Keeping High Speed, Metro Rail Safety and Performance on Track Good Power Quality is the Driver

Published On: Dec 05, 2020


Metro Rails are all set to take the center-stage of mass urban transportation in India’s fastest growing cities. But the performance and safety of the new and fast emerging metro rail infrastructure in India will only be as good as its supporting infrastructure and systems. Safe and reliable electrical power is at the core of keeping Metro Rails running. And Power Quality is at the core of ensuring safe and reliable electrical power, thereby keeping the performance of Metro Rails on track.

INTRODUCTION

India is rapidly expanding its high-speed electric rail network and metro network in recent years. The policy of developing smart cities with modern urban transportation has given a boost to Metro Rails. Formation of National High-Speed Rail Corporation Limited (NHSRCL) in 2016 aims to finance, construct, maintain and manage the High-Speed Rail Corridor in India. The Railway Ministry has planned for 100 per cent electrification of Broad-Gauge railway routes by the year 2023 as per the following planning:

  • 6,000 RKM in the year 2020-21
  • 6,000 RKM in the year 2021-22
  • 6,500 RKM in the year 2022-23
  • 5,265 RKM in the year 2023-24

The Indian Railways has also introduced several semi-high speed trains with speed ranging from 160 km/hr to 180 km/hr. in the last few years.

The Central Organization for Railway Electrification (CORE) states the Indian Railway Electrification has grown from 2015-16 to 2019-20 in increasing trend and resulted in a sum of 16889 RKM by 31st March 2020 which is 345% increase against previous five years IR data (2010-11 to 2014-15 – 3793 RKM).

Overall, the thrust on electrification, high-speed and Metro rails are the driving forces for India’s rail infrastructure. However, this growth is not without its share of problems and challenges.

Electrical infrastructure challenges in India’s Metro and High-Speed Railways

Many cities including Hyderabad, Delhi continue to face the issues of electrical technical snags in Metro Rails. What’s worrying is the time to resolve these issues still runs into hours as compared to snags lasting for only few minutes – which is the norm in many developed countries. Instances of passengers being evacuated and forced to walk between stations are common. Also, this slows down the Metro Services as only one line stays operational – thereby further putting the stress on the system and creating a built-up of passengers on the stations during peak hours. The reputation and reliability of India’s newly built infrastructure is at stake.

While the issues concerning these technical snags are wide ranging – from snapping of wires to breakdown of vacuum circuit breakers, harmonics and other power quality issues are also in observation as the potential root causes for many technical snags. Subsequently, several examples of technology or equipment upgrade are also being undertaken to improve the performance of Metro Railways. In 2018, Delhi Metro upgraded electrical equipment on elevated corridors for better reliability and performance. Other Metro Rail corporations are expected to follow the trend in recent future.

Power Quality Challenges in Electrical Network

Modern electric trains have a nonlinear load of a dynamic nature, which in turn creates its own challenges in the evaluation of power quality problems. Presently, Indian metro rails use 25KV AC power supply which is transmitted at a third rail or overhead. The AC power supply results in considerable power losses.

A number of studies have concluded that reactive power emerges as a major cause of power quality issue in high-speed electric trains. The present article examines various power quality issues faced by high-speed trains and the measures for their mitigation.

POWER QUALITY ISSUES IN HIGH SPEED AND METRO RAILWAYS

Indian railway traction power system uses 25 kV AC. Typically, the power supply for railway traction system is provided by the state utility – a three phase source at 132/220 kV. The traction Over Head Equipment (OHE) require a 25 kV supply and therefore only two phases are taken and step down to single phase 25 kV through transformer at traction substation. This 25 kV is fed to the OHE from feeder then to locomotive via pantograph. Arrival of the locomotive at the substation is a dynamic load. When there are several locomotives at the Traction Power Substation (TPSS) operating at a time, there is a voltage drop at OHE. Because of this, the motor operations get inefficient as a large amount of current is drawn from line. Due to the excessive load, there are instance of nuisance tripping of circuit breakers.

Reactive Power and Power Factor

Reactive power is the power that sustains the electric and magnetic fields of AC equipment. It’s a measure of the energy exchange between the power source and the reactive components of the load. Reactive power is calculated using power factor. Low power factor indicates high reactive power in the system. If the power factor of TPSS is low, the power transmission of the high-speed railway is affected.

The capacity of several components is reduced. This includes the generator set, power transmission and transformation equipment, and other electrical equipment. At the same time the cost of generating and transmitting electricity rises. The power and voltage losses in the transmission network rise. These losses increase because the apparent power and corresponding power increase if the power factor is lower than a specific level.

Under rated power demand configurations, the power factor of high speed railways is observed to be high. Output power factor tends to be low in case the operation density and power demand of the trains is low, or the output power is a low power demand region. The same happened when the Lanzhou–Xinjiang and Yunnan–Guizhou high-speed railways were newly started. The power factor of the trains was found to be lower than 0.85.

Harmonics

Harmonics are sinusoidal voltages or currents having frequencies that are multiples of the fundamental frequency at which the supply system is designed to operate. Harmonic currents are generated by the load and flow around the circuit via source of impedance through all parallel paths. Harmonic voltages too appear throughout the installation.

Harmonics damage critical components of the system such as signaling and communications equipment. The voltage and current distortion are further increased by resonance resulting in increasing loss for the power grid. Apart from damaging electrical equipment such as transformers, capacitors, arresters, and circuit breakers among others, resonance may also distort electromagnetic field. This leads to the rise of electromagnetic radiations which impact the communication system and other sensitive electromagnetic facilities. What’s more, the transformer core is saturated in the event of a resonance, affecting the accuracy of power measurement.

Impact of PQ issues on Rail Potential

As High-Speed Trains and Metro Rails set into operation, there is a possibility of leakage current when the rail comes in contact with the ballast bed. The rail potential increases as this leakage current travels back to the substation. This increase in rail potential could be dangerous for passengers while also impacting the communication system. Also, the mutual inductance between the rail and the catenary also induces current which flows through the rail impedance, increasing the rail potential. Electrical railways use single-phase high-voltage, high-current AC power to run. This is an unbalanced power system, and high-voltage and high current create a changed magnetic field which impact the communication system.

Low Frequency Voltage Fluctuations

Low frequency voltage fluctuations might cause the locomotive to lose the ability to move. The problem can occur under drive conditions. LVF first occurred in Norway in 1996. After this, it has also occurred in many places in China. LVF is generated when the frequency of the power grid and that of the TPSS are different. Static converters are applied for conversion of power, and studies have found that their characteristic frequency causes further LVF. There are two types of LVF. In type 1, the frequency is 1.2 ~ 1.9 and type 2 LVF where voltage frequency is l3 ~ 7 Hz. Type 2 LVF is defined as a vehicle-grid interaction problem. LVF occurs when the number of trains in the station is large.

PQ ISSUES OBSERVED AT JAIPUR METRO RAIL CORPORATION (JMRC)

The section below presents a summary of PQ Analysis performed at JMRC and presented in the research paper “Study and Solution of Reactive Power Compensation, Harmonics Mitigation and Load Balancing for Power Factor improvement of Jaipur Metro Power Supply System using Active Harmonic Filters” by Bharat Lal Mali M. Tech. Scholar, Regional College for Education Research and Technology Sitapura, Jaipur and Dr. Pramod Sharma Professor, Regional College for Education Research and Technology Sitapura, Jaipur.

JMRC TPSS

  • Power supply at 132KV at its Receiving Sub Station (RSS)
  • Two RSS at Mansarovar (MSOR) and Sindhicamp (SICP).
  • 132KV supply is step down to 33KV, 3-Phase to feed auxiliary load of all metro stations and 25KV single phase to feed traction load for train operation

The measurement summary, as observed in the study, is given below for both the RSS locations:

The study observes:

  • The JMRC load is dynamic in nature due to traction load.
  • A majority of the loads connected to the system are nonlinear; This generates harmonics in the system.
  • The single-phase traction load causes load unbalancing
  • The use of cabling network causes generation of excess reactive power which in turn leads to a very poor power factor (leading).

Clearly, as demonstrated in the case above, PQ issues are a key concern at the JMRC TPSS. It is a high probability, that similar or more severe issues could be present in other Metro Rail TPSS facilities, but only left undiscovered due to lack of measurement. Such studies must be taken up on priority in consideration of compliance to PQ regulations that are being rolled out in multiple states. Continuous monitoring of PQ issues can lead to accurate solutions to mitigate the risks from harmonics, power factor and other key PQ issues.

PQ MITIGATION IN TPSS OF HIGH SPEED AND METRO RAILWAYS

Power quality issues cause harm not just to the TPSS of the train but also to the upstream power grid. The IEC and the IEEE have proposed a number of standards to maintain the power quality in high-speed railways. When it comes to measures to mitigate PQ issues, a various filters and compensation methods are used.

Passive Filters

Three types of passive filters are used to mitigate PQ issues in high speed railways: detuned filters, single-tuned filters and damping filters. Detuned and single tuned filters work to mitigate harmonics of a specific frequency. Damping filters are taken into consideration to eliminate harmonics of higher order. Due to the presence of a variety of harmonics in the TPSS system of railways, several filters are paralleled. However, this requires large space, incurs high costs, and increases the losses of fundamental wave. Responding to these issues, researchers have proposed use of double tuned filters which can tackle a number of different frequencies at the same time, accomplishing the goal at a lower cost.

Active Filters

In high-speed railways, the reactive power or the TPSS changes frequently and thus, the fixed capacitor compensation ends up under-compensating or over-compensating instead of reaching the required power factor. Here, the reactive power compensation units need to be dynamic. The static var compensator (SVC) is deployed to achieve this. SVC is a combination of various capacitors and reactors such as fixed capacitor (FC), thyristor switched capacitor (TSC), thyristor-controlled reactor (TCR), controlled reactor (CR)

Steinmetz Circuit

The locomotives have a high-power demand and are single-phase loads. The loading is balanced in three phases by using the Steinmetz compensation circuit (SCC). The circuit is comprised of balance circuit and the reactive power compensation circuit. The reactive power is compensated by the variable capacitor paralleled in the load phase. The components of the balance circuit include variable reactance and capacitor.

EMERGING PQ RISK MITIGATION TECHNOLOGIES

PQ issues in TPSS are a subject of continuous research and development. Several new combinations of solutions are being tried and validated on an ongoing basis. Some of the promising and relatively established solutions are discussed below:

Railway Static Power Conditioners

The Railway Static Power Conditioner is comprised of an inverter and inverter transformer. It interchanges power between two circuits to compensate for voltage fluctuations and the unbalanced load in 3 phases. It also works to compensate the harmonics current emanating from trains.

The RPC controls the output current of two converters to transfer power between two power supply arms. Hysteresis and fuzzy controller are used in most cases to implement the control strategy. The PI controller is used to control the DC link capacitor voltage to gain a constant DC voltage. A testing of the RPC in Japan’s Shinkansen was found to be capable enough to minimize unbalance and voltage fluctuations even when the short circuit of the power system was not large enough.

Hybrid Power Quality Control System

HPQC is deployed mainly to tackle harmonics and reactive power. It comprises of a multipurpose balance transformer, three LC branches, and a three-phase full bridge converter. HPQC can also suppress the influences of load fluctuation and resonance leading to an improvement in the harmonic filtering performance of the PPF. However, the HPQC can reduce the capacity of the converter.

CONCLUSION

India is rapidly urbanizing, and new metros and high-speed trains are being set-up around the country. The Delhi Metro Rail Corporation (DMRC), for instance, has been one of the fastest expanding Metro rail system in the world. It carries 1.5 million passengers in a day who travel an average of 17 km on the train. But repeated instances of technical snags and breakdowns have led to power outages for long periods on all its lines. In many areas, the standards and recommendations adopted by Delhi Metro are followed by the other Metro Rail Corporations in the country. DMRC too recognizes that there are issues and is taking steps for resolution. However, the focus on good PQ through continuous monitoring still does not feature as an area of focus for most Metro Rails in India.

The standards guiding the Metro Rails point to use of electrical equipment or components that aid good PQ. Recommended use of Copper in 33kV cables, contact wires, Bus Duct, Catenary wire, Power cables, Contact wires, RSS Earthing (use of buried copper flats), Copper conductors as per International Standard IEEE80 etc. can all be counted as steps aiding good PQ. But PQ is also a dynamic phenomenon and therefore must be measured and monitored for continuous improvement. The PQ Regulations which are being rolled out in different states further necessitate the continuous monitoring and measurement. Continuous monitoring of PQ, as seen from the JMRC case study, is a key driver to improve the reliability and efficiency of the Metro Rails.

For the High Speed and Metro Rails, good PQ is not a destination but a journey that must be taken sooner than later.

REFERENCES

  1. Standardization/Indigenization of Electrical & Electromechanical Metro Rail Components – http://mohua.gov.in/upload/uploadfiles/files/STANDARDIZATION-INDIGENIZATION%20OF%20ELECTRICAL%20AND%20ELECTROMECHANICAL.pdf
  2. Power quality issues in high speed railway systems – https://www.tandfonline.com/doi/pdf/10.1080/23248378.2016.1169228
  3. Powering the world’s metro and tram systems – https://library.e.abb.com/public/cd3f69264ecff0c0c12578f7005ec569/Powering-the-worlds-metro-and-tram-systems.pdf
  4. PQ challenges in modernization of railway transport in India – https://apqi.org/archives/1875
  5. Critical Issues related to Metro Rail projects in India – https://www.researchgate.net/publication/255710610_Critical_Issues_Related_to_Metro_Rail_Projects_in_India
  6. Hyderabad Metro – still grappling with problems – https://timesofindia.indiatimes.com/city/hyderabad/hyderabad-metro-still-grappling-with-snappy-wires-tech/articleshow/66531428.cms
  7. Power Quality in Railway Traction and Compensation by Combining Shunt Hybrid Filter and TCR – https://www.semanticscholar.org/paper/Power-Quality-in-Railway-Traction-and-Compensation-Navaneethakrishnan-Chandrasekaran/467e8ce3545764047f672dc0a075a2d64883980d?p2df
  8. Study and Solution of Reactive Power Compensation, Harmonics Mitigation and Load Balancing for Power Factor improvement of Jaipur Metro Power Supply System using Active Harmonic Filters by Bharat Lal Mali and Dr. Pramod Sharma – https://ijisrt.com/assets/upload/files/IJISRT19JUL125.pdf.pdf
  9. A Review on Power Quality Problems and its Compensating Technique in Railway Traction System – http://www.jetir.org/papers/JETIR1604056.pdf
  10. Traction Systems, General Power Supply Arrangements and Energy Efficient Systems For Metro Railways – http://mohua.gov.in/upload/uploadfiles/files/Report%204%20Traction%20System.pdf
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