Effect of monopolar earth-return operation of grounding electrode on natural gas pipelines

When the monopolar ground-return operation mode is adopted in the high voltage direct current (HVDC) transmission lines during its commissioning and failure, a great amount of current, up to thousands of ampere, flows into and out of the grounding electrodes and consequently brings stray current interference to nearby metallic fabrics. In this paper, the interference on the Phase I natural gas pipelines of Guangdong Natural Gas Grid Co., Ltd. was investigated by measuring the distribution and change of electric potential at different positions of the pipeline under different grounding currents in the monopolar ground-return operation mode of Yulongling grounding electrode. Moreover, the effects of mitigating measures were assessed by analyzing the change of the interference potential. The influential laws of grounding current on pipelines were figured out on the basis of electric current analysis. It is indicated that the interference potential of the pipeline is in proportion to the earth current of the grounding electrode. When the pipeline is under the interference, its polarized potential is beyond the standard remarkably, and consequently it is faced with the risks of corrosion and hydrogen embrittlement and the risk points are located at the insulating joints. In the monopolar ground-return operation mode of HVDC transmission system, the interference voltage on the pipeline is up to 140.5 V, so personnel and equipment are risky. When the station grounding grid is used for mitigation, it has the remarkable effect on weakening the interference on the pipeline only at the connection positions, but the negative effect at the connection positions far from the grounding grid.