenary free section between two stations); the charge of OESD between stations (in parallel with tram motoring) to decrease the charging dwell time at stations and to help in achieving the operational timetable; the thermal effect of the additional load on the overhead contact system (OCS) when the tram is charging between stations; the sizing of OESD and WESD for emergency feeding in a metro system. The authors investigated the use of the WESD as a source of energy for the electrical car charging points to reduce the car pollution and carbon emissions. Presented in the paper is the enhanced multi train simulator with WESD prepared for the analyses conducted. The paper describes the DC electrical solver and WESD control method. A validation of the software has been conducted in regard to the substation voltage, WESD energy balance and WESD control.
In this paper, a dual-stage modeling and optimization framework has been developed to obtain an optimal combination and size of wayside energy storage systems (WESSs) for application in DC rail transportation. Energy storage technologies may consist of a standalone battery, a standalone supercapacitor, a standalone flywheel, or a combination of these. Results from the dual-stage modeling and optimization process have been utilized for deducing an application-specific composition of type and size of the WESSs. These applications consist of different percentages of energy saving due to regenerative braking, voltage regulation, peak demand reduction, estimated payback period, and system resiliency. In the first stage, sizes of the ESSs have been estimated using developed detailed mathematical models, and optimized using the Genetic Algorithm (GA). In the second stage, the respective sizes of ESSs are simulated by developing an all-inclusive model of the transit system, ESS and ESS management system (EMS) in MATLAB/Simulink. The mathematical modeling provides initial recommendations for the sizes from a large search space. However, the dynamic simulation contributes to the optimization by highlighting the transit system constraints and practical limitations of ESSs, which impose bounds on the maximum energy that can be captured from decelerating trains.
Battery‐powered electric buses are increasingly being used around the globe. This work investigates four technological concepts for the rollout of electric buses from a technical and economic perspective: very fast and moderate opportunity charging, overnight charging and trolley hybrid buses (power supply via catenary/battery). The investigations fund on real demonstrations in four European cities. They were carried out in direct cooperation with the respective public transport operators to obtain realistic results. The focus of this work is on an economic comparison based on the total cost of ownership (TCO), including all investment and operating costs in the bus service. Thereby, the battery represents a major asset and is examined more closely, especially with regard to the expected service life. The TCO of corresponding electric and diesel buses, scaled up to a complete line, are directly compared. This includes penalty costs for the emissions of noise and pollutants. Sensitivity analyses on the most relevant variables are conducted to take into account risks and uncertainties. It shows that electric buses can nowadays already be economically competitive under favourable assumptions, regardless of the concept. Trolley hybrid buses turned out to be the most cost‐effective variant in their respective country.
In modern electrified and rail-bound mass transit vehicles, a considerable part of the braking energy is still dissipated via resistors. This applies in particular to less connected and low frequented grid sections. For this purpose, wayside energy recovery systems can be used to store excess energy and release it during acceleration of nearby vehicles. They offer further advantages, such as a potential reduction in the power requirement of substations or additional safety and reliability. This paper provides an overview of actual demonstrations of various systems in public transport grids. The focus is on the most important technologies (namely: supercaps, flywheels, batteries and inverters), as well as the respective manufacturers and products. The achievable improvements, based on both real demonstrations and scientific studies, are identified and critically evaluated. Important points for a better assessment of the systems are worked out. A future forecast is given for each of the four technologies.
Today, in the railway sector there is considerable interest in studying the best ways of exploiting train braking energy, in order to achieve reduction in energy costs and better stabilization of grid voltage. Among the various on-board or wayside measures proposed, one of the most promising solutions is based on using Wayside Energy Storage Systems. A WESS is a storage installation which can be integrated into mass transit systems in urban areas as well as into long-distance railway lines. It can operate as a smart storage system able to provide relevant benefits in term of recovering surplus regeneration braking energy, voltage stabilization, reduction of peak power demand. For these purposes, an effective and flexible simulator is essential to provide all the elements needed to focus and compare feasible configurations and operations. This paper examines the problem of introducing the WESS technology in a real railway infrastructure. It proposes a method based on a simulator of the WESS system integrated into the infrastructure and carries out the results of dynamic simulations referring to real operating data of the system and vehicles. The results highlight the performance of the WESS in terms of energy and power exchange, also discussing of economic aspects.
Electrical modelling of rail tracks with multiple running trains is complex due to the difficulties in solving the power flow. The train positions, speed and acceleration are constantly varying resulting in a nonlinear system. In this work, a method is proposed for modelling DC electric railways to support power flow analysis of a simulated metro train service. The method exploits the MathWorks simulation tool Simscape, using it to model the mechanical and electrical characteristics of the rail track system. The model can be simulated to provide voltages at any position in the track and additionally, the voltages seen by any train. The model includes regenerative braking on trains, this is demonstrated to cause overvoltage in the feeding line if it is higher than the power demand of the other trains at that time. Braking resistors are used to protect the network from overvoltage by burning the excess energy. Through the implementation of Energy Storage Systems (ESSs), it will be possible to improve the energy efficiency and remove timetabling restrictions of electric railways by effectively controlling the rail track voltage. The paper proposes several methods to validate the model.
The use of supercapacitors (SCs) to store regenerative braking energy from urban rail trains is able to achieve a good energy saving effect. This paper analyzes the current balance method of stationary energy storage devices (ESDs). At the beginning of the paper, the mathematical model of the DC traction power system, which includes trains, ESDs and traction substations, is established. Next, based on this, the SC state-based control strategy (SCSCS) is proposed, which can adjust the charging voltage of the ESD according to the SC voltage and current, then the charging current of the ESD can be reasonably distributed under the voltage difference of ESDs, and the SC voltage and current stress can be reduced. In order to determine the optimal controlling parameters, the optimization model is proposed and solved by the genetic algorithm. The analysis of the case study also shows the effectiveness of the proposed control strategy and optimization algorithm. Finally, the rationality of the proposed strategy is verified by experiments.
The voltage solution of DC railway traction power networks is classically obtained via the current injection (CI) method, which is based on solving a sequence of nodal voltage equations. Specialized techniques, which build on the CI method, have been proposed for simulating limited network receptivity due to voltage rise constraints and nonreversible substations. These techniques may require a multitude of power reduction steps for modeling the local controller operation of trains in regenerative braking mode, and they consequently lead to a large computational effort. This paper proposes a sensitivity based approach for computing the regenerative train power that can be returned to the network without causing over voltage. In case of non-receptive substations, each regenerating train is switched to a voltage-current source model and the CI method is used to further adjust the power that can be received by the network; a two-phase approach is used to compute the regenerative train resistance without recourse to iterations. The proposed method is tested on network models with branched lines, detailed return circuits, and having up to 144 trains. The computational performance comparisons show that the proposed method for simulating local controllers can be significantly faster than the classical power reduction method.
Multi-train modeling and simulation plays a vital role in railway electrification during operation and planning phase. Study of peak power demand and energy consumed by each traction substation needs to be determined to verify that electrical energy flowing in its railway power feeding system is appropriate or not. Gauss-Seidel, conventional Newton-Raphson, and current injection methods are well-known and widely accepted as a tool for electrical power network solver in DC railway power supply study. In this paper, a simplified Newton-Raphson method has been proposed. The proposed method employs a set of current-balance equations at each electrical node instead of the conventional power-balance equation used in the conventional Newton-Raphson method. This concept can remarkably reduce execution time and computing complexity for multi-train simulation. To evaluate its use, Sukhumvit line of Bangkok transit system (BTS) of Thailand with 21.6-km line length and 22 passenger stopping stations is set as a test system. The multi-train simulation integrated with the proposed power network solver is developed to simulate 1-h operation service of selected 5-min headway. From the obtained results, the proposed method is more efficient with approximately 18 % faster than the conventional Newton-Raphson method and just over 6 % faster than the current injection method.
In this paper, we investigate the design of stationary ESSs based on supercapacitors (SCs) for metro network (MN). We implement a simulation tool in order to estimate the power flow among the metro vehicles and the ESSs through the MN. A new formulation of the ESSs siting and sizing optimisation problem is proposed and solved using particle swarm algorithm. The optimisation process minimises the energy supplied by the electrical substations and the whole ESSs capacity taking into the electrical line constraints in the metro network simulator. Several simulations are performed in order to design the number, the position along the track and the required capacity of the SCs. Using commercially available modules, a realistic design solution is compared with the theoretical one obtained by solving the optimisation algorithm. Finally, we estimate the energy delivered from the electric substations and the energy saved using the real ESSs configuration.
This paper presents a control strategy for the power flow management of a wayside energy storage system based on a supercapacitor technology installed in a tramway network. The control is based on the management in real time of voltage levels at catenary point connections in order to optimize the energy flow among the running tramcars and substations with the goals of improving the energy saving and reducing the voltage drops in the supply network. The suggested control algorithm was experimentally validated using a laboratory-scale model to verify the effectiveness of the strategy. A lithium-ion capacitor module with 72 series-connected laminated cells of the JSR Micro ULTIMO type was used. Thus, an emulation of a real tramway network in the city of Naples (ANM) was implemented to evaluate the potential impact in terms of costs/benefits of the next installation on the ANM network.
In this paper, a new procedure based on a back- ward/forward sweep (BFS) algorithm for solving power flows in weakly meshed dc traction networks is presented. The proposed technique is able to consider the trains as nonlinear and non- smooth (nondifferentiable) voltage-dependent loads or generators. This feature permits the inclusion of the trains’ overcurrent protection and the squeeze control. With the use of the mentioned controls, the conventional power flow problem becomes a voltage constrained power flow problem, and the interaction between the trains and the network can be accurately modeled. However, the train control induces a highly nonsmooth voltage-dependent load characteristic, causing convergence problems in most of the derivative-based algorithms. The proposed algorithm is faster, more robust, and more stable than the derivative-based ones. In addition, the authors present all of the formulation in a compact matrix-based form by means of the graph theory application and the node incidence matrix.
The paper aims to contribute to the use of electric double layer capacitor (EDLC) sets for boosting voltages of contact lines in urban and suburban railway traction systems. Different electrical configurations of contact lines are considered and investigated. For each of them, proper mathematical models are suggested to evaluate the electrical performances of the contact lines. They give rise, also, to sample design procedures for the sizing of the most appropriate energy storage systems, to be distributed along the lines, for boosting line voltages and avoiding undesired voltage drops. A numerical example based on the "Cumana" suburban Naples railway network is presented to give an idea of the weights and sizes of electric double layer capacitors needed to boost the voltage of a sample contact line. In particular, three different EDLC systems, for a overall installed energy of 9.6 kWh, have been placed nearby the stations presenting the highest voltage drops during the most representative situation of trains' service. The new voltage drop is equal to 32% of that obtained in absence of EDLCs.
The paper suggests an energy management control strategy of wayside Li-ion capacitor (LiC) based energy storage for light railway vehicles (LRV). The installation of wayside supercapacitor (SC) storage devices, as widely recognized, allows the recovery of the braking energy for increasing the system efficiency as well as a better pantograph voltage profile. A new type of SC, LiC, interfaced with dc-interleaved converter has been presented. This technology has an energy density comparable to batteries and power density much higher than the batteries. The authors propose a control strategy based on the maximum kinetic energy recovery throughout braking operations of the running vehicles. The stored energy comes back to the vehicles during the accelerations. The strategy stays on the knowledge of the state of charge of LiC device and the actual vehicle speeds. In particular, the control algorithm evaluates, in real time, the actual value of LiC voltage and current references on the basis of the vehicles inertial forces and acceleration estimations, taking into account the power losses of the system. Experimental tests made on electromechanical simulator, equipped with a 136-V, 30.5-F LiC module, fully confirm the validity of the suggested control. Finally, experimental characterization of LiC module has been achieved.
Recently a great interest has been paid in the relevant literature to the use of energy storage systems for the performance
improvement of electrified light transit systems. In this context, the main targets are the increase of the energetic efficiency
and the reduction of pantograph voltage drops. Therefore, it can be very interesting the determination of the optimal characteristics
of a storage device for satisfying these objectives, both in stationary and onboard case. In this paper, this problem is approached
for a sample case study, by showing that the optimal design of a stationary storage device can be regarded as a classical
isoperimetric problem, whose solution is very attractive in order to determine also the optimal allocation of the storage
device. For more complex configurations of the transit system, the methodology presented can be extended by solving a constrained
optimization problem, which in a quite general manner is capable of matching all the assigned technical requirements. The
reported simulations confirm the validity of the proposed design approach.
KeywordsEnergy storage–Light transportation systems–Optimization methods–Supercapacitors
This article will assess the installation of stationary super capacitor based energy storage systems (ESS) along a metro line for energy savings purposes. The influence of the ESS size and distribution along the line will be studied taking into account different traffic conditions.The ESSs will be configured with regards to energy content, voltage variation, maximum current and power losses. To carry out the study, an 'effect-cause' or 'backwards looking' model of the light rail vehicles and the electric network has been developed in Matlab/Simulink. A power flow controller to handle the energy flow in function of the network voltage and ESS state of charge will be proposed.
Lithium-titanate-oxide (LTO) batteries are one of the most promising technologies for various types of future applications in electric mobility, stationary storage systems and hybrid applications with high-power demands due to their long cyclic stability and superior safety. This paper investigates the cyclic and calendar ageing of 43 same-typed LTO cells considering 16 different operation conditions under variation of state of charge (SOC), temperature, depth of discharge, cycle SOC range and current rate. The ageing results are presented and the relative shift in incremental capacity is analysed in order to detect degradation mechanisms, separate the influence of degradation enhancing parameters and attribute them to their origin source. Our results show that the cells exhibit a two-stage ageing mechanism with stagewise increasing degradation gradient. In the first ageing stage the anode is limiting the amount of extractable capacity while the capacity fade mainly results from cathode degradation. After a certain level of degradation is reached the cathode starts limiting the amount of extractable capacity, initiating the second ageing stage with stronger occurring capacity fading gradient. A capacity gain of up to 2.42% becomes visible for cells operated and stored in a range below 50% SOC. For these cells an extended three-stage ageing mechanism is shown to be more applicable. The degradation behaviour is then estimated using a machine learning approach based on a recurrent neural network with long short-term memory, for which the presented incremental capacity data is used as training input.
Lithium-ion batteries are widely used in transportation applications due to their outstanding performance in
terms of energy and power density as well as efficiency and lifetime. Although various cell chemistries exist,
most of today’s electric vehicles on the market have a high-voltage lithium-ion battery system consisting of cells
with a graphite-based anode and a metal-oxide cathode. These cells offer a high specific energy density that
enables long driving ranges at moderate costs. For applications where power density is the critical design criterion,
cells with lithium titanate oxide-based anode materials can be an alternative. These cells offer further
advantages such as improved cycle stability and good charge acceptance even at temperatures below 0∘C. This
paper presents different applications for high-power batteries in electrified vehicles and compares the requirements
for suitable battery cells. After an introduction to lithium titanate oxide as anode material in battery cells,
electrical and thermal characteristics are presented. For this reason, measurements were performed with two
cells using different cathode active materials and a lithium titanate oxide-based anode. Aging behavior is investigated
with lifetime tests performed under high-current cycling conditions at two different ambient temperatures.
Differences in the capacity loss rate are shown and lifetime considerations are presented.
Furthermore, an incremental capacity analysis is performed at different times in the aging study for a deeper
analysis of the aging effects occurring in the two cell types. Finally, cost considerations of lithium titanate oxidebased
battery cells with different properties are presented. Varied production volumes are considered and
production costs are compared with costs of state-of-the-art graphite-based high-energy battery cells.
Two of the key requirements in micro-hybrid vehicles are the reduction of CO2 emissions alongside extended fuel savings. These goals are being accomplished by the usage of start-stop-systems, electric boosting while accelerating and recuperation of kinetic energy during braking phases. The latter one is predominantly limited by the charge acceptance of conventional lead-acid starter batteries. Thus, sophisticated power nets are being developed and a new 48 V voltage level has been established in the last years. In comparison to the standard 12 V grid, this new level is characterized by high power capability in order to accept high charging currents during recuperative braking and also provides high discharge currents for boosting.
This paper characterizes lithium titanate oxide cells suited for high power applications, such as in automotive 48 V systems, and emphasizes restrictions using state-of-the-art modeling approaches in terms of state of available power and state of charge. Therefore, open circuit behavior is analyzed in dependency on temperature, hysteresis and relaxation. Furthermore, different equivalent circuit models are evaluated regarding their accuracy for different states of charge, temperatures and current rates. It has been found that the HPPC testing procedure is limited in characterizing high current behavior. Furthermore, it is shown that the previous long-term history has a significant impact on voltage responses at certain states of charge and current rates, which originates from an inhomogeneous distribution of charge carriers on the electrode surface.
In the context of direct-current electrified railway systems, power supply quality issues are a major concern for the railway network operators. Indeed, after signaling systems and track equipment, voltage issues may be the most challenging factor for the capacity of a direct-current railway line. These issues and the high cost of new substations motivate the implementation of wayside energy storage systems to support the system. Since an appropriate sizing methodology is a key step towards an energy storage system implementation in a railway line, in this paper a sizing methodology is presented in detail. As hypothesis, the sizing methodology is defined such that the storage system is able to support the railway line under projected conditions of the rolling stock traffic, along with other technical criteria relevant to the railway operator. A real-time simulation oriented, direct-current railway network modeling approach is proposed and exploited in this sizing methodology. Using this modeling method, the optimal energy storage sizing formulation is described. The objective function to minimize is the trade-off between energy storage capacity and charging power. The proposed sizing method is applied to a real railway line with known power supply quality issues. This case study is introduced by analyzing real voltage measurements on two key sites of the line over several days. Then, the sizing methodology is applied and the results are discussed for the study case, defining the minimal technical requirements to install. The obtained minimal technical requirements for the storage system are considered by the operator to take a cost-effective decision in contrast with the reinforcement of the line with new substations.
The present work proposes a Modified Current Injection method for power flow analysis, bespoke for the specific features of DC traction networks. It introduces major contri- butions when compared with the State-of the-Art solution. In addition to simulating the non-linear and non-smooth behaviour of the trains, the proposed algorithm can simulate reversible IGBT-based and non reversible diode-based substations present in the system. These latter substations are the most common type and they supply unidirectional power flow from the AC distribution network to the DC traction sub-system. Depending on the voltage level at the DC side, the substations will be either blocked or conducting. This non-linear behaviour increases significantly the system complexity. Another key aspect of the pro- posed method is that it is also valid for heavily meshed networks. Even when the DC systems are usually radial or weakly meshed, the connections between the DC and the AC systems create multiple meshes, increasing the network topological complexity. In order to validate the performance of the proposed solution, the behaviour of the algorithm is compared with the conventional derivative based Trust Region Dogleg and the Backward/Forward Sweep algorithm. This comparison shows convergence and speed improvements in the performance of the proposed algorithm.
The traditional concept of dc traction systems for light rail applications was based in a simple dc system that was fed by ac/dc noncontrolled diode rectifier substations connected to the ac distribution network. Low-energy efficiency and controllability were not a problem. However, with the massive implementation of regenerative braking technologies in light trains and trams, the development of an effective way to manage the recovered energy became an important issue. The regenerated power injected in the system by a train in braking mode could be used only in the case where another nearby train was in traction mode. Otherwise, the regenerated power was dissipated in the dc traction system or in the rheostatic braking equipment, decreasing the overall system energy efficiency. The need for more solutions that would allow more use of the regenerated energy and increased energy efficiency in the system was the driving force for new technological developments. First, proposals were for the use of reversible substations able to give back the dc energy surplus to the ac distribution system. Then, due to energy storage cost reductions, a combination of technologies such as insulated-gate bipolar transistor (IGBT)-based reversible substations with on-board and off-board accumulation systems evolved into a reality. The use of substation-level off-board accumulation systems can also avoid the installation of reversible infrastructure, providing a high degree of controllability over the dc traction system voltage. However, higher efficiency rates are achieved when the energy storage is installed on board. In such a case, the regenerated power is used in-situ, minimizing the transmission losses. One of the main benefits of the offboard accumulation approach is the fact that many trains equipped with regenerative braking systems can take advantage of a single device. On the other hand, the use of on-board accumulation leads us toward more flexible, efficient, and dependable systems. Therefore, some manufacturers are using these kinds of on-board accumulation systems to avoid the use of overhead line equipment and to reduce the visual impact, offering catenary-free solutions.
The present paper deals with the energy management of stationary Li-Ion Capacitor energy storage (LC-ESS) applied in DC electrified mass transit systems. These transportation systems can benefit substantially from wayside energy storage devices for the recuperation of the kinetic energy during brakings and the limitation of supply currents during motoring. The proposed control evaluates analytically the LC-ESS reference current on the basis of the actual vehicles kinetic energies, taking properly into account the several efficiencies of the energy conversion chain. Some numerical simulations with respect to a case study and to realistic railway vehicles operations are presented, pointing out the potentiality of the tailored control technique. The results obtained show that the control is very effective and that the final choice of the storage size is a trade-off between energy saving and equipment cost.
This paper presents a criterion for the design of the optimal capacities and locations of stationary supercapacitors for light rail vehicles. The design criterion is based on a single train simulator, which uses an electrical and mechanical model of a light rail vehicle travelling across an electrified track with stationary supercapacitors. The train power and current are obtained by the characteristics of the traction system. The optimisation problem takes into account the isoperimetric constraint that the average power supplied by supercapacitors during every cycle is nil. The optimisation process minimises the difference between the train voltage and the nominal line voltage, the electrical energy consumption of the train and the storage capacity. The suggested procedure gives information on how the storage capacity is affected by the position of supercapacitors along the track. The results obtained show that the minimal capacitances are obtained when supercapacitors are close to the substations for each section of the electrified line. Finally, the paper presents a simple comparison between practical designs of supercapacitors for each section of the line using commercially available modules.
The electric railway system is one of the most peculiar power systems of which the location and power of electrical load are continuously variable. The variance of the location and power of the vehicle changes the participation factor of each substation for the vehicle and the sign and magnitude of the load current, respectively. Especially, on the substation feeder, there is huge voltage uctuation generated by the regenerative energy due to the braking vehicles. This regenerative energy is closely related with the energy efficiency since the surplus energy cannot be utilized and dissipated in the resistor. To improve energy efficiency of the railway system and utilize the surplus regenerative energy, the application of energy storage has been studied. In this paper, a DC railway powerow algorithm considering storages is developed to analyze the railway system with storages and to calculate the optimal power and storage capacity of them. The Seoul Metro Line 7 is selected for the test system and simulated to verify the effect of storages. Also, the optimal power and storage capacity of each SCES is calculated.
This paper investigates the use of ultracapacitors as an energy storage system for metropolitan railway systems. The basic concept is to take advantage of the energy generated during the braking process of trains, which is otherwise wasted and transformed into heat. A power system simulation was created as a tool to analyze the behavior of a stationary ultracapacitor bank connected to the Metro system. Ultracapacitor modules were dimensioned adequately, power electronic elements were adjusted and further a control system was established. Amongst others the simulation demonstrates the impact of varying the distance between ultracapacitors and traction power substation. Furthermore the dependency of the ultracapacitor's technical specifications on charging and discharging behavior is shown. The simulation allows a large number of parameters to be modified through the easy-to-handle user interface. A scientific method is provided to analyze ultracapacitor assisted regenerative braking of metropolitan railway systems. As an example the Metro of Medellín in Colombia is applied and rounded out by an estimation of economic viability.
The paper focuses on an innovative energy management control strategy of a wayside supercapacitor (SC) applied in DC electrified light transit networks. In order to achieve minimum energy demand from the electric feeding substations, the target of the control is the optimal tracking of the storage device voltage subject to the minimization of the mean square of the supply currents and, consequently, the reduction of power losses along the line. Hence, optimization theory has been used for the analytical determination and implementation of real time control strategy. Therefore the proposed strategy has been verified and validated via numerical simulations with reference to realistic LRV operation and different headway among the operating vehicles. A comparison with a classical energy storage system (ESS) state of charge control has been also carried out, as well as the energy saving improvement has been highlighted and the energy storage system integration and challenges are addressed. The results validate the theoretical approach and prove that the suggested control strategy is quite effective, putting in evidence its potentiality and the possibility of an actual implementation.
Over the past several years, mass transit systems in the United States have been facing increasing demands on their power systems. This is due to the several factors: increased ridership, growing overall demand for power and limited expansion of power generation facilities due to environmental concerns. Finding the solutions for these problematic factors has increased the demand for additional power sources at minimal cost, an ability to recycle the regenerative braking energy created by braking trains and line voltage stabilization. These demands have been successfully addressed in a project that tested a directly connected high capacity nickel-metal hydride (Ni-MH) battery developed by Kawasaki. These controlled tests were conducted at the New York City Transit (NYCT) early in 2010. This paper presents the results of the tests on the Battery Power System (BPS) using our GIGACELL batteries at the Far Rockaway and Manhattan locations in the NYCT system.
The improvement of the energy efficiency is a key issue for electrified railway systems in order to reach an increased competitiveness compared to other transportation technologies in terms of both costs and environmental pollution. An important step towards this direction has been made with the increasing development of storage technologies. The installation of stationary or on-board storage technologies, as well known, allows the recovery of the braking energy of the rolling stocks for increasing the energy efficiency of the whole system as well as improving the voltage profile. In the paper an optimization procedure is proposed for choosing in the planning stage the fundamental characteristics of a stationary storage device. The optimal parameters of the storage system can be determined by taking contemporaneously into account the energy saving aspects, substation current minimization and reduction of voltage drops along the feeder. The considered storage system is based upon a bidirectional DC-DC converter with ultracapacitors. A sliding mode control is implemented for tracking the optimal trajectory determined by the previously mentioned optimal technique. Numerical applications put in evidence the effectiveness of the proposed procedure.
The solution of simultaneous algebraic equations describing a
DC-traction power system is based on an iterative technique because of
their nonlinearity. Two iterative methods specified for DC-load-flow
problems are described and their mathematical and geometrical
explanations are presented. The methods have been tested for DC-fed
traction power systems and comparisons made of the computational time
and iteration count. The DC-traction software which has been developed
enables the situation with trains operating in regenerative-braking mode
to be simulated. A condition in which convergence problems are to be
expected occurs when there is a surplus of power produced during
regenerative braking. In this case two different modelling approaches to
represent regenerative trains in the simulation are investigated. A
representative DC-fed-traction power system with train regenerative
braking was simulated and the preferred method is selected by analysing
the simulation results obtained and the computation times expended
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