Study On Corrosion Mechanism Of Grounding Current In HVDC Transmission System To Pipeline

With the development of national economy and the stimulating demand for energy,the construction of High Voltage Direct Current(HVDC)transmission lines and natural gas transportation pipelines is developing at a high speed in China.Due to space constraints,high voltage transmission systems may affect the safety of gas pipelines.The HVDC transmission system will produce very large grounding current in monopolar operation and form stray current which interferes with buried pipeline and lead to pipeline corrosion,pipeline ancillary equipment interference and damage.In order to avoid the catastrophic accident caused by pipeline corrosion,to ensure the safe operation of the pipeline,it is of great significance to study the mechanism and protective measures of grounding current to pipeline corrosion.As the stray current corrosion is essentially electrochemical corrosion,this thesis carries out experimental study on direct current electrochemical corrosion on buried steel pipeline to explore the influence of DC current generated by electric field and current density on the corrosion of pipeline.On the basis of experimental study and analysis,the numerical simulation of electric field distribution and current density in the periphery of HVDC grounding is carried out to reveal the influence of the grounding current on the pipeline corrosion and put forward the corresponding protective measures.The major research contents include the followings:(1)The cause of grounding current of grounding electrode in HVDC transmission system and its influence on pipeline safety are analyzed.The operation mode of HVDC transmission line and the DC electric field caused by monopolar operation under abnormal working condition are introduced.Stray current caused by the HVDC transmission lines lead to the buried steel pipelines corroded and the cathodic protection equipment of the station and valve room damaged are also analyzed.At last,combined with the practical engineering,the interference of grounding current to gas pipeline is analyzed(2)Experimental study on electrochemical corrosion of natural gas buried steel pipeline by DC stray current.Through the electrochemical corrosion experiment,the effect of DC electric field and direct current density on the corrosion of metal specimen are studied.The polarization curves,AC impedance spectra,corrosion morphology and corrosion rate are analyzed.The result shows that the corrosion rate increases with the increase of applied current and DC current density.(3)The numerical simulation analysis for the corrosion of natural gas pipelines caused by the HVDC grounding current is carried out.Combined with practical engineering,based on numerical simulation,the electric field distribution around the grounding and grounding type(cathode,anode grounding)to influence the current density and the influence of grounding and pipe distance to the ground current density are investigated.The result shows that 1)with the increase of the current,the minimum potential pipeline decreases,and the maximum current pipeline linearly;2)the pipeline earth potential far beyond the limits of the relevant standards when the DC grounding monopole operation.(4)The protection technology of pipeline in HVDC interference area is studied.Combined with the practical engineering,the general principle and the specific protective measures are put forward,and the protective action of isolation measures on stray cxurrent interference pipeline is mainly studied.The result shows that 1)the isolation measures will cause large DC interference when the grounding electrode as the anode,and the interference influence of DC for the isolation measures is relatively small when the DC grounding electrode as the cathode.2)Cathodic protection in the two sections of the isolated area can effectively compensate the lack of isolation measures.In short,through the experimental and numerical simulation study,the effect laws of corrosion of gas pipeline caused by the HVDC ground electrodes is researched,the laws of distribution and the influence area of the DC electric field caused by the HVDC grounding current are explored,and the protection measures of the gas pipelines in interference area of the HVDC ground electrode current are presented.The research production of this thesis can provide technical guidance for reducing the contradiction between the HVDC transmission system and the buried steel pipeline in the developed area,and avoiding the electrochemical corrosion caused by stray current,and ensuring the safety of pipeline operation.