ead ac vs third rail dc s
running rails and the electricity is transmitted to the train by means of a sliding shoe, which is held in contact with the rail. A third rail system offers a
number of benefits, including: eradicating the impact of electromagnetic interference on
electrical components reducing maintenance costs because power supply equipment is virtually maintenance-free with only regular
inspections and cleaning required, and offering high efficiency - a 750V dc system offers a power supply transmission efficiency average of 92-94%. As a solid composite rail running along the track, a third rail is more rugged than an overhead contact wire and has a longer life expectancy. The system similarly benefits from high reliability because it is fed on both sides by rectifiers from adjacent substations, and it can offer lower comparative initial costs than an ac system. Rolling stock used on these systems also tends to be cheaper because no transformers are installed onboard which also reduces the weight of the vehicles and increases capacity for passengers. Inevitably there are a number of disadvantages with third rail, including unavoidable gaps in the power supply at points and level crossings. Speeds are also restricted to 160km/h due to the technical limitations of the system, while on lines electrified at 750V dc peak-time line capacity is limited to 60,000 passengers per hour per direction. Stray currents are also possible, although
IRJ February 2013
improvements in technology are managing and controlling this factor. Some metros have found that adjusting voltages can mitigate some of the disadvantages of third rail traction, particularly its capacity limitations. As a result a number of new build projects, including Ahmadabad Metro, are considering electrifying at 1.5kV dc which is capable of carrying more than 60,000 passengers per hour per direction. A study performed for Dubai Metro found that a 1.5kV dc third rail traction system can also cost up to 13% less than a 750V dc third rail system. One cost saving measure is the ability to locate fewer substations at greater intervals which reduces maintenance costs.
Some metros have found that adjusting voltages can mitigate some of the disadvantages of third rail
Typical substation spacing is roughly 1.5 - 2km for 750V dc systems, and 3 - 4km for 1.5kV dc systems although this distance is dependent on power demand, operating headways, system design, and land availability. Increasing the voltage can similarly result in a reduction in collector shoe maintenance costs, while stray currents, electromagnetic interference, and energy use are all reduced, and performance improves increasing peak-hour capacity. While adjusting voltages and the type
of traction system used can improve capacity on a metro system, it is the frequent station stops and terminal operations which are the major factors in determining line capacity and train headways. Even if the traction system is capable of handling the capacity required for longer trains, it is the length of the train itself which tends to increase headways to more than two minutes because of the time it takes to transfer from up track to down track at the end of the trip. As a result reduced headways are only achievable on systems where shorter trains are used. Hence the benefit of providing greater per hour per direction capacity by using 25kV ac traction is lost in a practical application on a metro system. Introducing a communications-based signalling system could be one way of reducing headways for longer trains, although on underground lines ventilation and smoke control system regulations can restrict train length because they require a distance of 200-300m between trains to allow effective and safe air circulation. It is within these parameters that one needs to look at the traction system applied and conclude what is the most suitable application. Delhi Metro, which is using a 25kV ac
overhead system and is recommending that other metros in India follow suit, has proposed shortening the length of trains that will be used on the third phase of its network to provide a headway of 1min 50s. While the traction system may be able to cater for more than 60,000 passengers per hour per direction under this plan, the system as a whole will not support higher capacity due to the shorter train lengths required to achieve lower headways. As we have found, a 750V dc third rail system regularly caters for a six-car metro train operating at two-minute headways. Planners would therefore be wise to consider all of the options available with respect to the specific demands of their system when designing new metros, and not to dismiss what some mi ght consider as old fashioned third rail systems. In some instances the efficiency savings that these systems offer could be of greater value than the promise of increased capacity provided by overhead traction. IRJ
35
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52