Electric railway traction Part 7 Electromagnetic interference in traction systems

This last article describes some classic problems of interference in electric railway traction systems together with mitigation and modelling techniques.

by R. J. Hill

Background Armed with a knowledge of traction noise source characteristics and equipment susceptibility, it remains to identify how electromagnetic int,?rference (EMI) appears in traction systems. Noise can appear in receivers by current, voltage, electric or magnetic field coupling, the principle coupling mechanisms being

0 conductive (galvanic): when two circuits share a commori conductor or impedance, a ne't disturbing voltage, equal to that gei7erated across the common part of the circuit by the source, appears in the receiver as noise

0 inductive: changes in magnetic flux cutting an electric circuit will generate an induced Ldildtvoltage, causing a noise current to flow with magnitude depending on the circuit and source impedances; the1 disturbing magnetic flux can arise from a number of sources either directly or via parallel earth-return or isolated circuits capacitive, which can be either static, where components or systems at high voltage form a Fiotential divider in conjunction with capacitance to earth and other nearby metallic structures, or dynamic, where rapid changes of voltage on circuit components, such as charging and discharging of capacitors and switching of semiconductors, may produce CdVldt currents flowing to earth

0 radiative, from very high frequency effects including pantograph-catenary arcing and fast current or voltage switching in converter electrical components.

Electromagnetic compatibility (EMC) assurance is the prclcess which requires apparatus to functbn satisfactorily under the

presence of disturbances. Today, a good understanding of traction system EM1 is available, and the correct operation of safety railway signalling and non-vital communications systems can be assured in the presence of interference generated by harmonics from power-electronic controlled traction drives and substations. Examples of mitigation measures include:

0 frequency separation of traction power and track signalling circuits

0 reduction of AC induction between catenary and open-wire telegraph lines caused by catenary, rail and earth currents by appropriate cable screening, termination and positioning

0 suppression of arcs drawn by traction power current collection systems which cause interference to broadcasting receivers.

This article describes some practical noise problems in traction systems, with emissions spanning the complete frequency spectrum. The production of low-frequency (LF) noise from conductive and inductive coupling of currents and fields flowing in the traction system is described in terms of simple circuit and field models. Radio frequency (RF) noise is usually assessed by practical measurement, for which the current state of the art is reviewed. The article is concluded by a summary of the effect of European Community EM1 limits.

Practical noise problems and mitigation Appropriate remedial measures against

EM1 can be devised if there is an understanding of traction noise source characteristics and equipment susceptibility, together with the physical phenomena involved. This section, with the aid of simple

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1 Earth currents in DC traction systems: transmission-line model with rail voltage, rail current and accumulated ground current profiles for electrically floating substation

models, reviews some classic EMC difficulties in traction systems with typical mitigation procedures.

Earth currents in DC railways DC propulsion current returns to the

substation along the running rails. Because the track is laid on the ground surface, has finite series resistance and is not perfectly insulated from the ground, part of the propulsion current also flows through the ground. Leakage current leaving the rails in the vicinity of the train and re-entering near the substation can

e cause electrochemical corrosion damage to metallic structures such as building reinforcements, pipelines and cables

e create high rail-to-earth voltages forming possible safety hazard step and touch potentials

e increase the energy losses in the power transmission path

0 make fault currents difficult to detect and clear.

Some effects adversely influence each other: for example, improving the rail-ground insulation will lower stray earth currents but increase the rail-earth voltage difference.

A subset of the track multiconductor transmission line (MTL) may be used to model rail-earth voltages and leakage

currents. Consider the rail-earth system as a leaky transmission line with series resistance and leakage conductance with respect to remote ground. The distribution of rail voltage and current may be calculated from the transmission line equations

where y = !(RG) is the propagation constant, R, = d(R/G) is the characteristic resistance and the constantSA and 5 are found from boundary conditions. A measure of the corrosive effect may be obtained by integrating the accumulated leakage current per unit length along the conductor for typical train service profiles. Fig. 1 illustrates the form of the curves for rail potentials, rail current and accumulated leakage current for a uniform line, constant ground conductivity and a single traction vehicle.

The rail-to-remote-earth resistance and grounding philosophy are important when considering the effects of catenary- raiI/ground faults. These may be particularly hazardous, and special protective measures are necessary t o earth the DC traction negative if its voltage with respect to ground under fault conditions exceeds a predetermined value. Several solutions have been proposed to the problem of fault protection involving the use of power semiconductors.

Inductive interference in telecommunication circuits

Historically, mutual magnetic coupling between parallel electric traction, power and telecommunication lines has been an important problem which first became apparent in the context of interference to telephone speech circuits. Coupling arises because of the close proximity and mutual impedances between sets of conductors (Fig. 2); the presence of the conducting earth provides a common reference point, enabling individual earth-return circuits to be defined. There are two principle induction effects: each cable experiences an induced longitudinal EMF which can produce safety hazard voltages, while asymmetries between the cables in a terminated pair, from unbalance in their earth impedances or physical positioning, results in a transverse EMF which can produce significant audio frequency noise in telephone circuits and low-level data transmission and power control cable systems. CCllT guidelines stipulate maximum noise levels for both conditions (Table 1 ) .

telecommunication line at the traction power frequency may be calculated from vector addition of the separate effects arising from magnetic flux around each of the conductors carrying traction current. Voltages can be

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The value of induced voltage in a

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Table 1 Permitted CClTT levels for induced voltage in electric railway lineside cables

ages continuous longitudinal voltage non-continuous (fault) longitudinal voltage

cabled adjacent public utility telephones cabled railway telephones open wire telephones

430 V f

hted

considerable: for example, if the mutual impedance between a cable and a catenary is 0.3 Wkm, a fault current of 1 kAwill produce a voltage over 10 krn of (0.3 x 1000 x I O ) = 3 kV. The ratio of transverse to longitudinal induced voltage is used as a measure of screening effectiveness. It may be estimated using Carson's formulae: for example, in Fig. 3, assuming the grcund current flows a t an equivalent depth of 330 m, with the cable pair a t 10 m and 10.1 m from the catenary, the ratio would be ;]bout 3 x 1 O-3.

In an AC electric *ailway, the outward path for traction current is the catenary, and the return path includes the rails and earth. The earth current a so contains an inductive component which is the sum of effects from the rails and catenary. The ground current loop, excited a t the traction power frequency but also 'carrying harmonics generated by the traction equipment, will inductively couple with parallel cable circuits and form an interference noise source. The presence of the earth loop is important - although the caten a ry-rai I separation can lead to significant i iduction alone, without precautions the earth current loop will dominate even thoiJgh it carries less current.

The size of the inljuction loop can be reduced by forcing the return current to flow through a feeder located physically close to the overhead contact wire, either by adding booster transformers (BTs) and a return conductor to the AC feed (a method used extensively in 16.7 tiz traction systems), or by autotransformer (AT) feeding with the rails connected a t the centre. In the BT system, the rail current is zero only a t the actual rail-feeder ccmnections, between which return current flows along the rails with a significant fraction entering the earth. With no trains on the section, the earth current is greatest ;It the BT location, so some coupling to open-wire and shielded cables still exists through the residual earth current loop.

Furthermore, the magnetising inductance is responsible for current unbalance between the return feeder arid contact wire, encouraging more (current to return through the earth.

In the AT system, the earth current is almost zero a t the AT locations and maximum a t the mid-points. However, magnetising current flows in the catenary- return circuit a t all limes, thus contributing to background induced noise in lineside circuits. Furthermore, the AT leakage reactance introduces a small voltage drop in the AT output which results in unbalance in

the transformer current, encouraging trains to draw power from adjacent ATs and increasing induced noise through the additional earth loop currents created.

Established immunisation procedures have been developed to reduce telecommunication circuit and receiver sensitivity by prescribing the use of twisted pair circuits, coaxial cables, isolating transformers, longitudinal inductors, balancing circuits, frequency selective working and screening. The principle of screening is to introduce another conductor into the circuit effectively earthed a t both ends. The induced current flowing along the screen opposes the excitation current, hence reducing the induced voltage on the target

7

2 Transmission-based signalling systems such as installed on the London Docklands Railway must be designed to eliminate interference from DC traction harmonics and parallel power lines

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3 Inductive coupling in parallel earth-return circuits and the principle of cable screening

4 Inductive coupling in parallel isolated circuits: the third-and-fourth rail power system

conductor. In the circuit shown in Fig. 3, the screening factor is

(3 )

whereZ,, and Z, are the earth-return, unit length self impedance of circuit i and mutual impedance between circuits i and;, respectively. Eqn. 3 may be used to estimate the EMF induced in lineside cables. For

telecommunications circuits, the screen and conductor I are physically close, so ZZs= Z,,, whereas for lineside signalling cables the screen is near conductor 2, so Z,, = ZI2. For concentric cables the sheath is the screen and L,, = MZ3. AC losses in the sheath/earth circuit are due to flux common to the screening and disturbed circuits, so the screening factor is approximately equal to Rdc/Zse, where R,, is the DC resistance of the sheath. It is prudent to choose materials with low Rdc, increasing Z,, by adding inductance to the screen-earth circuit, increasing the conductivity of the sheath material (from lead to aluminium) or using steel tapes wrapped around the sheath.

to real situations, the practical nature of the traction environment must be taken into account, since effective screening is dependent on a number of factors including the non-zero earth resistance of the screen, the effect of modern insulated cables, the imposition of earth rods (which must be installed with due regard to the exposure length) and the presence of rails (which also act as screens).

To apply the simple model described above

Inductive noise from traction power circuits Traction power circuits can exhibit

significant mutual inductive coupling with parallel, isolated circuits due to the long length of exposure. In third-and-fourth-rail power systems, such as London Underground, this is manifested as loop coupling between traction and signalling circuits; power supply and traction harmonics circulating in the power loop will couple with the track circuited running rail loop (Fig. 4).

knowledge of the disturbed circuit impedance and the mutual inductance between the two independent circuits. The voltage induced in a track circuit of length X,, due to a harmonic current I,, a t frequency o is

The induced current can be estimated from

The disturbing current may be found knowing the track circuit impedance. The mutual impedance is inductive at LF and may be estimated from the geometrical relationship

Mab.12 = & 1% d ld2b H/m (5) 2~ d,, dze

where d, is the distance of conductor i from conductorj. Practical measurements on London Underground in the frequency range 100 Hz to 2 kHz for track circuits of length up to 600 m reveal a mutual inductance of about 0.31 pH/m for the same track and 0.06 pH/m for parallel tracks.

Mutual coupling can also occur to track conductors from either track circuit or traction power currents. UIC-ORE has carried out tests t o define the coupling

262 POWER ENGINEERING JOURNAL DECEMBER 1997

between the various track cable patterns and the rails, and klas specified appropriate intervals for periodic cable transpositions to reduce EMI.

Coupling also exists with train carried equipment such as cab signalling antennae which receive signals from rail or track conductor currents Usually a pair of cab signalling receivers are series connected to reject common-mode interference. However, asymmetry of the two receivers with respect to the power rail pclsition inevitably gives rise to the reception of Tutually-coupled interference. It has Deen found that the best rejection of the power rail signal is about -1 2 dB.

Magnetic flux coupling from vehicle power systems

In multiple-unit trains, traction power equipment is generally mounted beneath the vehicles in close prcximity to the track. The propulsion equipment consists of current- carrying components such as iron and air- cored reactors for filters and commutation chokes, traction motors, braking resistors and cables. These components produce strong changing external fields and direct magnetic induction from these sources may induce noise in track-connected equipment as the vehicle passes.

Problems of this nature have been detected in low-power carrier-only AF track circuits with low-impedance receivers. Consider the 1ocalis.d receiver induction loop formed by a pair of wheelsets, the rails and a track circuit termination shown in Fig. 5. Induced noise from traction equipment may be estimated from Ilux measurements by integrating the total flux change within the loop as the vehicle passes. The noise in the

receiver may then be found from the receiver loop equivalent circuit impedance.

Conducted interference in track circuits

circuits is illustrated in Fig. 6. Propulsion current flows along the power rail and returns to the substation through the parallel combination of the running rails, and track circuit current flows around a loop comprising the running rails. In most cases, the traction return current does not divide equally between the running rails - the DC value differs due to impedance variations from asymmetries such as track curvature, and, in a third-rail power system, the AC components differ because of different mutual impedances between the power rail and each running rail. A net disturbing voltage, equal to that generated across the common part of the circuit by the traction current, appears in the track circuit as a noise

5 Direct inductive coupling from vehicle- mounted stray magnetic fields

The problem of conducted noise in track

6 Conductive coupling in third-rail power systems and track circuit coupling model

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~

source. The problem is exacerbated in signalling systems where one rail is continuous for the traction return current t o flow, with the other jointed for track ’ signalling ~ here the entire traction return current appears as the disturbing voltage.

A simple model to estimate the effects of conductive coupling may be devised analytically using a circuit based on the representation of Fig. 6. The purpose of the model is to estimate the current in the track circuit receiver a t the operating frequency due t o unbalanced audio frequency harmonics from the power rail flowing in the receiver.

The traction circuit feeding impedance and the unbalance in the running rails due to asymmetries in the mutual coupling with the power rail may also be estimated using the model. Since the track MTL impedances and admittances and the terminating bond impedances are known, it is a simple matter to solve the circuit for worst-case track parameter values, either analytically with approximations or using electric circuit simulation.

It is important to have a sensitivity for the magnitude of the phenomena involved. The physical geometry of third-rail traction power systems may be used to estimate critical parameters. For example, for a typical system such as in Fig. 6 with distances d,, = 673 mm, d , , = 1500 mm and d,, = 21 70 mm, and an effective rail radius of 50 mm, the traction circuit feeding inductance becomes 0.9 pH/m, which compares well with the measured value of 1.25 ,uH/m.

harmonics between the running rails, 7 Static and dynamic Unbalance in the traction return current electric coupling

occurring because of the difference in mutual coupling between each running rail and the power rail, is expressed through the ratio of LF harmonics as

assuming the track circuit terminations have low impedance compared with the track series impedance, so that the running rail longitudinal voltages along the traction return circuit will be identical.

is about 30% for power frequency track circuits, which has been confirmed by reported measurements at 46% for the Singapore MRT.

Interference monitoring units In normal operation, converter-fed

traction drives are designed t o limit harmonic emissions t o values calculated with reference t o the known impedance and sensitivity of signalling equipment. However, malfunction of the inverter equipment (such as failure of a gate drive unit causing a GTO to remain off, or input/output failure causing a delayed switch) can produce noise with unpredictable and potentially hazardous characteristics. Railway operators may therefore require rolling stock t o be fitted wi th interference current monitoring units to guarantee the emission levels at critical signalling frequencies; if the levels are too great (usually of the order of 1 A), the inverter firing circuits will be disabled.

The numerical value with the simple model

Electric coupling Electric coupling in traction systems can

arise from static or dynamic effects (Fig. 7). Static coupling occurs when components of the traction system at high voltage, such as the overhead catenary in AC railways, produce electric fields which induce charge on nearby conductors. If there is close proximity to metallic structures such as lineside fencing, bridges and building reinforcements, the induced voltage may be very large, particularly if foundations are not adequately earthed. Because the susceptance is normally low, static capacitive coupling is rarely significant as a disturbing noise source. However, it is necessary t o ensure that touch voltages do not exceed safety levels, and a rigorous earthing philosophy is necessary for all poorly earthed fences and other structures near the railway.

significant in isolated third-and-fourth-rail electrified DC systems where the power rails are well insulated, but the equipment case and vehicle body are earthed through the running rails. Repetitive charging and discharging currents will f low in the earth and can appear as transient EM1 at multiples of the converter modulation frequency. This type of interference has been detected during regenerative braking. Practical

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Dynamic electric coupling may be

2 64

measurements a t frequencies between 8 and 20 kHz have shown that transient rail current values can reach 10 A.

kvaluati'ng condu'ti\'e and indu'tive effects in signalling and COPlmUnicationS systems. Development of the:,e models using

Ra dia tive coupling RF coupling from 5,ources in traction

systems can affect equipment far from the railway, including broadcast receivers and sensitive instrument! in laboratories and hospitals, whereas LF fields can interfere with computer equipment operated close to the railway. The moving nature of the sources, the structurli of train bodies, the presence of more than one train, the traction duty cycle arid the arrangements for return of traction current can all affect emissions and the nciise received and make screening d ifficu It.

The question of RFI is a t present receiving attention because of the necessity to limit emissions according to international regulations. Accurate methods of predicting RF noise are not available, and com prehensive measurement programmes have been undertaken on several European railways, and in the LISA and Japan. The difficulty of site meawrements due to the presence of reflectincj objects has led to the development of a set of standard practical test procedures, witt- interpretation of the results as cumulative amplitude-time curves. Measurements are made with an interference meter or spectrum analyser according to the CISPR 16 standard, with loop, biconal or log periodic antenna calibrated between 10 kHz to 1 GHz, and with frequency bands giving + 5 dB accuracy. Antwnae are mounted 10 m from the track 'centreline a t a specified height. Below 5 MHz,, measurements are of the near field where the magnetic field component dominates. Here, antennae can be either loop, or hoizontally or vertically polarised dipoles.

Measurements of ow-frequency electric and magnetic fields have also been reported, driven by the need to respect biological EM radiation limits. Eleclric fields are measured with a single-axis disdacement current type sensor operated remotely. Maximum values have been detected in traction systems a t about 4.3 kV/m on the track centreline, with 3 kV/m in public areas. Magnetic fields are measured by a three.-axis magnetic field sensor or fluxgate m,agnetometer performing vector summation. Maxima are about 450 A/m peak and 220 A/m average at 50 Hz, reducing to average values of about 130 N m a t 300 Hz and 90 A/m a t 600 Hz.

8 harmonic spectra of cab signalling signal with no noise, substation harmonics. and substation and traction drive harmonics

Simulation of EM1 in DCtraction system: simulation MTL model and

EMC assurance, modelling and simulation

Traction system EPAC assurance should involve practical site testing assisted by interpretation of the physical phenomena using theoretical modelling. Analytical models incorporatin13 simplifications have Droved Darticularlv useful in the Dast for

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9 Finite element computer simulation is one promising electromagnetic field research area. Fig. 8 shows a catenary plots for: (a) Catenary to electrified track MTL system model, with running rail impedance; inverter-fed locomotive noise source, rectifier (b) power rail to far running rail admittance substations and AF signalling receiver

equipment. Solution automatically integrates the effects of the different coupling mechanisms (in this case conductive coupling from the overhead DC catenary with inductive coupling from adjacent-track crosstalk), and the separate influences of the power supply and traction harmonics are visible. Simulation can also provide sensitivity analyses for critical conditions such as traction converter modulation, multi-train operation and traction line parametric variations.

electromagnetic field modelling, with the objective of predicting static and dynamic magnetic and electric field distributions for various excitation conditions. The example shown in Fig. 9 is of a two-dimensional low- frequency field model solved using finite- element techniques to obtain track equivalent parameters for circuit simulation data.

A further current research area concerns

International co-operation and standards

International co-operation on EMC has a

long history. For example, work was carried out in the 1970s on the effects of using thyristor-controlled traction in railway traction drives sponsored by the UIC-ORE. Moreover, the CCITT has for some time issued directives containing design information pertinent to the protection of telecommunication lines from electric railway traction systems. Recently, EC legislation has been enacted specifying general EM emission limits and this has led to the proposal for the various national railways t o agree a common European standard. The work has been mandated to CE N E LEC technical committee TCI IO, with TC9X involved in the standardisation of railway electrical systems. The outcome will be a set of EN 501 21 series standards which will propose limits for emissions around moving traction vehicles, taking into account existing measurements. CENELEC is also proposing standards for the measurement of electric and magnetic fields in terms of antennae, bandwidth and position with respect to the track.

Appropriate levels for electric and magnetic fields for biological limits have also been the subject of some debate. At present, I N I RC (I n tern'a t iona I 'Non - I o n izi n g Rad ia t i on Committee) guidelines suggest a maximum magnetic field gradient of 10 m N m 2 at 50 Hz. The normal electric and magnetic field limits are 10 kV/m and 400 A/m for occupational exposure and 5 kV/m and 80 N m for public exposure. At higher frequencies, an equivalent plane wave power limit applies according to the equation:

P,, 6.94 1 o - ~ E2 + 20 H' (7)

the limits from which are calculated in terms of an allowable induced current in the body (in mA) including a point contact limit, a specific energy absorption rate (in W/kc$ and time-integrated power densities (in J/m ). For RF fields, the above formula gives frequency- dependent occupational and general limits, maximum magnetic field limits being 1630 A/m at 50 Hz, 543 A/m at 300 Hz and 272 A/m a t 600 Hz.

Further reading 1 CCITT Directives concerning the protection of

telecommunication lines against harmful effects from electric power and electrified railway lines (Geneva, 1989, 9 volumes). SUNDE, E. D.: 'Earth conduction effects in transmission systems' (Dover, New York, 1968) 90/336/EEC Council Directive of the 3rd May 1989 on the approximation of the laws of the Member States relating to electromagnetic compatibility, OfficialJournal o f the European Communities Legislation, 139, 23.1 5.89, pp.

2

3

9-26

OIEE: 1997

Dr. Hill is Reader in ElectricTraction Systems with the Department of Electronic & Electrical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK. He is an IEE Fellow.

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