Energy Storage Devices in Railway Systems

Martyn Chymera, Alasdair Renfrew, Mike Barnes

University of Manchester, UK, School of Electrical and Electronic Engineering, Manchester M60 1QD +44(0)161 306 2843 martyn.chymera@postgrad.manchester.ac.uk

Keywords: Energy Storage, Power Quality, Traction, Modelling.

Abstract

Efficiency and performance are key factors in railway systems. This paper discusses a key performance limiting factor, voltage regulation and uses tools to analyse the voltage regulation and power consumption in a railway network. The potential use of supercapacitors to improve voltage regulation and efficiency is described, with simulation results used to demonstrate the potential benefits of utilising energy storage in a railway system.

1 Introduction

Railway systems can be described as electrical networks, with moving and changing loads. These moving and changing loads are demanding on the electrical supply and pose voltage regulation problems. Trains can operate within a range of voltages; however the performance of a motor is limited by the voltage, and hence significant changes in voltage levels limit the potential speed of a train.

The voltage drop across a transmission line is proportional to the current. Each train drawing a current on a system contributes to the voltage drop hence increases in traffic density increase voltage regulation problems. If the voltage falls below a minimum, trains are unable to operate.

The power demand is proportional to the traffic density; this is limited by the substation capacities. The traffic density of a railway system must be limited to ensure voltage levels remain within specified levels for operations and substation current limits are not exceeded.

To increase the capacity of a railway system, investment in additional substations is often required. By increasing the number of substations the required additional power is provided. Using more substations can allow the substation spacing to be decreased, hence using shorter lengths of transmission line, and reducing the line impedances; this reduces the voltage drops and improves the voltage regulation. Voltage regulation can also be improved by using lower impedance transmission lines; this also helps improve the system efficiency, however cost and weight issues often outweigh the benefits of changing the transmission lines.

Escalating energy costs have become a serious concern in railway systems. Reducing energy consumption is a key

priority to rail system operators. Regenerative braking was introduced to reduce energy consumption. Regenerative braking is only effective if other trains are available to use the regenerated energy. Regeneration also causes further voltage regulation issues, introducing voltage surges. Successful implementation of regenerative braking requires the use of resistor banks to remove excess energy or inverting substations to regenerate onto the local distribution network.

Recent developments in energy storage devices, particularly supercapacitors and flywheels have made energy storage a viable technology to apply to railway systems. Energy storage devices can be used to tackle the issue of poor voltage regulation and help improve energy efficiency by storing regenerated energy from braking. This paper explores the use of energy storage in mass transit systems, using Blackpool Tramway as a case study.

2 Railway Electrical Network Analysis

Railway systems are complex electrical networks. Like a distribution network a railway system contains a transmission system: overhead lines or conductor rails, and loads: trains. However these loads are moving, hence changing position, the train loads are changing as speed and acceleration varies.

Figure 1 Simplified Electrical representation of a double end fed DC railway section

Figure 1 shows an electrical representation of a simple railway section. RS1 and RS2, represent the overhead line impedances. RR1 and RR2 represent the return conductor impedances. As the train moves these impedances change. As the train motors, coasts and brakes the current drawn vary.

The voltage across the train can be determined, by applying network analysis to the system, equation (1). The train voltage, TV is dependent on the impedances and train current,

as these change the train voltage changes.

1 2 1 2 1 2

1 2 1 2 1 2

S S S S R RT T T

S S S S R R

R R V V R RV I I

R R R R R R

⎛ ⎞ ⎛ ⎞= + − −⎜ ⎟ ⎜ ⎟

+ +⎝ ⎠⎝ ⎠ (1)

Voltage regulation is particularly a problem in mass transit systems where trains are accelerating and braking regularly. More significant voltage drops are experienced on systems using lower voltages, as higher currents are drawn; this is often experienced on mass transit systems which are commonly use low voltage DC supplies (550V, 630V, 750V).

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Figure 2 The tram voltage profile of a tram moving through a double end fed section on the Blackpool Tramway

Figure 2 shows the voltage levels measured on a centenary tram on the Blackpool tramway. The voltage profile shows significant variation in the voltage level. These measurements were made during the off-peak season, where a few trams were present in a section. During the peak holiday seasons, single sections can contain more trams, and hence further variations in voltage levels can be experienced.

The performance of an electric motor is inhibited when the terminal voltage falls. Hence a reduction in the overhead line voltage inhibits the performance of a train and therefore significant voltage drops are undesirable.

3 Regenerative Braking

Rheostatic braking is achieved by connecting resistor banks across the motor terminals. The motors generate a braking current which is dissipated through the resistors. The braking force can be controlled by controlling the current using a chopper. The braking energy is dissipated as heat, in cold conditions; this is beneficial as it can be used to heat the vehicle, at other times it is dissipated into the surroundings. If

the system is tunnel based the generated heat can contribute to tunnel overheating [1].

Regenerative braking generates electrical energy onto the overhead line. This energy can be used by other trains. By regenerating onto the overhead line, overall system efficiency gains can be made. The currents generated by regenerative braking can cause voltage swells. This introduces a further power quality problem. For operational purposes, the overhead line voltage is limited; regenerated currents have to be control so that this limited is not exceeded. The effectiveness of regenerative braking is dependant on the receptivity of the system. If no other trains are motoring within the section, the regenerated energy cannot be used, and the energy has to be dissipated through resistor banks.

The issues associated with regenerative braking; particularly on DC systems can be avoided by using energy storage devices. The energy storage devices can store regenerated energy on board trains or at the track side, hence reducing the magnitude of voltage swells. On board stored energy provides an additional power source for acceleration, hence reducing the acceleration currents drawn from the overhead line and therefore reducing the magnitude of voltage sags.

4 Energy Storage Devices

Significant developments in energy storage devices have recently been made, particularly for electric vehicle, power system and aerospace applications. For use in railways systems in conjunction with regenerative braking, an energy storage device with large power density would be required. Supercapacitors, flywheels, and SMES would be suitable for railway applications.

Supercapacitors consist of two solid electrodes in a liquid electrolyte. An ion permeable separator is used to electrically insulating the electrodes, but allowing ions of the electrolyte to pass through. Supercapacitors store charge at the interface of the solid electrodes and the electrolyte, forming a double layer [2]: a capacitance is formed by the two monolayers. The distance between the charge layers is only a few atomic diameters, hence a capacitance much greater than that achieved by conventional capacitors is possible. A double layer is formed at each electrode. Flywheels store energy in the form of rotating inertia. Using magnetic bearings and containing the flywheel in a vacuum has reduced losses, enabling flywheels to store energy for longer periods more efficiently [3].

Superconducting Magnetic Energy Storage (SMES) devices store energy in a magnetic field [4]. By applying a DC current to a coil, a magnetic field is created, storing magnetic energy. When the DC potential is removed, the energy is released. By using low loss superconducting coils, high amounts of energy can be stored in the magnetic field. SMES are used to improve power quality in distribution networks [5].

5 Modelling of Railway Electrical System

A railway system model has been developed at the University of Manchester for the purpose of power quality analysis [6]. The model is used to simulate the effect of train movement on the overhead line voltage levels.

DYNAMICSELECTRICAL NETWORK

SIMULATOR

TRAM MOVEMENT

INPUT

Figure 3 Modelling approach

Figure 3 shows the model used. The inputs to the model are the train position profiles. The dynamics block model is used to equate the forces acting on the vehicle, the rolling resistance, aerodynamic drag and gravitational force; this can then be used to determine the required driving force.

_ cosDriving drag rolling resistnaceF F F mg maθ= + + + (2)

The train current is then determined from the required torque. The motor current-torque characteristic is used to determine this. Motor current-torque curves are plotted for different voltages. Interpolation between the voltage curves is used to derive the required value of current.

The electrical network simulator uses steady state network analysis to determine the required system voltage levels. The impedances are determined from the train positions.

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Figure 4 Measured speed profile of a Blackpool Tram

Simulations of electric vehicles are often performed using standardised driving cycles[7]. These driving cycles are designed for automotive applications, driving cycles of cars differ significantly from that of rail vehicles. A recorded speed profile has been used for the analysis presented in this paper. The speed profile, Figure 4, was recorded using a GPS receiver, on a centenary tram on the Blackpool Tramway. A tram travelling from Starr Gate to Blackpool Pleasure Beach was simulated using this speed profile. An electrical representation of the test section is shown in Figure 5.

RSS1 RSS2

RNS1 RNS2

VS

RR1 RR2

GRID

Sub-station

Figure 5 Electrical Representation of the test section

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Figure 6 Simulated Current Profile of a Blackpool Tram

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Figure 7 Simulated Pantograph Voltage Profile of a Blackpool Tram

Figure 6 and Figure 7 show the simulated current and voltage traces respectively. The effect of a single tram was simulated. The simulation results show the current pulses drawn, and the voltage drops that coincide with these current pulses. Significant voltage drops are observed particularly when the distance of the tram from the substation is greater, this occurs at the start of the simulation. Additional trams on the system would increase the magnitude and the frequency of the voltage drops.

6 Energy Storage Devices in Railway Systems

Voltage regulation can be improved by adding energy storage devices to a railway system. This allows the utilisation of regenerative braking to be improved. Energy storage can be added track side or onboard trams themselves.

Figure 8 Tramway with energy storage at the substation

Figure 8 shows the schematic for placing the storage device, in this case a supercapacitor at the substation. The energy storage device could in principle be placed at any position along the track. Flywheels are used on the New York subway to strengthen the supply [8]. A storage device could be placed at the end of the single end fed section to improve the voltage regulation.

Adding energy storage to the track side requires energy to be transferred between the tram and the energy storage device using the overhead lines, this leads to transmission loses. These transmission losses can be avoided by placing the energy storage devices on-board vehicles. A high power density energy storage device such as a supercapacitor would be ideal for this application.

A variety of methods have been implemented to control systems with two energy sources, particularly on hybrid vehicles [9]. Storing the energy from regenerative braking and then using the energy in acceleration improves efficiency. However by using the overhead line to provide a steady power supply, and using the supercapacitor to provide the transient energy, allows the pulse loading to be removed and hence improve the voltage regulation. This can be achieved by averaging the power requirement over a specified time period. Typically a time period of 10s is used in electric vehicle applications, however acceleration times are typically longer in tram systems, and a time period of 30s would be more suitable.

This energy storage system can be implemented using 6 Maxwell Ultracapacitor 140F 48V modules connected in series, giving a total capacitance of 24F at 290V. Such a supercapacitor bank has a capacity of 2MJ. The mass of the supercapacitors would be 84kg and they would occupy 78l. In

addition to this a second DC-DC converter is required; this would add another 100kg to the mass of the system.

The supercapacitor bank can be modelled as an equivalent circuit with a voltage source, a equivalent series resistor (ESR) and an equivalent parallel resistor (EPR), Figure 9.

Figure 9 Equivalent Circuit for a supercapacitor

Figure 4 shows the measured speed profile of a tram on the Blackpool Tramway. This speed profile was used as a test profile to simulate the effect of using supercapacitor energy storage on board a tram.

Figure 6 shows the current profile of a tram without the use of energy storage. The current profile contains large current peaks, with amplitudes exceeding 250A. This current peaks results in significant variation in voltage levels. Figure 7 shows the Voltage regulation, significant voltage sags are experienced in the first part of the simulation, where the tram is at the end of a single end fed section. The simulation results only represent a single tram in operation; the operation of multiple trams would magnify the voltage fluctuation.

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Figure 10 Tram Current Profile with energy storage on board the tram

Figure 10 show the current profile achieved by using the supercapacitor bank on board the tram. A smoother current profile is observed and the size of the current peaks are reduced. Using the supercapacitors significantly reduces the

magnitudes of the voltage sags, providing a preferable pantograph voltage profile, Figure 11.

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Figure 11 Pantograph Voltage profile with energy storage

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Figure 12 Supercapacitor State of Charge During Operation

Figure 12 shows the supercapacitor state of charge during the tram operation. The initial state of charge was 80%. The graph shows the supercapacitor is utilised between 30% and 80% of its potential storage. The supercapacitor bank maximum power output is related to the state of charge, when state of charge reduces, the power available reduces, hence the state of charge has be maintained above a minimum.

Better utilisation of the energy storage could be made. Development of the control system use during operation can improve this utilisation.

7 Conclusions

The benefits of using energy storage have been outlined. A model has been used to simulate the use of supercapacitors on board traction vehicles. The simulation results have showed significant improvements in the overhead line voltage regulation.

The paper demonstrates the use of a simple control system to utilise the supercapacitor, this shows significant improvements to the voltage profile can be achieved, by

further developments to the control system further improvements could be achieved.

Acknowledgements

The research was conducted as part of an EngD research project carried out at the University of Manchester. The EngD is funded by the EPSRC and HILTech Developments Ltd. The authors would particularly like to thank Peter Brown, Chief Electrical Engineer and Blackpool Transportation for providing data on the Blackpool tramway and their trams and for the use of their system to verify simulation results.

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