Method for OCS current capacity calculation in AC railways

Abstract

The current capacity of an overhead contact system (OCS) is the maximum continuous working current that the OCS can carry when no conductor current exceeds the maximum allowed current. It’s a basic parameter when an electric railway is being designed. It influences the power supply capability of a traction power supply system. The higher demand of the OCS current capacity for a high-speed or heavy-haul railway is put forward due to the much heavier traction loads. A reinforcing feeder is normally mounted in the front half of the feeding section near the substation to meet the need of the current capacity for some heavy load lines. This reinforcing feeder is connected in parallel with the contact wire and the messenger wire every 500m. The OCS current capacity is determined by the current capacities and the current distribution relation of the parallel conductors constituting the OCS. At present, when the OCS current capacity is being calculated, only the influences of the conductor-earth circuits are considered, while those of the return conductors are not[1-3]. The OCS is not regarded as part of the traction network, which results in a model error. A new method for OCS current capacity calculation, which considers the return circuit influences, is proposed based on the multiconductor transmission line model in this paper. The OCS current capacity and the current distribution ratios are calculated for a direct feeding system and an autotransformer (AT) feeding system. II. CIRCUITS IN TRACTION NETWORK A. Direct feeding system Fig 1. shows a typical direct feeding system. The OCS supplies the current to the locomotive. The rails and the return wire are connected in parallel to form the return conductors. Part of the return current flows through the earth. The ratio of the earth current to the total return current relates to the railto-earth resistance, the position and installation method of the return wire, the earth resistivity and other factors. The value is 15%~20% generally.