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Compact Lines – The Future of Transmission Line Design?
Examining key design challenges, solutions and benefits of Insulated Cross Arm transmission lines.
Key Takeaways
Background
With demand for power expected to grow by more than 50% by mid-century[1], utilities worldwide are under pressure to improve grid capacity and resilience. For many, upgrading existing corridors to transmit more power is the most convenient option. However, in recent years, restrictions on transmission lines have resulted in limitations on knocking and rebuilding larger towers in existing corridors.
Utilities looking to build new transmission lines from scratch face similar challenges. Space is at a premium, so securing servitudes for new lines is becoming increasingly difficult, expensive and subject to conditions, resulting in costly delays.
Factors such as visual impression, interference with infrastructure, proximity to airports and impact on wildlife habitats all play a part in dictating the permissible structure size, meaning engineers often need to develop a more creative approach to transmission line design to achieve the necessary capacity within the given limitations.
Transmission Line Compaction
Traditional transmission lines were designed very conservatively using wide spaces between phase conductors, reducing the risk of phase-to-phase flashovers and minimising surface voltage gradients.
However, by utilising high-strength insulators, shorter span lengths, and improved tower designs, engineers can compact transmission lines, allowing them to fit into smaller right-of-ways (RoWs). This reduces visual impact and land acquisition overheads. In addition, compact line construction often requires less labour and building materials, decreasing costs further. [2]
Compact Tower with Insulated Cross Arm
Insulated Cross Arm technology is one of the more popular compact line applications, ideal in areas with a vertical height restriction. Due to the high loads transmitted to the insulators, mechanical performance needs to be strong[3].
Long Rod insulators are the optimal insulator type for Insulated Cross Arm applications due to their increased creepage distance and ability to be manufactured in a single unit.
Case Study: The Significant Cost Reduction Potential of Compact Lines
Compact Transmission Line Diagram. Source: PPC Santana.
Eduardo Hilsdorf, Sales Director, PPC Santana:
“In 2024, PPC Santana undertook a test project on the economic benefits of compact transmission lines using Insulated Cross Arm technology. This came about after discussions with Eletrobras, Brazil’s main transmission utility, during which they asked for definitive figures on the cost of building compact lines. We ran a technical project, which revealed that compact lines reduced land requirements by 15% and tower weight by 9.75%. This not only reduces the cost of RoW procurement, but also the cost of construction materials and work required to build the tower. Along with this, we found that supply interruption was also reduced. Because we are using less land, the environmental impact of our activities is much lower than with traditional transmission lines.”
| Benefit | Capex | Opex | ESG/Co2 | Electrical Performance |
|---|---|---|---|---|
| 15% RoW/land cost reduction | ✔ | ✔ | ✔ | |
| 10% tower structure material reduction | ✔ | ✔ | ✔ | |
| 7% reduction in foundation depth | ✔ | ✔ | ||
| 36% decrease in backflashover line shutdown | ✔ | ✔ | ||
| Possibility to uprate the voltage with same row | ✔ | ✔ |
Performance Considerations of Compact Lines
Mechanical Considerations
When designing compact transmission lines, choosing the correct insulator components at the tangent structure is critical to restrict conductor movement in windy conditions.
Lightning Performance of Compact Transmission Lines
The reduced structure height of compact lines means that they are naturally less likely to be exposed to lightning.
Insulator Specification and Design for Compact Lines
Choosing an insulator with the optimal dielectric strength will ensure the proper handling of voltages and any overvoltages that occur.
Porcelain and composite Long Rods are the most commonly utilised insulators for compact lines.
In systems where slow-front overvoltage levels are generally rather low, insulation choice is often dominated by pollution performance requirements. Considering the restricted space available for the insulators on compact lines, it is necessary to limit any uncertainties in insulator selection and dimensioning by following a detailed design approach.
Inappropriate insulator specification can incur significant future costs, so great care must be taken to ensure that all operational, mechanical and environmental factors are considered during insulator design.
When considering the choice of insulators, it is important to look at the following performance aspects:
- Life expectancy and total cost of ownership.
- Pollution flashover performance.
- Costs associated with maintenance.
The lifespan of porcelain long rod insulators is generally longer, and their maintenance needs remain stable over time, unlike composite alternatives, which often require more upkeep as they age. However, to determine the ultimate insulator material, a full analysis of line requirements and environmental factors should be carried out.
Conclusion
Compact lines using Insulated Cross Arm technology are a cost-effective, spatially efficient and more sustainable alternative to traditional transmission line design. With electricity demand continuing to rise, engineers should consider the benefits of compact lines to maximise capacity and space while reducing the total cost of ownership.
Although their design requires careful consideration of mechanical stability, electrical performance and insulation coordination, by addressing these factors early in the design process, utilities can implement reliable, resilient, and visually unobtrusive transmission solutions that meet modern power demands while minimising land use and environmental impact.
*The information provided in this content is for informational purposes only and should not be considered professional advice. We make no warranties or guarantees, express or implied, and are not responsible for any losses or damages resulting from your use of this information.
[1] Reconductoring vs Rebuilding: SCE’s Experience, Mustafa Ali, T&D World
[2] Compact overhead DC lines – a technical brochure. Cigre.
[3] EPRI Transmission Line Reference Blue Book.
