3 - Month Progress Report

Transcription

1 University of Birmingham School of Electronic, Electrical and Computer Engineering 3 - Month Progress Report What are the energy storage device challenges for the dc electrified railways of the future? Tosaphol Ratniyomchai Supervisor: Dr. Pietro Tricoli; Dr. Stuart Hillmansen DATE SUBMITTED:

2 ABSTRACT In modern electrified railways, trains are stopped by regenerative braking of electric motors using a suitable control system. In this case, the kinetic energy is transformed into the electrical energy by the traction drive. The effective application of regenerative braking requires the presence of extra components, such as resistance on board and/or energy storage devices (flywheels, batteries and double layer capacitors). In this research, energy storage devices have been considered for application to the dc electrified railway. To begin with, the characteristic of each type of energy storage devices has been studied and the requested functionalities have been explained by the Ragone plot in terms of energy and power density. The main features of energy storage devices suitable for the dc electrified railways are compared by the analysis of previous studies. Presently, the initial stage of work for 3 months has been devoted to analyse a simple model of the dc electrified railway with an auxiliary energy storage device. This example has been considered and designed for understanding the energy flows between the electric substation and the energy storage devices. As a result, energy storage devices can support the dc power source by discharging energy during train motoring. Besides, during braking mode, the regenerative energy is obtained by the traction drive and this energy can charge the energy storage devices for the next cycle of train journey. The model and function of the dc electrified railways will be studied with the aim of being close to a real train system, in terms of electric substations, energy storage devices, DC/DC converters and traction loading. Another objective is to develop a model of any possible faults occurring in the dc electrified railways, with and without energy storage devices. This problem will be addressed limiting the effects of the energy storage devices and enhancing the protection of the dc electrified railway. This stage of work will be carried out within 9 months.

3 Table of Contents ABSTRACT... i Table of Contents... ii 1 A summary of the project aims: research hypothesis Literature review The initial stage of work: The simple model of the dc electrified railways with the auxiliary energy storage devices Introduction and modelling Simulation result and discussion Conclusion Reference Web page Outline plan of work... 9 Appendix A: Risk assessment form ii P a g e

4 1 A summary of the project aims: research hypothesis Which one of the energy storage devices is suitable for the dc electrified railways? The best one of energy storage devices, operating within the dc electrified railway has to be quick to charge and discharge energy and capable of a high number of charge and discharge cycles. This can guarantee a long lifetime when storage devices are installed in the system. The model and function of the dc electrified railway have to be close to the real train system in terms of electric substations, energy storage devices, DC/DC converters and traction loading. What are the effects of energy storage devices when a fault occurs in the dc electrified railways? The energy storage devices are actually power sources when the fault happens, so they can actively supply energy to the fault. The limitation of this energy from the energy storage devices and the protection of any part of the system will be analysed and possible technical solution will be proposed. The modelling and simulation are carried out in order to solve this problem and also the experiments are designed by scaling down the whole system for make a comparison with the simulation results. 2 Literature review Presently, there are many research on energy storage devices (ESD) for electric railways. Most applications of ESD are presented in dc electrified railways. Several studies have shown that the use of ESD could reduce voltage drops and absorb the regenerative power from the trains. In addition, some studies suggested that energy saving could be improved by ESD. However, there are many types of ESD available for dc electrified railways. Therefore the comparison in terms of energy and power density is necessary to understand, which ESD is more suitable for these traction systems. The ESD already used for dc electrified railways include batteries, superconducting magnetic energy systems (SMES), flywheels and electric double layer capacitors (Okui et.al, 2010). Besides, power converters (DC/DC converters, or inverters) are very important to control effectively the charge and discharge of ESD. In addition, they are the intermediate connection between the electric substations and ESD and harmonise their voltage and current levels (substation side and ESD side). Fig. 1 shows the relationship between duration of charge and discharge of ESD and requested functionalities. A Ragone plot shows the available energy of ESD for fixed power, as shown in Fig. 2 (Christen and Carlen, 2000). Each type of ESD is typically placed in a different area. The unity slope is related to characteristic times of ESD. Some batteries have a high energy density and have typical discharge time of about 1 hour, such as sodium sulphur batteries, while some new batteries, such as lithium-ion, nickel metal-hydride and lead-acid have a high power density and can be discharged in 1 second to 10 minutes only. On the other hand, flywheels, SMES and capacitors have higher power density and typical charge and discharge times between P a g e

5 seconds and 10 seconds. In general, a period between 10 seconds and 10 minutes is strongly demanded for charge and discharge if ESD has to be used for application in dc electrified railways. There is clear evidence to suggest that capacitors (supercapacitors, electrolytic capacitors and electric double layer capacitors), flywheels, SMES and some types of batteries (nickel metal-hydride batteries) are suitable devices for the application to dc electrified railways. Similarly, some research suggested that this equipment can quickly charge and discharge and also they have a very high number of charge and discharge cycles. This can guarantee a lifetime long enough for dc electrified railways, especially supercapacitors, whose lifetime is about 10 6 charge and discharge cycles long (Iannuzzi and Tricoli, 2012). Fig.1. Relationship between duration of charge/discharge Fig. 2. Ragone plot and requested functionalities (Okui et.al, 2010) (Christen and Carlen, 2000) Recently, ESD have been installed aboard or wayside depending on the technical solutions (Iannuzzi and Tricoli, 2010). A suitable coach was redesigned for placing ESD aboard for reducing as much as possible the distance between ESD and the traction electrical drive. This aboard allocation has been mostly suitable for improving the power demand of the dc electrified railways both in accelerating and braking modes. As an alternative, wayside ESD could be installed in the stations along the power line, such as flywheels. The aspect of the electrical power use in the electrical railways is that the peak power is much larger than the average power. Energy saving was one of the key that showed the performance of ESD. There was some evidence that aboard ESD could reduce a consumed energy up to 30%, on the basis of measurements made on a light rail vehicle prototype, and also peak power demand from the line about 50%, compared to a modern regenerative light rail vehicle (Steiner and Scholten, 2005). Recently, there are many studies on ESD with the dc electrified railways, especially supercapacitors. Other studies on control algorithms of onboard supercapacitor with speed control of the electrical motors of metro trains suggested that this control algorithm could save up to 38% of kinetic energy of the train (Iannuzzi and Tricoli, 2012). In addition, this research pointed out a reduction of line power, voltage drops during the acceleration and surge voltages during the braking. Similarly, some studies have 2 P a g e

6 been reported that supercapacitor could limit overhead line currents and voltage drops at the train pantograph (Iannuzzi and Tricoli, 2010). 3 The initial stage of work: The simple model of the dc electrified railways with the auxiliary energy storage devices The simple model for the dc electrified railways with auxiliary energy storage devices has been considered and designed for studying the basic energy sharing between electric substation and ESD. The simulation result can show the connection between them and allows the understanding of the concept of the ESD state of charge. 3.1 Introduction and modelling The main structure of the dc electrified railways includes the power grid with rectified circuit, called Electric Substation (or ESS), the inverter circuit with the induction motors, called Traction Electrical Drive and also the optional auxiliary energy storage devices with dc/dc converter, called Energy Storage Devices (or ESD). The basic 3 rd rail dc electrified railways with ESD in shown in Fig. 3. Since the real model of the dc electrified railways is quite complex and difficult to understand, so a simple model of the dc electrified railway with auxiliary ESD has been considered and shown in Fig. 4. Fig. 3. Basic 3 rd rail dc electrified railways with ESD i d r d i out i L ( t) v d v out i in v in v sto Fig. 4. Simple model of the dc electrified railways with auxiliary ESD 3 P a g e

7 Firstly, the power grid with rectified circuit or ESS can be modelled by the Thevenin s equivalent circuit with a voltage source,, and its internal resistance,. Actually, is not constant but it is variable depending on the train movement. However, the simple model in this study will neglect this effect, so is considered constant. Secondly, the induction motors with inverter circuit have the function of the traction loading of the dc electrified railways, as for the simple way they can be considered by the dc current source,, which is function of time. Lastly, the auxiliary ESD can be modelled by a constant dc voltage source,, connected to the third rail by means of an ideal dc/dc converter. The relationship between input voltage,, and output voltage, and also input current,, and output current, are given in (1) and (2): = (1) = (2) Where n is a proportional constant of the input and output of the ideal dc/dc converter. From Fig.4 it can be found the summation of the voltage in the loop of the Thevenin s equivalent circuit and the ideal dc/dc converter by the Kirchhoff s Voltage Law (KVL) as in (3) = + = + (3) Thus, the current from the dc power source,, can be found in (4) = (4) The summation of the current at node a in the Fig.4 can be found by the Kirchhoff s Current Law (KCL) as in (5) = + (5) Therefore, the output current of the ideal dc/dc converter,, can be found in (6) = (6) The electrical parameters used for the simulation are reported in Table 1. From Table 1, is obtained by the internal resistance multiple by the traction length and is typically voltage of the British dc electrified railways; is the voltage of ESD taken from the examples illustrated in previous studies. Initial SOC of ESD is taken from a typical value of a low speed train, which is suitable for the simple model (For the next challenge, the speed and mass of train will be considered). The proportional constant n is a proper value for the traction loading condition in that moment. Table 1. Electrical simulator parameters. Parameter Unit Quantity 3 rd rail resistance (track length 1 km) Ω/km Internal dc voltage source 750 Internal dc voltage of ESD 390 State of Charge of ESD (SOC) h Proportion constant n P a g e

8 3.2 Simulation result and discussion The load current simulated is made of three mode of train movement, including motoring, coasting and braking, as shown in Fig. 5. The motoring mode starts from train standing at 0 seconds until 30 seconds, when train accelerates up to the cruising speed. The coasting mode starts from 30 seconds until 45 seconds and is characterised by constant speed. The peak of the load current is 4000 A at 25 seconds, but it drops to 100 A when the train runs at constant speed because the torque is required to win friction only. Then the breaking mode starts from 45 seconds until 75 seconds, in this situation the train will decelerate and stop at the end. In this mode, the load current will regenerate back to the ESS Coasting Mode Breaking Mode i(t), (A) Motoring Mode t(sec) Fig. 5. Load current in three mode of train movement A il(t) iout(t) id(t) 3 x Pout(peak) = MW Pout(rms) = MW 1000 i(t), (A) P(W) t(sec) Fig. 6. Road current, dc/dc converter output current and dc power source current. -2 Pin(t) Pout(t) Poutrms(t) MW t(sec) Fig. 7. Input and output power of dc/dc converter and rms value. Certainly, if this system had not ESD, all regenerative energy would come directly to the dc source. However, this is not possible because dc substations are not bidirectional in power. The energy must be dissipated in braking resistors, without any possibility of recuperation. In this simulation with ESD, the constant n is involved with dc current source and load current. The upper and lower areas of load current are divided by the average value, 5 P a g e

9 which makes these areas equal. Thus n= and dc current source is equals to A, as shown in Table 1 and Fig. 6, respectively. Fig.6 shows that the dc current source is constant, so the load current is supplied by ESD and the substation whenever the load current is larger than the average level. The rms input and output power of the ideal dc/dc converter are shown in Fig. 7 instead SOC = kwh MaxEsto = kwh SOC = kwh 2.5 x EL(t) Capacity of Esto = kwh 1.5 Esto(Wh) EL(Wh) MinEsto = kwh Esto(t) t(sec) Fig. 8. Energy of the energy storage devices from started the train until stop t(sec) Fig. 9. Load power of the electrified railways. Total energy losses = kwh Charge and discharge stages of ESD are shown in Fig. 8. In this figure, a discharge stage is represented by a decreasing line and this means that ESD is actively contributing to the train supplying. On the other hand, a charge stage is represented by an increasing line and this happens during the regenerating braking. The State of Charge (SOC) of the energy storage devices is set to 1 kwh for the initial value which is the same level as when the train is stopping. The maximum and minimum of the energy stored in ESD can be used for its design and is approximately equal to kwh. This means that the net value or the energy capacity of ESD is also about kwh which is actually SOC of ESD. The start and stop position of the train, therefore, SOC must be the same value. The energy consumed by the train from start until stop is equal to kwh, as shown in Fig. 9, and is representative of all the losses of the power train. The summation of power losses in the traction system is shown in Fig. 10. Fig. 10. The scheme of power and energy flow in the traction system 6 P a g e

10 3.3 Conclusion The simulation of a simple dc electrified railway with ESD and ideal DC/DC converters has been analysed with reference to a single cycle of train movement. The optimisation of the constant n for ideal DC/DC converters has been considered. This value has considerable influence on the state of charge of ESD, which must be the same at the start and the end of cycle. As a conclusion, it has been shown that ESD can be helpful in supporting the dc power source at motoring mode of the dc electrified railways. In addition, at braking mode, the regenerative energy can be used to recharge ESD for the next cycle of train journey. 4 Reference Christen, T. and Carlen, M.W (2000) Theory of Ragone plots. Journal of Power Sources, vol. 91, issue 2, pp , December. Iannuzzi, D. and Tricoli, P. (2010) Metro Trains Equipped Onboard with Supercapacitors: a Control Technique for Energy saving. in Proc. Int. Symp. Power Electronics., Elect. Drives, Autom. Motion SPEECAM, pp , Pisa, Italy, June. Iannuzzi, D. and Tricoli, P. (2012) Speed-Based State-of-Charge Tracking Control for Metro Trains with Onboard Supercapacitors. IEEE Transaction on Power Electronics, vol. 27, no. 4, pp , April. Okui, A., Hase, S., Shigeeda, H., Konishi, T. and Yoshi, T. (2010) Application of energy storage system for railway transportation in Japan. The 2010 International Power Electronics Conference and Publication, pp Steiner, M. and Scholten, J. (2005) Energy storage on board of railway vehicles. The 2005 European Conference on Power Electronics and Applications and Publication, pp P a g e

11 5 Web page Tosaphol Ratniyomchai, PhD PhD student Group : Birmingham Centre for railway Research and Education Supervisor : Dr. Pietro Tricoli, Dr. Stuart Hillmansen txr122@bham.ac.uk Biography Tosaphol was born in Suphanburi, Thailand in He took his Bachelor of Engineering (BEng) degree in Electrical Engineering (First-Class Honours) and Master of Engineering (MEng) degree in Electrical Engineering from Suranaree University of Technology in 2003 and 2006, respectively. He was then awarded a full scholarship to start his PhD from the Royal Thai Government in the field of energy storage devices with the electrified railways at the Birmingham Centre for Railway Research and Education. Tosaphol is a Lecture in the School of Electrical Engineering at Suranaree University of Technology, Nakhon Ratchasima, Thailand, since December 17, Research interests FACT and Custom Power Renewable Energy and Power Grid integration Current project What are the energy storage device challenges for the dc electrified railways of the future? Publications none Contact : +44 (0) [ Personal Research Group School University ] Last updated by Tosaphol Ratniyomchai on June 14,2012 Maintained by Tosaphol Ratniyomchai 8 P a g e

12 6 Outline plan of work Activity 1. Literature review: reading paper from the publication and other electronic data base about ESD with dc electrified railways. 2. Characteristic study of ESD. 3. Study the simple system of the dc electrified railways with ESD. 4. Preparing the 3-month progress report. 5. Study the model and function of the dc electrified railways to be close to the real train system in terms of electric substation, ESD, dc/dc converter and traction loading. 6. Study the theory and finding the model of any cases of fault occurring in the dc electrified railways 7. Study the possibility of fault in dc electrified railways and the operation of the protection devices in terms of the cause and effect. 8. Study the effect of the ESD on any part of the dc electrified railways when the fault happening and also finding the solution and devices to protect or limit this situation. 9. Modelling and simulation from studying and preparing and designing for the experiment. 10. Preparing the 9-month progress report Months (Started from April) P a g e

13 Appendix A: Risk assessment form Electronic, Electrical and Computer Engineering Risk assessment for generically assessed operations You do not need to carry out a written risk assessment in order to do the operations detailed below, provided such operations are done within rooms designated for that purpose and within the Gisbert Kapp building (1) Using equipment that has been purchased and provided by the School Using equipment that has a valid tested for electrical safety label Using a bench power supply that provides less than 30 Volts between terminals or earth Working on a project that USES voltages below 30 Volts (whether generated or transformed ). Working on a project that produces/uses Radio frequency signals below 1mW/cm 2 Using a P.C. solely to write software/documentation Not using Chemicals (Including proprietary substances such as glue, cleaning solvents or water) Using lasers classified as Class 1 Only Soldering in a designated area, with fume extraction & using colophony free solder Only using small hand-tools to construct electronic circuits (Pliers, wire cutters, and screwdrivers) Working with items at temperatures between 5 degrees and 30 degrees Celsius Not working with revolving or moving parts Any work outside these parameters must not be carried out until a risk assessment has been made. In some cases the risk assessment may be simple clarification from your supervisor or the person responsible for the working area, for example, does a piece of battery operated equipment require testing for electrical safety. In ALL other cases a full risk assessment will need to be carried out detailing the Hazard and likelihood of the risk, this may need to take into account legislation such as the Electricity at work regulation, The COSHH (Control of substances Hazardous to Health Regulations etc) and the School / University Health and Safety policies. Further details may be found by following the Health and Safety Link at:- By signing this form you are a) Stating that all of your work conforms to the above parameters b) Stating that you have read the relevant School and University Health and Safety Policies. This form must be signed or a written Risk assessment carried out and presented to your supervisor before you start any work and a new risk assessment made whenever circumstances change Access into swipe operated laboratories will not be allowed until a written risk assessment is produced. Hot-seated Undergraduate laboratory N219 is generally designed solely for the use of UG & MSc students who have signed this form and do not require a written Risk assessment. However some activities outside these parameters such as the usage of small amounts of adhesive may be allowed following a risk assessment. UG & MSc students operating outside these parameters must work in a designated research area and must display a copy of their risk assessment prominently at their working position. I agree that I have assessed my work and it only involves those operation stated above I understand that should my work change such that I am operating outside the above parameters I will need to re-asses the risks. Signed: Date Student Signed Date Supervisor (1) See for a room list 10 P a g e