Analysis of Rail Potential and Stray Currents in a Direct-Current Transit System

The diode-grounded scheme for stray-current collection in some systems, such as the Taipei rapid transit systems (TRTS), has been constructed to gather the stray current leaking from the running rails and avoid corrosion damage to the system as well as the surrounding metallic objects. During operation of the TRTS, a high potential between the negative return bus and system earth bus at traction substations, referred to as rail potential, has been observed on the Blue line between BL13 and BL16. Since the Blue and Red-Green lines have their running rails and stray-current collector mats in junction at the G11 station, the TRTS suspects that the impedance bond at G11 is the cause of rail potential rise. This paper presents the results of field tests for studying whether the impedance bond at G11 of the tie line has an impact on rail potential and stray currents in TRTS. The results show the rail potential can be reduced by disconnecting the impedance bond at G11 of the tie line so that the negative return current of the Blue line cannot flow to the rails of the Red-Green Line, and vice-versa. In addition, rail potential and stray currents occurring at a station of the Blue line are numerically simulated by using a distributed two-layer ladder circuit model. The simulation results are compared with the field-test results and they are consistent with each other.