Metro train is a type of high-capacity public transport generally found in metropolitan areas. The important factors which attract the passengers are mainly travel time, ticket price, punctuality, comfort level and safety. Metro train system not only save the journey time but it is also a comfortable, cheaper and safest mode of transport. Most metro trains are electric multiple units and electric power to metro train is commonly delivered by a third rail or overhead wires. Advanced metro trains have a microprocessor controlled three phase propulsion equipment system and regenerate electric energy during braking mode. In conventional railway locomotives, DC Series motors in series / parallel control are used as traction drive because it has inherent features of developing high starting torque at the starting and relatively lower torque at higher speed. DC Series motors require more maintenance and have higher initial cost as compared to equivalent induction motor. Advanced locomotives / metro trains are fitted with three phase induction motor drive and Variable Voltage Variable Frequency (VVVF) control is used to develop the high starting torque through the induction motor at the time of starting. Basic concept of conventional railway and advanced high speed railway system is described in 'Technical Information' section of this website. Metro train is more energy efficient as compared to road transport. Energy consumption of metro train can be reduced further by adopting suitable performance parameters. This website provides online simulation software tools to evaluate the effect of metro train parameters on its performance. Following demo simulation software tools are provided online to evaluate the performance of metro train with graphical presentaion of results :
In the production of metro trains having a microprocessor controlled three phase propulsion equipment, cost of the following heads is almost fixed and independent of propulsion equipment rating (i.e. traction motor, converter-inverter, traction transformer) :
Rating of propulsion equipment of metro train viz. traction motor, converter-inverter, traction transformer depends upon the performance required during traction as well as regenerative braking mode. Higher electric energy regeneration during braking mode lead to higher ratings of traction motor, converter-inverter & traction transformer; which result in higher initial cost of propulsion equipment only. On the other hand, the cost of electrical energy regenerated during the life span of metro train i.e. 30 years will be much higher compared to the increased initial cost of propulsion equipment.
Initial cost of microprocessor controlled metro train is much higher than that of conventional metro/EMU train having same number of motor coach and trailer coach. Maximum allowed adhesion value governs the upper limit of tractive / braking effort which can be developed through each power axle. Hence, the requirement of motorised axle / motor coach in an advanced metro train will be more to achieve higher acceleration, which further increase the initial cost of metro train. On the other hand, metro train with more number of motorised axles can develop adequate regenerative braking effort in complete range of the speed to achieve the required service brake deceleration. Use of more number of motorised axles also allows the regeneration of more electrical energy during braking mode. Thus, the effect of higher initial cost of microprocessor controlled three phase propulsion equipment may be compensated through the cost of regenerated electrical energy. If the trailer coaches of microprocessor controlled metro train are replaced with motor coaches to achieve higher acceleration and regeneration, its effect on initial cost of metro train will be comparatively less as the cost of train control software, train level control electronics and coach structures have already considered. On the other hand, if cost of regenerated electrical energy due to additional motor coaches is much higher than the depreciation, maintenance cost and the interest on increased initial cost due to motor coaches; then it is beneficial for the operator to adopt the metro train configuration having more motor coaches. As economy is one of the most important factors while designing any transportation system, thus the most economic design of advanced metro train is that for which the depreciation, maintenance cost, the interest on additional capital cost invested in various fields for operation of advanced metro train system can be recovered annually through the cost of electrical energy regenerated during braking mode. Simulation studies show that with 66% to 75% motorized axle, kinetic energy of the moving metro train can be transformed into the electric energy by applying only regenerative braking (except at low speed) during the braking mode. However, the operation of a metro train with only regenerative braking during the braking mode i.e. Eco-driving increases the journey time from 3% to 6% of the respective allout mode journey time. Eco-driving also reduces the maintenance cost of the pnumetic brake system.
The modified Kelvin's law is also based on analogous concept and used to find out the most economical conductor size of a transmission line. A transmission line can be designed by taking into consideration of various factors out of which economy is the most important factor. Most of the part of the total transmission line cost is spent for conductor. Thus it becomes significant to select the most economical conductor size of a transmission line. If the cross-sectional area of the conductor is decreased, the total capital cost of the conductor decreases but the line losses increase. On the other hand, if the cross-sectional area of the conductor is increased, the transmission line losses decrease but the total capital cost increases. The fixed charges of transmission line include the depreciation, the interest on capital cost of conductor and maintenance cost. The cost of electrical energy wasted due to losses during operation constitutes running charges. The most economic design of the transmission line is that for which total annual cost viz. fixed charges and running charges is minimum. The Kelvin's law suggests that the most economical conductor size of a transmission line is that for which the annual cost of energy loss is equal to the depreciation, maintenance cost and annual intermest for that part of capital cost which is proportional to the conductor size.
Time-table of train services usually provide certain time buffer or recovery margins in order to maintain punctuality of the train. Net energy consumption of metro train also depends on the slack time margin of time-table and the same can be utilized to adopt energy efficient driving pattern of metro train in a section. Elasticity of average energy consumption with respect to buffer time is very high i.e. slightly increased buffer times lead to strong reduction in energy consumption. Therefore, optimization of time-table based on the performance parameters of different types of rolling stocks will provide saving in net energy consumption from day one without any additional investment. The advantage of driving pattern can be further enhanced by adopting appropriate shape of tractive effort graph, regenerative braking effort graph based on the train weight, intermediate station distance, maximum service speed, service deceleration, time-table of given suburban section etc. Hence, appropriate performance parameters of metro train will result in more energy saving & economic operation of metro train in the life span.
During the operation of metro trains, energy consumption per passenger-Km depends upon the load factor at that time. Load factor is the ratio of occupied seat-Km over offered seat-Km. During the peak hours of traffic, time-table should not have much slack time margin to provide fast services. During the off-peak hours of traffic, the value of energy consumption per passenger-Km may too high and the same can be reduced by changing the number of cars per train or providing much slack time in time-table of off-peak hours. The extra slack time given in off-peak hours time table may be utilized for energy efficient driving pattern to reduce the energy consumption of the train by using Automatic Train Operation (ATO) system or Driving Advice Systems (DAS).
Sometimes, it is found that the requirements mentioned in the contract agreement / technical specification for designing the propulsion equipment of rolling stock are kept much stringent as compared to the actual loading conditions during the service of rolling stock. This results into overrated propulsion equipment design. Purchaser Railways considers it as safety margin. In actual, such overrated design of the propulsion equipment increases the overall weight and cost of rolling stock . Simulation studies explain that a lot of energy is wasted in carrying this extra propulsion equipment weight in the complete service life of rolling stock. Traffic study of most of the metro services indicates that the peak passengers loading conditions are occurred for few hours only in a day. Rest of the time, passengers loading conditions during services are observed easier than peak hour's conditions. Usually, peak environment temperature timings also do not clash with the peak passengers loading timings. This also provides the margin during the services from the peak loading design conditions. Therefore, a detailed study of traffic, schedule time, driving pattern and peak loading conditions is necessary before finalisation of design conditions of the rolling stock to avoid overrated propulsion equipment.
The behaviour of loads of electric railway power supply system (i.e. moving trains) and their location vary regularly in a wide range with respect to speed and time respectively. Power consumption of an accelerating train increases with speed and attains the maximum value during power zone of tractive effort graph ( generally between 25 to 60 kmph ). A train consumes comparatively low power at maximum service speed and at the time of starting. Advanced trains regenerate electric power during braking mode and the regenerated power also varies with respect to the speed of train. Thus, electric railway power supply system has additional moving distributed electric generation sources in the form of trains. The existing infrastructures of electric railway power supply system have its individual continuous and short time current ratings. Maximum efficiency of the existing electric railway power supply system can be achieved by avoiding operation of equipment in short time rating zone. Therefore, the time table for trains operation should be prepared in such a way that the start time of trains from different stations supplied through one traction power supply sub-station should not matched with each other to limit the peak power requirement during accelerating mode of the trains. One way of utilizing the existing infrastructure optimally is to maintain a balance between electric power required during traction and electric power generated by trains during braking. Time table for metro trains operation can be adjusted in such a way that most of the regenerated energy during braking mode can be consumed by the other moving trains in the same block section of traction power supply system. With the ever-increasing computer capacities, these kind of studies become more and more attractive to increase the efficiency of energy use. Therefore, time-table for trains operation should be prepared by time-table makers in consultation with the train designers, track engineers and power suppliers for economic operation of train.
It is a common opinion that a lot of saving in journey time can be achieved by increasing the maximum service speed of the long-distance trains. In reality, saving in journey time depends on many other factors also such as number of speed restrictions on track, average distance between stoppages, locomotive type, acceleration value in the complete speed range, number of coaches and actual operation of train at maximum service speed on the specific route. The net energy consumption of the train also increases with the increase in service speed of the train. On other side, saving in journey time can also be achieved by increasing the acceleration of the train in the complete speed range; however, no additional energy will be required in this case. Train having low acceleration takes less power but for long duration from OHE to achieve the maximum service speed. However, train having higher acceleration takes more power but for short duration from OHE to achieve the maximum service speed. Therefore, in both cases, the energy consumption (= power * time ) of train will remain same. Trains having higher acceleration will achieve maximum speed rapidly and will result into saving in journey time with the same energy consumption. Train operation at higher speed (around 180kmph or more) on existing railway track is only beneficial if the number of speed restrictions on track is minimal and the average distance between stations is around 100km or more. The most of the electrical energy taken by the train from OHE supply during acceleration period accumulate in the form of train’s kinetic energy. The kinetic energy of the train increases in square of the service speed. On every speed restriction, a lot of kinetic energy is wasted in the form of heat at wheels. Speed restrictions on track also increase the journey time significantly. Therefore, there is a need to quantify and estimate the effect of higher speed and speed restrictions on the journey time and energy consumption for specific route. Train operator should give the priority to the maintenance work of the track so that the speed restrictions can be removed. For maintenance work related to speed restrictions on track, the tendering process should be completed rapidly because on every day, a lot of energy is wasted on each speed restriction during train operation. There should be provision of penalty on the tenderer, if maintenance work is not completed within the specified time. The effect of energy and time losses due to speed restriction should also be included for assessment of the penalty.
Procurement strategies are one of the major factors determining future lanes of technology development. Manufacturers only produce what they can sell and only develop what they are confident they can sell in the future. A number of innovations that could reduce life-cycle-cost of rolling stock are not developed by manufacturers because the purchasing strategies of rail operator do not create any incentive to do so. Life-cycle-cost and other energy related quantities may be effectively optimized by creating incentives for manufacturers. Operator may offer incentives to the manufacturers for designing of energy efficient train on the basis of train energy consumption per seat-Km for a given route & time-table. Effect of train parameters on its performance is described in 'Performance Parameter' section of this website. Purpose of this website is to aware about railway technology, facilitate the train simulation tools and provide assistance in finalization of metro train performance parameters.
References : Reports of International Union of Railways (UIC), France; Royal Institute of Technology (KTH), Sweden; Railway engineering books and technical papers available on the Internet.
Write us for train simulations in following modes:
1. Allout run mode for minimum journey time,
2. Eco-driving mode (with only regenerative braking, except very low speed),
3. Train operation as per given time-table for optimum energy consumption,
4. Optimization of time-table for optimum energy consumption based on the performance parameters of all the rolling stocks operating on a given route.
The value of train speed, acceleration/deceleration, journey time, power supply current, energy consumption, tractive/braking effort, train resistance and speed restrictions will be provided at a sampling distance of 1m along with graphical representation of results. Details of train weight, train length, train resistance formula, tractive effort vs speed graph, braking effort vs speed graph, gradient and curves, speed restrictions, power supply voltage, propulsion equipment efficiency and time-table will be required for train simulations.
This website is under development and will be updated soon
Abhishek Kumar Singh
IIT Guwahati, Assam, India
Assist by: D.P. Singh (Electrical Engineer)
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