# Rio Madeira HVDC System: Commissioning of the Ground Electrode for Bipole 2 at Porto Velho
###### 1 Introduction  
The IEC [1], Cigre [2], and EPRI [3] guidelines are the main references for the complete cycle of HVDC ground electrode design, including the pre-design activity of site selection and the post-construction commissioning procedures. An HVDC transmission system comprises two converter substations, interconnected by the HVDC line, each one requiring a separate grounding electrode for its DC switch-yard, which usually is located from 15 km to 100 km away and connected through the electrode line. The electrodes usually conduct the unbalanced current of the bipole, about 20 A to 40 A, into the ground. In case of the loss of one pole of the HVDC line, the energy can be transmitted by the remaining pole with ground return, using grounding electrodes for the injection into the ground currents that may reach almost 4 kA. Therefore, the ground electrodes add reliability to the energy transmission system. HVDC electrodes are wide-area grounding systems, designed with cathodic protection technology. They can be considered the most complex of grounding systems, because they are the only kind of grounding that is not built at the place of the grounded installation, requiring a search for the best electrode site. The site selection requires thorough research, based on geographical, geological, and geophysical surveys, to find the site that will allow for the best electrode performance. The site selection surveys shall be carried out within a radius of some tens of kilometers around the converter substations at the two ends of the HVDC line. The best site at eachend is the one with the geoelectric structure that presents the lowest resistivities from the soil surface down to at least mid-crust [4; 5; 6]. HVDC electrodes are wide-area groundings and the high DC currents travel deep into the ground, comprising a wide interference area. The higher the ground resistivity, the deeper the currents will penetrate the ground and the wider the interference area will be. Therefore, the design of an HVDC ground electrode requires the construction of a deep geoelectric model, which, in the case of a high-resistivity subsurface, will comprise half Earth's crust. [PERSON] et al. [7] shows that a cratonic structure may require a geoelectric model down to 20 km depth to allow for the calculation of the electrode resistance within a 5% misfit. Therefore, the development of a 1D geoelectric model of the Earth's upper crust, as viewed on average from a specific area at the ground surface, about one square kilometer wide, is a basic requirement for the site selection and design of an HVDC electrode. The geoelectric model for this application has specific requirements; it should be detailed within the shallow ground layers, where the electrode is buried, and should be deep, down to mid-crust, to allow for the calculation of the electrode electrical performance. For the construction of the geoelectric model, a comprehensive geophysical survey shall be conducted, with different techniques for probing the shallow and deep crust layers. The processing of different geophysical datasets to construct a geoelectric model that is valid from the soil surface down to a few kilometers deep is a complex task and not usually within typical geophysical procedures [8; 9]. This paper summarizes the test electrode procedure applied for the confirmation of the new site selected for the Rio Madeira HVDC electrode, bipole 2, at Porto Velho, as well the results of the pre-commissioning and commissioning measurements. As defined by CIGRE [2], \"Commissioning covers all measures which need to be taken, to assure the correct functioning of both single components of the equipment and the station as a whole\".