Stray current measurement at the tramway infrastructure in Ostrava, Czech Republic

In most transit systems, rails are used as return conductors for the current from the vehicle to the electrical substations. If the rails are not fully insulated from the ground, some of this current would leak and become stray current, causing stray current corrosion on the rails and metal objects (such as pipelines) in the immediate area. It is very difficult to measure stray current directly, but stray currents can be calculated by measuring other parameters. Stray currents were measured on a 1.3 km section of tramway infrastructure in Ostrava. The potential between rail and earth was measured on the basis of the standard EN 50122-2, where two reference electrodes were placed at an appropriate distance from the tram track at three measuring points in the ground - the first point was located at the beginning of the section, the second in the half of the section and the last at the end of the section. Rail currents were measured at two measurement points - the first point at the beginning of the section and the second point at the end of the section. Using the rail-to-earth potential and the equation from the standard EN 50122-2:2011, the rail-to-earth conductance per length was calculated. The conductance per length was also calculated using Ohm's law, where the current difference is a difference between two measurement points. Since the results obtained using the standard and Ohm's law did not agree, a detailed analysis of the tram section was performed and electrical drainage was found. The drainage represents an electrical connection of the protected metal structure in the area of the tram track by a cable with stray current source. Through the drainage, the stray currents are directly returned to the rail. In this measurement section, the drainage has influenced the current difference between the measurement points - without drainage, this difference would be much smaller.

Study on safety operation of medium voltage distribution network

Aiming at the problem of safe operation in the medium voltage distribution network with neutral point grounding through small resistance, the distribution network model was built, and on this basis, MW-level photovoltaic (PV) system, doubly fed Induction generator (DFIG) wind turbine, permanent magnet synchronous generator (PMSG) wind turbine and energy storage device was connected respectively in this paper. The ground current and earth potential rise was studied when the power was delivered by mistake. The simulation results show that in the system with neutral point grounding through small resistance, whether it is the traditional or the active distribution network, the three-phase power delivery method is more able to ensure the personal safety of power grid operators. However, The access to distributed generator (DG) in the distribution network will have a greater impact on ground current and earth potential rise in the case of three-phase power delivery. Especially when the ground current and earth potential rise will increase after DG connected to distribution network, which is still likely to pose a threat to personal safety and needs further research.

Calculation model for the earth potential of HVDC ground electrode based on image recognition of surface-layer soil moisture

Changes in soil electrical parameters can affect the distribution of earth potential in high-voltage direct-current (HVDC) ground electrodes when climatic conditions changes. This paper proposes a model to describe the relationship between surface-layer soil moisture and surface-layer soil resistivity under short-term climatic influence, by using image recognition technology. Based on the relatively stable resistivity of lower soil layers, a soil model more reflective of the actual operating conditions is established for soils near the ground electrode, and a finite element method is adopted to calculate the earth-surface potential (ESP). The experimental results indicate the following: (1) Compared with other measurement methods, image recognition of surface soil resistivity is a low-cost, real-time, online, and accurate method; and (2) changes in surface-layer soil moisture affect both ESP and step voltage. These effects are large in the case of high resistivity for the soil layer where the ground electrode is buried. This large fluctuation in step voltage particularly results in a potential safety hazards during ground electrode operation. Therefore, in order to ensure personal safety and obtain more accurate electrical parameters, it is necessary to consider the effect of natural climate on the soil surface resistivity. Finally, the value of the step voltage can be observed using image recognition, this also provides a new method for the safety monitoring of the DC ground electrode.