Published On: Jan 16, 2019
As India takes a leap forward in modernization, especially, electrification of railways while aiming to significantly expand it's network of High Speed Railways, it's the right time to counter check the readiness of the underlying electrical network.
In 2016-17, the total route length of India’s Railways stood at 67,368 Kms with 25,367 Kms of it electrified (37.65%). Indian Railways electrified more than 2100 Kms of its tracks in 2016-17, one of the highest in its history. Even with this phenomenal progress, the Indian railway network compares as relatively small to the rail network in China with close to 120,000 Kms of railway routes and 19,000 Kms of it as high speed electrical route. Indian railways has also embarked on several initiatives that to increase the speed and safety in railways. These include everything from manufacturing of high speed coaches to introduction of 12,000 HP and above capacity electric locomotives, greater automation of infrastructure, especially signaling and upgrade of telecommunications. Additionally, several cities in India are also adding Rapid Transit (Metro Trains) to their transportation infrastructure. As of September 2018, India has 515 kms of operational metro lines and 381 stations. A further 500+ km of lines is under construction.
Here it is important to understand that all the electrification and increased speeds are also a perfect breeding tracks to throw up Power Quality issues. As India approaches rapid modernization of railways, in terms of speed, energy utilization and safety, Power Quality challenge will continue to rise and potentially threaten to derail the ambitious initiatives.
This blog highlights the PQ related challenges in modernization of transportation, with specific focus on the railways, by drawing in from experiences, research and related data of global as well as Indian initiatives to modernize railway networks.
Dubai Metro – A study of measurement of AC Side Harmonics of a DC Metro Railway observed required compliance with IEEE Std 519; however, on close inspection of the data and in comparison with previously published data, an odd behaviour of the 7th harmonic appearing larger than the 5th was noticed. The cause was assumed to be significant asymmetry of the rectifier feeding transformer.
The growth and rapid adoption Metro Transits, the new high-speed train is observed to have resulted in significant distortions in electrical network voltage and current in both traction power supply system (TPSS) as well as connected power system used to power these modes of transportation. The modern trains have a nonlinear load of a dynamic nature, which in turn creates its own challenges in the evaluation of PQ problems.
A comparison of various transportation modes quickly reveals that railways have always safer, cleaner and perhaps the most efficient. With greater speed and electrical power to run, the benefits seem to only add-up further. However, power quality problems have always been an important concern when it comes to railways and the extended electrical network.
Additionally, in recent times Railways have also joined national aspiration to develop their own solar generation points. For instance, the Delhi Metro has a 500 kilowatt peak (kWp)-capacity solar plant at the Dwarka Sector 21 metro station and plans to expand the capacity to 250 kWp by adding more such stations. Indian Railways on launched first solar-powered DEMU (diesel electrical multiple unit) in 2017. Further, the Railways plan to source 500 MW solar energy through roof top solar panels in over 400 Railway stations and other Railway buildings. While the Solar push for Railways is a welcome move from environmental perspective, given its scale, the PQ issues arising out of Solar powered grids could be significant.
On a very broad level the range of major PQ issues concerning railways, may it be conventional electrical locomotives, High Speed Trains or Metro Rail include the following:
Presence of harmonics
The power electronic devices in Locomotives, essentially constituting the non-linear loads, create harmonics and result in excessive heat generation as well as power loss. The IEEE Standards 512-1992 state that the THD must be less than 5%. This standard is also expected to be met by the Power Traction systems.
Resonance, observed when the harmonic currents from High Speed Trains match one of the system neutral frequencies, can lead to instability of tractive systems of the locomotives and in worst cases lead to damage of signalling and other sensitive communication equipment.
Negative sequence currents
In a balanced system the three-phased voltage and current are symmetrical with the same amplitude and the phase difference of 120° to each other. An unbalance system is a source of Negative Sequence Current (NSC). For the traction power system, the load from the train is a single phase load. The unbalance in the system may lead to built-up of high NSC in the power grid, posing risks to devices and reliability of the system. The NSC can adversely affect the transformer performance, functioning of protective devices such as relays, motor performance etc. to further amplify the PQ problems in the process.
Reactive power
The presence of inductive reactance in traction transformers motors and other equipment, traction power systems absorb high reactive power. The reactive power in a traction substation varies with the density and operation of these locomotives working in the network. A detail study of reactive power at the traction substation helps to understand the mitigation techniques for the reactive power.
While several other PQ related issues run through the electrical networks of the railways, most of these are connected to the one of the above.
As railways in India accelerate modernization, there are important lessons to learn from those countries who have been there and done that. Greater attention, awareness and measures to address the PQ issues are key to ensuring the success of modernization of Indian railways. Alongside customer service, reliability, efficiency and availability of the backbone of Railway transport, that is the electrical network, must also be ensured for safe and profitable operations.
Several studies highlight the need for a measured view of PQ challenges in electrical railways and the means to mitigate the same. What’s important is to pick the right set of tools and measurement frameworks to assess the impact of PQ and create a healthy electrical network that supports power systems for modernization of infrastructure and the transportation technology itself.
You're in the right place! Just drop us a message. How can we help?
Or see contact page