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Benefits of Upgrading the Overhead Line of a DC Railway Line in the Netherlands

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Highlights:
* Major improvements were seen in the upgrade from 500 to 800 mm².
* Upgrading to 1000 mm² is still attractive, but payback period and internal rate of return are less favourable.
* Energy consumption of the track decreased by 6%.
* Optimisation of conductor size should become standard in design of traction power supply systems.
* Such optimization requires a simulation study.

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Benefits of Upgrading the Overhead Line of a DC Railway Line in the Netherlands

  1. 1. Benefits of upgrading the overhead line of a DC railway line in the Netherlands a simulation case study November, 2001 Frederik Groeman, KEMA
  2. 2. About the European Copper Institute The European Copper Institute is a joint venture between the world’s mining companies, represented by the International Copper Association, and the European copper industry. Its mission is to promote copper’s benefits to modern society across Europe, through its Brussels office and a network of eleven Copper Development Associations. In fulfilling its mission, ECI manages a broad range of information and education activities. Dissemination to target audiences is carried out through the national Copper Development Associations located in the Benelux, France, Germany, Greece, Hungary, Italy, Poland, Russia, Scandinavia, Spain and the UK. About LEONARDO Energy LEONARDO Energy (LE) is a programme managed by ECI, involving over 100 partners in various projects related to electrical energy. LE focusses on Quality of Supply, Electrical Safety and Sustainable Electrical Energy. The programme targets professionals, press and regulators involved in the electrical energy sector. It promotes best practice in electrical engineering and energy regulation. Copyright c KEMA/European Copper Institute. Reproduction is allowed provided that the material is unabridged, and the source acknowledged. After publication, please send a copy to ECI for the attention of the Publications Office. Disclaimer While this document has been prepared with care, ECI, KEMA and any other contributing institutions give no warranty in regards to the contents and shall not be liable for any direct, incidental or consequential damages arising out of its use. European Copper Institute Tervurenlaan 168 b10 B-1150 Brussels Belgium +32-2-7777070 Email: eci@eurocopper.org Website: www.eurocopper.org
  3. 3. Contents 1 Summary 6 2 Introduction 8 2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2 Project objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3 Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3 Catenary cross-section 10 3.1 Energy (cost) saving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2 CO2 reduction and emission trading . . . . . . . . . . . . . . . . . . . . . . 11 3.3 Increased energy savings by regenerative braking . . . . . . . . . . . . . . . 12 3.4 Traction system performance: higher voltage and more power to the trains 13 3.5 Travel time reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.6 Limitations and drawbacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4 Case study 16 4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.2 The study case track . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.3 The railway traffic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5 Simulation results 18 5.1 Energy losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2 Traction system performance: supply of higher voltage to the trains . . . . 23 5.3 Travel time reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 6 Appraisal of the energy saving 30 6.1 Economical benefits without CO2 emission cost . . . . . . . . . . . . . . . 30 6.2 CO2 reduction and economical benefits including CO2 emission cost . . . . 31 7 Conclusion 34 7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 A Low-voltage power supply systems 37 3
  4. 4. List of Figures 3.1 Description of the catenary wire system . . . . . . . . . . . . . . . . . . . . 11 3.2 Power limitations as a function of voltage (example) . . . . . . . . . . . . . 14 5.1 Momentary energy use and losses in the study case track - 500 mm2 . . . . 19 5.2 Momentary energy use and losses in the study case track - 800 mm2 . . . . 19 5.3 Momentary energy use and losses in the study case track - 1000 mm2 . . . 19 5.4 Momentary relative energy losses in the study case track - 500 mm2 . . . . 20 5.5 Momentary relative energy losses in the study case track - 800 mm2 . . . . 20 5.6 Momentary relative energy losses in the study case track - 1000 mm2 . . . 20 5.7 Energy balances for the three study cases (100% = power input) . . . . . . 22 5.8 Relative losses for the three study cases (100% = power output at trains) . 23 5.9 Minimum and maximum train voltages as a function of the train – 500 mm2 24 5.10 Minimum and maximum train voltages as a function of the train – 800 mm2 24 5.11 Minimum and maximum train voltages as a function of the train – 1000 mm2 24 5.12 Improvement of mean useful train voltage for all 359 trains passing the study case track in 24 h . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.13 Time-location diagram for the study track on an hour with heavy load (500 mm2 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.14 Number of trains the travel time of which is reduced by increasing the overhead wire cross-section – Difference 500-800 mm2 . . . . . . . . . . . . 29 5.15 Number of trains the travel time of which is reduced by increasing the overhead wire cross-section – Difference 500-1000 mm2 . . . . . . . . . . . 29 6.1 Payback period (PBP) in years of an increase of the catenary cross-section as a function of emission cost . . . . . . . . . . . . . . . . . . . . . . . . . 32 6.2 Internal rate of return (IRR) for upgrading the catenary as a function of emission cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 7.1 Possible adaptation of voltage limits for a 1500 V DC system in order to increase energy efficiency (measured at the location of a train, during normal conditions) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 A.1 Schematic representation of a LV railway system) . . . . . . . . . . . . . . 37 4
  5. 5. List of Tables 3.1 Voltage limits for a 1500 V DC system according to EN 50.163 (measured at the location of a train, during normal conditions) . . . . . . . . . . . . . 13 5.1 Total power supplied and losses at the study case track – 24h . . . . . . . 21 5.2 Comparison of different cross-sections with the base case – 24h . . . . . . . 21 5.3 Energy saving with respect to the base case – 24h . . . . . . . . . . . . . . 22 5.4 Minimum voltage along the study case track . . . . . . . . . . . . . . . . . 25 5.5 Time gains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.1 Annual energy saving with respect to the base case . . . . . . . . . . . . . 30 6.2 Annual energy cost saving for the study case track . . . . . . . . . . . . . . 31 6.3 Profitability of overhead wire reinforcement of the study case track – per km 31 6.4 Emission reduction in the study case track . . . . . . . . . . . . . . . . . . 32 6.5 Annual energy cost saving for the study case track . . . . . . . . . . . . . . 32 5
  6. 6. Chapter 1 Summary In low-voltage railways, transmission losses can be significant. These losses can be reduced by increasing the conductor cross-section of the overhead line. Large amounts of energy can be saved, earning back the extra cost of the increased conductor cross-section plus reinforced gantries. Increasing the overhead line cross-section also brings other advantages. By reducing the electrical resistance between a train and the feeding substation, the traction power supply becomes “stronger”, offering higher power available to trains: trains can accelerate more quickly, which will give train operators more possibility to comply with the timetables e.g. in case of a delayed departure. In other words: the performance of the traction power supply is improved. It is, however, very difficult to estimate the degree of improvement without carrying out detailed simulations. The objective of the study described in this report is to quantify and to demonstrate energy savings and other benefits by means of a detailed simulation case study for a 52,5 km-long heavily loaded Dutch railway line, supplied with 1500 V DC. In this study, the standard cross-section of 500 mm2 was first increased to 800 mm2 , next to 1000 mm2 . The study into an upgrade to 800 mm2 per track yielded following results: • 5% energy saving on the total traction energy consumption, excluding savings from increased regenerated energy • If the support gantries of the overhead lines were to be upgraded anyway, the pay- back period for the extra cost would be at most 9,6 years • This ’project’ would have an internal rate of return of at least 9,7% • The mean useful voltage of most trains passing this track would increase by more than 50 volt, especially those trains that suffer from lower voltages • 14% of all trains running in the study case track would benefit a net time gain of 10 seconds or more. If CO2 emission rights were traded at e 33/tonne, the payback period would decrease to 6
  7. 7. www.leonardo-energy.org CHAPTER 1. SUMMARY 6,7 years and the internal rate of return would rise to 14,6%. For higher emission right cost, the payback period would decrease even further. Upgrading the track to 1000 mm2 instead of 800 mm2 would lead to even higher energy saving, but the main improvements are made in the upgrade from 500 to 800 mm2 . Upgrading to 1000 mm2 still is attractive, but both the payback period and the internal rate of return are less favourable than upgrading to 800 mm2 . The losses in the return path become dominant. This raises the question whether it would not be worthwhile to consider decreasing the return path resistance instead. Following recommendations are made in this study: • optimisation of the conductor size regarding energy losses should become a standard step in the design process of traction power supply systems. Such an optimisation requires a simulation study as the phenomena occurring are too complicated for hand-calculations • standardised minimum voltage levels of EN50163 (the traction power supply voltage standard), the TSI Energy and the like offer the possibility to design traction power supply systems with high losses and limited possibilities for regeneration of braking energy. These voltage levels deserve to be reconsidered from the point of view of energy saving. November, 2001 – Page 7 of 38
  8. 8. Chapter 2 Introduction 2.1 Background Electric railways form a large energy user, covering approximately 50 TWh/year, or 21/2 % of the total annual electricity consumption in the European Union (source: OECD). The associated costs are a significant part of railway exploitation, amounting to almost e3 billion annually. Especially in low-voltage (DC) systems, as used in many parts of Europe, energy losses between the public electricity network and the trains make a significant contribution to the total energy costs. Generally speaking, a lower system voltage correlates with higher losses. The Dutch railway system, for instance, is electrified at 1500 V DC, and the total energy losses in the railway power supply system are in the order of 10% or over 100 GWh/year. The losses in the overhead wires take a significant share of these losses. Presently, new railway systems are usually electrified with significantly higher voltages, for instance 25kV or 15 kV. Thanks to the high voltage, energy losses in such traction power supply systems are much lower than in low-voltage systems. In countries with an existing low-voltage system, the costs of upgrading the entire railway system to a high voltage are high, in the order of e2 million per track kilometre, excluding the costs for upgrading rolling stock. Considering this cost barrier, it is probable that significant parts of the railway network will continue to operate at low voltage for several decades. A straightforward possibility to reduce energy losses without abandoning the existing power supply system is to increase the conductor cross-section of the overhead line. Large amounts of energy can be saved, earning back the extra cost of the increased conductor cross-section. Increasing the overhead conductor cross-section also brings other advantages. By reducing the electrical resistance between a train and the feeding substation, the traction power supply becomes “stronger”, offering higher power available to trains: trains can accel- erate more quickly, which will give train operators more possibility to comply with the timetables e.g. in case of a delayed departure. In other words: the performance of the 8
  9. 9. www.leonardo-energy.org CHAPTER 2. INTRODUCTION traction power supply is improved. It is, however, very difficult to estimate the degree of improvement without carrying out detailed calculations or measurements. 2.2 Project objectives The objective of the study described in this report is to quantify and to demonstrate energy savings and other benefits by means of a detailed simulation case study for a heavily loaded Dutch railway line, supplied with 1500 V DC. 2.3 Approach A detailed simulation has been carried out for a railway line with ELBAS-SINANET R software, with subsequent analysis to determine the effects of increasing the net cross- section of the overhead line regarding: • energy saving and CO2 reduction • energy cost saving • voltage improvement (performance of the traction power supply) • driving time improvements. Railinfrabeheer (the Dutch railway infrastructure administrator) has kindly granted per- mission to use typical data on the traction power supply system and the railway network. This report is a sequel to the publication ’Optimal reduction of energy losses in catenary wires for DC railway systems’1 . November, 2001 – Page 9 of 38
  10. 10. Chapter 3 Effects of increasing the catenary cross-section This chapter gives an introduction to the benefits and drawbacks of increasing the catenary cross-section of low voltage railways. 3.1 Energy (cost) saving The obvious benefit of increasing the conductor cross-section is energy saving. This energy saving is realised in two additional ways: • The power loss of a current I passing through a resistance R equals I2 R. Increasing the cross-section of the overhead line decreases its resistance (e.g. by ∆R) and thus the losses (by I2 ∆R). • Trains basically act as a constant power load, as the power is regulated to obtain or maintain a certain speed. For all types of loads, reducing the resistance of the power supply implies an increase of the terminal voltage of the load. Constant power loads will reduce their current, thus reducing the transmission losses. In railway systems, this implies that reduction of the overhead line resistance does not only reduce the losses in this overhead line itself (by more than I2 ∆R mentioned above) but also the losses in the return circuit (rails), feeder cables, substations etc. 10
  11. 11. www.leonardo-energy.org CHAPTER 3. CATENARY CROSS-SECTION Catenary or overhead wire system details The key element of the overhead or catenary wire system is the contact or trolley wire, which transfers the electrical power to the train's pantograph. In order to prevent a sag of the overhead wire, it is hung to the so-called messenger or catenary wire, using short wires called droppers. In order to reduce the resistance of the overhead line, a feeder may be added. In most cases, the overhead line of a Dutch railway consists a double copper contact wire 2x100 mm2 , a 150 mm2 copper messenger wire and a copper 150 mm2 feeder wire. The relatively large cross-section of the wire is related to high currents flowing in the 1500V-overhead line (often 3000 A or higher). The contact wire(s) and the messenger wire take part in a complicated mechanical interaction during train passages and are carefully optimised for mechanical stability and lifetime. Therefore, the primary choice for reducing the overhead line resistance is adding or strengthening feeder lines. Feeder Dropper Messenger wire Double contact wire Overhead line Figure 3.1: Description of the catenary wire system The energy saving can be directly associated with the avoidance of CO2 emission in power plants. This subject is treated in the next section. The energy saving mentioned above also leads to cost savings, but this relationship is less straightforward. This is due to the fact that the electricity rate to be paid by the railway companies is not only determined by the amount of energy (i.e. the kWhs), but also by the peak load. How the peak load changes if the overhead line cross-section is increased, is a complicated issue as the changed voltage profile (section 2.4) causes the trains to drive differently, i.e. to be at a different position at a given point of time, which changes the summation of the loads of individual trains at a substation. Due to this effect, the substation peak power may decrease or in some cases even increase. 3.2 CO2 reduction and emission trading Benefits of energy saving are not only the avoidance of energy cost, but also the avoidance of CO2 emission and a contribution to the reduction of global warming. The CO2 emission November, 2001 – Page 11 of 38
  12. 12. www.leonardo-energy.org CHAPTER 3. CATENARY CROSS-SECTION depends on the average share of fossil fuels in the fuel mix of power plants. In the Netherlands, the CO2 emission associated to the use of electricity is assumed to be 0,6 kg CO2 per kWh of electricity used (the average for Europe is 0,4 kg/kWh). Presently, CO2 emission is free of charge, but emission limits are underway. To allow for economic optimisation, emission trading schemes are under discussion, and by that time, CO2 emission rights will have a price. The price will depend on actual market conditions. The price for CO2 emissions will add to the cost of energy. For this study, a price of e 33 per tonne of CO2 will be considered2 . The extra cost of energy due to the emission rights is equal to the CO2 emission per kWh times the price of the emission, i.e. about 20 e/MWh. 3.3 Increased energy savings by regenerative braking Many trains employ electrodynamical braking in addition to mechanical braking. The kinetic energy of the train is then converted to electricity by the traction motors. This electric energy may be dissipated in resistors or fed “back” into the overhead wire. Elec- trodynamical braking is very attractive compared to mechanical braking, especially on tracks with long slopes and at trains that are stopping frequently. Its advantages are a strong extension of the maintenance interval of the mechanical brakes, and energy saving if the electricity generated is supplied to the overhead line. In the latter case, the term “regenerative braking” is used. Regenerative braking may lead to large net energy sav- ings, often more than 10% of the total traction energy consumption in railway systems and more than 30% in metro systems. In mountain railways and metro systems, electrodynamical and in many cases even re- generative braking has been in used since the beginning of 20th century. An important prerequisite for regenerative braking is receptivity of the overhead wire for the power supplied. As, with only scarce exceptions, DC feeding substations are not able to supply power back to the public electricity network, a regeneratively braking train can only supply power to the overhead wire if another nearby train can absorb this power3 . The problem with regenerative braking is that there will always be a voltage drop between the regenerating and the other train. If the actual voltage level in the traction power supply system is too high, the regenerating train could push the voltage over the upper limit, i.e. the maximum allowable system voltage. In order to prevent this, a train will try to feed just so much power into the overhead wire that the upper voltage limit is not exceeded. The power that can not be supplied back to the overhead wire is dissipated in resistors or in mechanical brakes. Reducing the overhead wire resistance does not only reduce the transmission losses be- tween a regeneratively braking train and another train, but, more significant, it reduces the voltage drop across the overhead line. The resulting increase of regenerated energy November, 2001 – Page 12 of 38
  13. 13. www.leonardo-energy.org CHAPTER 3. CATENARY CROSS-SECTION may lead to energy savings in the same order of magnitude as the energy saving in the overhead line mentioned in the first section of this chapter, or higher. 3.4 Traction system performance: higher voltage and more power to the trains The function of the traction power supply system is to supply the trains with power at an acceptable voltage quality. The European standard EN 50163 defines limits to minimum and maximum voltage levels of 1000 and 1800 V for steady state conditions, see the table below. However, railway infrastructure owners usually design their networks for minimum voltages above the minimum set in EN 50163, i.e. at more strict requirements, in order to be able to cope with extreme load conditions. Permanent Non-permanent Highest voltage limit 1800 V 1950 V, 5 min. Lowest voltage limit 1000 V N/A Table 3.1: Voltage limits for a 1500 V DC system according to EN 50.163 (measured at the location of a train, during normal conditions) The voltage should not become too low for the following reasons: • as mentioned before, a train acts as a constant power load, rather than a resistor- type of load. If the voltage is low, a train will try to draw more current from the system. This, however, pulls the voltage down even further due to the ohmic voltage drop across the resistance of the catenary system. The stability limit of the system is reached if components (catenary wires, switchgear, rectifiers and transformers) are becoming overloaded due to excessive currents • due to the current rating of the equipment and to prevent a breakdown of the system, each train limits its traction current to a predetermined value. This current limitation also limits the power available to the train – if e.g. the voltage is reduced by 10%, the power available to the train also drops by 10%. For very low voltages, typically below 1200 V, the current limit itself is reduced to increase system stability, giving even more dramatic power reductions. In many cases this latter reduction is carried out by the driver, in modern trains such a limitation is automatically performed. The figure below gives a typical example of the limitations concerned, with a 4000 A limitation for voltages above 1200 V, linearly decreasing between 1200 and 950 V. The settings of these limitations will probably become standardised in the future. These power limitations are most important during acceleration, where trains will accelerate slower at low voltage conditions. This may lead to delays. November, 2001 – Page 13 of 38
  14. 14. www.leonardo-energy.org CHAPTER 3. CATENARY CROSS-SECTION 0 1000 2000 3000 4000 5000 900 1000 1100 1200 1300 1400 1500 Voltage [V] Current[A] 0 1200 2400 3600 4800 6000 Power[kW] Current Power Figure 3.2: Power limitations as a function of voltage (example) Especially at weak points in the network, an increased cross-section increases the voltage level. With the voltage level, the power available to the trains will increase, too. 3.5 Travel time reduction Thanks to the increased power available to trains, quicker acceleration will be possible. This may enable faster timetables. The main benefit may be expected, however, in conditions when the train traffic is severely delayed. During these conditions, the power supply system sees its limits (the lower one for voltage, the upper one for current) and trains experience significant extra delays due to the low voltage. An increased overhead line cross-section the power supply will not solve the delays, but at least decrease the proliferation of delays. 3.6 Limitations and drawbacks Adding extra feeder wires or increasing their cross-section adds to the mechanical loading of the support structures (gantries plus their foundations). This loading consists of two components: • the weight of the wires, including ice loads • wind load. Crucial for the economical feasibility of an overhead wire with an increased cross-section is the question whether the strength of the support structures is sufficient to carry the extra loads. If existing support gantries are adequate to carry an increased feeder cross-section, the extra cost for a feeder wire pays itself back by the energy saving. November, 2001 – Page 14 of 38
  15. 15. www.leonardo-energy.org CHAPTER 3. CATENARY CROSS-SECTION If existing support gantries and their foundations need to be replaced by stronger ones, this cost will not be earned back by energy saving alone. If, however, the support structures need to be replaced anyway, choosing a stronger support portal and foundation gives only small additional (differential) costs. Also, if overhead wires need to be replaced anyway (regular maintenance) and the support structures do not need strengthening, the differential cost may be favourable. The same applies to electrification of new or non-electrified existing railway tracks. For this study, the differential cost figures will be used. November, 2001 – Page 15 of 38
  16. 16. Chapter 4 A case study in the Dutch railway network 4.1 General The Dutch railway network is operated by Railinfrabeheer. The entire network (approxi- mately 4000 single-track km electrified) has been modelled in ELBAS-SINANET software. A part of this network has been used for the case study. Following cases have been con- sidered: • Present catenary conductor cross-section (500 mm2 Cu along the entire track) • Increased catenary conductor cross-section (800 mm2 Cu along the entire track) • Strongly increased cross-section (1000 mm2 Cu along the entire track). For this study case, the entire simulated network was reinforced to 1000 mm2 . For the 800 and 1000 mm2 simulations, all track data, timetables, train data, . . . have been taken from a network planning study carried out for Railinfrabeheer by KEMA-ELBAS in 2000Q1 (which was based on 500 mm2 cross-section of the catenary). 4.2 The study case track The catenary system has 500 mm2 cross-section per track. For the 800 mm2 and 1000 mm2 variants, the catenary has been upgraded at a 52,5 km long piece of railway of this double- track railway (105 single-track km). This section of the railway is referred to as “study case track”. The study case track is fed by 7 substations, including the substations that are located at the ends of the study case track. There are 7 passenger railway stations, one freight- only railway station and two intersections with single-track lines with relatively low traffic density. 16
  17. 17. www.leonardo-energy.org CHAPTER 4. CASE STUDY 4.3 The railway traffic The traffic at the study case track is very busy, with up to ten trains per hour per track during rush-hours, and up to 300 trains each day. The daily traffic quantity is up to 12 millions ton-km. The traffic can be characterised as mixed, national passenger trains being dominant, but international and freight trains, with their relatively large power demand, also form a considerable load to this railway. The traffic has almost reached the maximum capacity of the track, which is mainly determined by speed differences between train types and the required safety distances between trains (block system). The simulation has been carried out for 24 hours on a busy day, using a timetable for 2001. Trains departed according to the timetable (i.e. the normal situation), but during the simulation the traffic interaction between trains (e.g. keeping the right distance) and the interaction between trains and the traction power supply (e.g. power limitations due to low voltage) were taken into account. The possibility of some train types to apply regenerative braking, i.e. feeding electrical energy back to the overhead wire (and other trains) during braking, was not taken into account (all braking energy converted to heat at the train). The reason for this is, that the original 500 mm2 study did not consider regenerative braking. November, 2001 – Page 17 of 38
  18. 18. Chapter 5 Simulation results 5.1 Energy losses For the study case track, the momentary total power delivered to the track is plotted against time in graphs 5.1 - 5.3 (upper curves). In the same graphs, the momentary total losses in the track are depicted, too (lower curves). In the curves, it can be clearly seen that there is no train traffic on this track between 2:00 and 05:30 at night. The sharp variations are due to acceleration manoeuvres of individual trains: despite the averaging effect by summing the power of the substations, the peaks are clearly visible. The energy losses calculated are composed of: • losses in the catenary wires • losses in the return path (the rails) • losses in the cabling between the substations and the track. Not included are following losses: • The losses in the substation itself for e.g. auxiliary equipment • losses in the transformers and rectifiers • losses in the medium-voltage cables between the utility and the (track-side) 1500V substation. • Losses in the trains. In order to give an impression of the energy efficiency of the traction power supply system, and to show the energy savings clearer, figures 5.4 - 5.6 show the relative losses (i.e. the losses in the catenary, rails and feeders) as a function of time. These curves are a division of the losses and the input power depicted in graphs 5.1 - 5.34 . 18
  19. 19. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS Figure 5.1: Momentary energy use and losses in the study case track - 500 mm2 Figure 5.2: Momentary energy use and losses in the study case track - 800 mm2 Figure 5.3: Momentary energy use and losses in the study case track - 1000 mm2 November, 2001 – Page 19 of 38
  20. 20. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS Figure 5.4: Momentary relative energy losses in the study case track - 500 mm2 Figure 5.5: Momentary relative energy losses in the study case track - 800 mm2 Figure 5.6: Momentary relative energy losses in the study case track - 1000 mm2 When comparing the graphs, the energy loss reduction is clearly visible (the graphs have the same scaling). Also visible is that peak losses up to 25% occur. As this is the average loss, the peak losses at certain sections of the track are even higher. November, 2001 – Page 20 of 38
  21. 21. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS The totalled energy consumption of the track and losses during the 24-h simulation are given in the table below: Total power supplied to the track Losses Relative losses 500 mm2 411 MWh 50 MWh 12,1 % 800 mm2 393 MWh 34 MWh 8,7 % 1000 mm2 385 MWh 29 MWh 7,6 % Table 5.1: Total power supplied and losses at the study case track – 24h The power supplied to the track is measured at the DC substations (i.e. exclusive of losses in the substation itself, but including the losses in the power cables between the substation and the track). In the table below, the differential figures for power supplied and the losses are shown. The figures in brackets indicate the relative reduction compared to the 500 mm2 case. Power supplied to track Losses 800 mm2 -17,9 MWh (-4%) -15,6 MWh (-31%) 1000 mm2 -25,7 MWh (-6%) -20,6 MWh (-41%) Table 5.2: Comparison of different cross-sections with the base case – 24h The reduction of total power delivery to the track is higher than the reduction of the losses in the track thanks to the effect that the voltage at the trains is increased (see next section), and some trains are able to accelerate quicker. As some train types have significantly lower conversion efficiencies (between overhead line and wheels) when driving slowly, this also leads to energy saving (2,8 MWh for the 800 mm2 case). The higher voltage at the trains also causes the trains to draw less current at a given power. Some of the power delivered to the study case track is delivered from adjacent sections, especially if a train is close to the edge of the study case track. Energy saving at the study case track hence also leads to some energy saving on adjacent tracks. Energy saving in the track also leads to energy saving in the power system supplying that track. For energy cost saving, it is adequate to take into account the losses between the utility supply point and the DC busbar, i.e. losses in the medium-voltage cables, the traction transformers and the rectifiers. The components mentioned have a relative loss figure of typically 21/2 %. The losses in these components will decrease by 0,5 MWh in the 800 mm2 case. The resulting total energy saving is given in the table below: November, 2001 – Page 21 of 38
  22. 22. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS Overhead line case study track Overhead line other tracks Trains Substations and cabling Total energy saving 800 mm2 15,6 MWh 0,9 MWh 2,8 MWh 0,5 MWh 19,7 MWh 1000 mm2 20,6 MWh 1,2 MWh 3,1 MWh 0,6 MWh 25,5 MWh Table 5.3: Energy saving with respect to the base case – 24h The total energy saving is significant, although the step from 800 mm2 to 1000 mm2 brings relatively less advantage. Energy balance and relative losses The figure below gives the energy balance of the simulated cases for the study case track. The AC power input of each case is used as a reference (i.e. 100%). With an increasing cross-section, the shrinking share of the overhead wire losses is clear. Less visible is the reduction of the other losses, which is approximately one tenth of the energy saving in the catenary wire. By definition not shown in the figure is the reduction of the power demand of the train thanks to the better voltage, which would add approximately one tenth more to the energy saving. 75% 80% 85% 90% 95% 100% 500 mm2 800 mm2 1000 mm2 MV cables DC substation DC feeders Overhead wire Return path Net at train Figure 5.7: Energy balances for the three study cases (100% = power input) The figure above shows that more power becomes available to the trains. The reduction of losses relative to the power used at the trains is shown in the next figure. November, 2001 – Page 22 of 38
  23. 23. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS 0% 2% 4% 6% 8% 10% 12% 14% 16% 500 mm2 800 mm2 1000 mm2 MV cables DC substation DC feeders Overhead wire Return path Figure 5.8: Relative losses for the three study cases (100% = power output at trains) In the figure, it can be seen that the energy losses in the catenary drop below the energy losses in the return path if the overhead wire cross-section is 1000 mm2 . This raises the question whether it would be attractive to reduce the resistance of the return path. 5.2 Traction system performance: supply of higher voltage to the trains The voltage profiles along the track are shown in figures 5.9 - 5.11. Each graph contains two pairs of curves, i.e. the minimum and maximum voltage at a given location (x-axis) having occurred during the 24h simulation. Each pair corresponds to one track of the double-track railway. The horizontal axis shows the position in km according to the reference system used, and hence does not start at 0 km. November, 2001 – Page 23 of 38
  24. 24. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS Figure 5.9: Minimum and maximum train voltages as a function of the train – 500 mm2 Figure 5.10: Minimum and maximum train voltages as a function of the train – 800 mm2 Figure 5.11: Minimum and maximum train voltages as a function of the train – 1000 mm2 November, 2001 – Page 24 of 38
  25. 25. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS The maximum voltages correspond to the unloaded condition where 1800 V is present along the entire track. The minimum voltages correspond to the peak load conditions encountered during the 24h simulation. The higher peaks of these minimum voltage curves correspond to the locations of substations or switching stations (where the overhead lines of both tracks are cross- coupled). In the middle between two substations, the minimum voltage is significantly lower. The lowest voltages occur at kms 52-56 of the track with a minimum voltage of 1100 V in the base case. The minimum voltages are shown in the table below. Case Minimum voltage at 52-56 km Minimum voltage at other sections in study case track (indicative) 500 mm2 1099 V 1200 V 800 mm2 1144 V 1300 V 1000 mm2 1170 V 1350 V Table 5.4: Minimum voltage along the study case track As voltages below 1200 V lead to significant power limitations of trains (see chapter 2), this low voltage may lead to delays between 52-56 km. Increasing the overhead line cross- section significantly increases the minimum voltage, as shown in the table below. The main improvement is achieved at the first upgrade from 500 to 800 mm2 . At the remaining part of the study case track, voltages are higher, although values down to 1200 V are occurring between all other substations. These voltages may still lead to some delays in some cases, as the power is somewhat restricted (see figure 3.2). An upgrade to 800 mm2 leads to a satisfactory voltage profile with minimum voltages of 1300 V or higher, except for one location, where the voltage profile occasionally drops below 1200 V. Another method to analyse the voltage improvement is by regarding the “mean useful train voltage” for each train, that is the average voltage that that particular train experienced during traction (i.e. excluding braking). For those trains that passed the study case track during the simulation (which were 359 trains), the improvement is shown in the graph below. Along the horizontal axis are the relevant trains, sorted by the mean useful voltage in the 500 mm2 case. The first data series (the solid line) shows the mean useful voltages for these 359 trains in the 500 mm2 case. The scattered series with the rectangles and the triangles show, per individual train, the mean useful voltage at 800 and 1000 mm2 , respectively. November, 2001 – Page 25 of 38
  26. 26. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS 1400 1450 1500 1550 1600 1650 1700 0% 25% 50% 75% 100% Percentage of trains Meanusefulvoltage[V] 500 mm2 800 mm2 1000 mm2 Figure 5.12: Improvement of mean useful train voltage for all 359 trains passing the study case track in 24 h This graph clearly shows that: • those trains that suffer from the lowest mean useful voltage benefit the most. • at 800 mm2 cross-section, the mean useful voltage is increased by 50 volts or more, at 1000 mm2 by approximately 100 volts or more5 . This applies to more than half the trains at the study case track. 5.3 Travel time reduction The graph below shows the train traffic at both tracks of the study case track between 21:00 and 22:00. The vertical axis shows time, the horizontal axis shows the location and the acronyms of the stations and the nodes. Feeding substations are not shown in this graph. A line running from top-left to down-right represents an eastward train, and a line running from top-right to down-left represents a train in the reverse direction. The slope corresponds to train speed, a vertical line means standstill. Train numbers are used for identifying trains within the simulation and do not correspond to the official train numbers used by the railway company. November, 2001 – Page 26 of 38
  27. 27. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS 40 45 50 55 60 65 70 75 80 85 90 21:00:00 21:10:00 21:20:00 21:30:00 21:40:00 21:50:00 22:00:00 2102 2113 2117 2120 2122 2134 2138 2153 2156 2158 2166 2172 2174 2180 2187 2192 2211 2215 Figure 5.13: Time-location diagram for the study track on an hour with heavy load (500 mm2 ) Low-voltage conditions correspond to locations, where many trains are close together at a certain point of time, especially if one or more trains are accelerating. The shaded area corresponds to a time-location area, where time losses due to low voltage conditions are significant6 . Two heavy freight trains (marked 2113 and 2117), two inter- regional trains (marked 2153 and 2156) and one local train (2174) draw power in their electrical vicinity. The travel time for these trains between the stations at 46,5 and 54, as well as the improvement thanks to the increased overhead wire cross-section is shown in the table below. November, 2001 – Page 27 of 38
  28. 28. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS Train nr Between Travel time [s] Time gained with re- spect to base case km km 500 mm2 800 mm2 1000 mm2 800 mm2 1000 mm2 2113 46,5 52 244 244 244 0 0 52 54 87 86 86 1s (1%) 1s (1%) 54 61 292 292 292 0 0 2117 46,5 52 242 238 238 4s (2%) 4s (2%) 52 54 95 87 87 8s (8%) 8s (8%) 54 61 293 291 291 2s (1%) 2s (1%) 2153 46,5 52 209 206 205 3s (1%) 4s (2%) 52 54 52 52 52 0 0 54 61 177 177 177 0 0 2156 46,5 52 194 181 182 13s (7%) 12s (7% 52 54 52 52 52 0 0 54 61 176 176 176 0 0 2174 46,5 52 184 183 184 1s (1%) 0 52 54 100 97 97 3s (3%) 3s (3%) 54 61 228 227 227 1s (0%) 1s (0%) Table 5.5: Time gains The table shows that the time gain can be more than 10s, thanks to the better voltage7 . The time gain values mentioned above take into account, that a train may have to wait until another train has left the section ahead. This reduces the time gain. It should be noted, however, that the time gain of a train may be partially or completely offset in the next railway section, if the train has to wait there for another train. Therefore it is more realistic to consider the travel time along the entire study case track. For the total 24-h simulation, many trains would take advantage of an upgrade from 500 mm2 to 800 mm2 . 49 trains (14% of all trains running on the study case track) would benefit a time gain of 10 s or more along the entire study case track. 95 trains (27%) would benefit a time gain of 5-9 seconds. More than half the trains would not take advantage of the increased cross-section, mostly because a time gain in one section of a few seconds is offset by a time loss in another section. In a few cases (<2%), trains are delayed due to the changed travelling of other trains. An upgrade to 1000mm2 brings a slight improvement of the 800 mm2 situation, increasing the number of trains that win 10s or more to 73 (20% of all trains running on the study case track). November, 2001 – Page 28 of 38
  29. 29. www.leonardo-energy.org CHAPTER 5. SIMULATION RESULTS 0-4s 57% 5-9s 27% 10-14s 10% <0s 2% 15-19s 4% Figure 5.14: Number of trains the travel time of which is reduced by increasing the overhead wire cross-section – Difference 500-800 mm2 0-4s 52% 5-9s 24% 20-24s 1% 10-14s 13% <0s 3% 15-19s 7% Figure 5.15: Number of trains the travel time of which is reduced by increasing the overhead wire cross-section – Difference 500-1000 mm2 November, 2001 – Page 29 of 38
  30. 30. Chapter 6 Appraisal of the energy saving The energy saving resulting from an increased overhead wire cross-section brings several benefits, which are assessed in this chapter. 6.1 Economical benefits without CO2 emission cost The energy savings lead to cost savings. A cost model has been used to estimate the cost savings due to the energy saving. The annual energy use and energy (cost) saving have been estimated by extrapolating the 24-h simulation results to one year. 500 mm2 800 mm2 1000 mm2 Annual energy use of study case track 58,9 GWh/a 56,3 GWh/a 55,2 GWh/a Annual energy saving w.r.t. base case – 2,6 GWh/a (5%) 3,7 GWh/a (6%) Annual energy cost saving w.r.t. base case – 121 ke/a 209 ke/a Table 6.1: Annual energy saving with respect to the base case The costs of upgrading the track to 800 or 1000 mm2 depend not only on the material cost for the copper added (estimated at e 7,5k per single track km for an upgrade to 800 mm2 and e 12,5k for 1000mm2 ), but also on the strength of the support gantries. The study case track is fitted with gantries, that would probably need reinforcement or replacement if the overhead wires would be upgraded to 800 mm2 or 1000 mm2 . Replacement of these gantries only for saving energy cost is not an option. Should the gantries need to be replaced already for another reason, the support gantries could be designed for one gauge stronger at relatively low extra cost. A rough estimate of this differential cost is e 3500 per single-track km for a one-step increase of the gantries 30
  31. 31. www.leonardo-energy.org CHAPTER 6. APPRAISAL OF THE ENERGY SAVING (sufficient for two 800 mm2 overhead lines) and e 7000 per single-track kilometre for a two-step increase of the gantries (sufficient for two 1000 mm2 overhead lines). 800 mm2 1000 mm2 Investment costs 1,16 Me 2,05 Me Benefits 121 ke/a 209 ke/a Payback period 9,6 years 9,8 years Internal rate of return (20 years) 9,7% 9,3% Table 6.2: Annual energy cost saving for the study case track The table makes clear that, if the support gantries are replaced, it is very attractive to carry out a significant reinforcement of the overhead wire8 . The payback period is attractive and the internal rate of return is high. The table below shows the same above quantities, per single-track km, i.e. divided by 105. 800 mm2 1000 mm2 Energy use (500 mm2 : 561 MWh/km/a) 536 MWh/km/a 526 MWh/km/a Energy saving 24,4 MWh/km/a 35,0 MWh/km/a Energy cost saving 1150e/km/a 2000 e/km/a Investment needed 11.000e/km 19.500e/km Profitability Internal rate of return Payback period 9,7% 9,6 years 9,3% 9,8 years Table 6.3: Profitability of overhead wire reinforcement of the study case track – per km In reality, the figures above will be more favorable, as the effect of regeneration of braking energy has not yet been taken into account. 6.2 CO2 reduction and economical benefits including CO2 emission cost If the overhead wires were reinforced, the energy savings would also result into a reduction of CO2 emissions in fossil-fuelled power plants. For the Netherlands, an average value of 0,6 kg CO2 per kWh is a common figure. The emission reductions (excluding the emissions due to the extra material use) are shown in the table below: November, 2001 – Page 31 of 38
  32. 32. www.leonardo-energy.org CHAPTER 6. APPRAISAL OF THE ENERGY SAVING 800 mm2 1000 mm2 Energy saving 2,6 GWh/a 3,7 GWh/a Emission reduction 1550 ton/a 2200 ton/a Same, per single-track km 15 ton/km/a 21 ton/km/a Table 6.4: Emission reduction in the study case track If the emission cost would be e 33/tonne, the net energy cost would increase by e 20/MWh. The energy cost saving from table VIII would then be increased as follows: 800 mm2 1000 mm2 COSTS 1,16 Me 2,05 Me BENEFITS 172 ke/a 260 ke/a PAYBACK PERIOD 6,7 years 7,9 years INTERNAL RATE OF RETURN 14,6% 12,3% Table 6.5: Annual energy cost saving for the study case track The table shows that, if emission cost apply, it is very attractive to carry out a significant reinforcement of the overhead wire. The payback period is attractive and the internal rate of return is high. The increase to 800 mm2 yields a higher rate of return and hence may be more justified than the increase to 1000 mm2 . The graphs below show the payback period and the internal rate of return as a function of the price of CO2 emission. 0 2 4 6 8 10 12 0 25 50 75 100 Emission cost [EUR/tonne] PBP[years] 800 mm2 1000 mm2 Figure 6.1: Payback period (PBP) in years of an increase of the catenary cross-section as a function of emission cost November, 2001 – Page 32 of 38
  33. 33. www.leonardo-energy.org CHAPTER 6. APPRAISAL OF THE ENERGY SAVING 0% 5% 10% 15% 20% 25% 0 25 50 75 100 Emission cost [EUR/tonne] IRR[%] 800 mm2 1000 mm2 Figure 6.2: Internal rate of return (IRR) for upgrading the catenary as a function of emission cost In reality, the figures above will be more favorable, as the effect of regeneration of braking energy has not yet been taken into account. November, 2001 – Page 33 of 38
  34. 34. Chapter 7 Conclusion 7.1 Conclusions A simulation case study has been carried out for a 52,5 km long piece of the Dutch railway network, reinforcing the overhead line from 500 to 800 or even 1000 mm2 per track. The case study shows that it is quite attractive to upgrade the cross-section of the overhead line significantly. For the 800 mm2 case, following results were obtained: • The energy consumption of this track would decrease by 5% or 2,6 GWh/a, corre- sponding to 1550 ton/a CO2 reduction, excluding savings from increased regenerated energy • Energy cost would decrease by e 121.000 annually • Energy losses in the overhead wires, dominant in the 500 mm2 case, have become only slightly higher than the losses in the return path • If the support gantries of the overhead lines were to be upgraded anyway, the ad- ditional investment cost would be e 1,2 million. The payback period for this extra cost would be 9,6 years • This ’project’ would be quite attractive with an internal rate of return at 9,7% • If CO2 emission rights were traded at e 33/tonne, the annual cost savings would increase to e 172.000. The payback period would decrease to only 6,7 years and the internal rate of return would rise to 14,6%. For higher emission right cost, the payback period would decrease even further • The worst-case voltage at this track would increase from 1100 to 1150 V • The mean useful voltage of most trains passing this track would increase by more than 50 volt, especially those trains that suffer from lower voltages • 14% of all trains using the studied track would benefit a net time gain of 10 seconds or more 34
  35. 35. www.leonardo-energy.org CHAPTER 7. CONCLUSION Upgrading the track to 1000 mm2 instead of 800 mm2 would lead to following results: • The energy consumption of this track would decrease by 6% or 3,7 GWh/a, corre- sponding to 2200 ton/a CO2 reduction, excluding savings from increased regenerated energy • Energy cost would decrease by e 209.000 annually • Energy losses in the overhead lines become lower than the losses in the return path • If the support gantries of the overhead lines were to be upgraded anyway, the ad- ditional investment cost would be e 2,1 million. The payback period for this extra cost would again be 9,8 years. This ’project’ would be quite attractive, too, with an internal rate of return at 9,3% • The worst-case voltage of this track would increase from 1100 to 1170 V • The mean useful voltage of most trains passing this track would increase by more than 100 volt, especially those trains that suffer from lower voltages • 20% of all trains using the studied track would benefit a net time gain of 10 seconds or more Generally, the main improvements are seen in the upgrade from 500 to 800 mm2 . Up- grading to 1000 mm2 still is attractive, but both the payback period and the internal rate of return are less favourable than upgrading to 800 mm2 . The losses in the return path become dominant. This raises the question whether it would not be worth to consider decreasing the return path resistance instead. In reality, the figures above will be more favorable, as the effect of regeneration of braking energy has not yet been taken into account. 7.2 Recommendations On busy tracks of low-voltage railways, it is very attractive to optimise the catenary cross- section. It is strongly recommended to add energy optimisation as a design criterion for DC railway system catenaries. As it is very difficult to predict the energy savings and other benefits by means of hand calculations, and in view of the significant possible cost savings, it is recommended to optimise the catenary cross-section using detailed simulation studies. Key moments are major overhaul and modification projects comprising the replacement of overhead wires and/or support structures. The simulation study also raises another issue. By allowing a large voltage range, the international standard EN50163 offers the possibility to design traction power supply systems with high losses and limited possibilities for regeneration of braking energy. It is recommended to investigate the possibility of shrinking the range of steady-state voltages. The same recommendation applies to the TSI Energy9 . November, 2001 – Page 35 of 38
  36. 36. www.leonardo-energy.org CHAPTER 7. CONCLUSION As an illustration only, a possible adaptation of voltage limits from an energy-efficiency point of view is given in the figure below. 800 1000 1200 1400 1600 1800 2000 EN50163 Reduced losses Voltage[V]* Overvoltages Non-permanent voltages Normal steady-state voltage range Non-permanent voltages Undervoltages Figure 7.1: Possible adaptation of voltage limits for a 1500 V DC system in order to increase energy efficiency (measured at the location of a train, during normal conditions) November, 2001 – Page 36 of 38
  37. 37. Appendix A Low-voltage power supply systems In railway systems electrified at 1500 V DC, the traction power is supplied from the public medium-voltage network. medium-voltage cables bring the power to the railway track. in substations adjacent to the track, the voltage is transformed to low voltage level by a transformer and converted to direct current (DC) by a rectifier. The positive terminal of the rectifier is connected to the catenary, the negative terminal of the rectifier is connected to the rails. The overhead wire and the rails bring the power to the trains. The figure below gives a simplified schematic diagram. Substations are spaced 5-20 km. All substations feed their power in parallel to the track. Hence, the power demand of a train is dominantly supplied by the nearby substations, but substations farther away also take their share, albeit a lower one. Rail Public electricity network Transformer Catenary Pantograph of the train Rectifier Substation Substation Figure A.1: Schematic representation of a LV railway system) 37
  38. 38. www.leonardo-energy.org NOTES Notes 1 Optimal reduction of energy losses in catenary wires for DC railway systems An ECI-KEMA publication, reference 98430138-TDP 00-12709, July 2000 2 This price may seem high at present (2001), but could become reality in the near future. If the legislation becomes implemented, the upper limit of the market price will be the penalty for producing too much emission. The European Parliament has proposed a penalty of about e 50,= per tonne CO2 in 2005, rising to even e 100,= in 2008. 3 Another possibility would be to store the regenerated braking energy in energy buffers like flywheels or battery systems. This option is not yet very common. 4 Between 2:00 and 05:30 at night, there is no train traffic. The peaks in this period are due to traffic at other tracks 5 The values displayed in figure 6 are slightly pessimistic for the voltage improvement. The reason is that figure 6 shows the average voltage along the entire track of the train, which in many cases is longer than the study case track. Assuming little change in the remainder of the network, in the study case track itself, the improvement is even higher. 6 The main time gains are indeed achieved at the section with the lowest voltage (kms 52-56). But also at other locations, significant time gains are achieved: the improvement from 1200 to 1300 V discussed before does have a favourable effect. 7 The mean useful voltanges for these trains are given below: Train ID 500 mm2 800 mm2 1000 mm2 2113 1488 V 1534 V 1576 V 2117 1493 V 1523 V 1574 V 2153 1472 V 1529 V 1561 V 2156 1481 V 1538 V 1571 V Indeed, these trains appear in the lower portion of figure 5.2, 5.2, 5.2 8 Based on 105 km of single track, upgraded at 7.5+3.5=11 ke/km for 800 mm2 and 12.5+7.0= 19.5 ke/km for 1000 mm2 9 EC Directive 96/48 – Interoperability of the trans-European high-speed rail system. Draft Technical Specification for Interoperability – “Energy” Sub-System November, 2001 – Page 38 of 38

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