# Decreasing the Rail Potential of High-Speed Railways Using Ground Wires
## 1 Introduction  
In electrified railways, the rail not only serves as the running track of the train but also participates in the overall process of traction return current flow. Strictly speaking, it is not a simple grounding conductor, and the connection between the rail and the ground is not completely insulated. Instead, there is leakage conductivity that causes some of the current on the rail to flow into the ground [1]. Then, the current returns back to the rail in places near the substation and return point. The maximum value of the rail potential, which may cause a series of hazards to railway system operation, arises at the position where the train runs and at other positions where the current returns [2]. In the electrical discipline, the rail potential can be reduced by increasing the rail-ground leakage conductance [3]. However, in the signal discipline, in order to ensure the reliable operation of the signal track circuit, the ground leakage conductance should be as small as possible. From this point of view, understanding how to better cooperate between railway electrification engineering and railway signaling engineering is also a key issue in the study of ground return [4]. As an important path for traction return, the rail should have its potential within a safe range guaranteeing the safety of train operation [5]. Generally, a high-power traction load, i.e., a high-power electric locomotive, absorbs a large traction current from the traction network, which in turn leads to a large rail return current [6, 7]. Furthermore, under the condition of a high driving density, a great amount of rail current will be formed by thesuperposition of multi-locomotive currents, resulting in a high rail potential. It is necessary to take certain measures to reasonably control the distribution of the rail potential and rail current, preventing a large step voltage and contact voltage, and ensuring the safe and reliable operation of electrified railway systems [8; 9]. Although China's railway development began later than that of other countries, China has gradually become the country with the highest high-speed railway mileage in the world. High-speed railways have the characteristics of a large traction current, large rail leakage resistance, etc. It is necessary to constantly improve the problems existing in the traction return system to adapt to the development of railway systems [10]. For the distribution problem of traction return flow, ref. [11] mainly studied the influence of the wire diameters of different through-ground wires on the distribution of traction return current proportions and analyzed the current proportion distributed by each wire in the return current path through modeling. Reference [12] focused on the increase in the rail current and potential, analyzing the distribution of the traction current in the track. France, Germany and Japan are countries that developed their high-speed railways earlier and are typical representatives of advanced railway electrification technology [13; 14]. The grounding mode of French railway lines is direct grounding with integrated ground wires. The return process is completed jointly by rails, return wires, ground wires and choke coils. Equipotential bonding is achieved through the full transverse connection of longitudinal conductors. The full transverse connection distance between two track circuits is generally not less than 1 km, which is also convenient for rail break inspection. German railways also adopt the direct grounding mode, with integrated ground wires. The equipotential bonding line, rail and return line of each longitudinal conductor complete the return process together. The rail has longitudinal and horizontal electrical connections. In order to obtain a smooth return path and ensure the normal operation of the track circuit, the conductor in the integrated earthing system is grounded and connected to a grounding strip. The grounding system of the Japanese railway has the functions of lightning protection and reducing electromagnetic induction.