Electrical and electromagnetic investigations for HVDC ground electrode sites in India

High Voltage Direct Current (HVDC) power transmission systems require setting up of specially designed ground electrodes at terminal ends of the transmission line to close the circuit with an earth return path. The design parameters of these electrodes need the information about the electrical conductivity structure within a radius and depth of several km of the site in order to ensure that the injected current penetrates deep enough into the earth. Further, detailed conductivity structure, up to 100–200 m depth, of the electrode site covering an area of less than a sq. km is also needed to ensure safe limits for the step and touch potential at the site. Electrical and electromagnetic methods are very useful tools for this purpose. However, artificial source variants such as deep direct current (DC) resistivity sounding pose logistic problems for target depths greater than a couple of kilometers and in inaccessible areas. We have employed magnetotelluric (MT) and electrical resistivity tomography (ERT) tools to investigate the detailed deep and shallow electrical conductivity structure, respectively, of several potential sites in India for a ±800 kV, 6000 MW HVDC multi-terminal system. Investigations of a site in Assam revealed the presence of highly resistive crustal rocks at the depth of about 1.3 km beneath a thick pile of conductive sediments, rendering the site unsuitable even though the shallow conductivity was favorable. At another site identified after analyzing available geological and geophysical data, we inferred the presence of conductive structure up to at least 4.0 km depth. ERT investigation at this site revealed a favorable conductive structure except for the presence of a 20-m-thick near-surface resistive layer. This information has been useful for the electrode design. We suggest that a strategy suitably combining MT and ERT is useful in the selection of ground electrode sites.