Selection of Traction System Voltage for rail transport

Last updated on February 22nd, 2014 at 01:15 am

Many Traction systems have been developed over the period depending on dominant  technologies at that time and traffic demand. Traction system for urban, Sub-urban and main line transport developed over the period are at 750V DC, 1500V DC, 25kV AC, 2x25kV and 50kV AC. 750 and 1500 V DC are mainly for urban transport whereas 25kV and 2x25kV for all types of transport. 50kV is exclusively for stand-alone dedicated heavy haul long distance freight transport.

There is considerable advancement in technologies and the primary issue is to select the system based on its  power handling capacities. Power Demand depends on Peak Hour Peak Direction Traffic (PHPDT) in case of Urban Transport, Frequency of train operation, Maximum and Average speed, Level of Acceleration, Load of the train, gradient, regeneration etc.

It is the capital cost of the infrastructure and rolling stock as compared to saving in terms of Energy to be considered for the purpose of deciding the suitability of voltage level.

Decision making Process

Cost benefit analysis, simulation studies and many other analyses can be done for deciding the suitability of a technology to suit a situation but it is always the common sense which finally prevails in deciding the subject looking at long term benefit of the investment. Common sense is based on

• Previous experience
• Aesthetics
• Sustainability
• Longevity of technology
• Maintenance free content
• And most importantly shrewd marketing of the technology company

Initial Cost of traction power system shall depend upon the following

• Maximum power demand of load
• Level of redundancy – interruption of even seconds matters in case of urban transport
• Land Cost particularly for Traction Sub-station and sectioning Post
• Availability of technology and equipment at Competitive price

Study of Traction System Costing

A detailed study of the the initial cost of traction system is done by the committee headed by Sh Satish Kumar Ex.Director DMRC nominated by Ministry of Urban Development based on the tendering system for the ongoing works over different Metros and Indian Railways.

750 V DC

600-750 V DC system was the most initial traction system developed mainly for direct use of Traction voltage with resistance control of traction motor and minimum of on board equipment. With the advancement of technologies using IGBT based VVVF control of traction motor, on board under-slung equipment is unavoidable. Kolkata and Bangalore Metro are at 750V DC introduced in 1984 and 2011 respectively. Kolkata Metro was built with the primitive technologies of 70s i.e. use of Steel third rail with top contact, non-air conditioned rakes with tunnel air conditioning and non-regeneration. Bangalore Metro is using Al composite third rail, with air conditioned coaches and VVVF control of traction motor and regeneration.

Based on the total cost of traction system, subtracting capital cost of auxiliary supply and adding inflation at 5%, the present day cost per Km. Is Rs. 13.55 Crs. Similarly for Kolkata metro, the average cost per Km. works out to be Rs. 13.18 Crs.

The most important advantages of 750 V DC are aesthetics, low tunnelling costs, low wear and tear of current collection system, ease of maintenance  etc. As regards disadvantages, high operating currents, low level of regeneration as regenerated power to be used by the system only and phenomenon of stray current causing damage to metallic components and track structure.

1500V DC

There is no experience of 1500 V DC of any of the Metro in India. 1500 V DC is existing over Mumbai Sub-urban area undergoing conversion to 25kV AC and there is no experience on any Metro Line.  1500 V DC is generally with Overhead conductor. DMRC has awarded a contract for 1500V DC traction system which later on decided to be on 25kV AC. This is similar to the situation, wherein 3000V DC was chosen as traction voltage for Eastern Railways and decided later on to be built at 25kV AC. It was noticed that the construction cost at 25kV AC was 1 Cr. /Km less as compared to quoted cost of 1500 V DC traction system. Considering the inflation, the traction system cost for the underground and elevated is worked out as 12.26 and 10.51 Crs./Km respectively.

Fundamental advantage of 1500V DC system is reduced current to almost half, therefore, saving in terms of energy.

25kV AC

There are three metros namely DMRC, Chennai and Hyderabad following 25kV AC traction system. The power supply for station, air conditioning and ventilation, illumination, pumping etc is taken from the ASS for which the power supply is drawn from TSS. Therefore, the cost of TSS involves Auxiliary transformer and associated system. This is different to the system generally followed by Indian Railways were TSS is exclusively for traction purpose.

Note: DMRC cost is based on the tender finalised in Feb.2007  and brought at par by uniform inflation rate of 5% whereas that of Chennai is of  2011. It is observed that inflationary components in the overall cost may only be 25% and large component depending upon opportunity profit.

25kV AC traction system is followed over Indian Railways as well. The cost of electrification is Rs. 1.2 Crs/ Route Km  of double line  (including cost of sub-station and depends also on length of transmission line involved) . There are many factors which make the 25kV AC traction system 6 times costlier for metro and are summarised as follows:

1. Feeder line is through buried cable network as compared to overhead transmission lines for Indian Railway Network.
2. Inter substation distance is only 15 Km as compared to 50 Km for IR network
3. Gas Insulated Substations are used due to space constraints
4. Mast span length is 27 to 45 meters as compared to 63 meters for Indian Railways
5. Rigid OCS is used in the underground. The cost of rigid OCS alone is Rs. 1.75 Crs/Km.
6. Metro system is designed for higher level of reliability using special type neutral sections, gas ATD at critical locations, overhead protection cable, return conductor and booster transformer, special desing of earthing and bonding.

Advantage and disadvantage of 25kV Traction system are summarised in the post

2×25 kV AC

This system is prevailing only on a short stretch of Bilaspur division of South East Central Railway. It was estimated that the power demand on this line will be very high and therefore 2x25kV system shall be installed for experimental purpose. The electrification at 2.25kV was done with the technical support of Japan with many of the items imported from Japan. Evaluations could not be done as the traffic density did not touch as estimated. The cost of electrification adds towards feeder wire, auto-transformer at SSP etc. and is around 40% additional as compared to 25kV AC.

50 kV AC

There is no experience of 50 kV AC system in India. The system is stand-alone for heavy haul long distance trains on electric traction. The advantage as compared to 2x25kV AC is saving in electrification cost due to feeder wire, auto-transformer etc and reduced current handling at loco pantograph and transformer level.  Disadvantage is higher insulation level and clearances.

Conclusion

The cost of transferring power from a stationary  to moving frame varies from 1.2 Crs/route KM of double line section of Indian Railways  to 13 Crs./route Km of Metro System at 750 V DC. It is important for every electrical engineer particularly traction engineers to understand the reasons for the differences as given in the post.

Source: http://moud.gov.in/sites/upload_files/moud/files/Report%204%20Traction%20System.pdf