Search

The converter transformer is the most important piece of equipment in ultrahigh voltage direct current (UHVDC) transmission systems. The rectifying valve side of the converter transformer is dealing with composite voltages, containing both ac and dc components. Therefore, the characteristics of the partial discharge (PD) in oil–paper insulation of such rectifying valves are different from those of rectifying valves subjected to pure ac or dc voltage. In this study, theories of hydrodynamic drift-diffusion and bipolar charge transfer were used to build the needle-plate electrode model with a paperboard depression, and experiments were used to verify the feasibility of the simulation method. The electric field and net charge distribution that can reflect the PD were analyzed. Besides, the effects of instantaneous voltage, the proportion of ac–dc components, and depression parameters on the development characteristics of PD were also discussed. Simulation results show that applied voltage, interface charge density, and model parameters can all affect the electric field strength, which in turn influences the development of PD in the depression. This research reveals the process and influencing factors of PD during the electrical aging process, which not only helps to optimize the oil–paper insulation at the rectifying valve side of the converter transformer but also provides a basis for further explaining the PD mechanism. [ABSTRACT FROM AUTHOR]

• The characteristic analysis for the influence of the grounding pole current on the pipeline shows that the high-power UHVDC project that began in recent years impacts pipelines that are within 180 km from the grounding pole. It was found that the stray current and PSP offset are related to the extreme value, while the distance to the pipeline, the grounding pole current, the distance between the insulating flanges at the two sides, the operation of the potentiostat, and their impact mechanisms are very complex. • The characteristic analysis for the influence of geomagnetic storms on pipelines shows that the GIC and PSP offsets are related to the rate of change for the geomagnetic field dH/dt. The pipeline GIC is quasi-DC, and the dynamic DC interference has a more significant impact of pipeline corrosion. Determining the effects of the GIC on pipeline corrosion and prevention are the goals of future research. • Comparisons of the influence and effects of the two interference sources on the pipeline shows that their difference is in the underlying mechanism, the range and intensity of the influence, the corrosion rate of the stray current, and the PSP offset. The influence intensity of the grounding pole current is greater, but the duration is shorter and its influencing range is smaller. The geomagnetic storm interference has the characteristics of a long duration and large influence range. The influence of ±800 kV ultra-high voltage direct current (UHVDC) transmission monopole operation on buried oil pipelines was compared between the 6250 A grounding pole current conditions of the Zalute-Qingzhou Project on December 24, 2017 and the Shanghai Temple-Shandong Project on January 2, 2017. Besides, the influence of geomagnetic storms on the safety of buried oil pipelines were also analyzed based on monitoring data from the geoelectric field of the China Anqiu geomagnetic station on September 9 and 29, 2017. The results show that the influence of the DC grounding pole current on the pipeline is related to the distance between the ground electrode and the pipeline, the operating state of the potentiostat, and the spacing of the insulating flange on the pipeline. The geomagnetically induced current and pipe-soil potential offsets for the geomagnetic storm intruding the pipeline are related to the rate of change for the geomagnetic field. There are differences in the stray current and PSP generation mechanisms for the intruded pipeline caused by the two interference sources: the intensity and influence range of the interference source, and the annual average corrosion rate of the pipeline. Compared with the DC grounding pole current, the geomagnetic storm has a greater interference effect on buried pipelines. [ABSTRACT FROM AUTHOR]