1-s2.0-s0968090x07000861-main

Available online at www.sciencedirect.com

Transportation Research Part C 16 (2008) 515–534

www.elsevier.com/locate/trc

Review

An overview of electromagnetic compatibility challengesin European Rail Traffic Management System

Surajit Midya *, Rajeev Thottappillil

Division of Electricity, Angstrom Laboratory, Uppsala University, Uppsala, Box 534, S-75121, Sweden

Received 15 May 2007; received in revised form 5 November 2007; accepted 6 November 2007

Abstract

In Europe, the railway industry is rapidly getting transformed from traditional mode of public transportation to a veryfast, more reliable, long distance and cross country operation. A new concept, called European Rail Traffic ManagementSystem (ERTMS) is originated to make this transition smooth, reliable and compatible among different countries. Elec-tromagnetic interference and compatibility (EMC) issues play a major role on the overall system design and performanceof this. In this paper, an overview of the operational principles and major components of ERTMS and other modern rail-way systems are discussed in detail with an emphasis on possible EMC issues. Radiated and conducted interferences orig-inated from different sources and their consequences on different subsystems and components are discussed and analyzed.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Rail transportation; Rail transportation communication; European Rail Traffic Management System; Electromagneticinterference; Electromagnetic compatibility; Arc discharges

Contents

0

d

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5162. Railway signaling systems: a brief overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516

968-0

oi:10.

* CoE-m

2.1. Traditional signaling using track circuits and axle counters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5182.2. Brief of ERTMS system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5192.3. Different levels of ERTMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519

3. EMC challenges related to railways signaling and communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521
3.1. Different railway standards related to EMI and EMC issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5213.2. Interference with railway signaling systems and equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522
90X/$

1016/j.

rresponail add

3.2.1. Interference with track circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5223.2.2. Interference effects on axle counters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5233.2.3. Interference issues with the Eurobalise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5243.2.4. Interference with the leaky coaxial cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524

- see front matter � 2007 Elsevier Ltd. All rights reserved.

trc.2007.11.001

ding author. Tel.: +46 18 471 5801; fax: +46 18 471 5810.resses: [email protected] (S. Midya), [email protected] (R. Thottappillil).

mailto:[email protected]

mailto:[email protected]

516 S. Midya, R. Thottappillil / Transportation Research Part C 16 (2008) 515–534

3.2.5. Interference with the electromagnetic braking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
3.3. Interference problems due to pantograph arcing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
3.3.1. Previous work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5263.3.2. Physics of sliding contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5273.3.3. The situation in winter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5273.3.4. Investigations by OHL Ice Working Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5273.3.5. Radiated interference from pantograph. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529

3.4. Interference from power electronics and drives systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 530
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532

1. Introduction

In the recent past, globalization on the European railroad transportation system had a great impact to theoverall railway system and technology. Unlike the road transport or air transport, the technical specificationsof railway transport in Europe were different in different countries for various historical and political reasons.There were differences in terms of track width, signaling system, traction feeding voltage and frequency, nav-igational standards, etc. So, the interoperability between different countries was difficult. With numerousmergers in the railway industry and the need to operate trains through different countries, a need for a railwaysystem with cross country operability was building up. The ERTMS is the outcome of this. The main objectiveof this system as mentioned by de Tillere et al. (2003) was to set a standard for Europe in terms of onboardand trackside equipments like power feeding, signaling, communication, train control, complete certificationprocess etc., to provide and improve the safety of the overall system for high speed trains with only few min-utes headways between trains, enhancing the operational reliability and capability for carrying more trafficwith same infrastructure, reducing the maintenance cost, making it more efficient and enabling the cross coun-try functionality for both freight and passenger trains. Although ERTMS had started in Europe at the begin-ning, the overall technology and concept is adopted by many other countries across the world because of itssuperiority over the conventional railway system.

Electromagnetic noises, generated within the system or coming outside the system often hampers the overallsystem performance and sometimes create interference with the nearby civilian systems as well. Keeping inmind the huge size and complexity of the railway system, it is often difficult to identify the noise sourceand the coupling path by which it is affecting the victim. In this paper, operating principles of the critical com-ponents of the ERTMS are briefly described from the EMC point of view to have a better understanding ofthe overall operation of the system, which will help in identifying the source, victim and the coupling path.Another important challenge of ERTMS and modern railway is to ensure the availability of the different radiobased services because of high speed and more traffic than the installed capacity. With increasing speed andcapacity, it will get more and more challenging to ensure passage of all the required information through dif-ferent subsystems and components within a very short time span.

Section 2 describes different types of railway signaling systems and important components associated withit, an overview of ERTMS, its major components and different levels in brief. Section 3 provides informationabout major EMC challenges to railway signaling and communication both at component as well as overallsystem level. Section 3 also describes the major problems faced by the railway engineers associated with therelated standards, major components, the detail analysis about the nature and possible consequence of theimportant problems related to EMC.

2. Railway signaling systems: a brief overview

The main task of signaling system is basically to protect the train from collision with other trains, guide thetrain to maintain a desired speed, i.e., preventing it from undesired speeding or slowing, avoid derailing of the

S. Midya, R. Thottappillil / Transportation Research Part C 16 (2008) 515–534 517

train from the track changing positions till the connecting track gets ready. Pollack (1996) discussed about theevolution of the signaling system which can be divided in following five major divisions:

� Dark territory: Tracks have no visible signals and trains move within the territory following the time table.� Manual block signaling: A visible signal is displayed at the entrance of the block and hence, the cabin crew

knows whether the train can proceed or not.

Track circuit

Resistor

battery (Transmitter)

ReceiverCircuit

Green light circuit

Red light circuit

Signal feeding battery Track circuit feeding

Toggle Switch

Coil of trackcircuit relay

current (grounded)S Rail for return

I rail with insulated joints

Length of the track circuit

+

Resistor

Track circuit

battery (Transmitter)

Power supply circuit

Receivercircuit

Signal feeding battery

Red light circuit

Green light circuit

Train

Track circuit feeding

Toggle Switch

Coil of trackcircuit relay

S rail for returncurrent (grounded) I rail with insulated joints

Length of a track circuit

Fig. 1. Basic layout of the track circuit and corresponding (a) green and (b) red signal. (For interpretation of the references in color in thisfigure legend, the reader is referred to the web version of this article.)


� Automatic block signaling: This is based on the use of different types of track circuits to detect the traininside some particular block. Fig. 1a and b show a simple track circuit and its operating principle. This typeof signaling does not require any manual intervention and the signal gets activated once the block occu-pancy is changed.� Centralized traffic control: The occupancy of all the blocks are tracked from a central location and the sig-

nals are activated to ensure that maximum traffic throughput is achieved for a given situation. Also, thisensures that proper priority is given for fast trains.� Cab signaling: Rails of the track are used as the transmitting medium for signaling. An onboard display

inside the cab ensures the visibility of the signal irrespective of the atmospheric visibility condition. Follow-ing trains are immediately notified about the clearance and hence provide better track usage.

In traditional fixed block signaling, only one train can be present at one block and the physical location andlength of the track is fixed. Faster trains require longer distance to stop and hence, track circuit length shouldbe long enough to provide proper safety. This reduces the capacity of the railway lines. Communication basedsignaling (CBS) is mainly based on a radio link, which is at the core for both voice and data communication.In CBS, apart from the exact location of the train, many other information like speed, health monitoring data,etc. are also transmitted to the central control unit. With the pinpoint location of the train known, railways arein a position to implement the moving block system. In moving block signaling, precise location of each trainwill be known continuously using various onboard, line side electronic equipments and communication sys-tems and a safe distance (estimated from previous knowledge) will be maintained within which the traincan stop itself safely. This headway distance is maintained continuously as the train moves through the com-munication with the central control center and this can reduce the headway distance significantly as discussedbriefly by Pope and Harman (2007).

This has greatly increased the throughput of a railway track without much increase of the infrastructure,which would have been expensive, time consuming and difficult to implement in many cities. Pollack (1996)mentioned that at Sau Paolo metro, with the existing infrastructure, the headway can be reduced to 66 secondsduring busy hours using this moving bock techniques, maintaining the same operating speed.

In the near future when train detection will be a new responsibility of the onboard computer and it will bedisplayed at the driver’s panel. This requires the combination of the received train data with the status data,collected from the existing trackside equipments and the route information from the central control unit. Thenthe locomotive can provide control commands to the local wayside equipments. The train data will be contin-uously transmitted from the wayside control unit to the central control unit. Under normal operating condi-tions, all the information will be displayed in the locomotive and the wayside equipments will be automaticallyactivated to their permissive condition. The existing signaling will then work as a backup system. In Auto-matic Train Control (ATC), high speed trains like Shinkansen or TGV which travels so fast that the driveralmost has no time to acknowledge the trackside signals, the brakes of the train will be applied automaticallydepending on various conditions for safety reasons. ATC also controls the speed limit by checking the runningspeed and the permitted speed of the train and accordingly applies the brake and releases it once the speed iswithin the permitted limit. When the driver fails to apply brake or information from the trackside equipmentscannot be retrieved, the Automatic Train Stop (ATS) system stops the train automatically.

2.1. Traditional signaling using track circuits and axle counters

In a basic track circuit, there are three components, a transmitter, a receiver and a relay, connected to atriple pole switch, which in turn is connected to the red and green light signaling circuit as shown inFig. 1a and b. The track circuit relay coil is energized when there is no train and the green light glows. Presenceof a train shorts the two rails through the axle and the current drops in the track circuit relay coil, making itde-energized and making the red light glow as shown in Fig. 1b. The transmitter could be fed by audio fre-quency signals (usually between 1500 and 2600 Hz range) as in Indian railways, or by a near DC voltage inthe range of few volts, typically 3 V, 9 V, etc. with 0–2 Hz as in countries like, Sweden, Denmark, Luxem-bourg, UK, etc., or fed by power frequency, i.e., AC with even harmonics as in countries like Switzerland,etc. Usually the electrical polarity of the feeding voltage is altered in each consecutive block to prevent a block


falsely powering the next block. The reason for choosing DC and even harmonics is the belief that in an ACrailway these components cannot be present, which has been proved to be wrong later by Bormann (2003),Pollack (1996), Buhrkall (2005), Midya et al. (2007) and some other groups.

Similar to the operation of track circuit, an axle counter detects the passing of a train in a block. These areinstalled at both the ends of each block with a counter head. As each axle passes the counting head at one end,the counter increments and when it passes the other end, the counter head starts decrement. When the netcount is zero, the track is consider as empty. Usually its done by a centrally located computer, called evalu-ator. The information from the detection points of the train is transmitted by either telecommunication cableor wireless connection to the evaluator. It is mostly used in places like wet tunnels etc., where ordinary trackcircuits are not reliable, in tracks having electrically uninsulated steel sleepers which prevents the operation ofthe track circuit, and in longer sections of a track to save several track circuits.

2.2. Brief of ERTMS system

As described by many researchers like Page (2001) and de Tillere et al. (2003), major subsystems and com-ponents of ERTMS are:

� European Vital Computer (EVC), an onboard computer for controlling and managing all the information.� Signaling related technology, called European Train Control System (ETCS).� GSMR radio communication technology, between the vehicle and the central control station through the

GSMR towers with 876–915 MHz in uplink and 921–960 MHz in downlink for voice and datacommunication.� Radio Block Center (RBC), a centralized signal encoder module.� Eurobalise active and passive transponders, which lay in between the tracks and send information to the

onboard balise transmission module (BTM) at 4.234 MHz, once powered by 27.115 MHz microwave from it.� Euroloop, an inductive loop based localized communication system for semi-continuous mode of commu-

nication, where the line side signals get transmitted through a leaky coaxial cable laid across the track to anonboard antenna, similar to the onboard Eurobalise antenna. The uplink from track to train uses thespread spectrum modulated signal, spread over approximately 1.8–7.2 MHz. In UiC (2000), euroloop radiocommunication inside the tunnels at frequencies of 50–100 MHz is briefly discussed.

2.3. Different levels of ERTMS

To ensure a smooth transition from previously used national railway technology to ERTMS, three levelsare defined namely, levels 1, 2 and 3 as shown in Fig. 2a–c, respectively. Most of the countries are at level1 now and are getting transition to level 2. This transition is not exactly the same way everywhere:

� Level 1, as shown in Fig. 2a, is a fixed block spot transmission system (i.e., the driver machine interface isupdated only when the train passes the signal). To avoid the replacement of the entire existing signalingsystem, it is overlaid on conventional railway signaling but designed to deliver ATC/ATS functionalityand cab signaling. Information about infrastructure related data, viz., speed limits, balise linking distance,track information, i.e., gradient, etc. are programmed within the LEU, which determines the aspect of thesignal to be displayed and passes a serial message to the Eurobalise containing these information. Thedrawback of spot transmission is that a train approaching a red signal will be forced to apply brake evenif the aspect changes to green as the train comes closer. The mitigation of this problem, called in fill, isachieved either by placing additional balises on the approach of a signal, or euroloop is used for providingupdated information, or GSMR is used for communication to allow the driver to maintain the speed bysome modification in the ATP overspeed protection algorithms as mentioned by Bloomfield (2006). Trainsare detected by track circuit and axle counters. Based on the received ETCS data, odometer data, safe speedprofile and the breaking parameters, ETCS subsystem performs the ATP function. The informationrequired to run the train will be displayed at the driver’s panel.

Fig. 2. Overview of the ERTMS system at different levels (adopted from Page, 2001; de Tillere et al., 2003).


� Level 2, as shown in Fig. 2b is also a fixed block but continuous transmission system. This uses the GSMRradio network to communicate with the wayside equipments and provides complete fill in in each block.Balises are still being used, mainly as position reference. Optical signaling is not required at this leveland trains are detected in the same way like in level 1 by track circuits or axle counters. GSMR providesbi-directional transmission of all real time operational data regarding train, i.e., the train speed, location,


etc. Unlike the level 1, level 2 has the infrastructure data centrally within RBC and it uses the odometrydata for the location of the train. With the information from the local interlocking blocks (IXL), RBC alsochecks the interlocking status and block occupancy ahead of each train and calculates the movementauthorities. Then it transmits these information via the GSMR communication.� Level 3, as shown in Fig. 2c, is a moving block, continuous transmission cab signaling based on GSMR. All

information between trains and wayside equipments, i.e., train position, train integrity, etc. are transmittedvia GSMR. No optical signal, track circuits or axle counters are required. Balises are still used as positionreference and to initialise and recalibrate the odometer in level 3 as well. RBC uses the train position andinfrastructure status to decide on movement authorities.

3. EMC challenges related to railways signaling and communication

The beginning of EMC research on traction was to control the return current. There are many previousinvestigations and articles on both AC and DC railways on this by researcher like White (2007b), White(2007a), White (2006a), Hill and Cevik (1993), Hill (1994a), etc. Still, because of its complexity involved, thereare many EMC related problems even today as reviewed by researchers like McCormack et al. (2006), White(2006b), Leeming et al. (1992), Hill (1997a) and Hill (1997b), while Bourne et al. (2003), Ogunsola (2003) andOgunsola and Pomeroy (2003) etc. had described some practical management related issues dealing with rail-way EMC issues. Since railway is getting modernized dramatically over the last decade or so, its a challenge tokeep the older existing services intact as well as starting newer systems and following the EMC standards,which are getting more and more strict in respect of unwanted electromagnetic emissions. There is a needfor a proper understanding of the various issues related to EMC, both with the older systems as well as newersystems like ERTMS and their compatibilities with the non-railway surroundings and will be discussed in thissection. Major sources of EMI in a railway system are:

� Rolling stock: It includes propulsion drives, choppers/three phase inverter drives, circuit breakers, staticconverters, other facilities like, air conditioning unit, lighting, etc., electrical braking units, etc., corona dis-charges and breakdown of the onboard electrical equipments, arcing in the pantograph, etc.� Power supply unit: It includes substation equipments, switching, substation power electronics, load unbal-

ancing in the three phase system, etc.� Environmental sources like, lightning, electrostatic discharges, geomagnetic induced current (GIC), etc.� Infrastructure: Improper earthing, trackside equipment failure, etc.� Track: Since it carries low frequency signaling information as well as traction return current, coupling with

any other sources, e.g., nearby railway, DC components from different sources, etc. create interference.

The EMI victims includes:

� Signaling and communication equipments, including both power and communication cables, lines, etc.� Radio communication.� Traction power system network.� Automatic control system.� Trackside equipments.� Utilities in the surroundings of the railway, e.g., TV, radio and other wireless communication.

Apart from direct effects of interference to the signaling, there are many indirect effects of the interferenceproblems as well.

3.1. Different railway standards related to EMI and EMC issues

Railway is a largely distributed complex network comprises of various types of moving and static electro-magnetic noise sources and unfortunately compliance of the present standards do not give a guarantee of


trouble free satisfactory performance of the system. There are standards to deal with various EMC issues forboth different railway applications and its effects on the surroundings like, CENELEC international standardsEN50121:2000 (parts 1–5) specifying EMC for railway applications, code of practices by GM/RC 1500between the railway and the neighborhood, CISPR publications by subcommittees A-I, various standardsof different countries, etc. Also, there are other specifications like, the Railway Industry Association specifiessusceptibility levels of 20 V/m over a frequency band of 27–500 MHz as mentioned by Allan et al. (1993) andKonefal et al. (2002), EN50121 insists to have a minimum distance of 10 m from the centerline of the outertrack, which is often difficult to maintain in many cities and often not enough to keep the surroundings frombeing affected from the railway generated interferences, etc. It is reported that even when the train is 1 kmaway, it affects some applications at few hundreds of kHz (Bartlett et al., 1999). Because of the wide variationsin the nature of the various interference sources like, pantograph arcing, power electronics and drives systems,etc., and lack of proper experimental and theoretical investigations, these standards are often found to beinsufficient to protect various applications both within the railway and outside civilian systems from beingaffected from interferences as often reported by many researchers and engineers. Hence, there is a clear needfor proper investigations which can lead to more suitable standards.

3.2. Interference with railway signaling systems and equipments

In general there could be two kinds of problems due to interference to the railway signaling and commu-nication systems, viz., false occupancy, i.e., though there is no train, the signaling system will falsely show redlight, or false unoccupancy, i.e., although there is a train within the track, the signaling system will show greenlight. False occupancy can cause delay in the operation of the railways, whereas false unoccupancy can causecollisions and accidents, which may cause disasters.

3.2.1. Interference with track circuits

Since the rails carry both the return current of the power supply as well as the track circuit current, its verymuch vulnerable to receive interference from various sources. Hill (1990b) discussed about various safety andreliability features of synchronizable digital coding in railway track circuits and possible design to eliminate

Track circuit currentSignal lamp currentDC component of arcing current

Track circuit

Train

Power supply circuit with DC component Arc

Receiver

Track circuit feeding

Green light circuit

Red light circuitbattery (Transmitter)

circuit

Toggle Switch

circuit relayCoil of track

current (grounded)S rail for return

I rail with insulated joints

Signal feeding battery

Resistor

I int

Interference curentdue to pantograph aringIint

Length of a track circuit

+ − +−

Fig. 3. False signaling in track circuit due to DC component of the arcing.


interferences (Hill, 1990a). Interference to DC fed track circuits due to the DC components originated frompantograph arcing, DC railways running nearby, geomagnetically induced current, etc., are quite commonand frequent in some countries. This type of interference to track circuits may result in false unoccupancysince the interfering current couples itself to the track circuits as shown in Fig. 3. For power frequency fedtrack circuits, false occupancy can occur if the track circuit relay gets de-energized by some interference cur-rent, antiphase to the normal track circuit. False unoccupancy can occur when the track circuit relay gets ener-gized even when the train is still in the track circuit block. But it requires more current to energize the relayrather than de-energizing it and also the interfering current has to overcome the shunting effect of the trainaxles. Joos et al. (1998) mentioned that in general track circuits are more susceptible to false unoccupancycompared to false occupancy and hence causes delay in the train schedule.

White (2006b) discussed about various interference sources of the audio frequency fed track circuits like,switching circuits of the modern converters/inverters, fundamental and multiples of the fundamental fre-quency of the chopper, four quadrant thyristor converter, inverter drive and the resonant frequencies ofthe electrical and mechanical systems, mechanical sliding of the pantograph and sometimes the power supplyswitching, etc. Hill (1997a) and Konefal et al. (2002) reported about some false signaling in North America in1965 in the audio frequency fed track circuit because of interference from harmonics generated by the chopperunit. In case of a audio frequency (AF) fed track circuits, if unmodulated current of correct frequency getsinduced in the track circuit, this may result in false occupancy. False unoccupancy occurs when the interferingsignal of correct frequency and modulation rate gets coupled to the track circuit. But as Joos et al. (1998) men-tioned, in this case also the shunting effect of the vehicle axles have to be overcome and hence track circuits aremore susceptible to false unoccupancy compared to false occupancy.

3.2.2. Interference effects on axle counters

Although in mainland Europe, axle counters are proved to be more reliable compared to track circuits, itsometimes fails because of power failures, electromagnetic interferences, etc. and may result in accidents, likethe seven tunnel accident in UK in 1991. This was because of improper resetting of the counter, which some-times require manual intervention. van Alphen (2004) and Bloomfield (2006) mentioned that axle counters canget affected by the magnetic fields close to the rails and also from the magnetic fields emitted from the train or

Noise region of interest

ZX

Y

Balise antennas

Fig. 4. Balise installation and possible region with potentially strong EMI capability (adopted from Pozzobon et al., 2003).

Fig. 5. Radiated interference from the pantograph arcing, catenary wires and other radiating source and the interference from the tractiondrives to different components.


from the return current of the railway. Electromagnetic rail brakes can also create interference and false count-ing as discussed in detail in Section 3.2.5.

3.2.3. Interference issues with the Eurobalise

The onboard antenna, called balise transmission module (BTM), is very sensitive to vertical magnetic field.Sometimes even horizontal magnetic field and electric field components can cause interference too. From Figs. 4and 5 it can be said that vertical H field component may come from several sources like harmonics of the tractioncurrent, from the high frequency current components of the horizontally laid cables in the close proximity of theonboard antenna, current from the rails, etc. Although the microwave transmission is based on the vertical H com-ponent, sometimes horizontal components i.e., Hx and Hy , especially while tilting or cables with an inclination/bend, can cause interference to this. Pozzobon et al. (2003) reported about this type of interferences and mentionedthat the influence of interfering E field is not specified for the antenna module and needs further investigation.

Sufficiently strong random bursts of short duration can cause wrong bit demodulation which may createdifficulties in decoding the balise telegram and may lead to operational failures as reported by Pozzobonet al. (2003) and many other practicing railway engineers. Both trackside and onboard balise antennas are sub-jected to analysis and tests for magnetic fields due to harmonics of the power system and transients. But, it hasbeen observed and experimentally verified by many engineers and reported by Pozzobon et al. (2003) thatoften there are troubles in the balise detection module because of interference due to switching of the maincircuit breaker, etc. Today, most of the traction drives (>90%) use resistive breaking to dissipate the energywhile breaking/retardation as mentioned by Joos et al. (1998). The faster and sharp pulses originated fromthese variable speed electrical drives/power electronics modules and switching can cause interferences evenin the GHz range as reported by many researchers like Stemmler (1993), Jahns and Blasko (2001), Jooset al. (1998), Chen (2000), Wisniewski et al. (2001), etc. Often many railway engineers and researchers reportthat the electrical connectors to the breaking unit can cause interference with the different onboard radio basedservices, balises, etc. as shown in Fig. 5.

3.2.4. Interference with the leaky coaxial cable

The radiating leaky coaxial cable uses magnetic coupling between the wayside subsystem and the vehicle fortransfer of information. Although there are standards and tests as mentioned in UiC (2000) on the loop trans-


mission module (LTM) receiver, onboard antenna and the coupling factor between them to check the inter-ference levels, there are possibilities of interferences as discussed by Nakamura et al. (1996) and Pozzobonet al. (2003). This is particularly when the field strength becomes too low or there are some obstacles inbetween etc. Also, there are possibilities of interference with vehicles other than the intendant one.

3.2.5. Interference with the electromagnetic braking

In electromagnetics brakes, the breaking force comes from the electromagnetic attraction between the trackand the brake plate. Generally a high current is applied to the coils of the strong electromagnets within thebrake shoes, although in some cases permanent magnet is used. Instead of being economic and having severaladvantages like, regenerative braking, independent of the coefficient of adhesion between the wheel and therail, the main problem with electrical breaking was its less reliability because of the complex circuitry andstrong interference problems with some electronic equipments located adjacent to the rail due to its strongmagnetic field and ferromagnetic material as discussed by Kroger et al. (1999) and Tolksdorf (1974). Today,many high speed trains like ICE train in Germany, Adtranz X2000 on Sweden, some trains operating in hillareas, etc. are fitted with electromagnetic track brakes to be used for emergency purposes only. The strongmagnetic field causes interference problems with the electromagnetic axle counter and other similar sen-sors/devices. Also, when applied and hence lowered down to the rail, these brakes can mislead the axle coun-ter, which may count it as wheel. This will stop the train automatically without any reason as discussed byKroger et al. (1999). The heavy current required for the braking operation and the transient caused by itsswitching can saturate the rails and can create interference problems.

3.3. Interference problems due to pantograph arcing

In the sliding contact between the contact wire and the pantograph, arcing is a common phenomena. Thisbecomes more predominant with higher speed as observed with the test run of V150 TGV on 3rd April, 2007in France as well as in many other high speed trains and in winter as observed in many cold countries. Thisarcing generate radiating electromagnetic waves in a wide band. Bartlett et al. (1999) reportedly measuredEMI at UHF band at a distance of 0.12 km away from the train line. The major problems from the panto-graph arcing can affect the railways in two ways, one, in conducted EMI to the traction power and signalingsystem and the other, radiated EMI to the wireless communication system. This radiation can cause interfer-ence with both the railway systems and non-railway systems in the near vicinity like, TV reception, radio com-munication, etc. as mentioned by Bartlett et al. (1999). This type of arcing in a sliding contact distorts the idealsinusoidal waveform of the power supply and creates assymetry and hence a difference in the voltage dropbetween the positive and negative half cycle of the AC waveform. This voltage drop is higher in the negative

Fig. 6. Propagation of DC current into the railway power feeding network with booster transformer (adopted from Kiessling et al., 2001).


half cycle compared to the positive half cycle of the power supply as reported by Bormann (2003), Bormannet al. (2007), Buhrkall (2005), etc. which results in a net DC EMF with the pantograph as positive electrodeand the catenary as negative electrode as shown in Fig. 7. This kind of assymetry also generates even harmon-ics, interharmonics and subharmonics as described in IEEE Industry Applications Society (1981, 1992) stan-dards and by many other researchers. This DC component and the harmonics propagate in the entire railwaysystem including rail tracks, track circuits, other locomotives in the same track, substation supply transformer,vehicle transformer, etc. as shown in Fig. 6 and create false signaling as shown in Fig. 3, saturation of thetransformers, corrosion problems, etc. Also, Buhrkall (2003b) reported that in some new models of trains like,OTU EMU, etc., the DC supervision system gets tripped, which results in delay and sometimes cancellation oftrain operation. The severity of this phenomenon depends on several parameters and in general, is predomi-nant in winter.

Brillante et al. (1998) reported that in the presence of multiple pantographs (‘0 series’ Shinkansen trainshave 16, 12 and 6 pantographs parallel and the last few show clearly visible arcing because of the induced oscil-lation due to applied forces by the preceding pantographs), and some heavy freight trains with multiple trac-tion engines, the situation is bit complex. Buhrkall (2005) mentioned that the following pantographs provide areturn path to the DC component and harmonics and the magnitude of the DC generated from the arc in thefollowing pantographs are lesser compared to the preceding ones as shown in Fig. 9. The magnitude of the DCvoltage generated on the other pantographs depends on some other parameters like, number of following pan-tographs, the electrical load and some other circumstances like, removal of ice by the preceding pantographs,etc.

The arc mechanisms of this type of sliding contact, both in the absence and presence of the ice and the phys-ics behind this DC component generation is not well understood so far. In this subsection, a brief of the pre-vious investigations, physics of sliding contact and the need for further investigations with the changingscenario due to ERTMS are discussed.

3.3.1. Previous work

Some experimental investigations and numerical simulations on the pantograph arcing were done by Kla-pas et al. (1976), Buhrkall (2005), Galdi et al. (1998), Brillante et al. (1998), Tellini et al. (2001a,b) and Gian-netti et al. (2001). Their investigations gave some characteristic information about the arcing, the variation ingap between the contact wire and pantograph, variation in the duration of attachments and detachments,interference level, etc. Because of the complexity of the problem and limitation of experimental facilities, mostof these experimental works were limited to few hundred kHz to few MHz frequency band and they madesome simplified assumptions to make the laboratory investigation and computational analysis possible at uni-versity and research laboratory levels. In the contexts of ERTMS and modern railways, some of these assump-tions and simplifications like, lower train speed by Klapas et al. (1976), lower frequency measurements byBrillante et al. (1998), only static attachment and detachment of the pantograph and contact wire insteadof sliding contact by Tellini et al. (2001a,b), nature of the arc, which in reality is different than the test setup

Arc generated DC current flow in the entire circuit

Arcing at the pantograph

Fig. 7. The net DC component generated by the pantograph arcing and its propagation into the traction system (adopted from Buhrkall,2005).

Fig. 8. Burning arcs attached to the rotating wheel (source: Bormann, 2003).


by Tellini et al. (2001a,b) and Klapas et al. (1976), etc., demands more realistic and detailed investigations toget a more clear picture of the interference levels at operational railway sites.

3.3.2. Physics of sliding contact

In a sliding contact, usually the arc tends to attach itself to a particular point, called cathode spot or anodespot depending on the polarity, on the contact wire and then as the current collector moves, the arc gets elon-gated and finally, it is extinguished as shown in Fig. 8a and b. It attaches itself to a new spot again and con-tinues the same way. Usually, when the speed is low, the arc may continue for several half cycles of the supplyvoltage before getting quenched, i.e., before current becomes zero. When the speed is high, the arc becomesquenched near the zero crossing of the applied voltage as observed by Bormann (2003) and Bormann et al.(2007). If the speed is very high, the arc will be quenched well before the zero crossing and then it may reignitesagain within the same half cycle depending on the gap length, voltage at that instant and train speed. Themechanical oscillation and the AC current waveform interacts in such a way that there is a very rapid tran-sition from a highly conducting arc to a very low or almost a zero current. This superimposed nature of thetwo waveforms makes the situation worse from the EMI point of view by generating very sharp switchingpulses.

3.3.3. The situation in winter

Although arcing from pantograph occurs in all the seasons because of the particular nature of the slidingcontact and vertical oscillation of the train, it becomes more critical and visible to the bare eyes in winter, espe-cially below the freezing point. When temperature is above zero, usually there is a thin film of water whichmakes a sliding contact smoother. This phenomena was investigated and reported by many scientists likeHolm (1946), Shobert (1976, 1993), Slade (1999), etc. In winter this water film gets frozen and hence thesmoothness and self lubrication of the sliding contact is gone. Moreover, the ice layer on the contact wire actsas a dielectric layer. These two phenomena work together to make the sliding contact far away from beingsmooth and we see the clearly visible arc moves with the pantograph along the contact wire. For high speedtrains, which generates visible arcing in normal weather, the situation becomes far worse in winter and thisdegrades the life and performance of the contact wire and the pantograph.

3.3.4. Investigations by OHL Ice Working Group

As a part of a European Union project, an Over Head Line Ice Team (OHL Ice Group) was formed, con-sisting both academicians and industrial experts to investigate this arcing phenomena and its consequences.The group conducted three field tests on four different types of trains at different test tracks with different sup-ply in different environmental conditions. On December 2001, they conducted experiments at Luxembourgwith BR185 locomotives and Z2000 EMU with 50 Hz supply at below freezing point and on March 2002,

Net DC current flow in the entire circuit

following unit

DC current flow from preceeding unit to the

Fig. 9. Flow of the DC current in the presence of multiple pantographs. Adopted from Bormann (2003) and Buhrkall (2005).


at Vasteras, Sweden with OTU EMU with both 16 23

and 50 Hz supply in artificial iced condition and at Cerhe-nice, Czech Republic, with Class 357 EMU in an ice simulated environment with 50 Hz supply. As reported byBuhrkall (2002), they observed that the DC component is independent of the RMS value of the fundamentalcomponent and the vehicle speed. They also measured the interference levels at both 16 2

3Hz supply with

100 Hz track circuit feeding and 50 Hz supply with 77 Hz and 83 13

Hz track circuit feedings to investigateon the possible effects. It was found that the level of the line current harmonics are proportional to the levelof the DC. Buhrkall (2003b, 2005) reported that they also varied the load angle and tested both AC–AC pow-ered vehicles having four quadrant converters with AC motors and AC–DC vehicles having phase angle con-trolled thyristor rectifier with DC motor. They concluded that although both types generate almost same levelof DC EMF, the first type is advantageous since it runs at a slightly lagging phase angle whereas the secondtype runs at a lagging phase angle by nature. They also conducted some experimental investigations by run-ning two different types of trains forming a multiple unit, i.e., two BR185 locomotives with AC motors, con-nected back to back and one Z2000 EMU with DC motors back to back at 25 kV 50 Hz supply. It was noticedthat the net DC emitting from two vehicles in a multiple formation is lower than a single unit. This is probablybecause the first pantograph removes the ice layer to certain extent and the following one acts as a return pathto the previous one as described in the Fig. 9 since the generated DC EMF is lower in the following panto-graphs. They also noticed that the returning of the DC current via the second locomotive was around 50%compared to the first one.

The OHL Ice group conducted some laboratory experiments at Vasteras ABB facility to investigate furtheron this matter. Fig. 11 shows a simplified diagram of the experimental setup. They reported a possible DCvoltage of upto 106 V depending on the voltage, the current, the distance between the pantograph and the con-ductor, the train speed (simulated experimentally by rotating the wheel) and the phase angle. Bormann (2003)

Copper conductor

Carbon collector

Conducting plasma

Conducting plasma

Burning plasma

V

V

V

V

VV

sinusoidal voltage input

Fig. 10. The origin of the resultant DC current in the pantograph arcing because of differences in potential drops in both the polarities.

Fig. 11. Experimental setup of the ICE team (source: Bormann, 2003; Buhrkall, 2003a).


and Bormann et al. (2007) discussed about the physics behind the origin of the DC component and discussedabout the difference in material properties to explain the difference in the voltage drop between the positiveand negative half cycle. Voltage drop in the plasma of the arc, as shown in Fig. 10 is higher when the panto-graph side is positive and lower when the contact wire is positive. Thus a net DC component of the order of5–18 V gets generated with static electrodes of different materials like that in the present case with the panto-graph as positive as shown in Fig. 10. This leads to a current flowing from the contact wire to the pantograph(Buhrkall, 2003b; Buhrkall, 2005; Bormann et al., 2007). The arc resistance and corresponding voltage dropdepends on the electrode material, arc length, arc radius and the current. This phenomena requires furtherinvestigation to estimate the level of voltages with various test conditions and influence of various parameterslike electrode material, voltage, current, arc length, arc radius and the wheel speed. Their investigations withthis experimental setup match with the results obtained during the site tests. Bormann et al. (2007) reportedthat it takes certain voltage to ignite an arc in a new spot. The ignition voltage of the arc depends on the elec-trode materials and the voltage and its polarity at that instant. The attachment and detachment process of thearc leaves burn marks on both the contact wire and the carbon and its aluminum frame as reported by Bor-mann et al. (2007) and this deteriorates the life and performance of the contact wire as well as the pantograph.They concluded that in general, the DC EMF increases with:

� increase in speed of the rotating wheel,� increase in voltage and current,� increasing gap between the contact wire and the pantograph.

When the speed is low, the DC EMF decreases with increasing phase angle, but when the speed is high, itincreases with increasing phase angle (Bormann, 2003; Buhrkall, 2003b, 2005).

3.3.5. Radiated interference from pantograph

As discussed in Section 2.2, ERTMS is very much dependent on the wireless communication and uses sev-eral frequency bands ranging from few hundreds of kHz to few GHz. Fig. 5 shows all the radiating sourceswhich include the arc itself, the contact wire, the pantograph with its aluminum support, the connecting wire


from the pantograph to the traction drives, etc. Because of the complex geometry and position of the radiatingstructures as shown in Fig. 5, its often very difficult to find the most suitable location for the vehicle antennasand different wireless based sensors and services. In 1999, an investigation was conducted mainly from Ban-verket (The Swedish Rail Administration) at Kiruna (north of Sweden) on the effects of pantograph arcing inthe presence of ice with the railway radio communication services. Significant level of EMI in the range of60 MHz–1 GHz were measured with a train speed of 100 kmph. The measurement antennas were placed atthe front inside the second vestibule, having a glass front window. But the measured data was bit noisyand influenced by test condition.

3.4. Interference from power electronics and drives systems

DC motors and the low frequency (e.g., 16 2/3) AC motors dominated the traction industry for decadesbecause of its over load capability and load–torque characteristics and are still being used widely as discussedby Hill (1994a,b), Steimel (1996a,b) and Stemmler (1993). Since the second half of the last century develop-ments in rugged semiconductor devices, high performance AC machine control algorithms along with highspeed digital processors to implement these and developments in material science leading to the developmentof low cost neodymium–iron (Nd–Fe) permanent magnets are having a significant impact on the tractiondrives system. AC adjustable-frequency drives started to dominate traction system worldwide, both for urbanlight rails and intercity heavy rails. France has pioneered in the use of high voltage power frequency supply forthe traction system in 1950s. Because of numerous advantages this is quickly adopted in many other countries.Another very important advantage of AC drives over DC is that for similar power and torque, AC motors arecompact with lesser weight and physical dimensions. Although synchronous machines were selected for TGVin at least two versions in 1993, squirrel cage induction motors came up in the dominant position for new gen-eration of traction vehicles.

Once the thyristors came into the market in 60s followed by line commutated controlled rectifiers, tapchangers in the AC supplied vehicles were replaced and collector motors with DC supply ruled the tractionindustry even with its drawbacks like, poor power factor, limited performance, high losses and maintenance.Turn on thyristors, with forced commutation were used to built step down choppers for controlling the col-lector motors with controllable pulsed DC voltage. But this technique was used only for lighter vehicles due toits limitations. Forced commutated inverters were used for feeding three phase induction motors with variablevoltage and frequency, i.e., V

f control. Once gate turn off thyristors (GTO) came into market in 80s, the expen-sive, voluminous and complicated turn on thyristors are gone and inverter fed three phase induction motorsstarted becoming a preferred choice. However, GTO traction inverters have their limitations in terms ofswitching frequencies. For PWM switching frequency, its limited to 500 Hz or less and require complicatedgate circuitry which reduces its reliability and increases the switching losses. Developments of high voltageIGBTs like, 3.3 kV–1.5 kA and 6.5 kV–0.6 kA, with higher switching frequencies in the range of 1500 Hz, sim-pler gate drive and higher efficiency made it preferred for newer drive units. Although DC drives will continueto evolve in some sectors, three phase voltage source inverters with six switch topology for variable speedinduction motor drive applications is becoming a preferred choice. This brings in many issues to be addressedproperly, like input power quality, effects of transients and fast switching and related interference issues, etc.

Modern IGBTs evolved to such an extent that it requires a minimum of snubber circuit to limit the high didt

and dVdt , or in some drives applications even without any snubber circuit at all. But the output of these snub-

berless inverters may contain switching rates more than 1000 A/ls and 10,000 V/ls, which may result in unde-sired and sometimes harmful consequences. High dV

dt switching rates can interact with the inverter outputcables and the machine windings because of large transient voltages across the outermost turns of the statorwindings, which can cause catastrophic failures of the stator winding insulation as investigated by manyresearchers like Kerkman et al. (1997), von Jouanne and Enjeti (1997), Aoki et al. (1999), Ogasawara andAkagi (1995), Jahns and Blasko (2001), etc. Longer cables make this phenomena frequent and hence somemachine manufacturers provide better insulation levels, at least for the outermost turns of the winding. HighdVdt switching rates can generate unbalanced charge build up in the parasitic capacitance coupling between themachine stator, rotor, inverter switches and the ground. These accumulated charges can discharge through thebearing in the absence of any other discharge path and can eventually cause bearing failure by creating serious


spitting of the bearing balls as experienced and reported by Jahns and Blasko (2001), Erdman et al. (1996),Busse et al. (1995), Bhattacharya et al. (1999), Link (1999) and Macdonald and Gray (1999). Manufacturerscame up with solutions like insulated bearings and grounded rotor shaft bearing, etc. Another problem withthe high di

dt and dVdt is that it increases the level of conducted EMI in the drive input lines and the ground. There

are some preventive measures that can significantly reduce this common mode EMI, like, adding commonmode inductors in both input and output lines together with small capacitors between each DC link busand the grounding which will prevent the high frequency currents from reaching the utility grid or machines,improved PWM switching algorithm, etc. Differential mode EMI can be mitigated by using active or passivefilters on the inverter output lines as reported by Jahns and Blasko (2001). But, this additional filer is alwaysbulky because of the high power rating and sometimes not a feasible solution. Most of the industrial drives(>90%), including the traction drives systems do not return the power back to the utilities till date, and thepower dissipation path often creates noises with the line side equipments, balises, etc. as shown in Fig. 5. Inter-estingly, Joos et al. (1998) showed that AC propulsion produces more interferences than DC propulsionbecause of the requirements of variable frequency operation of AC traction system.

Onboard power electronics, i.e., power converters/inverters are one among the major contributors of theinterference in a wide frequency band. A major drawback of the previous analysis of the interference causedby the power electronics and drives was that it used lumped circuit modeling, which does not demonstrateresults beyond few MHz. Whereas, Chen (2000) and Lombardi (1994) investigated and found that measure-ments and different types of modeling show interferences at higher frequencies. To avoid the complexity of theproblem, which is actually a very vast one, some researchers like Chen (2000) divided the entire system intosmaller subsystems and then analyzed the interference, which gives a much more clear picture of the actualsources causing interference. Wisniewski et al. (2001) reported that in Sweden, SEMKO tested six standardinverters with five of them had higher EMI radiation than the permissible limit in the European market.Between 1997 and 2000 they performed several tests on different commercially available variable speed drivesand measured considerably high radiation in 30–300 MHz by using a biconical antenna and from 200 MHz to1 GHz by using a log-periodic antenna on the actual installation site. Chen (2000) reported that at frequencyranges of few hundreds of MHz, the power stage of the inverter contributes more to the noise. Based on theelectrical layout of the system, these HF current components get coupled to different systems and subsystems.It is observed that IGBT imposes significant capacitive loading beyond few MHz to the signal source. Severalresearchers like Chen (2000) reported that the noise gets distributed through several independent paths like,through cables to the traction motor, through the motor winding’s stray capacitance to the chassis of the train,through the stray capacitance of the switching devices with the chassis and in many other possible ways.

Wisniewski et al. (2001) concluded that the level of the interference is high enough to cause bit-error rate ofthe digital communication systems. This shows that there is a potential risk to affect a wide variety of radioservices including digital communication services, mobile phone service, navigation system, commercialbroadcast, radio and paging services. The level of interference on the victim receiver depends on severalparameters like, the frequency of interest, distance from the drive system, cable length, type of cable beingused, configuration of the site and many other parameters.

4. Conclusion

Railway signaling and communication is quite sensitive to both conducted and radiated electromagneticinterference from various sources and have potential risks of false signaling. Old believes like DC or even har-monics cannot be present in an AC fed railway are proved to be wrong and hence, the track circuit feeding hasto be designed properly. Also, these interference problems deteriorate the health of different components andreduces the estimated life time and reliable operation of the railway. Both theoretical investigations and exper-imental observations found that all wireless and radio based applications related to railways are quite vulner-able to these electromagnetic noises generated from various sources. ERTMS is very much dependent on theradio based services for several applications and proper caution has to be taken in various forms to protectthese services from possible EMI. Thus, it is very important to investigate the level of interference and possiblecoupling mechanisms from all these sources to the radio based services to make the ERTMS and other modernrailway system more robust and reliable. Proper experimental and theoretical analysis of the noise sources will


give a precise knowledge of the EMI levels that various radio based services will be facing. Also the relatedstandards should be modified to consider the higher level of interference, which was not properly observedbecause of lack of proper experimental and simulation facilities. Once the ERTMS will be at level 2 and level3, the higher speed and lower gap between the trains will demand more reliable and robust signaling and com-munication without any potential threat from electromagnetic interference. Also, the consequences of inten-tionally radiated EMI has to be investigated properly to protect the ERTMS system from possible threats.

Acknowledgements

The authors would like to thank Banverket (The Swedish Rail Administration) and Bombardier Transpor-tation for providing funding for this work, Dr. Nelson Theethayi of Uppsala University and Mr. Stuart Shir-ran of Bombardier Transportation for their valuable feedback and Dr. Dierk Bormann of ABB CorporateResearch and Dr. Thorsten Schutte of Rejlers AB. for the valuable discussions and feedback.

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