High-Frequency Experimental Characterization of Impedance Bonds Used in Railway Systems

Eugenio Fedeli(1), Sergio A. Pignari(2) and Giordano Spadacini(2)

(1) RFI S.p.A., Istituto Sperimentale Rome, Italy, Email: e.fedeli@rfi.it

(2) Politecnico di Milano, Dipartimento di Elettrotecnica Milan, Italy, Email:{giordano.spadacini, sergio.pignari}@polimi.it

Abstract—The development of EMC models of complex railway systems requires accurate characterization of all the units integrated in the railway environment. In line with this aim, in this paper two methods are proposed for the characterization of impedance bonds (IBs) up to 100 MHz. The first method makes use of a signal generator and an oscilloscope for the measurement of open-circuit impedances, whereas the second foresees the measurement of scattering parameters via a Vector Network Analyzer. This experimental characterization of IBs proves to be useful for multiconductor-transmission-line (MTL) modeling of the railway infrastructure, aimed at analyzing its impact on the electromagnetic emissions radiated to the outside world.

Keywords-Impedance Bonds; Rail Track Circuits; EMC models

I. INTRODUCTION

The need for rolling stock interoperable over the countries of the European Union (where supply systems with different characteristics coexist), as well as the increasing use of high-powered electronic equipment, micro-controllers, and electronic devices, have implied more attention to EMC critical issues in railway systems. In this framework, mandatory requirements have been recently developed that should be fulfilled by every unit/system operating in the rail environment. In particular, experimental procedures and setups for the assessment of the electromagnetic emission radiated by the whole railway system to the outside world in the frequency range 9 kHz – 1 GHz are detailed in the European Standard EN 50121-2, [1].

Despite all the general aspects of the measurement procedure and limit values are clearly defined, measurements carried out according to EN 50121 do not allow for a detailed characterization of radiated emissions. Indeed, measurements foreseen by this standard do not permit separate identification of the contribution to the radiated electromagnetic field due to

rolling stock and contributions depending on the infrastructure in proximity of which measurements are performed [1]-[2]. This consideration represents the motivation for an ongoing research activity aimed at the theoretical analysis and experimental assessment of electromagnetic emissions radiated by a railway system, targeted to characterize and to distinguish the emissions related to the infrastructure from those directly due to rolling stock. In particular, the analysis includes a theoretical investigation on the radiation properties of the railway infrastructure, which is treated as a distributed-parameter system supporting both propagation and radiation phenomena [3]-[4]. Modeling includes all the wires of the supply system, the rails and ballast, as well as non-ideal behavior due to geometrical and electrical asymmetries of conductors, metallic structures located in close proximity with the tracks, etc. Aim of this work is to estimate the main properties of the distribution of noise-current along the conductors, and of the radiated electromagnetic field.

The EMC simulation of so complex systems requires an a priori characterization of all the involved units and devices. As a specific example, impedances of power coils, capacitors, and filters have to be properly determined in the frequency interval of interest for EMC. In line with this aim, in this paper two methods are proposed for the experimental characterization of impedance bonds (IBs), which are center-tapped coils largely employed in the return current circuit of 25 kV ac power supply systems [5]-[7].

The paper is organized as follows. In Section II, applications of IBs and their equivalent circuit are described. In Section III, a method for characterizing IBs in terms of open-circuit impedances up to 10 MHz is presented together with experimental results. Section IV reports measurements performed by using a Vector Network Analyzer (VNA). In Section V, a significant stretch of a 2 x 25 kV ac railway system is simulated and the impact of IBs on the distribution of noise currents along the line conductors is analyzed.

This work was supported in part by the Italian Ministry ofUniversity and Research under PRIN grant #2006095890.

978-1-4244-2737-6/08/$25.00 ©2008 IEEE

(a)

(b)

Figure 1. Connection of IBs to the rails: (a) double rail track circuit with insulated joints used in 3 kV dc railway systems; (b) double rail track circuit without insulated joints used in 25 kV ac railway systems.

Figure 2. Equivalent circuit of an IB at the operating frequencies

Figure 3. The ideal IB as a DM choke

II. THE DEVICE UNDER INVESTIGATION

A. Rail Track Circuits and IB Connections Track circuits are used in railway systems for the

purpose of detecting the presence of trains. The principle of operation exploits the electrical connection between the rails which is caused by the metallic wheels and axles of the rolling stock. Namely, track occupancy is detected when a train enters a section of railway and short-circuits the rails. Since the rails are also used as a conductor of the power supply system (i.e., for the return current), special center-tapped coils called IBs (also known as inter-rail reactors, or inductive connections) are used to achieve both the following aims: (a) maintain high impedance between rails, in order to prevent false-occupancy signals; (b) provide a continuous path for the return current.

The application of IBs in two different systems is exemplified in Fig. 1. In particular, Fig. 1(a) refers to the traditional double rail track circuit used in many countries (e.g., in the 3 kV dc Italian system). In this case, the track sections are separated by insulated rail joints. Pairs of IBs are used to allow the return current I to flow around insulated joints [see Fig. 1(a)], while preserving high impedance between rails at the operating frequency of the track circuit (e.g., at 50 Hz or 83.3 Hz in Italy).

Fig. 1(b) refers to the Italian 2x25 kVac, 50 Hz High Speed railway lines, implementing the new ERTMS Level II signalling system [6]. In this case, sections of track circuits are tuned at different audio frequencies (1.9-17 kHz) and do not require insulated joints between them. IBs are placed every 1500 m and the center tap is connected to a buried earth conductor. Accordingly, IBs provide high impedance between rails at the audio frequencies used for signalling, while allowing the 50 Hz train return current to leave the rails and flow in the earth conductor (which is a return conductor).

B. Equivalent Circuit at Operating Frequencies An equivalent circuit of IBs (valid at the operating

frequencies) is shown in Fig. 2. The half coils can be modeled as a symmetrical coupled inductor, characterized by the auto-inductance L and the mutual-inductance ML . The coupling factor k is close to the unity, i.e., LkLLM �� . Parasitic elements are drawn in Fig. 2 with gray lines. Namely, cR represent the resistance of coil windings, and the lumped capacitances

cC are intended to model the effect of inter-winding and winding-to-case capacitances. This IB equivalent circuit can be characterized via a two-port representation by assuming terminal 0 as reference node, and by defining port voltages 1V , 2V and currents 1I , 2I as shown in Fig. 2. In the ideal case (i.e., if parasitic elements are negligible), the constitutive relationship can be written in the frequency domain as:

��

���

��

���

��

�

���

2

1

2

1

II

LLLL

jVV

M

M� (1)

As outlined in Fig. 3, an ideal IB acts as a differential mode (DM) choke. Indeed, a rail-to-rail DM current dI(i.e., the current of track circuits) flows through the equivalent DM inductance )(2 Md LLL �� . Hence, rails are connected by a high-impedance reactor. Conversely, the common-mode (CM) return current cI flows through a very low CM inductance 0�� Mc LLL .

Figure 5. IB Trafomec S.p.A., model RM-01K8-AN Figure 4. Experimental setup for the measurement of open-circuit impedances.

III. MEASUREMENT OF OPEN-CIRCUIT IMPEDANCES

A. Experimental Setup The two-port representation of an IB in terms of

open-circuit impedances can be written as:

��

���

��

���

��

�

���

2

1

2221

1211

2

1

II

ZZZZ

VV

(2)

It is worth noting that by virtue of reciprocity 2112 ZZ � .Additionally, as the IB is constitutionally symmetric, it is expected that 2211 ZZ � .

The experimental setup sketched in Fig. 4 is used for the measurement of open-circuit impedances. This setup relies on the use of (i) a signal generator to feed the circuit with a sine wave; (ii) an oscilloscope with high-impedance channels (labeled with CH in Fig. 4); (iii) a high-precision shunt resistor sR of known value [8]. For each frequency of interest, the experimental procedure is composed of the following steps: (a) setting the frequency f on the signal generator; (b) acquisition of oscilloscope traces CH1, CH2, and CHM=CH3-CH1 (mathematical difference between channels), at least over a period fT /1� . The above procedure can be easily automated by exploiting software tools for computer control of instruments.

Voltages )(1 tvCH and )(2 tvCH acquired by CH1 and CH2 in time domain are used to estimate phasor voltages

1V , and 2V , respectively. Similarly, the time-domain current sCHCH Rtvtvti /)]()([)( 131 � is used to estimate phasor 1I . Finally, impedances 1111 / IVZ � and

1221 / IVZ � (since 02 �I ) are evaluated. By interchanging the IB ports, the same setup can be exploited for the measurement of 22Z and 12Z .

B. Processing of Time-Domain Measurements Since traces acquired via the oscilloscope may be

affected by a large amount of noise, as well as they may be slightly distorted or biased due to the non-linearity of

the device under test, correct estimation of phasors represents a critical issue affecting the accuracy and effectiveness of the proposed method.

To overcome this difficulty, sampling theory for periodic signals can be exploited to evaluate the Fourier coefficients 1a and 1b of the fundamental harmonic component

)2sin()2cos()( 111 ftbftatx �� (3)

of a periodic band-limited signal )(txCH having period fT /1� (i.e., a channel trace). Namely, these

coefficients are evaluated from the samples of )(txCH as

� �C

n

kCCH

C

nkkTxn

aC

/2cos)(2 1

01 �

�

� (4)

� �C

n

kCCH

C

nkkTxn

bC

/2sin)(2 1

01 �

�

� (5)

where CC nTT /� is the sampling period, and Cn is an integer number. For correct sampling, max2nnC � is required, where maxn is the maximum order of the harmonic components of the band-limited signal )(txCH .Once 1a and 1b are known, the phasor associated with (3) can be expressed as 111 jbaX � .

C. Results The IB under test is a Trafomec RM-01K8-AN

( mH2.1�dL ) used in the Italian High Speed railway line (see picture in Fig. 5). The experimental setup described in Fig. 4 has been implemented by using an HP 3314A Function Generator, a Tektronix TDS 3034 Digital Oscilloscope equipped with high-frequency 10X compensated probes, and a shunt resistor ��10sR .

Open-circuit impedances have been measured in the frequency range 1 kHz – 10 MHz. Experimental results are reported in Fig. 6-9 (markers).

103

104

105

106

107

1081

10

100

1000

frequency, Hz

mag

nitu

de, �

|Z11|

|Z11|

|Z22|

|Z22|

Figure 6. Magnitude of Z11 and Z22: direct measurement (markers); derived from S-parameters (solid lines).

103

104

105

106

107

108-90

-60

-30

0

30

60

90

frequency, Hz

phas

e, d

eg

Z11Z11Z22Z11

Figure 7. Phase of Z11 and Z22: direct measurement (markers); derived from S-parameters (solid lines).

103

104

105

106

107

1081

10

100

1000

frequency, Hz

mag

nitu

de, �

|Z12|

|Z12|

|Z21|

|Z21|

Figure 8. Magnitude of Z12 and Z21: direct measurement (markers); derived from S-parameters (solid lines).

103

104

105

106

107

108-180

-120

-60

0

60

120

180

frequency, Hz

phas

e, d

egZ12Z12Z21Z21

Figure 9. Phase of Z12 and Z21: direct measurement (markers); derived from S-parameters (solid lines).

IV. MEASUREMENT OF SCATTERING PARAMETERS

Measurement of scattering (S) parameters is the preferred method for the characterization of networks operating at radio frequency. According to Fig. 10, the two-port characterization of an IB in terms of the S-parameter matrix S is expressed by

��

���

��

���

��

�

���

2

1

2221

1211

2

1

aa

SSSS

bb

(6)

where 1a , 2a are the incident power waves, and 1b ,

2b are the reflected power waves at port 1 and 2, respectively [10].

Direct measurement of the S-parameters can be readily performed via a VNA. However, interfacing the input ports of such an electronic instrument (usually employing coaxial connectors such as Type N or SMA) with the terminals of power devices (such as the IBs under investigation) is not a trivial task [9]. In order to make this connection possible, a suitable interfacing device has been designed with a twofold aim: (a)

minimizing parasitic effects; (b) provide a controlled measurement setup up to 100 MHz. This structure is shown in Fig. 11 and exploits the metallic case of the IB for the connection of the ground to the IB center-tap.

Measurements were carried out in the frequency range 9 kHz – 100 MHz via a VNA Agilent E5071C. The open-circuit impedance matrix was obtained from the S-parameter matrix via the following relationship [10]:

-10 )()( S1S1Z �� R (7)

where �� 500R denotes the reference resistance used to define the S-parameters, 1 is the 22� unit matrix. Results are reported in Fig. 6-9 (solid lines) and are in very good agreement with those obtained in the previous section.

As expected, experimental results in Fig. 6-9 confirm the property of reciprocity (i.e., 2112 ZZ � ), and symmetry (i.e., 2211 ZZ � ) of the open-circuit impedances in the entire frequency range. According to the ideal element constraints in (1), in the operating audio-frequency range,

Figure 10. Two-port characterization of an IB in terms of scattering parameters.

Figure 11. Device for connecting the VNA ports (SMA coaxial connectors) to the IB terminals.

track #1

contact line #1feeder #1

z

aerial ground #1

feeder #2

ground wire #1

contact line #2aerial ground #2

track #2

ground wire #2 xtrack #1

contact line #1feeder #1

z

aerial ground #1

feeder #2

ground wire #1

contact line #2aerial ground #2

track #2

ground wire #2 x

Figure 12. Symplified representation of the cross-section of a 2 x 25 kV ac railway line. A total number of 12 wires, running above a lossy ground, is present. IBs are conneted every 1.5 km between the rails and the ground wire [see Fig 1(a)].

(i.e., for kHz20�f ): (a) the magnitude of all the open-circuit impedances increases linearly with frequency; (b) the phase of 11Z and 22Z tends to 90o; (c) the phase of

12Z and 21Z tends to -90 o.

Additionally, one can note that parasitic effects (e.g., the distributed inter-winding capacitances of the coil, the reduction of the magnetic permeability of the iron core, etc.) strongly influence the behavior of open-circuit impedances at higher frequencies.

V. IMPACT OF IBS ON NOISE CURRENT DISTRIBUTIONS IN A 2 X 25 KV AC RAILWAY SYSTEM

According to the considerations reported in Section I, the high-frequency characterization of power devices used in railway environments is needed to understand the impact of the electrical and geometrical features of the infrastructure on the radiated emissions. In particular, the impact of IBs on the distribution of noise currents (that is, the conducted disturbances along the railway line wires and the rails to which the radiation properties of the infrastructure are directly related) is investigated in this work.

In order to overcome the huge computational cost required by three-dimensional EM codes for the modeling of a significant stretch of a railroad (tens of kilometers), a computationally-efficient circuit model based on multiconductor-transmission-line (MTL) theory has been developed and validated in [4].

The aforementioned MTL approach is exploited here for modeling the 2 x 25 kV ac system used in Italian high-speed railway lines, whose cross-section is sketched in Fig. 12. Namely, twelve wires running above a lossy ground are present, i.e., two rails, a buried and an aerial ground wire, a contact line (catenary) and a feeder for each track. For the investigation of the infrastructure impact on radiated emissions [1], the train is represented as an ideal noise current source connected between the contact line and the rails. Electrical substations and autotransformers are modeled as lumped circuit along the MTL, whose parameters are inferred from the available literature for the low-frequency behavior, whereas models are refined by adding suitable parasitic capacitances to account for the high-frequency behavior [4]. IBs connect the rails and the ground wires every 1500 m [see Fig. 1(b)], and are modeled via the open-circuit impedance representation in (2).

Figs. 13-14 show an example of the noise current distribution related to a line section with length of 3000 m. The train is supposed to be located 1000 meters from the left end of the line. The plots show the ratio between the amplitude of the current along the railway conductors and the current injected in the contact line by the train, at 300 kHz. Results obtained by the simplified MTL model (blue solid line) are in good agreement with the outcome of the EM simulation (red solid line) performed via the commercial software tool FEKO [12] and used to validate the MTL model. As expected, the MTL model requires a much lower computational time with respect to the EM solver, and this confirms the potential of the proposed modeling approach.

In Figs. 13-14, it is worth noting the discontinuity in the current distribution due to the presence of the IB, which connects the rails and the ground wire [see Fig. 1(b)] at the position of 1500 m.

VI. CONCLUSION

In this paper, two methods for the experimental characterization of impedance bonds (IBs) at the frequency of interest for EMC have been proposed. The former makes use of a signal generator and an oscilloscope, for the measurement of open-circuit impedances up to 10 MHz, i.e., from audio frequencies to the HF band. The latter foresees the measurement of scattering parameters at the IB terminal ports.

0 500 1000 1500 2000 2500 30000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

position [m]

curr

ent r

atio

MTL modelNumerical model

ext. rail #2

ext. rail #1

Figure 13. Noise current distribution (f=300 kHz) in the external rails of a a 2 x 25 kV ac railway line (3 km length). The train (current source) is located at 1000 m and IBs are located at 1500 m.

0 500 1000 1500 2000 2500 30000

0.1

0.2

0.3

0.4

0.5

position [m]

curr

ent r

atio

Numerical modelMTL model

ground wire #1

ground wire #2

Figure 14. Noise current distribution (f=300 kHz) in the ground conductors of a 2 x 25 kV ac railway line (3 km length). The train (current source) is located at 1000 m and IBs are located at 1500 m.

The obtained data have been exploited for simulations of a 25 kV ac railway systems, aimed at investigating the impact of IBs on the distribution of noise-currents along the conductors.

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[1] EN 50121-2, Railway applications - Electromagnetic compatibility, Part 2: Emission of the whole railway system to the outside world, CENELEC, 2006.

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[7] A. M. L. Tortia, “Turin-Milan high-speed railway-line, 2x25 kV 50 Hz AC electrified. EMC problems in earthing of exposed conductive parts” in Proc. SPEEDAM 2006 International Symposium on Power Electronics, Electrical Drives, Automation and Motion, Taormina, Italy, 23-26 May, 2006, pp. S5-27–S5-32.

[8] T. Hasman, “Reflection and transmission of traveling waves on power transformers,” IEEE Trans. Power Del., vol. PWRD 12, no. 4, pp. 1684-1689, Oct. 1997.

[9] G. Liang, H. Dong, X. Wang, X. Zhang, H. Sun, and X. Cui, “High-frequency EMTP model of transformer windings,” in Proc. EMC Zurich 2006 Int. Symp. Electromagn. Compat., Singapore, Feb. 27-Mar. 3, 2006, pp. 662-665.

[10] S. Ramo, J. R. Whinnery, and T. Van Duzer, “Fields and Waves in Communications Electronics,” John Wiley & Sons, 1994.

[11] EM Software & Systems-S.A. (Pty) Ltd, FEKO Suite 5.3 User’s Manual, July 2007, Stellenbosch, South Africa, www.feko.info.