Projects:2021s1-13005 Determining Dynamic Line Ratings of Over-Head Transmission Conductors based on Line Tension

Power flow through relatively short lengths of overhead transmission lines is typically limited by the operating temperature of the conductor, where increased power flow equals increased Ohmic losses and heat gain. Elevated conductor temperatures result in increased sag and an increased rate of deterioration of the conductor’s mechanical strength. Conductor temperature must therefore be controlled to firstly avoid violating regulated ground clearances, which is a safety concern, and to secondly avoid premature conductor ageing, which is a reliability concern. South Australia’s transmission network operator and project sponsor, ElectraNet, is looking to implement dynamic line rating throughout their transmission network, as it offers an economic approach which maximises the current flow through their existing lines by taking advantage of more favourable conditions when they exist, while still maintaining the required safety and reliability operational requirements. This offers an economic benefit of enabling a greater capacity to be extracted from existing transmission line assets, thus easing line congestion, and deferring line expansion or upgrade costs.

The 2021 iteration of this project continues on from the work completed in the project of the same name in 2020 by Adrian Barone and James Smithson.

Project Team

Team Members

• Andrew Gross
• Michael Iuliano
• Taimur Abdullah Said Al-Sanaidi

Supervisors

Principal Supervisor: Wen Soong
Co-Supervisor: David Vowles

ElectraNet Sponsors

• Josh Smith
• Ellen Power

Background

Problem Statement

Transmission lines are traditionally operated conservatively and often below their full capacity. Maximum current flows, termed line ratings, are limited by the maximum allowable temperature of the conductor. This rating depends on the conductor material and is calculated from predetermined, static, worst-case ambient weather conditions. As power generation transitions to renewable energy sources, an outdated line rating method limits the utilisation of renewables, increases the likelihood of line congestion, and necessitates costly infrastructure upgrades or expansion. However, simply increasing current flow causes the line to run hotter, increasing the risk of permanent physical damage from overheating. A hotter line also stretches more, causing more sag and potentially violating regulated safety clearance limits.

A better line rating method is needed.

Problem Aim

South Australia’s transmission network service provider, ElectraNet, is investigating the use of Dynamic Line Ratings (DLR) as part of their risk mitigation strategy. The project aim is therefore:

• To calculate a DLR using a combination of thermal, weather-based modelling and mechanical, tension-based modelling.
• To validate the modelling results against historical line rating data supplied by ElectraNet to confirm that this economic approach is reasonable and feasible.

Method

Weather-based modelling uses thermodynamic principles and relies on measured ambient temperature, wind speed and wind direction.

• Heat generated from solar radiation and from current flow resistive loss is balanced by cooling from thermal radiation and convection from air movement.
• The balance of heating and cooling determines the allowable current, based on a maximum defined operating temperature.
• Natural variation of weather over long distances makes it difficult to determine the weakest line section.

SLR vs DLR Comparison.

Spatial Weather Variation, IRENA.

Heat Balance Factors, CIGRE.

Tension-based modelling uses the relationship between conductor temperature and tension and provides information on the line’s physical state.

• Conductors are typically continuous over multiple spans, therefore tension modelling can capture the average line condition over long distances.
• The weather-based model is still required to determine a line rating from tension-calculated conductor temperature.

Results

• The tension-based model provides more reliable results during low tension and high temperature conditions, making it ideal for critical operational periods.
• The weather-based model can alternately be used for determining a line rating, with conductor sag and clearance still available from tension-based modelling.
• The tension-based model performs better for line segments with longer spans, where the low point of the conductor falls below the lowest support elevation.

Conclusion

• A combined modelling approach provides a more robust line rating that simultaneously considers the line’s thermal capacity and sag over time.
• A DLR can have up to an 70% improvement in Winter and a 35% improvement in Summer over a static line rating.
• It is also able to capture non-conservative line ratings and sag that is non-compliant with regulations.
• Tension-based modelling does require a favourable line geometry that contains flatter and longer spans.