Challenge A: A more and more energy efficient railway
The Merseyrail Energy Monitoring Project
Edward Stewart, Paul Weston, Stuart Hillmansen and Clive Roberts
The University of Birmingham, Birmingham, U. K.
Abstract DC powered railway systems have significant losses in the power network. This paper presents results from a project that aims to measure simultaneously the power output from substations and power input to vehicles. Data have been recorded during initial instrumentation test runs on a vehicle and separately in two substations. It is possible to observe details in the current waveforms, such as notching currents, from the instrumentation located on board the train and at the substations. Further analysis of the data taken in one of the substations reveals the effects of different driving styles on the overall energy consumption.
1. Background Electric railway networks are either fed using high voltage AC or low voltage (750 to 1500 V) DC. Lower voltage DC systems often use line-side power conductors energised between 660 and 780 Volts nominal. Rolling stock deployed on these systems may take several thousand Amperes, and therefore minimising loop impedance of the power network is an effective way to limit the I2R losses. Generally, as the distance between a motoring train and a substation increases, so do the network losses. In addition, the line voltage and the touch potential of the return conductors may approach the acceptable upper limit at these locations. Whilst modelling of these systems is reasonably well established [1-3], there are few studies that report experimental results [4, 5]. This paper presents the results from an ongoing experimental study on a DC powered network. One train and two substations have been instrumented with custom made monitoring equipment. The intention is to analyse data recorded from train mounted equipment in conjunction with that recorded at substations. Using this combined information, it is possible to compute the instantaneous power flow, and therefore gain an insight into the losses in the system. To date, the systems have recorded data from both train and substations during the short-term initial instrumentation phases.
2. Experimental The energy metering systems used in this project have all been custom designed and built. Companies such as LEM who specialise in measuring electrical parameters also produce commercial energy metering systems. One example of these systems is the EM4T which is an energy meter designed specifically for railway traction systems. Systems such as these have been fitted to railway vehicles in the past. A report in the Railway Gazette International [6] describes a five month trial with energy metering equipment fitted to a vehicle in the Virgin Pendolino fleet (390049). The results from the trials were used to justify inclusion of the fleet in the reduced energy tariff associated with vehicles with regenerative braking capabilities [7].
These energy metering systems are designed in line with GM/RT2132, the UK railway group standard implementation of the European railway energy metering standard BS EN 50463, and the energy management systems standard, BS EN 16001. A second edition of GM/RT2453 (currently in draft) will update the mandatory data requirements for rolling stock to include energy metering data. These standards mandate energy results to be recorded every five minutes. This is adequate for billing and broad averaging purposes but does not provide the level of detail that is being sought in this study.
Challenge A: A more and more energy efficient railway
2.1. Train mounted equipment The rolling stock instrumented was the British Rail Class 508, which is a camshaft controlled train. Each train comprises 3 cars. At each end there is a driving motor car which contains the traction control system and four series wound DC traction motors driving each axle. The driver’s commands are common to each end of the train, but the camshaft controller also uses input signals from the passenger weight transducer, a current sensor and the tacho signal, thus each of end of the train requires separate monitoring. The experimental equipment consisted of a suitable arrangement of current, voltage, and additional sensors, all interfaced to a number of instrumentation nodes connected to a CAN bus data network. Figure 1 is a plan view of a Class 508 vehicle showing the locations of the instrumentation.
Class 508 plan view
Digital I/O (Control & Cam Signals)
Power Management (110 – 24 V) Data Logging Can / Fibre Optic Conversion GPS & Gyro IMU Tacho Laptop Data logging Power Measurement
Figure 1: Plan view of instrumentation locations
2.1.1 Train-mounted instrumentation details The instrumentation was sited in 4 boxes duplicated at each end of the train. The instrumentation is spread over 7 sites and 16 PCBs. There are a total of 11 microcontrollers running 6 firmwares. A CAN bus is used to link the instrumentation nodes, and an optional fibre optic interface is provided for out-of-service data monitoring purposes. Data are stored in a custom format on solid state devices. Traction rated current and voltage sensors have been used for the main train circuit with an additional current sensor used for the auxiliary loads.
Box A: This contains electronics for the energy monitoring components together with the main power conversion (from 110 V to 24 V) Box B: This contains the interface with the tacho test point signal and a data logger. Box C: This contains the driver’s demand and camshaft interface electronics. Box D: This contains the inertial measurement unit and a GPS receiver.
Table 1 shows the sampling and data rates used by the monitoring equipment.
Signal Sample Rate [Hz] Data Rate [Hz] Traction current 8192 256 Aux current 8192 256 Line voltage 8192 256 Driver’s demands 16 16 Camshaft position 16 16 Tacho 8 8
Challenge A: A more and more energy efficient railway
Passenger weight 1024 1 Temperature 1024 1 Body inertial measurement 1024 16 Bogie inertial measurement 8192 256 GPS position and time 1 1 Traction energy 8192 1 Auxiliary energy 8192 1 Table 1: Data sampling rates
2.2 Substation equipment The substation equipment was contained in one box. Interfaces were provided to current sensors and the bus bar voltage sensors. Figure 2 shows the location of the instrumentation fitted in the substations at Hoylake and Meols. Each substation contained four feeder cables feeding the up and down lines, and a cross bonded return conductor. The instrumentation box was fitted with a GPS sensor that was used to provide a satellite time reference which can then be used for data synchronisation purposes. All current and voltage transducers were sampled at a rate of 256 Hz.
Voltage Current GPS Also: Total return current at each substation
Figure 2: Substation instrumentation 3. Initial results
3.1. Train measurements Data from one end of a three-car unit were collected on the 11th November 2009. The train was driven in between scheduled services, and a number of trials were carried out following a major overhaul. The route covered the line between Hoylake and Chester. Figure 3 shows the data for one driving vehicle recorded during a number of brake tests. The line voltage (around 780 V and below) drops as the train takes current. The magnitude of the voltage drop is dependent on both the loop impedance and the substation transformer regulation, hence from this data alone it is difficult to estimate the network losses with any degree of certainty. The current data (shown in blue) displays the classic series-parallel-weak field operation of these types of traction drives. The driver’s demands are also shown (in black). In driving position 2 the traction controller keeps the motors connected in series. Only in driving positions 3 and 4 are the motors allowed to switch from series to parallel operation. The speed of the train is shown in addition.
Challenge A: A more and more energy efficient railway
Traction Current / Voltage / Speed / Handle Position 1200
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200 Traction current [A] / Voltage [V]/ Voltage / [A] current Traction 100 x position / Handle [mph] x10 Speed
0 1500 1600 1700 1800 1900 2000 2100 Time [s]
Figure 3: Initial data recorded during brake tests. Red line: line voltage. Blue line: traction current. Black line: driver's demand. Green line: vehicle speed.
The operation of the camshaft was also recorded. These data are shown in Figure 4. The camshaft shift signal can be seen to correlate with the step changes in current which occur when a lower resistance is switched into the motor circuit.
Traction Current / Handle Position / Camshaft Shift Signal 400
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50 Traction current [A] / Handle position x 100 / x 100 position Handle / [A] current Traction shiftsignal Camshaft
0 1686 1688 1690 1692 1694 1696 1698 1700 1702 Time [s]
Figure 4: Camshaft shift signal plotted together with the traction current for the driving vehicle
Challenge A: A more and more energy efficient railway
3.2 Energy consumption from train measurements It is possible to compute the energy consumed (at the train) by the driving vehicle by integrating the product of the voltage and current. These data are plotted in Figure 5 for the journey from Chester to James Street (note that these data, as before, are for only one of the two driving vehicles). The data has also been processed to give summary outputs every five minutes. This represents the outputs from commercial energy metering systems. The figure also includes a speed profile for the vehicle.
The figure shows the energy used by three different styles of acceleration. During the trials, the initial acceleration out of Chester consisted of a series of accelerations and braking stages at the maximum possible rate. As a result, the energy used during this acceleration phase was abnormally high although some voltage drop was detected which will have reduced the values somewhat. At approximately 1150 seconds, the vehicle accelerates out of a station. This is a normal acceleration albeit for an un-laden vehicle. Finally, at approximately 2300 seconds the vehicle once again accelerates from being stationary. At this time, the vehicle is entering the tunnel under the river Mersey which has a severe gradient. The vehicle only needs to use enough energy to overcome the rolling resistance.
180 Energy 160 5 Minute Energy Speed 140
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20 Energy [MJ] / 5 Minute Energy [MJ] / Speed [m/s] Speed / [MJ] Energy Minute /5 [MJ] Energy 0 0 500 1000 1500 2000 2500 3000 Time [s]
Figure 5: Cumulative energy consumption for the journey between Chester and James Street
The cumulative energy use shown in Figure 5 is also presented for five minute periods. This is indented to simulate the outputs of commercial energy metering systems developed in line with GM/RT2132. The commercial systems are intended to be used for billing purposes and so do not require the additional information present in the figure. In the first 600 seconds the train performs a series of acceleration and braking procedures which are of particular interest when considering the performance of the traction system; this is not evident from the five minute result.
Challenge A: A more and more energy efficient railway
3.3. Substation measurements The instrumented substations were recording data continuously During normal running the currents from the substations are complicated by the fact that trains cross over near the Hoylake substation. However, by looking at the first two trains of the day, which are half an hour apart, it is possible to identify which currents belong to which train. Figure 6 shows data from the Hoylake substation, roughly covering the first hour of operation. The four supply current results, corresponding to the four electrical sections recorded at Hoylake, are arranged from top to bottom in the order in which a train from Liverpool to West Kirby, and then back again, encounters them. The electrical sections, designated ‘325’, ‘326’, ‘327’ and ‘328’, are indicated on Figure 2.
Note that since powerlines 327 and 328 are open ended, when there is no train drawing power from these powerlines, they provide no current (apart from possible leakage current). They also do not show negative currents as there is no way for the powerlines to draw current back into the substation bus bars. In contrast, powerlines 325 and 326 can show negative currents when current is being drawn on 327 or 328 as current is provided from Meols substation and beyond.
2000 A1 1000 326 0 2000 B1 C1 C2 B3 1000 B2 327 0 D1 2000 D2
Current [A] Current 1000 328 0 2000 E1 E2 F1 F2 1000 G1 G2 325 0
0 500 1000 1500 2000 2500 3000 3500 Time [s]
Figure 6: Selected events observed at Hoylake substation
From the figure, it is possible to distinguish the entire journey undertaken by the first train of the day. This covers its approach to Hoylake substation, passage to West Kirby, change of direction and the return part of the journey as far as the second substation at Meols. The second train of the day can also be identified, particularly in the early stages of its journey up to its departure from West Kirby. From the appearance of the third train of the day, designated B3 in Figure 6, the frequency of trains is sufficiently high that it is very difficult to distinguish individual vehicles in the substation data. The appearance of the third train coincides with the continued motoring of the second train as it leaves West Kirby. Therefore, events D2 onwards may not be completely separable. All of the events marked on Figure 6 are described in Table 2.
Event Description A1 Train 1 motors travelling towards W. Kirby, approaching Hoylake substation
Challenge A: A more and more energy efficient railway
B1 Train 1 motors between Hoylake and W. Kirby, travelling towards W. Kirby C1 Train 1 motors, departing W. Kirby and approaching a pair of crossovers D1 Train 1 motors further, having switched over the crossovers. E1 Train 1 motors away from its Hoylake station stop F1 Train 1 motors away from its Manor Road station stop G1 Train 1 motors away from its Meols station stop (there is another substation here) B2 Train 2 motors on the approach to W. Kirby C2 Train 2 motors from W. Kirby station stop D2 Train 2 motors away from W. Kirby, after crossovers E2 Train 2 motors away from Hoylake station stop F2 Train 2 motors away from Manor Road station stop G2 Train 2 motors away from Meols station stop (where there is another substation) B3 Train 3 motors travelling towards W. Kirby, approaching Hoylake substation Table 2: Events seen from Hoylake substation
For the 13 minutes between events B1 and C1, train 1 is stopped at West Kirby station and is drawing auxiliary current only. When train 1 leaves West Kirby, the current previously drawn from Hoylake on powerline 328 switches to be provided from powerline 327. This point occurs as the train crosses over the crossovers immediately outside West Kirby station. Another point that can be determined is the point at which a train crosses the boundary between 327 and 325, outside of Hoylake substation. The current that was previously being drawn on 327 drops to zero and the current drawn on 325 changes from negative to positive. This point is within 75 m of the Hoylake station stop (there is only braking until the station stop). The journey between W. Kirby and Hoylake is examined more closely in the next section.
3.4 Energy consumption from substation measurements The journey from West Kirby to Hoylake is approximately 1.9 km in length. The currents drawn from Hoylake substation have been compared for the first train of the day on a number of days. Table 3 lists journey times, a metric for energy and the average current consumption for the journey for 8 days around May / June. The data are listed in ascending journey time. The amp seconds metric is the integral of current with respect to time and is roughly proportional to the energy consumption.
Journey Journey time Amp Average [s] seconds current [A] I 130 58340 450 II 136 45139 333 III 149 57294 385 IV 150 57422 384 V 150 53192 354 VI 154 59900 388 VII 157 67355 430 VIII 165 41891 253 Table 3: Data for the first train of the day taken on 8 days
Figure 7 shows the combined current drawn through powerlines 328 and 327 as the first train of the day (journey VIII) leaves West Kirby and travels towards Hoylake. The two electrical sections are indicated at the top of Figure 7. The transition between the two powerlines occurs after 33 seconds. The figure suggests that the driver does not reduce his power demand during the transition. There is, however, a large period of coasting shortly after the transition. The train arrives at Hoylake after 165 seconds, as indicated by the vehicle transitioning away from powerline 327. This example illustrates the longest journey time of the 8 examined cases, possibly due to the large coasting period following the power line transition. Not unreasonably, it also has the lowest energy cost of the 8 examined cases.
Challenge A: A more and more energy efficient railway
2500 328 327
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Figure 7: First train of the day leaving West Kirby – journey VIII
Although the journey that took the longest exhibited the lowest energy cost, it does not necessarily follow that the fastest journey must use the most energy. Figure 8 shows journey I from Table 3. Again, this journey was the first made on a particular day and occurred when only a single train was in the electrical sections associated with the Hoylake substation.
Despite coasting over the transition from section 328 to 327, as indicated by the drop in current several seconds before the transition, journey I was completed in 130 seconds. This is the fastest of any of the 8 journeys being considered. The energy consumption for journey I was moderately high, but not the highest by some margin.
Challenge A: A more and more energy efficient railway
2500 328 327
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Figure 8: First train of the day leaving W. Kirby – journey I
The journey with the highest energy consumption was journey VII. The combined current drawn from powerlines 328 and 327 for this journey is shown in Figure 9. The figure shows that the driver did not reduce his demands when making the transition from powerline 328 to 327, but that there was a brief period around 85 seconds into the journey where traction current was not being drawn. Despite a comparably short period of time spent coasting on the approach to Hoylake station, journey VII was the second slowest of all those examined.
Challenge A: A more and more energy efficient railway
2500 328 327
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Figure 9: First train of the day leaving W. Kirby – journey VII
Just as an inefficient driving style can lead to a high energy cost for a comparatively poor performance, an efficient driving style can deliver a good compromise between performance and energy consumption. Journey II is an example of one such efficient compromise. The journey is both the second fastest and the second most energy efficient of all 8 journeys considered.
Figure 10 shows the combined current drawn through powerlines 328 and 327 for the 136 seconds spanning the duration of journey II. The journey exhibits three bursts of acceleration with coasting between them and an additional period of coasting on the approach to Hoylake station. One of the periods of coasting is a short period while the vehicle transitions between the two power lines. In addition to the coasting, the current drawn when the vehicle is acceleration is comparatively low, and much of the time spent motoring is done in sections of the traction curve where the motors are operating in their efficient region.
Challenge A: A more and more energy efficient railway
2500 328 327
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Figure 10: First train of the day leaving W. Kirby – journey II
Simply using coasting does not ensure a good balance between energy efficiency and journey time. Figure 11 shows the same combined current drawn through powerlines 328 and 327, this time for journey VI. The figure shows only two regions where traction current is drawn, these combine to approximately 45 seconds which is less than a third of the 154 second journey time. The rest of the figure shows the vehicle to be coasting. Despite this, journey VI used the second most energy and was one of the slower journeys examined. Although a lot of coasting was used, the current drawn when the traction system was active was substantially higher than for the other journeys. Journey VI is thought to represent a bang-bang type of driving style.
Challenge A: A more and more energy efficient railway
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Figure 11: First train of the day leaving W. Kirby – journey VI
The auxiliary current drawn by a vehicle supports a variety of systems. These include the heaters (both for the car and the driver’s cab), the motor/alternator set and the compressor. The 8 journeys that have been examined all occurred early in the morning near the end of May so the heaters, which draw a significant portion of the auxiliary current, were switched on. An auxiliary current of approximately 70 A can be seen when the vehicle is not motoring, such as during coasting on the entry to Hoylake station.
In most of the examined journeys, the auxiliary current is a significant portion of the average current, however, journey VIII appears to have a lower auxiliary current. This may be due to the heaters being switched off, and may contribute to journey VIII having the lowest energy cost.
Journey VIII lasted for 165 seconds. Using the approximate value of 70 A observed during the other journeys, it is possible to estimate the difference in energy consumption as 11550 amp seconds. Adding this additional auxiliary energy requirement to the energy observed for journey VIII gives a total ‘energy’ cost of 53441 amp seconds. This value would make journey VIII the third most efficient out of the 8 journeys considered.
4. Origin of system losses The losses in both the train and the network can be significant. Because the Class 508 is a camshaft controlled train, there are control resistors that are switched into the circuit. These are then progressively switched out as the motor terminal voltage increases in line with the vehicle speed. The use of starting resistors in both series and parallel operation means that the efficiency varies in a non-uniform manner throughout a typical duty cycle. A detailed analysis of the train losses will be carried out as part of the ongoing project.
The power network losses are largely due to the Ohmic losses in the conductor and return rails. Figure 12 illustrates a typical feeding arrangement for a DC system where the current paths are shown. There are also losses in the high voltage AC feeders to the substations. Often, the rectifier is modelled as a Thévenin source with an equivalent internal impedance.
Challenge A: A more and more energy efficient railway
Whilst this is adequately represents the I-V characteristics of the substation, the loss calculations should take into account the regulation that will be provided by the substation transformer (when the voltage across the secondaries reduces). Typical loop impedances are 10 to 80 mΩ.I It is straightforward to estimate the power losses at these points in the network as a function of the current drawn.
Interconnected DC network DN
IL=I1+I2 I I 1 2 PC I 1 I2
I IR2 R1 T/RR
I I E1 E2 Figure 12: Typrical feeding arrangements for a DC power network
The data collected to date does not allow a detailed analysis of losses in the power system. When further data become available, it will be possible to dynamically account for the losses in the system downstream of the substation rectifier. The work will be of interest to railway vehicle operators and infrastructure managers because the work will be able to test strategies for reducing energy consumption within the network. Additionally, because this is a long term trial, both dynamic and static losses from the system will be determinable.
5. Summary The operators of DC powered railways have renewed interest in understanding the system losses. This has been motivated by recent moves towards the requirement to fit DC vehicles with energy meters, and the desire to reduce energy usage. The interim results from this study indicate that the in-service monitoring is practical and the quality of the data will be sufficient to improve the understanding of the system losses in DC railways. The system can also be used to explore the effect of driver style on energy consumption, and can be used to identify areas where the system efficiency is low.
A number of journeys have been analysed in some depth, and significant differences in driver style have been observed. When more data become available, it will be possible to make recommendations for improvements in driving style which, if implemented, could provide a step change in energy consumption.
6. References 1. Mellitt, B., Goodman C.J. and Arthurton R.I.M.: “Simulator for studying operational and power-supply conditions in rapid transit railways” Proc. IEE., Col. 125, No. 4, April 1978. pp 298—303. 2. Ho, T.K., Allan J., Digby G., Goodman C.J.: “Modelling of signalling for an interactive on- line rapid-transit railway simulator” IEE 2nd Intl. Conf. 'Software engineering for real-time systems', Publn. 309, pp 209—213. 3. OpenPowerNet – “The New Co-Simulation Tool for Traction Power Supply” A Stephan, IFB GmbH (Railway Technology Institute), Germany J Chen, G Zhu, Tsinghua University, China. RTS2010 Birmingham April 2010. 4. Bae, C.H., Jang, D.U., Kim, Y.G., Chang, S.K. and J.K. Mok, “Calculation of regenerative energy in DC 1500 V electric railway substations”, 7th International Conference on Power Electronics, Daegu, Korea, pp 801—805, Oct. 2007. 5. Grillo, D., Landi, C., Luiso, M. and N. Pasquino, “An on-board monitoring system for electrical railway traction systems”, Instrumentation and Measurement Technology Conference (IMTC 2006), Sorrento, Italy, April, 2006.
Challenge A: A more and more energy efficient railway
6. Railway Gazette International, http://www.railwaygazette.com/news/single- view/view/metering-validates-regeneration-saving/browse/2.html, 7th August 2008. 7. J. Evans, “Energy monitoring on the Virgin Pendolino trains”, Railway Traction Systems (RTS 2010), IET Conference, Birmingham, 2010.
7. Acknowledgements The authors would like to thank the Department for Transport for funding the work. We also received considerable in kind contributions from the railway industry, including Merseryrail, Network Rail, RSSB, Angel Trains, ATOC and RIA.