Modeling and Monitoring Stray Currents for Existing Metro Systems as a Result of Using Voltage Limiting Devices

摘要

A system of VLD's (Voltage Limiting Devices) is installed in the Copenhagen Metro traction system. The purpose of having these VLD's is electrical safety. At every Metro station a "57-VLD" is installed. The VLD's will be switched on automatically if the potential at the running rail exceeds the potential at the earthing system by more than 90 volt. Such incidents are not very common according to Metroselskabet, and the duration of such short-circuiting incidents is short. In addition to the automatic activation of VLDs every night for five days a week, maintenance work is also carried out at specific parts of the Metro track system. This maintenance is scheduled where specific parts of the track system are taken out of service for about 5 hours. During these periods the VLD's in each end of the mentioned sections are switched on for the safety of the workers. Even when one VLD is switched on (possible scenario for the automatic activation), the track system is changed from being electrically "floating" to a system where stray currents may enter or leave the track system through the VLD. The paper presents 1) A risk evaluation of stray current corrosion risk for the Metro 2) a mathematical simulation of the electrical currents where a number of the "57- VLD's" are activated. 3) Monitoring results of the track system during periods where a number of the "57-VLD's" are activated. The monitoring results are in line with the mathematical simulation of the electrical currents and it is found that VLDs comprise a considerable corrosion risk that should be considered.