Transgrid PSCR: Addressing low spans on Line 1, Line 2, and Line 973/9GL

Market Network Service Provider
StageProject Specification Consultation Report
ConvenorAEMO NSP
Accepting submissions?Yes
Submissions close05/08/2024

Transgrid has published the Project Specification Consultation Report (PSCR) for addressing low spans on Line 1, Line 2, and Line 973/9GL. Publication of this PSCR represents the first step in the RIT-T process.

Overhead transmission lines are designed and constructed to achieve standard minimum electrical clearances to the conductor, i.e., a minimum distance between the ‘wire’ and any land, vegetation, or infrastructure around it. This ensures that safety and environmental risks from the lines are minimised.

Design of transmission lines considers a range of safety and environmental factors, including thermal expansion of the conductor (known as sag) and movement of the conductor position due to wind (known as blowout). Sag occurs where load on the conductor causes the conductor to stretch when it is hot, such that the conductor between two poles or towers of a transmission line hangs low. To account for sag, line design temperatures are set as the maximum temperature that a conductor may operate at while still achieving minimum electrical clearance. If the conductor at its lowest point exceeds the minimum electrical clearance specified in that line’s design, this is referred to as the line having ‘low span’.

There are a number of spans between towers on the above lines that do not currently meet the applicable design standards (i.e., exhibit ‘low spans’) and thus could pose safety and financial risks if left unaddressed. While these lines were identified as not meeting the original design standards, utilisation had historically been sufficiently low such that there was not a material safety or operational risk. However, as line utilisations have increased, operating temperatures on the lines have also increased causing line clearances from the ground to reduce.

One credible option (Option 1) was considered within the PSCR. Option 1 involves remediating the low spans on Lines 1, 2, and 973/9GL to the line design temperatures and will align all lines with AS/NZS 7000 (the current industry standard). Remediation is expected to involve mid-span structure installation, ‘dummy strain’ insulator arrangements and associated landscaping near the line.

The estimated capital cost of this option is approximately $19.07 million and there are not expected to be any additional annual routine operating costs (i.e., the cost under the option is the same as under the base case) since it does not affect the frequency of required inspections.

The proposed investment will enable us to continue to manage safety risk to ALARP, consistent with our obligations. Consequently, we consider this to be a reliability corrective action under the RIT-T. A reliability corrective action differs from a ‘market benefits’-driven RIT-T in that the preferred option is permitted to have negative net economic benefits on account of it being required to meet an externally imposed obligation on the network business.

Transgrid welcomes written submissions on materials contained in this PSCR. Submissions are due on 5 August 2024.

In accordance with the requirements of the Rules, a summary of the Project Specification Consultation Report is made available on the AEMO website. A copy of the PSCR can be obtained from Transgrid’s website or by emailing

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