A study on assessment of dropper life for conventional line speed-up

Challenge H: For an even safer and more secure railway A study on assessment of dropper life for conventional line speed-up K. Lee, Y.H. Cho, Y. Park, S.Y. Kwon, S.H Chang Korea Railroad Research Institute, Uiwang-City, Korea Abstract In steady state, tensile load caused by contact wire dead load is applied to droppers. At the moment a train passes, the droppers are slackened by pantograph uplift force. After the train passes through, variable force is applied to the droppers and tightened again due to contact wire deadd load. Increase of dynamic load is caused by pre-sag that increases the static applied to a dropper, and if dynamic load becomes too large, fatigue rupture may occur. Starting from April 2004, the 300km/h class Kyungbu High-speed Line has suffered dropper rupture at over 55 locations. Even on the express line of France, cases of dropper rupture have been reported. Accordingly, assessed in this paper is the influence of pre-sag to dropper life measured at the conventional Honam Line to increase speed and current collection performance. First, to estimate fatigue life of a dropper, a rupture mode and a P-N diagram was analyzed and obtained from a developed dropper fatigue tester. Then, the effect of the amount of pre-sag and train speed to dynamic load when a train passess was analyzed by a catenary-pantograph simulation program. Results of estimated variable force history were simplified by rainflow cycle counting. Then, dropper fatigue life was assessed by applying Miner's rule to the simplified force history and P-N diagram. The proposed dropper fatigue test method and fatigue life assessment procedures are expected to be an assessment guideline and securee safety for tension increases for speed enhancement in respect to newly developed droppers and reliability of droppers when pre-sag is applied. Introduction In electric railways, overhead contact lines compose an important interface that make contacts with pantograph(s) installed on rooftops of an electric train to supply power. Along with the opening of Kyung-bu High-speed Line, Korea has been researching many ways to develop high-speed overhead contact lines and increase speed of conventional lines.to speed up conventional lines to over 180km/h various methods such as ncreasing tension and applying pre-sag to contact wires are being studied[1]. For a simple catenary type, applying pre-sag improves current collection performance, resulting speed enhancement [2]. In steady state, tensile load caused by contact wire dead load is applied to a dropper. At the moment a train passes, the droppers are slackened by pantograph uplift force. After the train passes through, variable force is applied to the droppers and tightened again due to contact wire dead load. Increase of dynamic load is caused by pre-sag that increases the static applied to a dropper, and if dynamic load becomes too large, fatigue rupture may occur. Starting from April 2004, the 300km/h class Kyung-bu High-speed Line has suffered dropper rupture at over 55 locations [3]. Even on the express line of France, cases of dropper rupture have been reported [4]. With application of pre-sag, the static force acting on the first droppers is increased. If static force increases, dynamic force is also expected to increase.therefore, to increase commercial speed on conventional lines, it is essential to assess safety of droppers and dropper clamps when pre-sag is applied. In this study, we anticipated the safety of dropper cable and dropper clamp with pre-sag applied on the fatigue side of view.first, a rupture mode and a P-N diagram was analyzed and obtained from a developed dropper fatigue tester[5]. Then, the effect of the amount of pre-sag and train speed to dynamic load when a train passess was analyzed by a catenary-pantograph dynamic simulation program[6] verified by the field tests and etc. Results of the estimated variable force history were simplified by rainflow cycle counting. Then, dropper fatigue life was assessed by applying Miner's rule to the simplified force history and P-N diagram. Fatigue Test Results for dropper and clamp In steady state, tensile load caused by contact wire dead load is applied to a dropper. At the moment a train passes, the droppers are slackened by pantograph uplift force as Fig. 1. After the train

passes through, variable force is applied to the droppers and tightened again due to contact wire dead load. Fig. 1 Dynamic behavior of dropper A fatigue tester that can implementsuch dynamic behavior of droppers was developed[5]. As in Fig. 2, 3,a dropper cable and a dropper clamp are attached to a contact wire with a weight connected to the bottom side to exert load on the dropper. The below structure makes it possible to copy the dynamic behavior of droppers. Fig. 2 Dynamic behavior of dropper by the tester Fig.3 Drive method for the fatigue test Shape and materials of the droppers and dropper clamps for fatigue tests are shown below. Table 1 Characteristics of the dropper cable Conductor Dropper Nominal cross section area [ ] 12 Chemical composition [%] Bronze 72 Min. breaking load [N] 6,943 Linear mass [kg/m] 0.103 Fig. 4 Cross section of the dropper cable (1wire, 7strands: A = 0.65, 6wires, 7strands: G = 0.54, D = 5.19 ) Name Chemical composition [%] Table 2 Characteristics of the dropper clamp Dropper clamp Pb(below 0.05), Fe(below 0.10), Sn(7.0-9.0), Zn(below 0.20), P(0.03-0.35), Cu+Sn+P(above 99.7) Tensile withstand load [N] 2,942

Challenge H: For an even safer and more secure railway Fig. 5 Configuration of Dropper cable and clamp for Honam conventional line After conducting fatigue tests on conventional line droppers with the fatigue tester at 1Hz, all hangers of dropper clamps with different loads were ruptured regardless of whether if they were from the contact wire side or the messenger wire side as shown in Fig. 6.Shown in Fig. 7is a dropper clamp right before totally being ruptured by the fatigue test. Crack initiation can be clearly observed. In Fig. 7 (a) a crack was initiated from the left side of the hanger which is exposed to direct contact with the wire hanger and in Fig. 7 (b) signs of wear out can be observedfrom the hanger part.it can be determined from the specimensthat wear out caused by friction between hangers and wire hangers affects life of the dropper clamp. Mechanical abrasive wear reduces cross section of the contact surface concentrating stress at that point, eventually resulting rupture of the dropper clamp. Fig. 6 Fractured specimen after fatigue testfig. 7 Crack initiation of dropper clamp Fig. 8displays a P-N curve ofthe results of conducting fatigue tests in ambient temperature for droppers used in the Honam conventional line.the specimen which was ruptured at 96,075 cycles with 107.5daN was excluded. This result should beunreliable since the specimen was dislocated from the center of the weight when it was installed on the messenger wire side. Fig. 8 P-N Curve for the dropper using in Honam conventional line

Where, linear fit : Log(y) = -0.36929Log(x) + 3.96401 Dynamic Simulations between catenary and Pantograph Pre-sag, which is applied for speed enhancement of overhead contact lines, affects static force and dynamic force exerted on droppers depending on the amount of pre-sag applied. Therefore, static and dynamic forceson the first droppers that vary by the amount of pre-sag applied to overhead contact lines and train speed were calculated for conventional lines using the developed simulation program[6]. Parameters of the pantograph and conditions of the Honam conventional line that were used in the simulation are shown in Table 3 and Table 4, respectively. Table 3. Parameters of the pantograph [7] Parameter Value Mass of the pan-head [kg] m u = 8 Mass of the upper frame [kg] m m = 9.1 Mass of the lower frame [kg] m l = 23 Damping coefficient between the pan-head and the upper frame [Ns/m] c u = 0 Damping coefficient between the upper frame and the lower frame [Ns/m] c m = 60 Damping coefficient between the lower frame and the train roof [Ns/m] c l = 140 Static uplift force [N] P 0 = 70 Aerodynamic force [N] where V is the train speed in m/s f aero = 0.018V 2 Stiffness coefficient between the pan-head and the upper frame [N/m] k u = 9,000 Stiffness coefficient between the upper frame and the lower frame [N/m] k m = 1,200 Stiffness coefficient between the lower frame and the train roof [N/m] k l = 1 Table 4 Catenary condition for Honam conventional line Parameter Value Contact wire Tension of contact wire Messenger wire Tension of messenger wire System height Cu110 12,000 N Bronze 65 12,000 N 960 Considering the Honamconventional line conditions, 10 spans were modeled and 1 span from the center was processed as effective data. To study the influence of train speed and amount of pre-sag on dynamic force exerted on droppers, variable speedthe maximum speed of Honam conventional line, 150km/h and 180km/h for speed enhancement;and variable amount of pre-sag; no pre-sag, 1/2,000, 1/1,000 of 50m standard span length were applied as simulation cases, respectively. Since static force exerted on the first dropper becomes relatively large when pre-sag is applied to a span

with asymmetric contact wire height, the simulation was conducted speed by speed after adjusting dropper length to the worst case. Table 6. Simulation cases Case number Train speed Pre-sag Condition of contact wire height Case 1 Case 1.1 150km/h N/A Symmetric Case 2 Case 3 Case 2.1 150km/h 1/2,000 Case 2.2 180km/h Case 2.3 180km/h Asymmetric Case 3.1 150km/h 1/1,000 Symmetric Case 3.2 180km/h Case 3.3 180km/h Asymmetric Fig. 9 and 10are the results of calculating static force exerted on the dropper. It is observed from the results that static force exerted on the first dropper is the largest when pre-sag is applied. Fig. 9 Static force on the dropper (Case 2.1 & 2.2)Fig. 10 Static force on the dropper (Case 3.1 &3.2) The following figures show the results of calculating variations of force exerted on the first dropper right after a train passes through. The results of applying pre-sag of 1/1000 of span length for cases of 150km/h and 180km/h train speed are shown in Figs. 12, 13, respectively. And the result of applying pre-sag of 1/1000 of span length, 180km/h train speed, and asymmetric contact wire height is shown is Fig. 14. Comparing Fig. 11 and 12, with constant speed, the vibration level rises as the amount of pre-sag increases. Vibration levels also rise as speed increases. Maximum dynamic force as well as vibration level exerted on the first droppers was determined to be the highest at asymmetric contact wire height.also indicated in the results is that in Case 3.3; 180km/h and asymmetric contact wire height, maximum dynamic force and vibration level as soon as a pantograph passes through were the highest.

200 200 Force on the dropper, N 150 100 50 Force on the dropper, N 150 100 50 0 0 2 4 6 8 10 0 0 2 4 6 8 10 Time, s Time, s Fig. 11 Force on the dropper for Case 2.1Fig. 12 Force on the dropper for Case 3.1 200 2 0 0 F o rc e o n th e d ro p p e r, N 1 5 0 1 0 0 5 0 Force on the dropper, N 150 100 50 0 0 2 4 6 8 1 0 T im e, s 0 0 2 4 6 8 10 Tim e, s Fig. 13 Force on the dropper for Case 3.2Fig. 14 Force on the dropper for Case 3.3 Life Assessment The variable force exerted on the first dropper calculated from the simulations depending on train speed and amount of pre-sag provided in the previous section was simplified by the rainflow cycle counting. Then this simplified force history and P-N curve obtained from fatigue tests were applied to Miner s rule, a rule that describes linear cumulative damage to obtain the numbers of cycles(bf) that pantographs pass throughto resultdropper rupture. Calculation results are stated below in Table 7. Table 7 Calculation results of life evaluation Case number B f [cycle] Case 1 Case 1.1 22,781,145 Case 2.1 34,676,204 Case 2 Case 2.2 26,342,329 Case 2.3 12,821,771 Case 3.1 44,070,886 Case 3 Case 3.2 24,996,812 Case 3.3 10,856,424

At high-speed, as the amount of pre-sag increases(case 3), static force and dynamic force exerted on the first dropper also increases, shortening life of droppers. In comparison with Case 2.1; 150km/h train speed, small amount of pre-sag and Case 3.1; 150km/h train speed and large amount of pre-sag, the reason that the life of Case 3.1 is longer is determined to be that the degree of droppers being slackened differ, resulting small variations of dynamic force and that the trend of force being exerted also differ.from Case 2.2 and Case 3.2, under same overhead catenary conditions, variable force increases as speed is increased and increased variable force results shorter dropper life. From Cases 2.3 and 3.3, under same conditions, dropper life is expected to be shorter at a span with asymmetric contact wire height compared to a span with symmetric contact wire height. The case that was determined to have the shortest life is Case 3.3; span length 50m, 1/1,000 presag, asymmetric contact wire height which was ruptured at approximately 10,856,424 cycles. Even if dropper life of the Honam conventional line is assumed to be equal to contact wire wear life suggested in UIC 799[8]; 2,000,000 cycles, calculated life for the worst provided case is about 5 times longer. This 5 time difference should make applying pre-sag 1/1,000 of span length harmless enough for even cases with worse conditions that were not considered. Discussion and conclusions In this study, we anticipated the safety of dropper cable and dropper clamp with pre-sag applied on the fatigue side of view.first, a P-N diagram was obtained from a developed dropper fatigue tester. Then, the effect of the amount of pre-sag and train speed to dynamic load when a train passes was analyzed by a catenary-pantograph simulation program. Results of estimated variable force history were simplified by rainflow cycle counting. Then, dropper fatigue life was assessed by applying Miner's rule to the simplified force history and P-N diagram obtained by fatigue tests. In conclusion, the influence of increasing speed and pre-sag on the Honam conventional line droppers is reduced dropper life due to increased static and dynamic force exerted on the first dropper of a span. In addition, assuming identical span length, speed, and pre-sag, dropper life is expected to be shorter if the contact wire height is asymmetric compared with the symmetric case. By assuming parameters of the Honam conventional line and catenary conditions used in the case with 1/1,000 pre-sag of span length, minimum dropper life was assessed to be 5 times longer than the expected life that was provided. Therefore, it is determined to be safe enough to apply pre-sag at high speeds even if corrosion and other environmental conditions were to be considered. The proposed dropper fatigue test method and fatigue life assessment procedures are expected to be an assessment guideline and secure safety for tension increases for speed enhancement in respect to newly developed droppers and reliability of droppers when pre-sag is applied. Acknowledgements We gratefully acknowledge that this Advanced Material Tilting Train System Project has beensupported by Korean Ministry of Land, Transportand Maritime Affairs. References [1] Kiwon Lee, Yong Hyeon Cho, Young Park, Sam-Young Kwon, Improvements of Existing Overhead Lines for 180km/h operation of the Tilting Train, World Congress on Railway Research, 2008 [2] YongHyeon Cho, Kiwon Lee, Young Park, Bubyoung Kang, Ki-nam Kim, Influence pf contact wire pre-sag on the dynamics of pantograph-railway catenary, International Journal of Mechanical Sciences, 52, 1471-1490, 2010 [3] Tae-Hoon Lee et all, A Study on Fatigue Analysis of Dropper for High Speed Electric Railway, Korean Institute of Electrical Engineers Summer Conference, 2008(printed in Korean) [4] A. Bobillot, L.M. Cléon, A. Collina, O. Mohamed, R. Ghidorzi, Pantograph-Catenary: a High-Speed European couple, the 8th World Congress on Railway Research, 2008

[5] Kiwon LEE, Yong Hyeon Cho, Sang Hoon Chang, Chang-Sung Seok, A study on assessment of a fatigue life for a dropper in high-speed overhead line, International conference on Engineering Failure Analysis IV, 2010 [6] Yong Hyeon Cho, Numerical simulation of dynamic responses of railway overhead contact lines to a moving pantograph, considering nonlinear dropper, Journal of Sound and Vibration 315 pp. 433-454, 2008 [7] Honam line electrification: Final report - Col 4/6 Dynamic simulations, SYSTRA, 2002 [8] UIC799 - Characteristics of A.C. overhead contact systems for high speed lines worked at speeds of over 200km/h, International Union of Railways, 2002