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Mumbai's Metro-3 will be one of the few in the world to run on AC

The MMRDA has decided to run the Colaba-Bandra-Seepz Metro corridor on alternating current, rather than the cheaper and more viable direct current

It seems the authorities have ignored the wisdom of agencies running the most efficient Metros in the world, and chosen to run the 32-km-long Colaba-Bandra-Seepz (CBS) Metro corridor, or Metro line-3, on alternating current (AC).

While 97 per cent of the underground Metro corridors across the world run on direct current (DC), say experts, the CBS rail will run on AC, which is more intensive in terms of cost as well as labour.

Experts feel there may be a 15 per cent cost escalation in the project due to this, since it requires more digging for the rail to work on AC. Additionally, there can also be a problem of dumping the muck that would be excavated while tunnelling.

Euston to Watford line in London, UK, North London Line between Richmond and Gunnersbury, South West Trains’ Putney Bridge to Wimbledon, and rails in Russia and Chicago and New York are some of the Metros that run on DC.

Sources from MMRDA said that bidders for Metro line-3 are also in favour of the DC model. Incidentally, S P Khade, director (technical), MMRDA, while writing in a magazine specific to the railway sector, has said that the Delhi Metro’s decision to adopt 25KV-AC instead of 1,500V-DC was not a good idea.

The authority’s joint project director, Dilip Kawathkar, said, “Metro line-3 has been planned after a proper study by a team of experts, and based on this study it was decided that AC model is a better option.” The construction of the line will be carried out in seven phases at 14 different locations.

The project will see state-of-the-art technology in underground tunnelling, used for the first time in an infrastructure project in Mumbai. While the TBM (tunnel boring machine) technology will be used to dig the tunnels, construction of stations will be either cut & cover method or the NATM (New Austrian Tunnelling Method), as per site requirements.

Expertspeak
Anil Jangid, promoter-director, Leap Infraasys Private Limited, which provides engineering project management services to railways and metro rail systems, said, “The planning and design aspect of such a huge project needs proper assessment.

The project is estimated to cost Rs 25,000 crore; it may exceed this limit if planning at this stage is not executed efficiently. The choice of traction voltage here, AC instead of DC, is incorrect and unviable. 97 per cent of underground Metros around the world run on DC, which is a preferred and successful model.

The DC model has many advantages over AC in underground Metros.” He added, “Dumping the debris from the extra digging in case of AC would not only be expensive, but would also affect the endangered mangroves and the city’s ecology.”

13.9 lakh
No of commuters Metro-3 is expected to carry daily by 2021

27
No of stations on the CBS line

17 lakh
No of commuters Metro-3 is expected to carry daily by 2031

32km
Length of the CBS Metro line

Why DC?
>> Power consumption of DC (1,500V) is way less than that of AC (25,000V).
>> AC system needs a larger tunnel, requires 15% extra dirt to be dug out
>> Cost of boring the tunnel and handling the muck will be more in the AC model 
>> DC rakes have lighter axles due to the absence of transformer and inverter 
>> With identical traction motor, DC rakes tend to move much faster 
>> DC rakes can carry 200 additional passengers in eight-coach rakes 
>> Underground DC models have a low maintenance cost

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