Rapid transit


Traction choices: overhe V


With third rail dc and overhead ac traction offering numerous advantages and disadvantages, prospective rapid transit operators should carefully consider the type of traction system they adopt, argues Anil Yadav, general manager, electrical, at Rites, India, who is currently providing consultancy services to Bangalore Metro.


A


WIDE variety of electric traction systems are used on rapid transit systems around the world which have been built according to the type of railway, its location and the technology available at the time of installation. Most metros are operated with dc power either at 750V with third rail or 1.5kV with third rail/overhead contact line. Operating metros on 25kV ac overhead is a relatively new phenomenon and there is a lot of debate about the value of this adaption due to the importance of traction power to a system’s performance. A conventional electrification system


provides electrical power to the trains by means of the traction power supply, distribution, and traction power return systems. Third rail always uses dc power with a variety of voltages in use around the world including 600V on the Tokyo metro, 750V, which is the most common use, 825V in Moscow, 1.2kV in Berlin and 1.5kV in Guangzhou. Overhead traction has also evolved


from 1.5kV dc, 3kV dc, and 15kV ac in early applications to 25kV ac (or 2x25kV ac) which is now widely used and more often than not the traction system of choice for new mainline and high-speed railways. High-voltage ac electrification has also been applied on S-Bahn systems in Germany (15kV) and on part of the RER network in Paris where mainline commuter lines have been connected with new underground sections in the city centre. The fundamental difference between ac and dc is that on a dc network each substation includes transformers and


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rectifiers which condition the power to the relatively low voltage required for direct use by vehicle propulsion equipment. In ac systems the power is supplied by the substations directly without rectification. This necessitates further transformation onboard the rolling stock so the voltage is suitable for use by vehicle propulsion equipment. Both systems offer distinct advantages and disadvantages, but with Delhi Metro using a 25kV ac rigid catenary system both above and below ground, which is encouraging other new metro projects in India to follow suit, it seems appropriate to consider what they might lose or gain by copying this example.


Increasing capacity


With demand for rapid transit services on the rise, operators are constantly looking to increase capacity and improve the efficiency of their networks. The norm for most metro line peak services is 30 trains per hour, or two-minute headways, although there are some examples where this is exceeded. For example Paris Metro Line 14’s headways are as low as 85 seconds. In theory adopting a 25kV ac traction system could be one way of achieving greater capacity because it allows the operator to use longer trains more frequently. For mainline and high-speed railways 25kV ac is now the most proven and widely used system. It offers a number of advantages, including reducing the cost of power supply


equipment, improving efficiency, and using energy from braking more effectively which are all potentially attractive features to metro operators. Power supply efficiency on a line equipped with 25kV ac overhead contact wire is also 98% although this may vary depending on rolling stock. However, there are also several disadvantages of 25kV ac, particularly when applied in an urban metro environment. With the overhead contact line system more prone to failure, regular maintenance is necessary and requires a plan that is supported by a 24- hour maintenance team. The potential for electromagnetic interference and the impact of magnetic fields on properties and activities close to the line must also be considered, although special design features such as return conductors or booster transformers that minimise magnetic fields are now common. Tunnel construction that is suitable for overhead catenary will also be significantly more expensive than for third rail because of the larger profile required. This is one of the major factors why metros have traditionally tended to favour third rail. In addition, because the onboard transformer imposes significant weight on the ac- powered rolling stock, metro operators have typically found that dc systems are better suited for urban applications where relatively short station spacing requires frequent and high acceleration. On most third rail systems, the conductor rail is placed outside of the


IRJ February 2013


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