Characteristics and Behavior of Transient Current during Multiple Lightning strokes on a Train in Thailand by using ATPDraw

This study describes characteristics and behavior of transient current during Multiple lightning strokes on train’s pantograph of 2×25 kV AC, 50 Hz Catenary Contact System. 25 kV AC, 50 Hz Catenary Contact System on double-track elevated railway system which connects Suvarnabhumi Airport to central Bangkok (Phayathai, Makkasan) was used in the study. The characteristics of transient current in term of its magnitude, front times and tail times affected by Multiple lightning strokes were studied. The assumption of studies based on the return Multiple lightning strokes current ranging 1–200 kA, front time of multiple lightning strokes between 1–3 μs, elevated pole resistance of 50 Ω and grounding resistance of 5 Ω. The Behaviour of transient current when the pantograph is at the Mast (4 th Mast) for Case 1 and at the mid-span of Masts (3 rd and 4 th Masts) for Case 2 were also studied. The Catenary Contact System on elevated railway system with a train and multiple lightning strokes were modeled in ATPDraw software. Transient current of negative multiple lightning strokes showed flashovers across insulators in any magnitude and waveform for Case 1 and Case 2. The improved protection for Catenary Contact System should consider the effects of negative multiple lightning strokes.


Introduction
Until recently, Catenary Contact System has been the most traction power supply system to deliver power to electric train (1)(2) .However, most of the overhead catenary transmission lines suffer from the lightning problems (2)(3) .
Lightning strokes are the major source of transient current with random characteristics and behavior which may lead to power system failure (3)(4)(5) .The overvoltage due to transient current during lightning strokes was shown to be mostly likely caused by a stroke to phase conductor, shielding wire, and ground in line proximity (3,6) .But the stability of power system can be more affected when lightning strikes on conductors (6)(7) .Flashover occurs when the induced overvoltage exceeds lightning withstand voltage level of insulators (2) .Current research studies have explained lightning effects when strikes in the Overhead Catenary Transmission line (1,3,(8)(9)(10)(11)(12)(13) .
Thailand is among of tropical countries which experience thunderstorm days and lightning activities per year.The report of lightning statistics in Thailand from Marungsri et al. (14) showed that lightning often occurs in April-May but severely in June.The magnitude of 11-171 kA with positive polarity and -10 to -139 kA with negative polarity were reported.Positive lightning strokes were accounted for 5% while negative were 95% with the magnitude of -10 to -50 kA.Negative lighting was showed to have a higher possibility of a multiplicity of strokes per flash (15)(16)(17)(18) .Most of the studies in the overhead catenary system have been examined the effects of lightning strikes on the mast, conductors, and traction substation by using several transient programs for simulation (1,3,(8)(9)(10)(11)(12) .These studies have been done on single lightning strokes without considering the effects of multiple lightning strokes.Despite the fact that the lightning protection was studied, but it is important to understand the variation of transient current behavior during multiple lightning strokes in different waveforms while doing lightning protection analysis.
In this paper, characteristic of transient current in a 25 kV Overhead Catenary transmission line is presented by using ATPDraw software.The behaviors of peak current, when negative multiple lightning strokes strike on pantograph are investigated.

Background
The examined system uses overhead catenary transmission line on elevated railway system which is designed with catenary wire, return wire and auxiliary wire as indicated in Fig. 1.The supply voltage of the system is 25 kV AC-50 Hz as per IEC 60850:2014 (19) .An overhead catenary transmission line of 480 m length supported by 7 masts with 60 m spacing was selected for simulation.The supply voltage was applied on both end points of the line.Since lightning behavior is unpredictable, the strike on pantograph as part of the conductor was mostly considered.Case 1 was considered the strike on pantograph at the 4 th Mast (see Fig. 1(a)) and Case 2 between the 3 rd and 4 th Masts (see Fig. 1(b)).The magnitudes of -34 kA, -50 kA, and -100 kA with 1/30.2 µs, 1.2/50 µs, 2/77.5 µs and 3/75 µs waveforms were used as lightning sources (14) .The elevated poles and grounding resistances were assumed to be 50 Ω and 5 Ω respectively.

Overhead Catenary Transmission line
The details of Overhead Catenary Transmission line involved Catenary line (R1), return line (R2), Auxiliary line (R3) with running rails (S-rail, I-rail) and distributed-parameter line spans on both sides of the impact point were given Table 1 (12) .A Railway Transmission line was modeled by LCC_8 with JMARTI model in ATPDraw as shown in Fig. 2. Catenary line, return line, Auxiliary line includes S-rail and I-rail were considered in this study as in Mazloom (12) ; Achouri, Achouri, and Khamliche (8) with Autotransformer and Booster transformer as modeled by 1:1 ideal transformers in ATPDraw.Yang and Zhang (9) showed that Autotransformer and Booster transformer force the traction current to return through designated return conductors to traction supply.Also, it reduces stray current which may cause electromagnetic interference with electrical systems in the vicinity of the railway system.Mast configuration parameters of 2×25 kV AC, 50 Hz Catenary Contact System are given in Table 2 (13) .Table 2. Mast Configuration (13) .Table 1.Details of 25 kV Overhead Catenary Transmission Line (12) .

Multiple Lightning Source Model
Negative lightning stroke with magnitudes of -10 to -50 kA, its association with multiple strokes per flash averaging 3 to 4 strokes per flash with intervals of tens of milliseconds were the most reported lightning incidences in Marungsri et al. (14) ; Omidiora and Lehtonen (15,17) ; Martinez-Velasco and Aranda (16) ; Rodriguez-Sanabria, Ramos-Robles, and Orama-Exclusa (18) .Three strokes per flash were used with intervals of 1 ms.The 1 st stroke was modeled with Heidler ideal source, the 2 nd and 3 rd strokes were modeled with two slope Ramp Type 13 in ATPDraw based on the characteristic of the lightning strokes.Parameters of lightning sources are given in Table 3, and the waveform of the multiple strokes is shown in Fig. 3.
Three ideal sources were used for the multiple lightning stroke current, with time duration of 0.6 ms for 1 st stroke and 0.3 ms for 2 nd and 3 rd strokes each.

Mast Model
The mast was modeled by cylindrical geometrical steel column in single wave impedance model as recommended by IEEE and CIGRE in expression (1) (9,20) .The modeled parameters of the mast are shown in Table 4.

Train Model
The three-car train was modeled as electric locomotive which contains Pantograph, Locomotive transformer, Diode rectifier bridge and two DC motors as in Zupan, Teklić, and Filipović-Grčić (21) ; KaragÖz (22) .Figs. 4 shows the model of Train contains Pantograph, Locomotive transformer, Diode rectifier bridge and one DC motor with Railway Transmission line, Mast, Elevated pole, Ground and Insulators at the Mast and at the mid-span of Masts.Insulators of Mast were modeled with a branch of capacitor and voltage controlled switch as shown in Fig. 4(a).To model this insulator, Switchvc.supmodel was used in ATPDraw with voltage withstand capability as calculated values given in Table 1 (23) .Table 3. Parameters of multiple lightning sources (14)(15)(16)(17)(18) .

Simulation Results
A 25 kV Railway Transmission line with 7 masts in Case 1 and Case 2 were simulated in ATPDraw with multiple lightning sources on the pantograph where elevated poles and grounding resistance were set to 50 Ω and 5 Ω unchanged respectively.The magnitude, front time and tail time of negative multiple lightning strokes were studied as factors that cause a transient current which leads flashover across insulators.
As the impulse voltage withstand capability of insulator depends on the front time of lightning stroke current, the multiple lightning stroke current of -34 kA with 1/30.The magnitudes of mast induced voltages and flashover across insulators with multiple lightning strokes current for Case 1 and Case 2 are shown in Table 5 and Table 6 respectively.Key: S -flashover, Ax -Auxiliary line, Rt -Return line and Ct -Catenary line

Conclusion
This paper has studied the characteristics of transient current in term of its magnitude, front times and tail times of negative Multiple lightning strokes and its behavior when the pantograph is at the Mast (4 th Mast) and at the mid-span of Masts (3 rd and 4 th Masts).The analysis was made for the flashover voltage across insulators of a 2×25 kV,50 Hz AC Catenary Contact system used in Thailand.It was seen that shorter front time, resulted into higher mast induced voltage.Similarly, a higher magnitude of Multiple lightning strokes, induced higher voltages across mast insulators for case 1 and case 2. The Flashover occurred for all waveforms and current magnitude from -34 kA to -100 kA when pantograph was at the Mast and at the mid-span of Masts.Critical flashover voltage was found in case 1 compared to case 2. Therefore, short front time and high magnitude of negative multiple lightning strokes are key parameters which may be considered in lightning protection.
(a) at the Mast.(b) at the mid-span of Masts.
(a) At the Mast.(b) At the mid-span of Masts.

Table 4 .
Modeled Parameters of Mast.