Technical Papers and Presentations

HVDC electrodes are installed on HVDC transmission systems to provide a low resistance current return path during both monopolar and bipolar operation, using the earth and/or sea as the conductive medium. HVDC electrodes are in general less costly and have lower losses than dedicated metallic return conductors. Numerous HVDC electrodes have been installed now for close to 40 years without arising concerns for public health and or for the environmental impact. Yet the impact may be reduced further by selecting suitable materials, design and location, based upon the results of the geophysical survey and modeling, and an appropriate analysis of thermo and electrical distributions around the electrode and along the HVDC link.

The analysis requires the consideration of both local and remote impacts. For instance, local studies of the earth potential rise and surface potential gradient are used to define step voltages and transferred potential. The evaluation of the electric field strength around submerged coastal electrodes is used, among other things, to determine the impact in terms of chlorine gas production, to determine material selection, cable specifications, instrumentation required for an electrode station, electrode testing and commissioning. The analysis of the electric potential along the entire link and the evaluation of the resulting stray currents is used to determine the corrosion of surrounding structures, such as pipelines, railways and powerlines [1].

The electric voltage, current and temperature distributions for the HVDC link can be evaluated using multiple coupled 2D/3D models using the AC/DC Module of COMSOL Multiphysics^® simulation software, each describing the physics of the HVDC link at different scales. For the evaluation of the voltage distributions along the entire length of the HVDC link one may use a 2D model of the 500kmx500km area surrounding the installation describing the topography of the region and resistivity of the soil/water (in the case of subsea electrodes), in which the anode and cathode are represented by  electric current point sources, and pipes and metal structures can be represented by external lumped parameter circuits, coupled to physical points model via voltage coupling. In order to have a better resolution around the sources, the large scale 2D model is then coupled to smaller scale 10kmx20km 2D models of the area surrounding the electrodes.

To describe the current density on the electrode and in the immediate vicinity, one may use 1kmx1kmx500m 3D models of the electrodes and feed cables, in which the boundary of the smaller 3D model is coupled to the larger 2D model using the general extrusion feature. For coastal electrodes this model can be also be used to evaluate the joule heating generated and the convective heat loss from the electrode terminal and cable to the water. 

The results of the analysis can be used highlight potential health hazards (or lack thereof), and to define mitigation measures, test specifications, link reliability, maintenance intervals and finally electrode life cycle.