Review on Stray Current Distribution and Corrosion Characteristics of Cross-Water Metro Systems

The DC traction power system adopts the track as the return rail. When the track-to-earth insulation in the subway tunnel deteriorates, stray currents will cause electrochemical corrosion to tunnel steel structures and seriously affect the service life and safety of metro tunnels. Stray currents cannot be directly measured and can only be calculated. Therefore, a calculation model with a hollow circular cross-section structure was proposed, and the stray current distribution in tunnel steel structures was calculated. In addition, the effects of different rail-to-ground transition resistances and adjacent buried metallic pipelines on the stray current distribution of the tunnel steel structures were taken into account. The results show that the total amount of stray current dispersed into the tunnel steel structures and soil is similar. The stray current density distribution in each steel tunnel is related to its location. The total stray current carried by the steel structures of the bottom tunnel segment is 102, 15.7 and 3.1 times higher than that of the top, upper and lower side tunnel segments, respectively. The reduction in the transition resistance and increase in the distance of the train from the traction substation increase the total rail leakage current and have a small effect on the percentage distribution of stray current in tunnel structures. The buried metal pipeline parallel to the tunnel has a lower impact on the total stray current leakage, but can reduce the total stray current in steel structures and drainage net, enlarging the positive stray current scope of some tunnel steel bars, further increasing the stray current density on tunnel steel bars. The results of this study can be used to determine the degree of corrosion of the underground steel tunnels and thereby provide support for corrosion prevention.

By simplifying and limiting current circuit model of metro power supply, this paper firstly showed stray current production and its damage, then analyzed bilateral power supply system using resistor network theory, and constructed stray current analytical model of bilateral power supply, computer simulation in different conditions of stray current distribution was carried out, the results show that metro stray current is affected by substation pitch, railway portrait resistor, rail-to-ground transition resistor and other factors, the distribution of stray current is provided macroscopicly in a qualitative way. This gives theoretical basis on how to set the distance of substation. Ultimately it provides a good solution to reduce the stray current corrosion.

In a rail transit system, there is a constant leakage of current from the subway rails to the earth, and these stray currents have complex propagation paths and a wide range of influence. Since no stray current collection devices are installed at subway depots, some of the stray current leaking from the mainline will converge at the depot, seriously corroding the structural reinforcement and buried metal of the station, thereby jeopardizing the normal operation of subway trains and passenger safety. In this paper, a field-circuit coupling method is proposed to analyze the current leakage and distribution law of the subway mainline and depot. It is found that the failure of the gauge block at the mainline will trigger the maximum leakage of rail current. Additionally, it is observed that the stray current distribution at the depot is mainly influenced by the operating status of the one-way conduction device (OWCD) and the change of rail potential. These results validate the applicability and effectiveness of the field-circuit coupling method proposed in this paper and provide new technical support for the study of stray current leakage distribution in subways.

Stray current directly affects the regular operation of electrical equipment and facilities in the subway DC traction power supply system. Therefore, it is worthwhile to study the stray current distribution characteristics during train operation and the quantitative corrosion of buried pipelines. This paper introduces the traction characteristics of power carriages and power wheelsets of subway vehicles into the DC traction process. A finite element model considering the dynamic distribution of stray current under the actual operation of subway vehicles is established. The interference characteristics of stray current and the contribution of power sources under the multiparticle model are analyzed. The rail insulation damage caused by long service time and the quantitative calculation of rail and buried pipeline corrosion is considered. The model results show that the stray current in the buried pipeline under the multiparticle model is more accurate and more suitable for the protection in the actual subway. The quantitative corrosion of the buried pipeline is stronger than the partial insulation damage environment when the rail is not insulated. The rail and buried pipeline corrosion at both ends of the insulation damage position is relatively severe. The stray current distribution model established in this paper gives full play to the solution advantages of the finite element method and provides a new idea for the quantitative calculation of buried pipeline corrosion.

In DC metro systems, the stray current may cause many challenges for the nearby metal structures and the AC power systems, such as the corrosion of the buried pipes and the grounding grids of substations. Therefore, calculating the stray current and analyzing the distribution characteristics of the stray current are necessary for the design of the buried pipelines and substations. To obtain the stray current, considering the through-type structure characteristics of the return current system and the traction power supply system, a multiple power supply sections simulation model (multiple-section model) of stray current is proposed. In the proposed model, the train is equivalent to two dc current sources, and the dynamic results can be completed. Through comparing the rail potentials of simulation results and the field test data of the Shenzhen metro line, the proposed model is verified. And based on the proposed model of the Shenzhen metro line, the stray current distributions are obtained and analyzed. The results indicate that the stray current distribution has a certain periodicity which is approximately equal to the headway time of the train.

The stray current will be generated in the process of metro operation. It is of great significance to study the distribution characteristics of stray current in metro to realize the impact assessment of buried equipment. In this paper, the resistance network model of "rail-drainage net-structural steel reinforce-earth" stray current distribution under double-end power supply mode is established, and the analytical expression of stray current distribution is derived. The variation law of stray current distribution is studied when the locomotive current, the distance between locomotive and substation, the longitudinal resistance of track and the transition resistance of track and ground change. The three-dimensional model of shield metro is built by using MALZ module in CDEGS, and the correctness of the resistance network model is verified. The proposed method clarifies the distribution characteristics of stray current, which has certain guiding significance for effective suppression of stray current and corrosion prediction of buried metal structures.

Electrochemical corrosion happens when stray currents leak out of buried pipeline. This will threaten the safety of pipeline operation and operators. The distribution of stray current was studied by simulation system. The results indicate that stray current intensities have the same distribution with potential gradient Esx which parallel to the metallic pipeline. The distribution curves of stray current show regular symmetry. The stray currents increase gradually along the buried metallic pipeline. It reaches maximum at the pipeline midpoint. The potential gradient Esy distribution curves in y-direction show a hyperbolic shape. For the exits of faradic electric field, stray current intensity is maximum in the beginning and it flows into the pipeline. It flow along the pipeline and parallel to the pipeline at middle. After that, the stray currents begin to flow back to cathode and the stray current intensity reaches maximum at the end of pipeline.

Stray current flows through the buried metallic pipeline which exposed to electrochemical corrosion attack. The distribution of stray current is studied by Finite Element Method analysis model, the results indicate that stray current increase gradually along the buried metallic pipeline, reaching to a maximum value in an intermediary point, and then decreases gradually, has a regular bilateral symmetry. The distribution model of stray current is discussed by the inducted electric field theory.

Electromagnetic plasma accelerators which can generate high-density and hypervelocity plasma jets have been widely used in plasma physics research and application fields. An experimental platform of parallel-plate accelerator electromagnetically driven plasma is established in this paper, mainly including a parallel-plate accelerator, a power supply, magnetic probes, photodiodes, a current probe, and an oscilloscope. The current distribution and plasma velocity characteristics of a parallel-plate accelerator under static pressure are studied by using magnetic probe array and photodiode array. The working gas is synthetic air. A mechanical pump is used to pump the vacuum chamber to about 1 Pa, and then synthetic air is injected into the vacuum chamber to a target pressure. The power supply of the parallel-plate accelerator has a sinusoidal oscillation attenuation waveform with a total capacitance of 120 μF and a total inductance of about 400 nH. When the charging voltage is 13 kV, the discharge current is 170 kA and the pulse width is 23.5 μs. The discharge currents are 38, 100, 135 kA, and 170 kA when the pressures are 100, 200, 400 and 1000 Pa, respectively. The current distribution of the parallel-plate accelerator is concentrated, and the discharge mode is consistent with the snowplow mode, when the discharge current is small and the working pressure is high. As the discharge current increases or the working pressure decreases, a diffuse current distribution gradually appears in the parallel-plate accelerator. Two regions are formed, i.e. the plasma front region and the plasma tail region. The diffuse current distribution phenomenon is more remarkable when the discharge current is higher or the working pressure is lower. The plasma front current distribution proportion decreases and the plasma front velocity increases with the increase of discharge current and the decrease of working pressure. However, the plasma velocity proportion increased is much lower than the discharge current proportion increased or working pressure proportion decreased. When the discharge current increases from 38–170 kA, the plasma velocity increases from 25.0 km/s to 33.6 km/s, with the velocity increment being only 34.4%. The plasma front region is subjected to both the Lorentz force and the thermal pressure of the plasma tail region.

Leakage currents from DC traction power supply system of metro will reduce the lifetime of the buried pipeline. This study illustrates a real contribution in the analysis of dynamic stray current and the evaluation of the corrosion status of the buried pipeline. The metro locomotives dynamic operation process was established through traction calculation, and then analysed the dynamic influence of stray current distribution under transient finite element model. Finally, a comparison of corrosion influence of stray current is also presented. According to the simulation results, the distribution of stray current is affected by many factors, the relative position of the cathodic protection pipeline and unprotected pipeline has significant influence on stray current.

The railguns can be classified into three types by the interior surface shape of rails, which are the type of plane-rail, convex-rail, and concave-rail. In this paper, the characteristics of current distribution in rails and armature are analyzed and compared for those three types, during the rising, dropping, and the flat part of the exciting current source. This current distribution is simulated by the finite element software ANSYS. The convex-rail type gun is of the weakest opposite direction current phenomenon, and its current distribution on the rail section is the most even. The evenness of the current distribution can be adjusted by changing the height of the convex part. The current density at the corner of armature is very large in the plane-rail and the concave-rail types, which results in stress concentration and probably damage of armature or rail surfaces.

The rail guns can be classified into three types by the interior surface shape of rails, which are the type of plane-rail, convex-rail, and concave-rail. In this paper, the characteristics of current distribution in rails and armature is analyzed and compared for those three types, during the rising, dropping and the flat part of the exciting current source. The current distribution is simulated by the finite element software ANSYS. The convex-rail type gun is of the weakest opposite direction current phenomenon, and its current distribution on the rail section is the most even. The evenness of the current distribution can be adjusted by changing the height of the convex part. The current density at the corner of armature is very large in the plane-rail and the concave-rail types, which results in stress concentration and probably damage of armature or rail surfaces.

Once DC transmission line or pole bus-bar grounding fault occurs; the energy of neutral-bus arresters will be very great. So neutral-bus arresters are designed in type of multicolumn parallel. To avoid one column of the varistors assuming too much energy because of the current distribution inequality, the current distribution coefficient must be strictly controlled. The method of judging current distribution is to test the current distribution coefficient of the arrester at 500A under standard lightning and switching current. And compare it with 1.1 which met the requirements of national standard. However, the current amplitude and waveform of neutral-bus arresters in real fault conditions are so different. The current distribution characteristic may also be different. This article tested the current distribution, analyzed the influence of current amplitude and current type on current distribution characteristic. The results show that the current distribution coefficient decreases gradually, along with the increasing of impulse current ranging from 25A to 500A per column. The current type has no effect on the current distribution characteristics of valve multicolumn parallel at low current.

Stray current can cause severe corrosion on buried steel pipelines. To investigate the corrosion effect of stray current, originating from impressed current cathode protection system of pipeline, on interference pipeline, a numerical model based on boundary elements using commercial software COMSOL Multiphysics was developed to simulate corrosion potential and stray current density distribution on interference pipeline. The model geometry was comprised of the cathodically protected pipeline, interference pipeline and an auxiliary anode. The effects of crossing angle and crossing distance of the two pipelines, output current and location of anode, soil resistivity and coating surface resistivity were investigated. The results demonstrated that the impressed current cathode protection system substantially affects the corrosion potential and current density distribution of interference pipelines in vicinity of crossing. The crossing angle and vertical distance of two pipelines have no important influence while the anode output current, anode location, soil resistivity and coating surface resistivity have significant influence to stray current corrosion on interference pipeline.

Numerical simulation and experimental investigation of the stray current corrosion of viaducts in the high-speed rail transit system