Investigation of the time process of arc separation of the contact network positioner support structures

. The contact network is important for the traction power supply system to supply power to electric locomotives. It is significant to ensure that the contact network works properly. Whether the locator is intact directly affects the operation of the contact network. The traction power supply system has dynamic load vibration and wind dance vibration during operation. The contact network positioner and the wrist arm support structure appear to be separated, and there is a potential difference between the positioner and the support structure, which generates an electric arc and ablates the positioner and the wrist arm support structure device. Therefore, this paper selects the magnetohydrodynamic (MHD) mathematical model with reference to the arcing model modelling method. COMSOL Multiphysics software is used to simulate the arc transient process of the contact network locator support structure separation, analyse the electric field between the locator articulated parts, give the temperature distribution of the locator and the articulated parts, and analyse the influencing factors of the temperature distribution between the locator and the articulated parts. This provides guiding suggestions for the design of contact networks and the selection of positioners in engineering.


Introduction
The contact network equipment works in an open-air environment and often encounters the effects of severe weather conditions such as wind, rain, lightning and thunder without any other protective measures or back-up equipment.During the travelling of electric locomotives, factors such as uneven train tracks, insulation failures, component detachment, electrical coupling failures, and contact line ice cover will lead to the existence of air gaps in the bow network, and the high voltage and high field strengths on the contact network will lead to the breakdown of the air gaps in the bow system, thus generating bow network arcs [1,2].If the contact network equipment is damaged, the external power supply cannot provide energy to the locomotive through the contact network, resulting in the locomotive not being able to operate normally, and even bringing more serious harm.
At present, there are two main research ideas to study the arc model: one is to study the external characteristics of the arc mathematical model, the arc as a two-terminal network, ignoring the internal complexity of the physical process, only to study the relationship between arc voltage and current; the other is to study the internal physical characteristics of the mathematical model, it is the existence of the arc throughout the process of the physical phenomena as the basis for the study, the mathematical method to describe the formation of mathematical equations that can be solved, the method can be accurately described the characteristics of a particular occasion of the electrical properties.
In the study of the arc model, not only the mathematical model should be considered, but also the influence of the physical properties of the arc on the equations, and only the coupling of the two can lead to the result of an accurate arc model.With the development of MHD numerical analysis technology, people will consider combining with the gas equation of state, Ohm's law and other equations on the basis of MHD equations to further promote the development of a 3D arc MHD model.
In this paper, we have chosen a magnetohydrodynamic (MHD) model to simulate the electromagnetic and heat transfer module using the modelling software COMSOL Multiphysics.In this paper, we give the electric field mode curve diagram of the gap of the locator articulator, elaborate on the generation, development process and arc quenching process of the arc, observe its temperature distribution, and analyse the influence factors of the temperature distribution of the locator's hook and loop structure, including the arc current, airflow speed, arc length.

Electric arc modelling theory
Arc generation needs to be in a high-pressure environment, the essence of the gas discharge phenomenon, this gas discharge is self-sustaining discharge, and does not need a very high maintenance voltage can be maintained, a maintenance voltage of about 7V/cm. in the initial stage of the formation of the arc, the cathode and the anode have a large electric field strength between the cathode and the anode, cathode surface of the electrons into the arc gap into the free electrons, under the action of the electric field, the cathode surface emission of electrons and arc gap hair other particles collision between the two poles is full of charged particles between the two poles were pierced to form an electric arc.Under the action of the electric field, the electrons emitted from the cathode surface and other particles in the arc gap collide, the two poles are full of charged particles between the two poles, and the air gap between the two poles is pierced to form an electric arc.After the formation of the arc, the high temperature emitted makes the cathode surface thermally free, and more electrons on the cathode surface enter the arc gap to continue the process, so no high voltage is required to maintain the arc of the ignition of the incandescent [3].
The locator arc is a complex process of mutual coupling and change between multiple physical fields such as electric field, magnetic field, airflow field and thermal field, which affects many physical factors with strong randomness, and the theoretical study is more difficult [4].Traction network power supply system in the operation process due to the presence of dynamic load (electric locomotive or rolling stock) vibration, wind dance vibration, etc., in the combined factors may cause the contact network locator and wrist arm support structure separation of the situation, by the dynamic load electrical parameter distribution and the contact network impedance distribution parameter, the locator and the support structure of the voltage difference between the voltage increases dramatically, the air is broken, the formation of the electric arc [5][6][7][8][9][10][11][12].The arc root tends to be in motion, and as the separation distance increases, the arc is elongated and the temperature of the arc column rises, at which time thermal and photoelectron emissions are dominated.The arc is extinguished until the contact grid positioner and the wrist arm support structure come into contact with each other again.

Structural parameters of the contact network positioner
A typical contact network is shown in Figure 1.The positioner and hinge structure are shown in Figure 2. Other parameters are shown in Table 1.

Structural parameters of the contact network positioner
Combine the table to create a locator connection diagram as shown in Figure 3, and in COMSOL Multiphysics software, create a two-dimensional model of the locator as shown in Figure 4.In the material module, set the positioner and the articulated structure as copper (Cu), and the other parts as air (air).Hook and loop structure through current according to the actual situation choose 200A, hook and loop structure separation distance choose 1mm, arc resistance 0.05 ohm, while considering the wind speed of 1km / h.Define the hook and loop structure separation position for the boundary heat source, with the software that comes with the physical field control mesh mesh on the model.Calculate the locator and articulation, wrist arm support hook and loop structure in the separation process of the spatial electric field distribution, when the locator and the fixed device are apart, the two will be in the role of the voltage difference produce an arc, the arc maintains the voltage is only 7V/cm; locator and the fixed device separated from the arc did not produce when the two gaps between them in the process of maintaining the voltage difference, when it will produce a discharge, the discharge electric field of 3000V/mm.When the voltage between the two is 200V, the distance between the two is 2mm, and the two separations process electric field distribution local diagram shown in Figure 5 The electric field distribution curve of the air gap after the positioning device is separated from the fixing device is shown in Figure 6.According to the curve, the point where the electric field strength is zero is inside the metal, and the point where the electric field strength is not zero is the hook and loop structure air gap.The maximum value of the electric field strength is 80000V/m, and this is the location where breakdown and arcing are most likely to occur.

Generation and development of electric arcs
The simulation mimics the process of passing a certain amount of current under a certain gap and letting it naturally extinguish the arc after 1s.When the electric field strength is greater than 3000V/mm, the strong electric field emits electrons and generates an electric arc.Arc generated by the high temperature so that the electrons have enough energy to launch outward, the electrons in motion constantly collide with other particles, the formation of free electrons and positive ions, the gap is full of electrons and positive ions, the air was pierced to form an electric arc.When the current is no longer passed, the electric field strength decreases, and when it is lower than the arc maintenance voltage of 7V/cm, the air gap charged particles go free process dominates, and the arc tends to extinguish.The temperature distribution curve of the positioner hook and loop structure at different moments is shown in Figure 7.  Through the calculation and comparison, it can be seen that as the current increases, the temperature of the arc also increases, when the value of the current exceeds the safe operation of the range of 250A, the arc temperature rises rapidly, locally up to 5000 degrees Celsius, the surface temperature of the hook and loop structure is too high, if it reaches the melting point of the metal, resulting in the loss of heat on the surface, the generation of the tip, resulting in the emergence of the metal part of the corrosion damage to the ageing, reduce the mechanical characteristics of the hook and loop structure, affecting the safety.

Effect of airflow velocity on arc temperature
Considering the influence of airflow during the arc generation process, the temperature distribution of the arc through 200A current at wind speeds of 1km/h, 2km/h, 3km/h, 4km/h, and 5km/h is simulated as shown in Figure 9.
From the simulation results, it can be seen that the arc temperature decreases with the increase of the gas flow rate.The increase in airflow speed leads to the arc length being elongated, the arc resistance increases, the arc energy decreases and the temperature decreases under the condition of constant voltage.At the same time, the acceleration of the wind speed also accelerates the heat dissipation process of the arc to a certain extent, reducing the arc temperature, the

Effect of hook and ring separation gap on arc temperature
When the wrist arm support structure occurs a large vibration or other unfavourable factors combined, the equipotential device will appear in the phenomenon of hook and loop separation, the hook and loop structure separation distance determines the length of the arc, combined with the actual situation, the calculation compares the temperature distribution under the arc length of 1mm, 2mm, 3mm, 4mm, 5mm arc, as shown in Figure 10.With the increase of the separation distance of the hook and loop structure, the maximum value of the arc temperature gradually decreases, the separation distance of the hook and loop structure determines the length of the arc, and the long arc is higher in energy and temperature than the short arc.Stable arc burning arc resistance is greater, the more energy dissipated between the arc gap, the more conducive to arc extinguishing.

Conclusion
The high temperature generated by the locator arc will erode the locator and the articulated structure, which will cause serious harm to the train operation, so it is of great practical significance to take the locator hook-and-loop interstructural arc as the research object.
COMSOL Multiphysics software is used to build a locator hook and loop structure model, and the complex physical process of the arc is simulated.Some research has been carried out on the relationship between the various physical fields of the hook-and-loop structural arc.The following are the main research works: The electric field pattern of the gap of the hook and loop structure of the positioner was simulated.When a current exists between the hook and loop structures, an arc can be generated by the separation of the hook and loop structures, and the arc is considered to be maintained at a voltage of 7 V/cm until the hook and loop structures come into contact again.
Arc current, airflow speed, and arc length are important factors affecting the distribution of arc temperature.The larger the locator arc current is, the temperature of the arc is also increasing, when the current value exceeds the range of safe operation and reaches 250A, the arc temperature rises rapidly and locally reaches 5000 degrees Celsius.The greater the airflow speed of the surrounding environment, the arc length is elongated and the arc heat is reduced, while the airflow accelerates the heat dissipation of the arc to a certain extent, reducing the arc temperature.The larger the hook and loop gap, the longer the arc, the arc column resistance increases, the voltage remains constant under the conditions of the arc heat is reduced, the arc temperature is reduced.
The temperature in the hook and loop structure is lower than the arc temperature, which affects the physical properties of the surface of the hook and loop structure, and the temperature decreases within the hook and loop structure after the arc is extinguished.

Fig. 1 .
Fig. 1.Models of each component of the catenary.

E3S
Web of Conferences 458, 01020 (2023) EMMFT-2023 https://doi.org/10.1051/e3sconf/202345801020 3 Power plant analysis and the development process of the arc 3.1 Electric field analysis of the locator hook and loop structure

Fig. 5 .
Fig. 5. Cloud view of the electric field distribution after separation of the positioning device from the fixing device

Fig. 6 .
Fig. 6.Electric field distribution after separation of the positioning device from the fixing device.

Fig. 7 .
Fig. 7. Temperature profile of hook and loop structure at different moments.

Table 1 .
Positioner and hinge structure parameters