Micro-alloyed copper conductors offer an interesting alternative to steel-reinforced aluminum conductors for high-voltage overhead lines.
Changing market structures combined with increasingly distributed, variable power generation are giving transmission network operators substantial challenges. Renewable energy power stations are often built in remote areas; therefore, new lines must be built to connect them to the main grid and transport their production to the centers of demand. The growing share of renewable energy in power generation is creating a highly variable supply; reinforcement of the grid is necessary to cope with this variability. Operators seeking to increase line capacity are limited by the maximum operational temperature of the line conductor; above it, the conductor material shows excessive creep and the mechanical integrity of the cable can no longer be guaranteed. Finally, regulators are paying more and more attention to the energy efficiency of overhead lines and are shifting their main goal from minimizing investment cost to minimizing the life-cycle cost of the line.
A new conductor is needed to overcome these challenges and increase the energy efficiency, capacity and overload capacity of the line.
Copper instead of aluminum?
Overhead line conductors are traditionally aluminum, using either steel-reinforced aluminum (ACSR) or aluminum alloys to achieve the required strength. Recognizing copper as a material for overhead lines may come as a surprise, since it is a substantially heavier material. This is the primary reason why its use declined several decades ago in favor of ACSR conductors.
However, using an innovative and advanced micro-alloyed copper rather than the copper ETP prevalent in the past, retains all the advantages of copper (high conductivity, resistance to corrosion) and addresses the mechanical and thermal strength drawbacks.
Five advantages of micro-alloyed copper conductors
- The strength of the towers for supporting overhead lines is not primarily determined by the weight of the conductor, but by resistance against forces created by wind and ice. Micro-alloyed copper conductors offer a smaller cross-section, which reduces the drag coefficient. This makes them particularly well suited for overhead lines in cold and windy climates.
- The high annealing temperature of copper (> 300°C) makes it easier to apply surface coating on the copper conductor without being concerned about a potential change in the mechanical properties of the material. Hydrophobic surface coatings can prevent ice loading. This, combined with the smaller cross-section, significantly reduces wind and ice load, which is a decisive factor for determining the required strength of the towers.
- The ability to apply a coating on the copper conductor also brings another advantage: reduction in corona losses as well as energy losses and the associated noise levels. Corona losses can become a substantial source of irritation for passersby, especially in wet climates, adding to the negative image of high voltage overhead lines. Moreover, the coatings on each individual wire of the conductor result in even stronger corrosion prevention; copper conductors perform well in salty or polluted areas, where aluminum conductors tend to corrode easily.
- The lower electrical resistance of copper combined with a reduced skin effect result in significantly lower energy losses compared to ACSR conductors. Such energy losses comprise the main portion of the life-cycle cost, especially for the refurbishment of a line, but also for new lines. Consequently, even though the copper conductor requires a higher investment cost (approximately 70 percent), its life-cycle cost will in many cases drop beneath that of ACSR conductors.
- Micro-alloyed copper delivers a higher maximum operating temperature compared to that of ACSR conductors. This makes it possible to charge the conductor with overloads of at least 60 percent without compromising mechanical properties. Such an overload can help transmission network operators to comply with the N-1 safety criteria and cope with periods of high renewable energy production.
Proven by feasibility studies
Two feasibility studies were conducted by the Dutch consultancy agency DNV GL (KEMA). The first study examined the construction of new lines; the second investigated the refurbishment of existing lines. The studies concluded that the use of micro-alloyed copper conductors would be competitive (against traditional choices like ACSR) in the following cases:
- When life-cycle costs of overhead lines are taken into consideration: the higher conductivity of copper offers a lower cost of losses, reducing the life-cycle costs by more than 10 percent, with payback periods of around six years.
- In challenging weather conditions: windy, icy or a combination.
- In corrosive atmospheres.
For existing lines, DNV GL concluded that micro-alloyed copper conductors lead to lower losses and simpler installation.
- Capacity increase (ampacity): between 50 and 100 percent more than conventional conductors
- Better energy efficiency than ACSR and HTLS
- Safety: transports energy peaks thanks to its high-temperature properties; ability to cope with capacity overloads (N-1 or N-2)
- Better behavior in icy and windy weather condition due to its smaller diameter and possibility to use advanced hydrophobic coatings
- Fewer losses due to the combination of dielectric coating and copper
- No creep at high temperatures
- Easy to install; standard fittings available
- New overhaul lines (fewer towers)
- Competitive upgrading of existing lines, increasing their ampacity without investing in pylons and rights-of-way, with lower energy losses
- Areas with adverse climatology (snow, ice, wind)
- Lines connecting renewable energy plants to the main lines
- Corrosive atmospheres