Interference Rules On The Buried Pipeline Cathodic Protection System By AC Electrified Railway Stray Current

With the continuously rapid development of energy industry, electric power industry and railway transportation industry, the oil & gas pipelines will cross or parallel to the AC electrified railway and the high-voltage AC transmission lines in some developed regions or special geographical areas such as public corridor zone. The buried pipelines will occur AC interference corrosion inevitably by the AC stray current, because the railway cannot insulate against ground completely on account of the power supply mode and characteristic of the electrified railway traction power system, and the high-voltage AC transmission lines will generate electro-magnetic induction coupling. In the monitoring process of field pipeline cathodic protection condition, we found that the pipe to soil AC potential was very high, that some position of pipeline also occurred AC corrosion, that the pipeline coating and the cathodic protection system would be interfered by AC stray current as well, although the cathodic protection potential had reached-850 mV of the traditional protection criteria. These conditions are not good for the reliable service and integrity management of the pipelines. Therefore, it is very important to study the interference rules on the buried pipeline cathodic protection system by AC electrified railway stray current, and this will guide the stray current monitoring an drainage of the field pipelines.This paper designed the lab experiment to study the AC stray current interference, on the basis of electrified railway system composition and of the current path characteristics. We got the pipe to soil potential changing rules under different parallel distance, different parallel length, and different crossing angles, based on above-mentioned experiment and the field testing data. By the analysis, the curve of the pipe to soil AC potential and the parallel length appears ―S‖ type change, and the curve shape will distort because of different coating defect positions. The interference will reach maximum at the crossing point of the pipeline and railway, and the smaller crossing angle is, the greater interference effect will be.Simultaneously, the experiment analyzed the AC interference stray current effect on the cathodic protection system under different parameters, such as the coating defect area, the defect position, the interference current intensity, and the output voltage doze of the potentiostat. Stray current interference will lead to large IR drop, and this IR drop is greater than that without interference, which will greatly reduce the pipeline protection degree. On the basis of the experimental data, we analyzed the IR drop which has an important effect on the true protection potential, and gained the IR drop changing rules along with different parameters.