Effects of Vehicle Running Mode on Rail Potential and Stray Current in DC Mass Transit Systems

The ungrounded scheme with overvoltage protection device (OVPD) has been adopted in the construction of Nanjing Metro Line 1. During the operation of Line 1, a high rail potential was observed, which caused the OVPDs installed at some traction supply substations (TSSs) to keep turning on at any moment, and the stray-current leakage increased obviously. Since the magnitude of rail potential and stray current depends on the modes of a running vehicle, including accelerating, coasting, constant speed, or braking, Line 1 suspected that the vehicle running modes were the cause of a potential rise in the rail. This paper presents the results of field tests and numerical simulations to study whether different vehicle running modes have an impact on rail potential and stray current in Line 1. The results show that the rail potential in the acceleration and brake modes is significantly higher than that in the other modes, which are in the opposite direction. Moreover, within the stray-current interference, the maximum shifts of buried-conductor potential may exceed the limit in these two modes. In addition, the high rail potential and buried-conductor potential occur when the running mode changes from acceleration to coasting or the maximum brake power is reached. As a vehicle is running between two different TSSs, the rail potential is highest in its position.