Challenges, Design Considerations & Field Studies for Relocation of Earth Electrode Station- User’s Perspective

±500kV, 1500 MW Chandrapur-Padghe HVDC Bi-pole Link (owned by MAHATRANSCO) is under operation since 1995 and has been crucial to supply electricity from the coal-fired Chandrapur Super Thermal Power Station to India’s largest city Mumbai. The project has a 752 kilometers bipolar overhead line. The transmission voltage is ± 500 KV & the transmission power is 1,500 MW. There are two electrode stations, one located at Chincholi which is about 22 km from Chandrapur (rectifier) terminal and the other located at Anjur which is about 23 km from Padghe (inverter) terminal. These electrode stations are connected to terminal station through overhead electrode lines of 33 kV rating. The location of existing earth electrode station at Anjur is near to ocean creek, hence it possesses low resistance to remote earth and low power losses. The electrode station has shown good performance since commissioning of the HVDC system. Due to continuous expansion of infrastructure (Rail network, pipelines, high rise residential etc.) during last two decades has resulted in restriction of ground currents during monopolar operations and necessitated to shift the existing Earth Electrode at Anjur (Padghe, Mumbai end) to a new location. To optimize the overall cost, the shifting of existing electrode need to be carried out by retaining of neutral DC Switchyard equipment and other main circuit equipment.

This paper presents the various selection criteria, field studies which inter alia includes Magneto Telluric (MT) studies, shallow Electrical resistivity tomography (ERT) study and soil testing to be carried out for selection of land for Electrode station. The paper also describes the various challenges encountered during the selection process of suitable land near the Padghe converter station as the surrounding area is covered with basalt and rocks. Detail methodology of MT studies along with ERT field studies and their analysis and results have been presented in detail. The paper also covers the design criteria of electrode station, main technical parameters for electrode, selection of type of electrode, dimensions of shallow ring electrode, detailed design of Shallow ring electrode, current sharing between rings, current density, electrode resistance to remote earth along with other design parameters for the best available land to attain the desired resistivity. The construction activities for the execution of new electrode station are under progress. The field measurements shall be carried out after commissioning of new electrode station to validate the design parameters of electrode station. The field measurements shall also include the impact of electrode operation on nearby facilities like large metallic structures that are buried or in contact with the earth such as bridges, transmission lines, and substations with grounded neutral transformers would also be carried out after commissioning of electrode. This would allow any potential interference to be quantified and countermeasures to be applied, if required. The objectives of the subject paper are following-

Challenges in selection of electrode land for relocation

Geophysical investigations, Field Studies & MT Studies carried out for selection of suitable earth electrode site

Electrode detailed design

Way forward for construction and commissioning activities for the new Electrode station