AC versus DC is a subject of debate over the century for different applications of power. DC arrived first at the scene of illuminating the house in 1850’s with many patents to the credit of Thomas Edition. The use of AC for power supply distribution taken up by entrepreneur cum engineer George Westinghouse in 1880s and the war started between General Electric and Westinghouse because it was the business empire of DC equipment manufacturers and many patents losing its sheen. Source: http://en.wikipedia.org/wiki/War_of_Currents Finally, it was AC which won the war because of technical superiority and dominating the scene of the power supply distribution till today.
Debate of AC versus DC continues for Rail Transport
The DC series motor characteristics is the most suitable for traction purpose, and therefore, DC dominated early traction schemes and DC traction motor continued to dominate in Rail transportation. With the development of on-board Ignitron, Excitron tube and later solid state rectifier, AC arrived at the scene as the feeding voltage. Transfer of power at 25kV single phase to locomotive and then converting it into DC for traction motor was demonstrated by SNCF and Indian Railways also decided to follow 25 kV AC power supply standard in its Railway Electrification policy. But it also passed through a debate on technology benefits among Railway Electrical Engineers as narrated by Ex Member Electrical in the posts
Finally, Indian Railway decided to go for 25kV AC with the intervention of the then Minister of Railways Jagjivan Ram. Before deciding for conversion of DC system to AC system on Mumbai Sub-urban network, similar war broke out with few advocating against the investment for the conversion. The most important factors which have gone in favour of deciding for 25kV AC traction in Mumbai Suburban network were
- Uni-gauze network without the need for a change of locomotive thus saving on running time
- The current level at 1500V DC is very high and difficult to enhance passenger transportation capacity. The current level reduces by 1/20th times at 25kV making it easier to enhance capacities.
- Energy saving due to less traction distribution losses.
There were no debatable issues while deciding in favour of 3 phase traction motor in lieu of DC series Traction Motor and it went through smoothly
Discussion of Traction voltage for Metro System
This subject has again opened up debate while deciding for the suitable supply voltage for Metro Transport System. National Manufacturing Competitiveness Council (NMCC) organized a meeting on May 03, 2012 and one of the Agenda item was “Promotion of Manufacturing for Metro System in India as well as formation of Standards for the same”. In view of this, Ministry of Urban Development has taken initiative to form a committee for “Standardization and Indigenization of Metro Rail Systems” in May 2012. Subcommittees were formed to standardize system in Traction system, rolling Stock, Signalling System, Fare Collection System, Operation and Maintenance, Track Structure and Simulation Tools. The committee on Traction System is headed by Shri Satish Kumar Ex..Director/Elect/DMRC Convener, with experts from DMRC, MOUD, RDSO, CMRL, BMRCL, Industries etc. Terms of Reference for Traction Supply Sub-committee were
- Study of traction systems adopted by various metros around the world, including year of commissioning of these metros
- Study current trends, i.e. traction system being adopted by newly built metros (say last five years) and metros being built
- Establishing a relation between type of traction system and maximum PHPDT that can be catered to by 750V DC/1500V DC third rail and 25kV AC OCS
- Analysis of Capital cost of various types of traction systems for different levels of traffic for a sample corridor. Such analysis shall include
- Direct cost of traction power system
- The direct cost of rolling stock
- Weight reduction of rolling stock and consequent energy savings and impact on operating cost
- The cost impact of regenerative braking e.g. DC system may rewire additional investment in inverters for utilizing the regenerative energy
- Civil infrastructure cost (e.g. Cost impact of increased tunnel diameter)
- Study of Capital cost of electrification of the DMRC, BMRCL, CMRL, KMRC, Indian Railways, Kolkata Metro etc. and compare operating costs on both types of traction
- An analysis of energy savings on account of regenerative braking in DMRC, BMRCL, Mumbai Suburban and other relevant systems
- Thorough analysis of regenerated energy during braking in Mumbai Sub-urban with 1500 DC and 25kV AC system
- Identifying constraints in process of indigenous development and evolving strategy for placing development orders for assemblies/ systems/subsystems
- Prepare report covering above, including cost benefit analysis and recommendations of traction system.
Salient features of the Report
Kolkata, Kochi and Bangalore Metro adopted 750 V DC, whereas DMRC, Chennai, Jaipur and Hyderabad adopted 25 KV AC. With two traction system existing in India, debate DC versus AC for traction is likely to emerge with each one proving his point. The most significant result of the study is finally the acceptance of the suitability of any of the two depending from case to case. The issues debated thereon are:
Advantage 25 KV AC
- Higher voltage has the benefit of handling lower current. Lower current means
- Reduce losses
- Improved protection system
- Increased inter-Substation distance – an important factor in urban transport due to high land cost.
- Improved current collection at pantograph level
- Higher voltage helps in higher DC link and TM voltage level. This benefits achieving higher HP/Weight ratio.
- With the completion of Phase 3 and Phase 4 network of DMRC project, the traffic density of the existing network will swell, requiring running of the trains at an interval of 90 seconds. With such the traffic density, it has been simulated that 2x25kV system will be more suitable mainly due to
- Energy efficiency
- Avoiding installing additional sub-station
- Mitigating EMC/EMI problem and avoiding booster transformer
- The 2.25 KV system has been adopted by Seoul Metro on their Sin Bundang Line. Advantage DC Traction System Advantage AC Traction System
Advantage 750/1500 V DC
- Avoiding carrying dead weight of the transformer and front end converter all along resulting
- Reduced passenger transportation capacity due to dead weight of transformer and front end converter
- Increased cost of coach due to addition of transformer and front end converter
- Increased energy cost due to transport of unnecessary dead weight
- Higher reliability of Third Rail Current Collection due to less number of component and less time to attend breakdown.
- Lower capital cost of traction system as compared to 25kV AC
- Higher tunnel bore diameter, thus requiring higher capital cost
- With no OHE mast, better aesthetic of skyline
Discussion on disadvantages of 25kV AC Traction
DMRC has gone for 25kV AC Traction in 2002 after a lot of debate and studies by RITES and approval by Railway Board. Let us discuss the disadvantages and how it accounts at Macro level and actual field conditions.
- Cost of 25kV AC Motor coach is higher than 750V DC Motor Coach
|Voltage||Name of Metro||Cost Scenario I||Cost Scenario II||Remarks|
|750 V DC||BMRCL||9.99||9.41||2.88m wide, 1.0 m/s2, 67% motored axles|
|KMRCL||9.29||8.05||-do- + flexible PVC clauses for 66% components without any clamping|
|25kV AC||CMRL||8.74||8.28||2.9m wide, 0.82 m/s2 , 50% motored axles, flexible PVC clauses for 66% components without any clamping|
|DMRC RS2||9.26||8.73||3.2m wide, 0.82 m/s2 , 50% motored axles|
|DMRC RS3||10.09||9.97||2.9m wide, 0.82 m/s2 , 50% motored axles|
|RS10||8.58||7.91||3.2m wide, 1.0 m/s2, 67% motored axle and more energy efficient|
Scenario I is with taxes and including export benefits and scenario includes cost of spares but without tax benefits. From the above it is clear that the cost of 25kV AC rolling stock is now comparable with 750V DC rolling stock. It is in view of the large many factors accounting into costing such as
- Number of coaches involved in the procurement
- Specification i.e. acceleration, de-acceleration, scheduled speed,
- DC Link voltage
- Commercial Conditions like defect liability period, indigenisation clauses, price variation, delivery period, time frame for completion, ambient conditions etc.
- Risk factor as perceived by the contractor
- Capital Cost of the 25kV AC system when compared with 750V DC as per actual tendering process found to be favouring 25kV OHE system by as much as 15-35% under different scenario. This is mainly due to higher cost of Third rail and limited competition.
- 750 V DC third rail system is certainly reliable in view of less number of components, but over the period the reliability issues of the 25kV OHE system have been suitability addressed through redundancies, reliability measures etc.
- Regeneration is the most important feature of urban transport for its sustainability. The actual records of DMRC reveal an energy saving of 25% in 25kV AC traction system operating with acceleration of 0.82 m/s2 and maximum speed of 75 Kmph. Simulation studies indicate that the energy saving in 25kV AC system may go up to 35% with the use of higher acceleration of 1 m/s2 using 4M+2T rake.
- A 25kV AC system for underground system will need higher tunnel bore diameter up to 5.8m but in actual practice it is observed that M&P facilities required from 5.2 to 5.8m is similar and only makes a slight difference in the cost of tunnel boring for the 25kV traction system. This cost is also offset by reducing in cost due to lesser number of substation and other associated benefits of larger tunnel diameter. In fact, the optimum tunnel diameter is worked out not only on Traction system, but also on many other factors such as a) Dimensions of the coach b) Number of coaches in trains and length of train c) Minimum curvatures d) Type of evacuati0n (side or front) e) Traction Voltage f) OCS or Third Rail g) Soil Temperature
- 750V/1500 V DC third rail gives better aesthetics in urban city as compared to 25kV Traction system and while deciding 750 V DC to Bangalore, this was one of the major factors. Bangalore is blessed with large size tree right in the heart of the city and certainly 25kV system will call for cutting or trimming a few ones.
The Major advantage to AC system is ease of transformation to different voltage level and has always won the debate in its favor. Energy saving due to low-level of current is the long-term benefit and has always weighed in favor of AC traction. With technological advancements every day, it cannot be said that it will be AC only in the future. DC will find wide range application if voltage transformation could be made possible with efficiency and cost advantage in future. There is already a thought to provide DC grid supply at Information Technology Centre to meet the power supply need of Hardware thus saving energy to the tune of 7-8% towards UPS.
What is special of the report
The committee has given enough of insight material for the new metros to decide Traction system based on merits and demerits enumerated in the report. Every advantage and dis-advantage of a system has been discussed and debated with factual data, and that makes the report a wonderful worth reading document for every electrical engineer and traction planner.