7 aspects of understanding the Metro Rail-Traction System.

Discover the effort that goes into building an engineering marvel.

Metro, the most integrated means of transportation, often goes unappreciated and how it functions is still a mystery for a lot of people. The entire system is developed to relieve the severe stress on the cities and excessive pressure on its transport system. A meaningful and sustainable mass transit system hence becomes necessary to ensure proper transportation facilities are available for the people.

Photo by Random Institute on Unsplash

Through this series of articles, I would like to help you develop a sense of understanding of the general functioning and technicalities of a metro rail system.

Being an electrical engineer, I can help you understand the various technical and functional aspects of a metro. The primary electrical engineering components in a metro system include the rolling stock requirement, train operational plan, traction system, signaling & transmission system, substations & power supply, ventilation/air conditioning, and maintenance depot.

These are the seven electrical engineering aspects of Metro railways. In MRTS (metro rail transit system), all the above-defined operation is in electromechanical integration. In this article, we will explore the third aspect of metro railways, the Traction system.

Traction in layman's terms is simply the method to apply and ensure electrical power to our locomotive or metro rail. Traction in general has two main parts the first being the conductor that supplies the electric power supply and the other being the pantograph or similar structure for retrieving the electric power from the conductors. These main structures are supported by various other equipment like the generators that produce the electric power, the substation that receives the power, and the transformer that steps down the voltage from distribution voltage level(220, 132 kV) to Traction system level(25 kV), this components form the part of the substation and power supply part of metro functioning and will be discussed separately in a different article.

The traction system in general are divided into two different types that are-

1. 750V DC third rail system.
2. 25kV single phase OHS(overhead catenary system), also known as OCS.

DC Third rail system.

The DC third rail is among the most employed traction system in the world as it maintains the aesthetics of the city and is highly researched hence maintenance and construction is comparatively simpler. In this system, a separate third rail runs along the existing track structure carrying live DC electrical power and is retrieved by the metro by a small conductor attached at the base of the metro rail.

The third rail system (750V, 650V, 1200V) dc setup has the following features: -

DC third rail system components

MERITS-

1. The merit of the system lies in the fact that it doesn’t deteriorate the aesthetics of the city. The skyline of the city remains clear and the system looks neat.
2. This system is widely used all over the world and maintenance as well as wear and tear of the third rail is very less and repair is almost completely understood and well tested.
3. The tunneling cost is less due to a lower tunnel radius (2.2m) as compared to OCS.

DEMERITS

1. Due to large dc current and low voltage the power drop was very significant over long distances, this caused the necessity to have a greater
   a number of substations in order to maintain the specific power level, as desired by the metro.
2. The phenomena of stray current is evident in the DC third rail metro system. When the return current comes back through the rail a segment of
   the current [stray current] leaks into the material causing corrosion.
3. Having a live third rail in the metro track is also dangerous for the commuters as its completely exposed.
4. It's difficult to regulate the power level of dc system as compared to ac systems.
5. The problem of over bridgeable and non-bridgeable gap is also evident while constructing the third rail system over curves and bridges, as the entire system is rigid.
6. The level of regeneration of energy as in regenerative braking is very poor in the dc system, we have to use inverters and batteries to make it feasible.

25 kV Single-phase OHS(overhead catenary system)

Before understanding the OHS let us first understand some common terms and types of equipment required for a better understanding of the overhead catenary system-

1. Auto Tensioning Device- The function of ATD is to maintain a constant tension in OHE conductors i.e. contact wire and catenary’s wire under varying temperature conditions so that the profile of OHE is maintained for better current collection.

Auto Tensioning Device

TYPE OF AUTO TENSIONING DEVICES-

5:1 ratio Winch type ATD

3:1 ratio pulley type or Three

The ratio mentioned above is the pulley ratio.

2. Pantograph-

pantograph components

The pantograph is an apparatus mounted on the roof of an electric train to collect power through an overhead tension wire. It is a common type of current collector. Typically, a single wire is used, with the return current running through the track.

3. Overhead Catenary System Apparatus-

OHS components

Catenary Wire- This is a support wire which is not electrified its main purpose is to keep the contact wire perfectly rigid and stable. The catenary curve has a U-like shape, superficially similar in appearance to a parabolic arch this shape is such that it provides structural integrity.

Contact Wire- It’s the main electrified wire. It’s always kept intact using auto tensioning devices.

Droppers- it’s an insulating wire that holds the contact wire in place along with the support of the catenary wire.

The rest of the equipment including the mast the bracket tube steady tube are for support purposes.

Now coming on to the entire system The OHS system employs a live 25kV single-phase AC power carried by the flexible contact wire that is kept under tension by the auto tensioning device so that it always exists in its place. The live contact wire is supported by various supporting structures including droppers, catenary wire, and mast to make sure that it remains straight throughout. This is ensured so that whenever the pantograph placed on the roof of the metro that withdraws ac power from the contact wire, never loses the physical connection and hence ensuring a continuous supply of electric power to our metro rail.

OHS overall components

The ac overhead catenary system or OCS or OHS is better in comparison with DC third rail system because of the reason that: —

MERITS

1. A low number of substations required to maintain voltage drop levels, and it's easier to maintain high voltage for transmission using transformers.
2. No stray current phenomena is evident in this case.
3. The regeneration is very prominent in this case about 30 % of energy is recovered, using regenerative braking without the need for additional supplements like inverter and batteries.

DEMERITS

1. Demerits of the system lie in the fact that it deteriorates the cities skylines and affects the aesthetics of the city.
2. The cost of tunneling in the underground system increases since it requires more space about 2.6–2.8 m of tunnel radius for accommodating the overhead catenary system.

CONCLUSION

Metro, in itself, is a very vast aspect of engineering and is integrated into our lives. I can’t emphasize enough on its importance in the development of a nation and its functionality in supporting traffic flow.

From the article, I tried to give you a clear image of what kind of thought process goes through in creating such an engineering marvel, so next time you board a metro, don’t forget to appreciate the effort that goes into developing one. Make sure to check out other parts of the series to get a full understanding of the functioning of a metro.