# Rio Madeira HVDC System: Commissioning of the Ground Electrode for Bipole 2 at Porto Velho
## 6 Discussion  
The first finding of this project was that the test electrode proved to be an important tool for the evaluation of the selected site, despite the cost of building a 40 km long temporary electrode line. The preliminary test with the temporary line allowed for the electrode design with the certainty that the site was well selected and there was no risk of saturating the converter transformers. The test electrode was also useful for the preliminary tests of the electrode before its energization.  
It is worth noting that the electrode of bipole 1, at Porto Velho, is located almost 60 km away from the converter substation, while this new electrode 2 was built 40 km away from the same converter substation. The bipole 2 electrode site is located within a graben, a faulted and subsided fragment of a high-resistivity craticion basement, which contributed to a better geoelectric structure.  
The commissioning of the new electrode of the Rio Madeira bipole 2, at Porto Velho, was successful and allowed for important findings. The two soil-surface potential profiles were produced from a sequence of potential differences referenced to the electrode busar, with no reference to the remote ground. Therefore, prior establishment of an electrode resistance was required, to allow for the adjustment of the soil surface potential profile calculated during the design work. The electrode resistance, which is only well defined after the processing of the commissioning data, is thus an essential parameter to be defined by the commissioning procedures [11; 12; 13; 14].  
The electrode resistance could not be calculated by using the traditional fall-of-potential method, which requires a measuring line ending close to the remote ground, where the stabilization of the apparent resistance curve on the electrode resistance can be achieved.  
Figure 7: Measured (blue and red) and average curves (green): (**a**) the two soil-surface potential profiles; (**b**) the corresponding resistance curves.  
For the Porto Velho electrodes, the high-resistivity regional tectonic setting, associated with the border of the South Amazon craton, results in a wide interference area, which would require excessive soil surface potential survey, many kilometers away from the electrode, to achieve the stabilization of the resistance curve. It is nearly impossible to measure a very long soil surface potential profile due to the many obstacles that appear along the measuring line, such as closed forests, private properties, roads, and rivers (such as the one that limited the West measuring line).  
In this case, the measurements performed at the converter substation were used as an aid for the calculation of the electrode resistance. For the project here reported, a previous paper was available [10] that reported some useful data acquired during the commissioning of the original electrode. However, these data will not usually be available. Therefore, the geoelectric model applied for the electrode design shall be corrected according to the first approach electrode resistance. This starting value for the electrode resistance can be then used for the estimate of the unknown parameters of the expressions of item 5.2 of this paper, which can be calculated through computer simulation.  
The small deviation of 5% between the first approach resistance and the adjusted value is not significant, considering the strong coupling between the electrode and the converter substation grounding grid due to the high-resistivity regional tectonic setting (cratonic). The adjusted value was a bit lower than the first approach value, allowing for the conclusion that the first approach resistance will be an appropriate and conservative value for the calculated resistance of HVDC ground electrodes in general, independent of the tectonic setting of its location.