TasNetworks has published the Project Assessment Conclusion Report (PACR) for Managing safe and reliable operation of St Marys substation. Publication of this PACR represents the final step in the Regulatory Investment Test for Transmission (RIT-T) process.
St Marys substation is located on the east coast of Tasmania, south of the township of St Marys. The substation supplies approximately 6,636 customers along almost 130km of the north-east coast of Tasmania, stretching from Coles Bay in the south to Binalong Bay in the north.
St Marys substation currently operates with two 10 MVA transformers – T1 and T2 – that were commissioned in 1966. However, peak loading of the St Marys substation exceeds 10 MVA. As such, the load supplied from St Marys substation is non-firm.
TasNetworks has identified that the transformers at St Marys are approaching their end of life based on condition assessment. In the event of a failure in one transformer, the remaining unit would be forced to operate above its 100% nameplate rating during peak demand periods, jeopardising its operational limits. During peak demand periods, this scenario would force load shedding until a system spare is commissioned. In addition, these transformers do not align with current standard fire mitigation requirements and a fire incident could lead to the simultaneous loss of both transformer assets, increasing unserved load.
More broadly, the bushings of T2 are original and in the case of catastrophic failure the porcelain may shatter sending sharp projectiles across the switchyard. This is a safety concern to operators or adjacent in-service equipment. Finally, the transformer’s aged oil containment systems have deficiencies, which pose environmental risks in the event of oil leakages.
Addressing the condition issues of the transformers will enable us to manage reliability, financial, safety and environmental risks at St Marys substation. TasNetworks expects that addressing these issues will result in significant market benefits and, as such, we consider the identified need for this investment to be market benefits under the RIT-T.
We consider that there are four credible options from a technical, commercial, and project delivery perspective that can be implemented in sufficient time to meet the identified need. This PACR found that Option 1 is the preferred option at this stage of the RIT-T (consistent with the draft conclusion in the earlier PSCR).
Option 1 involves completely replacing both St Marys transformers, along with associated oil containment systems in the 2024-2029 regulatory control period.
The estimated capital expenditure associated with Option 1 is $7 million (in 2023/24 dollars). On a weighted basis, the net economic benefits of Option 1 are approximately $6.5 million. The works are estimated to take place between financial years 2023/24 and 2025/26, with practical completion and commissioning in the first half of financial year 2025/2026.
No submissions were received in response to the PSCR during the consultation period which closed in October 2024 and no additional credible options have been identified.
In accordance with the requirements of the Rules, a summary of this PACR is made available on the AEMO website. A copy of the PACR can be obtained from either the TasNetworks website or by emailing our regulation team via regulation@tasnetworks.com.au.