DC Component From Pantograph Arcing in AC Traction ... - IEEE Xplore

18

IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 53, NO. 1, FEBRUARY 2011

DC Component From Pantograph Arcing in AC Traction System—Influencing Parameters, Impact, and Mitigation Techniques Surajit Midya, Member, IEEE, Dierk Bormann, Thorsten Schütte, and Rajeev Thottappillil, Senior Member, IEEE

Abstract—Pantograph arcing in ac traction system generates transients, and causes asymmetries and distortion in supply voltage and current waveforms. These asymmetric voltage and current lead to a net dc component and harmonics that propagate within the traction power and signalling system and cause electromagnetic interference. This problem is enhanced during winter because of the layer of ice/snow on the overhead contact wire. The sliding contact becomes poor and a visible arc moves along with the pantograph. In this paper, it is shown how different parameters like traction current, line speed, power factor, and supply voltage influence the arcing, its characteristics, and the dc components. It is shown that the dc current component increases with increasing train speed and traction current, and reduces at a lower power factor. It is also discussed how the presence of an ice layer influences the arcing and the dc components. It is found that running the trains below the normal operating power factors is an effective choice to mitigate this problem. The findings presented in this paper could be beneficial to estimate the probable limit of the dc component at the planning stage so that proper precautions can be taken at the design stage itself. Index Terms—Arc discharges, electromagnetic compatibility, electromagnetic interference, rail transportation, rail transportation power systems.

I. INTRODUCTION ANTOGRAPH arcing in electrified railways is a common phenomenon. It is more intense during winter, thereby resulting in damage to pantograph carbon strips (need frequent replacement) [1]. Pantograph arcing distorts the sinusoidal waveform, generates a net dc component, harmonics (including even harmonics and interharmonics), and generates transients that cause high-frequency conducted and radiated electromagnetic interference. In this paper, we focus on the dc-component as-

P

Manuscript received December 12, 2008; revised September 13, 2009. Date of current version February 16, 2011. This work was supported by Bombardier Transportation and Banverket (The Swedish Rail Administration). S. Midya is with the Rail Control Solutions, Bombardier Transportation, Stockholm 11760, Sweden (e-mail: [email protected]. com). D. Bormann is with the Department of Power Technologies, ABB Corporate Research, Väster˚as 72178, Sweden (e-mail: [email protected]). T. Schütte is with the Rejlers Ingenjörer AB, Väster˚as 72130, Sweden (e-mail: [email protected]). R. Thottappillil is with the Electric Power Engineering and Design, Royal Institute of Technology, Stockholm 10044, Sweden (e-mail: rajeev. [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TEMC.2010.2045159

pects. DC components originating due to pantograph arcing can saturate the traction feeding and vehicle transformers. Moreover, a dc-fed track circuit for signalling applications can suffer interference due to those dc components [1]–[5]. All these create possible rail traffic delays, service disruptions, and a reduction in operational reliability. To understand the relationship between pantograph arcing and the dc component in better detail, an overhead line ice (OHL ICE) team was formed [2], [6]. Most previous work focused only on the electromagnetic interference (EMI)/electromagnetic compatibility (EMC) issues from pantograph arcing (under fair weather conditions) [7]– [15], without considering the winter effect and dc components. ¨ Buhrkall [3] and Ostlund et al. [1] reported measured dc current components in winter on specific train sets. Measurements on train sets at sites are difficult because of the complex large system and involved uncertainties, other interfering sources, difficulties in positioning the sensors, and freedom to control and regulate parameters like train speed, traction currents, etc. Such measurements are therefore train specific, and identification and comparison of critical parameters that govern the dc components during pantograph arcing are difficult. For these reasons, experiments were performed in controlled laboratory conditions and the subsequent results are presented in this paper so as to identify the main parameters that influence the dc-component generation during pantograph arcing. Since it is difficult to perform repetitive experiments with a layer of ice/snow between the pantograph and the overhead contact wire within the laboratory, it was decided as a first step to have an air gap between the electrodes (representative of ice layer dielectric separation). The paper is organized as follows. Section II presents the experimental and measurement setup. Section III describes the theory behind the polarity-dependent nature of pantograph arcing and dc component. Section IV presents the test results and key observations, followed by discussion in Section V. Section VI describes the situation in winter and mitigation techniques, followed by conclusions in Section VII. II. EXPERIMENTAL SETUP, OPERATION, AND MEASUREMENT DETAILS A. Design Considerations for the Test Setup Designing a test setup that closely replicate the interaction between the pantograph and contact wire of a railway system within the laboratory itself is a big challenge and possibly be the reason for only few existing systems in the world. While designing the setup, the major requirements were:

0018-9375/$26.00 © 2010 IEEE

MIDYA et al.: DC COMPONENT FROM PANTOGRAPH ARCING IN AC TRACTION SYSTEM

19

TABLE I DIFFERENT PARAMETERS AND POLARITY CONVENTION

Fig. 1. Experimental setup of the OHL ICE team (courtesy: Bombardier Transportation, ABB Corporate Research; source: [2], [6]).

1) physical dimensions of the setup: the setup should be small and fit within the test chamber; 2) the relative speed between the pantograph and contact wire should be controllable; 3) the zigzag length should be controllable; 4) the pantograph and the contact wire should be separated by a variable distance, similar to the case of a nonuniform dielectric layer of ice/snow; 5) the dielectric separation between the electrodes should be such that it maintains its dielectric properties unchanged during one test run (typically 10 s), and can repeated for more than one hundred test runs within a day. Considering all these, the setup, as shown schematically in Fig. 1, was designed and built by the OHL ICE team, which includes experts from railway and power industries, mainly from ABB Corporate Research and Bombardier Transportation. Here, the dielectric ice layer was represented by a variable air gap, which was agreed to be adequate enough, and in practice, by far the most feasible solution based on the requirement. A brief description of the setup and its operational details are described in the next section and further details can be found in [2], [5], and [16]–[18]. B. Experimental Setup and Its Operation Fig. 1 shows the schematic details of the experimental setup, details of which can be found in [2] and [5]. The main shaft hosting the wheel is insulated properly so that no current goes to the motor or other parts of the setup. A variable-speed motor rotates the wheel where the overhead contact wire (sometimes referred as line in this paper) is grooved into the periphery, providing the forward motion. Tests were conducted at a line speed up to 30 m/s. The pantograph is housed on a carriage. Another smaller motor drives a threaded shaft that moves this carriage back and forth, providing the zigzag motion. The back and forth zigzag motion and zigzag length is controlled by two limit switches fixed at the pantograph carriage, which reverses the direction of the rotation of the second motor. The setup

replicates the sliding contact mechanism between pantograph and contact wire within the laboratory. Similar to an actual traction system, the air gap between the pantograph and the contact wire varies because of: 1) mechanical oscillations of the springs at the base; 2) slight imperfections on the periphery of the wheel; 3) the slightly convex surface of the moving pantograph. The pantograph was arranged in two ways: 1) without any tilt, as in trains in normal operating condition (runs 140–152) and 2) slightly tilted (similar to the situation when a train is taking a turn) to create a variable dgap (runs 153–251) for a better understanding of the influence of the dielectric separation between the electrodes. Although most of the test runs were conducted with a carbon pantograph, copper and aluminum pantographs were used in a few test runs to investigate the material dependency. Controlled, measured, and estimated test parameters, and the polarity convention of the electrodes used in this paper are shown in Table I. Altogether 251 tests, both with ac and dc

20


Fig. 3. Layout of an overhead electrified railway showing the zigzag of the contact wire; adapted from [19].

above about 20 kHz. Test runs with ac supply were recorded with an ordinary video camera, which are quite informative, if not always conclusive. III. THEORETICAL BACKGROUND: POLARITY DEPENDENCE, ASYMMETRIES, AND DC COMPONENTS FROM PANTOGRAPH ARCING A. Polarity Dependence

Fig. 2. Circuit representation of the test setup and measurement circuit for ac supply (adapted from [2]).

supply and different test conditions, were conducted. The run numbers are those of the original lab notes [2] and are just a consecutive numbering. C. Power Supply A large rotating electrical machine, which delivers the electrical energy at the expense of its inertia, was used to supply the energy. This can provide almost a constant voltage for each test lasting for approximately 10 s. The HV supply is connected between the wheel and the pantograph, as shown in Fig. 2. Supply voltage was varied from 2–5 kV (rms) in steps of 1 kV. A large resistor and inductor panel, capable of handling both high voltage and high current, was used as connected loads to limit the current. D. Measurement Details U (t), I(t), vline , and the position of the pantograph were measured continuously for each test using a multichannel transient recorder (TRA 800 III) with a sampling rate of 100 kSamples/s. The recorded data for all the test runs have 512 periods and 2000 data points in each period. dgap and vpant were estimated from the position of the pantograph. The current was measured using a 10 mΩ shunt resistor. The voltage U (t) was measured using a 1000:1 resistive voltage divider, and the position of the pantograph by a linear displacement transducer. An isolation transformer with bandwidth 0–20 kHz was used to avoid grounding problems. This introduces some damping on signal components

Because of the zigzag configuration of the overhead contact wire, as shown in Fig. 3, vpant in the lateral direction is much slower than vline in the forward direction. Therefore, the arc root moves slowly along the pantograph surface and remains heated. On the other hand, the arc root moves faster along the overhead contact wire surface, is continuously changing position, and thus, is colder. Because of the sliding motion between the two electrodes, arc roots at both electrodes move along their respective surfaces at different speeds, having different electrothermal conditions [5], [16]. This leads to an asymmetry between both polarities of the U (t) and I(t) waveforms [4], [5], [16]. Both the pantograph arcing, its polarity-dependent nature, the U (t) and I(t) waveforms, and arc root movements are influenced by several parameters like vline , Irm s , U0 , and the presence of inductive load in the circuit [5], [16]. Based on these test conditions, the nature of the arc root movements can vary, and sometimes, the same arc root is maintained for several half periods [5], [16]. Fig. 4(a) and (b) shows the two main types of asymmetries in the voltage and current waveforms that we observed [5], [16]. B. Variation of I(t) Due to Arc Reignition We noticed that all test runs generate either of the two types of asymmetric I(t) waveforms with some variations in between [5]: 1) Fig. 4(a)—indicating dielectric reignition; 2) Fig. 4(b)—indicating dominant thermal reignition. Dielectric reignition has a typical signature pattern of zerocurrent regions of durations τnp and τpn , as shown in Fig. 4(a), just after the current zero crossing (CZC) [20]–[22]. To reignite the arc, a minimum reignition voltage is required, which depends only on the contact gap parameters like gap length, contact material, etc., and independent of the circuit parameters [20]–[22]. Fig. 4(a) also shows the instantaneous U0 (t) before the reignition, marked as U0 (t)|t=τ pn and U0 (t)|t=τ np . As the Irm s is


21

Fig. 5. Difference in τ np and τ pn is clear throughout test run 141, where v lin e = 11.18 m/s, Irm s = 17.60 A, and U 0 = 3006 V. Duration of τ np is always higher compared to τ pn . (a) Distribution of τ np and τ pn for 10 s. (b) Duration of τ np and τ pn as %.

Fig. 4. Two different types of asymmetry in the I(t) waveform: (a) zerocurrent regions (τ np , τ pn ) during CZC indicate dielectric reignition v lin e = 29.82 m/s, Irm s = 23.70 A, U 0 = 4008 V, R = 160 Ω, ωL = 0 Ω. (b) Changed slope of I(t) after CZC indicates thermal reignition v lin e = 29.32 m/s, Irm s = 30.10 A, U 0 = 3010 V, R = 85 Ω, ωL = 28 Ω.

increased and vline is reduced, there is a gradual transition toward thermal reignition [20]–[22]. As the nature of reignition becomes more and more thermal, the values of both τnp , τpn , and corresponding U0 (t)|t=τ pn , U0 (t)|t=τ np decrease [see Fig. 4(b)] [20]–[22]. Fig. 5(a) and (b) shows the distribution of τnp and τpn for the complete duration of run 141. The mean duration of zero current from negative to positive crossover (where copper conductor becomes cathode and pantograph becomes anode) is clearly larger than the reverse, i.e., τnp av > τpn av . C. Variation of U (t) Due to Arc Reignition The voltage drop U (t) between the two electrodes depends mainly on arc length (cathode and anode voltage drops are less than 20 V [23]), and hence, on the movement of the arc roots along the contact wire. These can vary to a certain extent within

the same test run because of the variations in dgap due to the slightly convex surface of the pantograph, its zigzag motion, surface irregularities, and the stochastic nature of the arc [5], [16]. Although we noticed wider variations in the U (t) waveform due to different modes of arc root movement and corresponding arc length, variations in the I(t) are not pronounced. The voltages U0 (t)|t=τ pn and U0 (t)|t=τ np are different in the two polarities, as shown in Fig. 4(a). In dielectric reignition [see Fig. 4(a)], this difference between U0 (t)|t=τ pn and U0 (t)|t=τ np is clearly noticeable. However, it diminishes as reignition becomes more and more thermal [see Fig. 4(b)]. The asymmetry in the U (t) is mainly due to the polarity-dependent arc root movement, affecting the voltage drop across the arc channel, as shown in Fig. 4(b) [5], [16]. D. Asymmetry and Corresponding DC Current Component Fig. 6 shows a simplified case with a sinusoidal voltage waveforms and an asymmetric current waveform similar to the one observed for dielectric reignition [see Fig. 4(a)] [5]. Equation (1) estimates the average value of I(t) for this case, where T is the time period. It should be zero for an ideal sinusoidal or T of (1) will be negasymmetric waveform. Since τnp > τpn , Idc tive, indicating a net current flow always in the direction from the contact wire to the pantograph, as shown in Fig. 7. When

22


Fig. 6. Asymmetry in the I(t) and U (t) waveforms due to different τ np , τ pn , and corresponding U 0 (t)|t = τ pn , U 0 (t)|t = τ p during CZCs. n

Fig. 8. Increased U 0 (t)|t = τ pn and U 0 (t)|t = τ p at lagging cos(φ) during n CZCs. U 0 (t) not to scale, τ np = 2.11 ms and τ pn = 1.12 ms. TABLE II DC COMPONENTS DUE TO DIFFERENCE IN τ np AND τ pn OF SINUSOIDAL WAVEFORMS

current. Fig. 7. Net dc component generated from the pantograph arcing in ac-fed traction system. The Id c propagates from the pantograph to the vehicle and follow the return path of the traction power system (adapted from [3]).

τnp

τpn

T Idc

the difference between and increases, increases (see Table II). Idc is estimated by taking an average of the measured I(t) for the complete duration of test (10 s) for all the test runs. T is small, Idc is mainly If the difference between Idc and Idc p due to the difference between τn and τpn , indicating the reignition is mainly dielectric in nature. Otherwise, it is mainly due to thermal reignition, where the major contribution is from the changed slope of I(t) in positive polarity, as shown in Fig. 4(b), with some contribution due to the difference between τnp and τpn , if any. A question is often asked by practicing railway engineers— what is the theoretical upper limit of the dc current component. For dielectric reignition, the worst hypothetical case could be a half-wave rectification, where τnp = T /2 = 10 ms τpn = 0. T /Im ax = −1/π ≈ −32%. Another theoretical upFrom (1), Idc per limit could be the case when reignition happen at Vm ax , i.e., when the electrical stress between the electrodes is highest. Substituting τnp = T /4 = 5 ms and τpn = 0 for this case T in (1), Idc /Im ax = −1/2π ≈ −16%. In reality, Idc > 50 A is experienced by Swedish Railways during winter, especially with freight trains carrying iron ore from Kiruna [5]. Considering a typical load of an electrical locomotive as 4–6 MW, Idc = −50 A is equivalent to −7.5% to −17% of the traction

T Idc

1 = T

=−

Im ax sin(ωt)dt

τ np

1 + T

T /2

T

Im ax sin(ωt)dt (T /2)+τ pn

Im ax cos(ωτpn ) − cos(ωτnp ) . 2π

(1)

E. Asymmetry and Corresponding DC Voltage Component The U0 (t)|t=τ pn and U0 (t)|t=τ np in both polarities are different corresponding to different τnp and τpn , as shown in Fig. 4(a). This, together with the polarity-dependent voltage drop across the arc channel (due to polarity-dependent arc root movement and hence length of the arc channel), leads to a positive dc voltage component Udc for all the test runs [2]. U0 (t)|t=τ pn and U0 (t)|t=τ np can be approximately estimated by U0 (t)|t=τ np = Um ax sin(ωt|t=τ np + φ) U0 (t)|t=τ pn = Um ax sin(ωt|t=τ pn + φ)

(2)

where φ = tan−1 (X/R). At unity power factor, φ = 0. Fig. 8 shows that at a lagging power factor cos(φ) = 0.9, the corresponding U0 (t)|t=τ pn and U0 (t)|t=τ np values would be higher if t = τpn and t = τnp were unchanged.


23

TABLE III CARBON PANTOGRAPH WITHOUT ANY TILT

TABLE IV VARIATION OF v lin e AND TILTED CARBON PANTOGRAPH

IV. TEST RESULTS: INFLUENCE OF DIFFERENT PARAMETERS ON THE NET DC COMPONENT

TABLE V VARIATION OF Irm s AT LOW v lin e AND TILTED CARBON PANTOGRAPH

We will present our analyses by categorizing the test runs based on variation of vline and Irm s , first with R load and later R–L loads. Since a carbon–copper sliding contact is most widely used, we will confine most of our analysis to test runs with a carbon pantograph. Later, we will present results of a few test runs with aluminum and copper pantographs to show the material dependency. In this section, we will present the test conditions (U0 , Irm s , vline , R, and ωL), estimated parameters (τnp av , τpn av , U0 (t)|t=τ np , T , and Udc ) for and U0 (t)|t=τ pn ), and the dc components (Idc , Idc T all test runs. Both Idc and Idc will be presented as % of the Irm s to have a relative comparison between different test runs.

TABLE VI VARIATION OF Irm s AT HIGH v lin e AND TILTED CARBON PANTOGRAPH

A. Test Runs With a Horizontal Carbon Pantograph In Table III, runs 140–141, 142–143, and 144–145 are grouped by variation of vline , whereas runs 146–147 are by variation of Irm s having an inductive load. Major observations with increasing vline are 1) τnp av and τpn av , and the corresponding U0 (t)|t=τ np and U0 (t)|t=τ pn increase; 2) the increase in τnp av and the corresponding U0 (t)|t=τ np are higher compared to the reverse; T increase. 3) both Udc , Idc , and Idc Addition of inductance and/or increased Irm s leads the following major changes: 1) a reduction in τnp av and τpn av , and corresponding U0 (t)|t=τ np and U0 (t)|t=τ pn ; T . 2) a reduction in both Udc , Idc , and Idc With increased U0 in runs 142–143 compared to 140–141, there is a reduction in τnp av and τpn av . B. Test Runs With Variation of vline and Tilted Carbon Pantograph After the pantograph is tilted to obtain a variable dgap , runs in Table IV show similar trends noticed in Table III. From the recorded videos, we noticed that the arcing was not consistent throughout the test runs and sometimes gets interrupted when dgap is largest [5]. This could be the reason for the variations. T , but their difference At low vline , Idc is always higher than Idc is reduced with increasing vline .

C. Test Runs With Variation of Irm s and Tilted Carbon Pantograph Tables V and VI show the influence of Irm s at high and low vline , respectively. Major observations with increasing Irm s are: 1) τnp av , τpn av , and the difference between them reduces; 2) U0 (t)|t=τ np , U0 (t)|t=τ pn , and the difference between them reduces; T follow the same trend, but occasionally 3) Udc , Idc , and Idc there are slight deviation; 4) Although Idc Ampere increases, Idc /Irm s reduces; T irrespective of the vline and the 5) Idc is higher than Idc difference increases with Irm s . D. Test Runs With Tilted Aluminum and Copper Pantograph A copper pantograph was used for test runs 197–200 and 223–251 and an aluminum pantograph for runs 201–222. Both

24


TABLE VII TILTED PANTOGRAPHS WITH R–L LOADS

U0 (t)|t=τ pn . Together with the difference between τnp av and τpn av , a changed slope of I(t) [see Fig. 4(b)] can enhance the T . asymmetry of I(t). Hence, Idc is higher than Idc As vline is increased, arc roots move more frequently to new and hence colder spots on the contact wire, implying a transition toward dielectric reignitions [see Fig. 4(a)]. There is a gradual increase in τnp av , τpn av and the corresponding U0 (t)|t=τ np , T decreases. U0 (t)|t=τ pn . The difference between Idc and Idc B. Influence of Irm s on the DC Component From Pantograph Arcing

τnp av , τpn av , and corresponding U0 (t)|t=τ np , U0 (t)|t=τ pn are generally higher for aluminum and copper pantographs compared to carbon. At higher vline and lower Irm s , this difference is higher and we noticed the highest Idc and Udc with copper pantograph. The difference gradually decreases with increasing Irm s and/or decreasing vline . E. Test Runs With R–L Loads Runs 194–196 are grouped based on the variation of vline , whereas runs 190–195 and 250–251 are based on the variation of Irm s (see Table VII). With R–L load, similar trends are noticed on the I(t) and U (t) characteristics. Compared to purely resistive loads, the major differences are: 1) τnp av , τpn av , and the corresponding U0 (t)|t=τ np , U0 (t)|t=τ pn are lower, similar to the effect of increased Irm s ; 2) τnp av , τpn av do not change noticeably with increasing vline , whereas increasing Irm s has a more significant effect; T increases with increas3) the difference between Idc and Idc ing vline . V. DISCUSSION In any continuously burning arc, to supply and maintain the current the temperature at the cathode arc root must be high. This can cause a time delay (τnp and τpn ) after the CZC and before the arc is reignited, even if the same arc channel as before the CZC is sustained. The arc root at the pantograph is already heated up (due to slow vpant ), whereas that at the copper contact wire is often new, especially at high vline , and thus cold. Therefore, we note a higher τnp , and corresponding U0 (t)|t=τ np (when the cold copper contact wire is the cathode), compared to τpn , and corresponding U0 (t)|t=τ pn (when the already heated pantograph is the cathode). In some test runs, especially with lower U0 (=2 kV) and lower Irm s , we have noticed that sometimes the arc does not reignite at all. A. Influence of vline on the DC Component From Pantograph Arcing At lower vline , the same arc roots (which are already heated up) are maintained at both electrodes for several half periods [5], [16]. This leads to thermal reignition [see Fig. 4(b)]. We note reduced τnp av , τpn av and correspondingly reduced U0 (t)|t=τ np ,

Similarly, increasing Irm s enhances the heating of the arc roots. This reduces τnp av , τpn av , and the corresponding U0 (t)|t=τ np , U0 (t)|t=τ pn . Hence, at lower Irm s , the difference T is small, and increases with increasing between Idc and Idc Irm s indicating a transition from dielectric reignition at lower Irm s to thermal reignition at higher Irm s . An increase in Irm s leads to an increased Im ax , and hence, Idc , as shown in (1). On the other hand, it also reduces τnp and τpn . These two effects on Idc are opposite to each other, and hence, there is a race between these two effects on the Idc . In general, we noticed that Idc increases with increasing Irm s whereas the ratio Idc /Irm s decreases. However, because of the race between the two effects, we noticed some exceptions as well. C. Influence of Inductance (or Power Factor) on Arcing Fig. 8 shows how a reduced power factor increases the U0 (t)|I (t) 0 at the CZC in both polarities. Addition of L in the circuit influences in two ways: 1) because of the lower power factor (i.e., increasing φ), U0 (t)|I (t)=0 is higher during CZC, as shown in (2) (see Fig. 8); 2) similar to the switching of an R–L circuit, the transient overvoltages are increased after the CZC. Both effects lead to a faster reignition of the arc by favoring a breakdown of the gap between the electrodes. We note a reduction in τnp av and τpn av , and the corresponding U0 (t)|t=τ np , U0 (t)|t=τ pn (see Table VII). Similar to the effect of increasing Irm s and reducing vline , the transition from dielectric reignition toward thermal reignition is enhanced with inductive load. T with an Therefore, we always observe that Idc is larger than Idc inductive load. At common ac traction voltage levels, i.e., at U0 = 15 kV or 25 kV, U0 (t)|I (t)=0 will be quite high and impress a higher electric field between the electrodes for a faster reignition after the CZC than in our laboratory tests, leading to a greater reduction of Idc . VI. CONSEQUENCES OF THE DC COMPONENT AND MITIGATION TECHNIQUES IN WINTER A. Situation in Winter In normal weather conditions, there is a thin film of moisture between the pantograph and the overhead contact wire, which makes the sliding contact smooth [21], [24], [25] and


25

Fig. 9. Details of the sliding contact between the pantograph and the overhead contact wire in winter (not to scale).

we do not observe intense arcing. In winter, this thin layer of moisture is frozen. Apart from this, depending on the metrological conditions, there could be a thick layer of ice/snow on the overhead contact wire [3], [26], [27]. Therefore, instead of a carbon–copper sliding contact, it becomes a ice/snow-layer– carbon sliding contact as shown in Fig. 9. This makes the slide between the two electrodes far from smooth and a visible bright glow of arc is noticed in almost all countries with cold weather when a train passes [5]. There are different types of ice layers that can be formed on the overhead contact wire based on the atmospheric and geographical location viz., hoarfrost, rime (soft and hard), glaze, and spray ice [3], [26], [27]. Depending on the formation and type, some of these are more sticky (hoarfrost), granular in structure (rime), harder (hard rime, glaze), and homogeneous in thickness (glaze) [3], [26]. When the pantograph moves through the overhead contact wire, the pantograph slides through this ice layer. To have the power flow from the overhead contact wire to the pantograph, there has to be electrical breakdown of this dielectric layer of ice. Compared to our case with a dielectric air gap, an ice layer has a higher breakdown voltage, which increases with decreasing temperature [28], [29], and hence, it requires higher voltages and longer times to be able to reignite the arc and supply the required current. Thus, based on the aforementioned properties and thickness of the ice layer, the breakdown voltage and time to reignite the arc (τnp and τpn ), and corresponding U0 (t)|t=τ pn and U0 (t)|t=τ np will vary to a certain extent. As explained before, the arc root on the pantograph surface is already heated because of slow vpant , whereas the one on the overhead contact wire covered by an ice layer is much colder. Therefore, the electrothermal asymmetry between the arc roots is even higher than in our case of an air gap. We can expect a higher τnp , and corresponding U0 (t)|t=τ np , compared to the reverse case. This will lead to a higher level of Idc compared to our investigation in normal weather conditions, which is also experienced by different railways in many countries during winter.

cle in the entire traction power and signalling system, as shown in Fig. 10(a), with a booster transformer feeding system, and in Fig. 10(b), with an autotransformer feeding system. Because of the small cross-sectional area, the rails often provide a higher resistive path than soil. Since dc prefers a low-resistive path, a larger portion of the dc current flows through the soil, and thus, enhances the possibility of interference. The dc component creates saturation problems with the traction feeding transformers and vehicle transformers, and there might be interference with the track circuit signalling system. It is often experienced that modern traction feeding transformers with an improved core material are more sensitive to this type of saturation problems. Buhrkall reported Idc = 20–30 A measured during an outdoor experimental investigation as a part of the ICE project with Bombardier Transportation in Luxemburg [3]. In some other experiments during winter in Sweden, Idc > 35 A was measured for freight trains [1]. Considering the fact that we used air as the dielectric gap between the pantograph and overhead contact wire, our results for Idc are reasonable. This would have been higher in the presence of a separating ice layer. The limit for Idc was increased from 10 to 25 A in Sweden and discussions are in progress to increase it even further because of the increasing speed and electrical load of trains [5]. It is a big challenge to estimate the possible Idc limits and its impact on the infrastructure at the planning stage for appropriate design and necessary precautions for different traction feeding systems, be it AT, BT, a combination of both AT–BT, or recently developed modified AT feeding system used in Norway.

B. EMC Issues Due to the DC Component

C. Mitigation of the Icing Problems

Idc almost always flows from the overhead contact wire to the pantograph, as shown in Fig. 7, and propagates through the vehi-

Several techniques have been used to mitigate the icing problems with the overhead contact wire.

Fig. 10. Propagation of a dc component and other harmonics in the entire traction power system with: (a) booster transformer and (b) autotransformer.

26


1) As discussed in Section V-C, an ac traction system running the train at lower power factor can make a reignition faster by causing an earlier breakdown of the ice layer. In a four-quadrant drive system, a desired power factor can be achieved by adjusting the firing angles of the switching devices and without addition of any inductance. 2) Since addition of inductance in the circuit generates high transients, as we experienced before [5], if properly chosen it can be also used to cause an earlier breakdown of the ice layer without tripping/disconnecting the regular supply. This is also a recommended practice in some dc railways in Sweden. 3) Scraping the overhead contact wire by running an old train in the early morning (widely used by many dc railways, although not very efficient way, it works most of the times). 4) Within cities, often a truck with a pantograph is used for scraping overhead tram or trolleybus lines. 5) Driving a circulating current between two feeding stations—an expensive and energy inefficient technique, but still used in some Deutsche Bundesbahn lines, Germany [30]. 6) Using some de-icing chemicals, like sodium chloride, calcium chloride, ethanol, etc. Mitigation techniques based on electrical control are emerging as the most effective choices in ac-fed traction system because of the advantage of the power factor control using a fourquadrant drive system. However, reducing power factor always comes at the expense of: 1) higher current, which may leads to overloading, and higher electrical losses and energy bill; 2) higher transients, which can trip the breakers, causing service interruptions. In dc-fed traction systems, this is not possible, and hence, other techniques are often used based on the requirements and resources.

reignition because of a colder contact wire, whereas slower line speed favors thermal reignition. The presence of inductance and driving the train at a lower power factor enhances the transition toward thermal reignition. In most of the cases, it is a combination of both dielectric and thermal reignition that determine the asymmetry, and thereby, the dc component. In general, the magnitude of the dc current component increases with increasing line speed. However, increasing current does not reflect so clear trend because of two opposing influences: increasing peak value and reduction of zero-current regions. The magnitude of the dc component can be reduced by making the arc reignition faster after the CZC. This is achieved by addition of inductance (i.e., driving the train at a lower power factor), which is emerging as the most effective mitigation technique against the icing problems with electrified ac traction systems. Further investigation on actual traction systems is necessary to estimate the possible magnitude of the dc component for different traction current, line speed, and power factor at different icing conditions. This will help to evaluate the impact on the different types of traction feeding systems so that suitable preventive measures can be taken at the planning and design stage. ACKNOWLEDGMENT The authors would like to thank ABB Corporate Research for providing the facilities, L. Liljestrand, L. Nordin, and M. Ali of ABB Corporate Research for designing and building the test setup and invaluable help with performing the tests and S. Shirran, G. Bohlin, P. Mellberg, and N. Theethayi of Bombardier Transportation, P.-A. Lindeberg and R. Byström for useful discussions and sharing information. The authors would also like to express their special thanks to OHL ICE team. Contribution of the reviewer is also acknowledged in organizing the paper to its present form. REFERENCES

VII. CONCLUSION In this paper, we presented an experimental investigation and analyses to understand the dc components in an ac-fed traction system due to arcing in the pantograph. The problem is gaining more importance recently due to both the rolling stock and railway infrastructure owners because of increasing traction load and train speed, especially during winter because of the layer of ice/snow on the overhead contact wire. Here, we have shown that the asymmetry in the current and voltage waveforms are mainly due to the difference in the electrothermal conditions at the arc roots. The arc root at the pantograph is already heated, whereas that on the overhead contact wire is continuously changing position, and thus, colder. We can distinguish two types of arc reignition after the CZC, viz., dielectric and thermal reignition. Which type will dominate depends on test parameters like traction current, line speed, and power factor. At lower current, it is mainly dielectric reignition. A gradual transition toward thermal reignition happens with increasing current. Higher line speed enhances dielectric

[1] [2]

[3] [4]

[5] [6] [7]

¨ S. Ostlund, A. Gustafsson, L. Buhrkall, and M. Skoglund, “Condition monitoring of pantograph contact strip,” in Proc. 4th IET Int. Conf. Railway Cond. Monit., Derby, U.K., Jun. 2008, pp. 1–6. D. Bormann, “Origin of dc component in railway line current upon operation with iced-over overhead line: Theory, experimental results and conclusions,” ABB Internal Report of the project: Investigation of dc current because of ice on contactwire, ABB,Väster˚as, Sweden, Ref. No. SECRC/PT/TR-2003/038, May 2003. L. Buhrkall, “DC components due to ice on the overhead contact wire of ac electrified railways,” Elektrische Bahnen, vol. 103, no. 8, pp. 380–389, Aug. 2005. D. Bormann, S. Midya, and R. Thottappillil, “DC components in pantograph arcing: Mechanisms and influence of various parameters,” in Proc. 18th Int. Zurich Symp. Electromagn. Compat., Munich, Germany, Oct. 2007, pp. 369–372. S. Midya, “Electromagnetic interference in modern electrified railway systems with emphasis on pantograph arcing,” Licentiate thesis, Uppsala Univ., Uppsala, Sweden, May 2008. L. Buhrkall. (2003, Jun.). Ice on the overhead line of ac electrified railways: Conclusions from the OHL ice team. The OHL Ice Working Group [Online]. Available: http://www.buhrkall.dk/ T. Konefal, D. A. J. Pearce, C. A. Marshman, and L. M. McCormack. (2002, Mar). Potential electromagnetic interference to radio services from railways. Final Rep. [Online]. Available: http://www.yorkemc.co.uk/research/railways/


[8] E. J. Bartlett, M. Vaughan, and P. J. Moore, “Investigation into electromagntic emissions from power system arcs,” in Proc. 1999 Int. Conf. Exhib. Electromagn. Compability, EMC York, York, U.K., Jul. 1999, pp. 47– 52. [9] D. Klapas, R. Hackam, and F. A. Benson, “Electric arc power collection for high-speed trains,” Proc. IEEE, vol. 64, no. 12, pp. 1699–1715, Dec. 1976. [10] V. Galdi, L. Ippolito, and A. Piccolo, “Arcing in AC railways: a mathematical approach,” in Proc. Int. Conf. Comput. Aided Des., Manuf. Oper. Railway Other Adv. Mass Transit. Syst., Lisbon, Portugal, Sep. 1998, pp. 891–904. [11] S. Brillante, P. Ferrari, and P. Pozzobon, “Modelling of electromagnetic emission from catenary-pantograph sliding contact,” in Proc. Int. Conf. Comput. Aided Des., Manuf. Oper. Railway Other Adv. Mass Transit. Syst., Lisbon, Portugal, Sep. 1998, pp. 881–890. [12] B. Tellini, M. Macucci, R. Giannetti, and G. A. Antonacci, “Conducted and radiated interference measurements in the line-pantograph system,” IEEE Trans. Instrum. Meas., vol. 50, no. 6, pp. 1661–1664, Dec. 2001. [13] B. Tellini, M. Macucci, R. Giannetti, and G. A. Antonacci, “Linepantograph EMI in railway systems,” IEEE Instrum. Meas. Mag., vol. 4, no. 4, pp. 10–13, Dec. 2001. [14] R. Giannetti, M. Macucci, and B. Tellini, “EMI measurements in line– pantograph contact discontinuity in railway transportation systems,” in Proc. 11th Int. Sysmp. Trends Electr. Meas. Instrum., IMEKO TC-4, Lisbon, Portugal, Sep. 2001, pp. 25–90. [15] J. Allan, W. Chan, Z. Shaoz, and B. Mellitt, “Low frequency and radio frequency electromagnetic compatibility for rapid transit railways,” in Proc. 5th Eur. Conf. Power Electron. Appl., Brighton, U.K., Sep. 1993, pp. 106–111. [16] S. Midya, D. Bormann, A. Larsson, and T. Schütte, R. Thottappillil, “Understanding pantograph arcing in electrified railways – influence of various parameters,” in Proc. IEEE Int. Symp. Electromagn. Compat., Detroit, MI, Aug. 2008, pp. 1–6. [17] S. Midya, D. Bormann, and T. Schütte, R. Thottappillil, “Pantograph arcing in electrified railways—Mechanism and influence of various parameters: Part I—With DC traction power supply,” IEEE Trans. Power Del., vol. 24, no. 4, pp. 1931–1939, Oct. 2009. [18] S. Midya, “Conducted and radiated electromagnetic interference in modern electrified railways with emphasis on pantograph arcing,” Ph.D. dissertation, Royal Inst. Technol. (KTH), Stockholm, Sweden, Jun. 2009. [19] F. Kiessling, R. Puschmann, and A. Schmieder, Contact Lines of Electric Railways: Planning, Design and Implementation. Munich, Germany: Publicis Corporate Publishing (Siemens), 2001. [20] A. Takahashi and M. Lindmayer, “Reignition voltage of arcs on double-break contacts,” IEEE Trans. Comp., Hybrids, Manuf. Technol., vol. CHMT-9, no. 1, pp. 35–39, Mar. 1985. [21] P. G. Slade, Electrical Contacts: Principles and Applications. New York, NY: Marcel Dekker, 1999. [22] D. Chen, X. Li, and R. Dai, “Measurement of the dielectric recovery strength and reignition of AC contactors,” IEICE Trans. Electron., vol. E88, no. 8, pp. 1641–1646, Aug. 2005. [23] Y. Yokomizu, T. Matsumura, R. Henmi, and Y. Kito, “Total voltage drops in electrode fall regions of SF6 , argon and air arcs in current range from 10 to 20 000 A,” J. Phys. D: Appl. Phys., vol. 29, no. 6, pp. 1260–1267, May 1996. [24] R. Holm, Electrical Contacts. Uppsala, Sweden: Almqvist and Wiksells, 1946. [25] E. I. Shobert, “Sliding electrical contacts,” in Proc. 39th IEEE Holm Conf. Electric Contacts, Pittsburgh, PA, Sep. 1993, pp. 123–134. [26] L. D. Minsk, “Icing on structures,” U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, NH, Tech. Rep., CERL Report 80-31, Dec. 1980. [27] T. Berger. (2005, Mar.). Icing on overheadlines of electrified railway lines—Metrological conditions. SBB Swiss Nat. Railway, Infrastructure, Interoperability, SBB Swiss Nat. Railway, Switzerland, Tech. Rep. [Online]. Available: http://www.buhrkall.dk/ [28] M. Farzaneh, S. Brettschneider, K. D. Srivastava, and S. Y. Li, “Impulse breakdown performance of the ice surface,” in Proc. 11th Int. Symp. High Voltage Eng., London, U.K., Aug. 1999, pp. 341–344. [29] K. S. S. Brettschneider and M. Farzaneh, “Ice surface discharge initiation,” IEEE Power Eng. Rev., vol. 22, no. 8, pp. 59–60, Aug. 2002. [30] E. Board, “Betriebsstörungen durch raureif,” Electrische Bahnen, vol. 106, no. 1/2, p. 97, Jan./Feb. 2008.

27

Surajit Midya (S’09–M’10) was born in India in 1976. He received the B.E. degree in electrical engineering from Bengal Engineering and Science University, Kolkata, India, in 1997, the M.S. degree in electrical engineering from the Indian Institute of Science, Bangalore, India, in 2004, the Licentiate Ph.D. degree in electrical engineering from Uppsala University, Uppsala, Sweden, in 2008, and the PhD degree in electrical engineering from the Royal Institute of Technology (KTH), Stockholm, Sweden, in 2009. He is currently at the Rail Control Solutions, Bombardier Transportation, Stockholm. From January 2004 to April 2005, he was a Diagnostics Technologist at the Global Research Center of General Electric Company (John F. Welch Technology Center), Bangalore, India. From 1997 to 2001, he was a Project Engineer in India with various organizations.

Dierk Bormann received the Ph.D. degree in physics from the University of Heidelberg, Heidelberg, Germany, in 1992. After some years of academic research in the fields of statistical physics, low-dimensional quantum systems, superconductivity and superfluidity, he joined ABB Corporate Research, Väster˚as 72178, Sweden, in 2000. He is also supervising research students at the Royal Institute of Technology (KTH), Stockholm, Sweden. His current research interests include transient modeling of electrical apparatus and machines, electrical breakdown, and arc physics. Dr. Bormann is a member of the American and German Physical Societies (APS and DPG) and has been a member of Working Group of the Cigré SC A2 (Transformers) since several years.

¨ Thorsten Schutte was born in Kassel, Germany, in 1957. He studied meteorology and physics at the Universities of Kiel, Germany, and Uppsala, Sweden and received the Ph.D. degree in lightning physics in 1987 from Uppsala University, Uppsala, Sweden. Since 1990, he has been an Associate Professor at Uppsala University. From 1987 to 1998, he was a Research Engineer at the ABB Corporate Research Center, Väster˚as, Sweden, where he was engaged in electrical insulation, transformer physics, and patents. After working with feeding systems for railways at Adtranz/Balfour Beatty Rail and neutral grounding systems at Swedish Neutral, since 2005, he has been a Senior Scientist at Rejlers Ingenjörer with railway feeding systems and consulting on electrical insulation issues as his main tasks. His current research interests include ground currents, electromagnetic interference, optimizing of railway feeding systems, liquid/gas interfaces exposed to electric fields, electrical breakdown statistics, and lightning physics.

Rajeev Thottappillil (M’88–SM’05) received the B.Sc. in electrical engineering from the University of Calicut, India, in 1981, and the M.S. and Ph.D. degrees in electrical engineering from the University of Florida, Gainesville, in 1989 and 1992, respectively. In 1995, he joined Uppsala University, Uppsala, Sweden, where he was a Professor during 2000–2009. Since September 1, 2008, he holds the Chair in Electric Power Engineering and Design, Royal Institute of Technology, Stockholm, Sweden. He has authored or coauthored more than 170 scientific articles in journals and international conferences. He was engaged in electromagnetic interference questions in large distributed electrical systems, lightning protection, and hardening of civilian systems to intentional EMI. His current research interests include power systems components for smart grids. Prof. Thottappillil was the Chairman of the EU COST Action P18 “Physics of Lightning Flash and its Effects” during 2005–2009. He is also a member of SC 77C of Swedish Electrotechnical Commission and IEC on High power transients, member of CIGRE WG C4.407 on lightning parameters for engineering applications.