As India takes a leap forward in modernization, especially, electrification of railways while aiming to significantly expand its 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.
TRACTION POWER IN RAILWAYS
Railways use a Traction Power System for supply of power to the electrical locomotive. Annually, approximately 30 billion units of electricity is consumed by Indian Railways, out of which 10.4 billion units are used for electric traction purpose. Indian Railways pays over Rs 5,000 crores (~USD 720 mn) every year on account of traction energy charges which constitutes about 20 percent of total revenue budget of Railways. Railways consume roughly about 2.5 to 2.8% of the Country’s estimated power.
LEARNING TRACKS FOR INDIA
The history of modernisation of railway infrastructure has a similar trajectory in almost every country. However, it’s the difference between handling, management and long term resolution of risks to safety, reliability of the transportation infrastructure that separate the winners from the average performers.
- The high-speed trains network is expected to be around 45-50,000 km in 2020.
- Countries with High Speed Railway network – Belgium, France, Germany, Italy, the Netherlands, Spain, the United Kingdom, China, Japan, Korea and Taiwan
- Countries establishing or growing High Speed Railway network in near future – Brazil, India, Morocco, Russia, Saudi Arabia and the United States
Here are some examples of how some countries, that have taken up the large scale electrification and High Speed Train route in the past, had to face challenges in context of PQ.
China’s High Speed Train network poses a challenge in harmonics
Since HSTs were put into operation in 2007 in China, a few harmonic and resonance problems have been encountered in the Traction Power Supply System (TPSS), which had never happened with conventional electric trains.
- When the Beijing–Harbin HSR was put into operation, the voltage of the power supply arms ﬂuctuated abnormally in Jixian South traction substation (TSS), which rose to 35 ~ 75 kV, and it is much higher than the normal voltage (27.5 kV).
- When the Hefei–Nanjing and Hefei–Wuhan HSRs were put into operation in 2008, serious high-frequency harmonic voltage and current distortions occurred with the CRH2E locomotives, which caused the explosion of the lightning arrestors of the locomotives and traction network.
- In 2010, serious voltage harmonic distortion occurred in Bo’ao TSS, which reached up to 24% total harmonic distortion and repeated many times.
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.
UNDERSTANDING THE RANGE OF PQ CHALLENGES IN RAILWAY TRANSPORTATION
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.
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.
- Indian Railways – Facts and Figures – http://www.indianrailways.gov.in/railwayboard/uploads/directorate/stat_econ/IRSP_2016-17/Facts_Figure/Fact_Figures English 2016-17.pdf
- India’s first High Speed Electric Locomotive – https://economictimes.indiatimes.com/industry/transportation/railways/all-you-need-to-know-about-indias-first-high-speed-electric-locomotive/12000-hp-engine/slideshow/63683454.cms
- Zhengyou He, Zheng Zheng & Haitao Hu (2016) Power quality in high-speed railway systems, International Journal of Rail Transportation, 4:2, 71-97, DOI: 10.1080/23248378.2016.1169228 – https://doi.org/10.1080/23248378.2016.1169228
- Measurement of AC side harmonics of a DC metro railway – http://www.academia.edu/31814400/Measurement_of_AC_side_harmonics_of_a_DC_metro_railway
- Study of Energy Utilization in a Metro Rail Limited Company by Bhagyalakshmi R, Dr. James K C https://www.ijedr.org/papers/IJEDR1704234.pdf
- Railways Code Manual for Traction Power – http://www.indianrailways.gov.in/railwayboard/uploads/codesmanual/ACTraction-II-P-I/ACTractionIIPartICh1_data.htm
- Survey of Harmonic Distortion in Indian Railway Electric Supply System: Results and Remedial Actions http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.581.6862&rep=rep1&type=pdf
- Solar Panels in Railway Stations – http://www.pib.nic.in/Pressreleaseshare.aspx?PRID=1528465