The challenges of sub-optimal substation site selection

Abstract

This paper discusses sub-optimal substation site selection and the challenges of developing an earthing design for the substation and transmission line, as well as the solutions adopted. The site selected was close to a major Telstra exchange and high pressure gas valve, on a system with a high fault capacity, poor soil resistivity and which is connected to a transmission line possessing limited OHEW length. The earthing design identified possible existing LFI issues beyond the project's geographical location. Both gas and water steel pipelines were running parallel to the existing transmission line, over a considerable distance. A detailed analytical model was prepared, which showed the significance of the proximity effect. Risk based and deterministic criteria were compared to establish the best safety criteria for this design. A preliminary site test (using deep electrodes) was conducted to verify soil resistivity test results. The contractor's test readings were found to be inaccurate and were re-done. Follow up testing reflected sound correlation between field and modelled results. To avoid underground asset mitigation, special and cost effective earthing design was adopted. It was discussed and agreed with Telstra and colleagues in the distribution section to minimise transfer voltage via Common Multiple Earth Neutral (CMEN). This case study illustrates that engaging major stakeholders is key to the success of complex projects.