Belgian TSO Elia selects ACCC conductors to double the load transfer capacity of a 150-kV line.
The development of transmission systems is essentially linked to the economic development of regions, expanding electricity markets, and the need to provide and interconnect all forms of power generation.
When seeking new rights-of-way, transmission system operators (TSOs), together with many other service organizations, often face increasing opposition due to environmental issues and the not-in-my-backyard syndrome. Therefore, innovative TSOs with considerable support from research organizations and manufacturers are using new technical solutions that minimize the environmental impact and remain compliant with national legislation and international design standards.
When faced with this challenge, Elia, the Belgian TSO (which is also 100% owner and 60% operator of the German TSO 50Hertz Transmission), considered the use of established solutions such as increasing the line voltage, increasing the conductor cross-sectional area, real-time monitoring and using conductors with a higher conductivity. None of these solutions offered a satisfactory answer. To meet the growing demand for energy and guarantee network reliability, Elia had to search for a cutting-edge solution.
150-kV Mol-Beringen Transmission Line
In 2007, Elia was informed that an industrial plant wanted to install a new 400-MW power-production unit and accordingly increase its electricity injection at a specific connection point on the Belgian transmission network by 2010. At the same time, another production unit 20 km (12.4 miles) farther away gave notice that its expected production would cease. Load flow studies completed for different system configurations examining the system reliability (N-1) indicated the 20-km-long double-circuit 150-kV network would be overloaded under certain conditions. These two circuits were strung with 298 aluminum conductor steel-reinforced (ACSR) conductors with a diameter of 22.4 mm (0.69 inches), a maximum load transfer capability of 162 MVA and a maximum operating temperature of 75°C (167°F), as specified by Belgian legislation.
The challenge Elia faced was to increase the load capacity of each circuit from 162 MVA to 360 MVA, subject to three specific constraints:
- Limiting reinforcement of the tower-foundation system
- Uprating one circuit with the second circuit in operation
- Limiting the environmental impact.
Elia considered all technical solutions before deciding that reconductoring the existing circuits was the best solution. Circuit reconductoring did not require the existing towers and foundations to be reinforced, and it was the only solution that enabled Elia to complete the project within a reasonable time frame. One existing design parameter that had to remain unchanged was conductor sag. The sag of the new conductor at its maximum operating temperature should not exceed the sag of the existing ACSR conductor at 75°C.
The outer diameter of the new conductor and the forces applied to the existing towers had to be less or equal to those applied by the ACSR conductor, and at 75°C load transfer capacity, the new conductor had to be better or at least equal to the previous conductor. Several new or existing technologies were examined to check whether they were suitable to meet the projected line specification and site conditions.
Since the 1990s, the accepted solution for uprating existing transmission lines has been to replace the existing conductors with a conductor with Z-shaped strands having the same overall diameter. This allowed a modest increase in the load transfer capacity using the same maximum conductor temperature without the need to reinforce supporting towers. However, this solution was insufficient to satisfy the capacity increase required by the 150-kV Mol-Beringen transmission lines. Elia launched a tender inviting bidders to propose advanced conductors to provide a solution for this problem.
Aluminum conductor steel-supported (ACSS), gap-type super thermal-resistant aluminum conductor steel-reinforced (GZTACSR), aluminum conductor composite-reinforced (ACCR) and aluminum conductor composite-core (ACCC) conductors all were considered for this project. After analyzing the cost-benefits of this major investment, Elia opted for the ACCC Lisbon 325 conductor because of its advantages:
- The load transfer capacity of the circuit increases by a factor of two using a conductor of similar cross-sectional area that has a maximum operating temperature of 180°C (356°F).
- No significant change to the loading on the existing supporting towers limiting the expenditure for reinforcements.
- While maintaining the same design of 150-kV towers, the rearrangement of the two existing circuits from super bundle to a low reactance and the lower sag arrangement reduces the magnetic field level at the annual mean current.
- Reduction of time for restringing the existing circuits with new conductors, resulting in the lowest overall costs when compared with other technical solutions.
This solution offered the lowest line losses and line construction costs, these being the most important considerations. Additionally, the length of ACCC circuits in service and the low execution risk convinced Elia to use this technology. To allow the use of high-performance conductors on the 150-kV transmission line between Mol and Beringen, Elia had to apply for special exemption of the regulations governing the conductor's maximum operating temperature granted for this pilot project.
Elia arranged an extensive test program for the ACCC Lisbon 325 conductor. Several tests were performed by independent test laboratories in Europe and the United States. The program included various electrical, mechanical and thermal tests according to international standards and Elia's technical specifications on the ACCC conductor and ACCC accessories.
The ACCC conductor was supplied by the Belgian manufacturer Lamifil. It was the company's second such European project, following the delivery of ACCC conductors for a wind farm in Spain in 2008. Lamifil started collaborating with CTC Cable in 2006 and has been selling ACCC conductors directly to end users since 2008. Line hardware, special tools and services also were supplied by Lamifil and CTC Cable. The line reconductoring was carried out successfully by Fabricom, a well-established Belgian contractor.
Project Pre-Assessment and Installation
During the preparation of the project, Elia evaluated all potential technologies. This included visits to the manufacturer's site, meetings with other utilities and meetings with the installers of the respective technologies. This project pre-assessment ensured, at an early stage, all potential difficulties and risks were evaluated and resolved.
Prior to installation, Elia arranged a one-day course presented by the manufacturer of the ACCC conductor. The course consisted of a theoretical presentation followed by a practical training to demonstrate and practice the making of a dead-end clamp and splice. At the start of the actual installation, on-site practical training was given to about 40 linemen. During the stringing of the first circuit, a representative of the ACCC conductor manufacturer was present to overview the installation and assist where necessary. Most stringing of the second circuit was done without supervision from the conductor manufacturer.
The design of the dead-end clamps and splices is quite different from the hardware used for conventional conductors, but the design ensures the installation of ACCC conductor is relatively easy. The possibility of making compressions on dead-end clamps on the towers with a light 60-ton press was considered to be a major advantage for safety and comfortable working conditions.
The main considerations when installing ACCC conductors are to respect the minimum bending radius and apply adequate back tension at all times during the conductor stringing operation. During the stringing process, plain metallic sheaves were used, which have to be examined for abrasiveness, debris and marks on the rollers to ensure no damage is caused to the soft aluminum outer layers of the ACCC conductor.
During installation, the ACCC conductor manufacturer monitored the conductor stringing procedure and installation of the accessories, and controlled the quality of work. All conductor compressions were subject to a visual inspection by Elia Grid Services prior to commissioning and immediately after commissioning; the transmission line was checked with thermovision. To secure the knowledge and expertise with respect to ACCC technology, Elia trained a full crew of its Grid Services-Power Links division.
Elia successfully completed its first project involving ACCC conductor. The existing ACSR conductor on both circuits of the 150-kV Mol-Beringen line was replaced by ACCC Lisbon conductor of a similar cross-section. The in-stallation of 125 km (78 miles) of ACCC conductor ran smoothly and was completed ahead of schedule. The 150-kV double-circuit transmission line has been back in normal operation since November 2009. It is now offering ample current-carrying capacity with the conductor temperature up to 75°C and has a long-term spare capacity for working under N-1 conditions.
Jean-Francois Goffinet (firstname.lastname@example.org) works as a senior expert on Power Links at Elia Engineering, and is a member of CIGRÉ and CENELEC. Elia owns the entire Belgian high-voltage grid (150 kV to 380 kV) and 94% (ownership and user rights) of Belgium's high-voltage grid infrastructure (30 kV to 70 kV). Elia is also 100% owner and 60% operator of the German TSO 50Hertz Transmission (40% of the shares are held by IFM).
Bart Pelssers is a senior project leader at Power Links at Elia Engineering.
|Conductor type||Operating temperature|
|GZTACSR 328||AT3 (ZTAL) Aluminum zirconium gap-type conductor||170°C to 210°C (338°F to 410°F)|
|ACSS 246||Annealed aluminum conductor steel supported||250°C (482°F)|
|ACCR 226||Aluminum zirconium alloy metal composite reinforced||150°C to 210°C (302°F to 410°F)|
|ACCC Lisbon 325||Annealed aluminum polymer composite supported||180°C (356°F)|
Characteristics of 150-kV Transmission Line
|Existing situation||Future situation|
|Type of conductors||298 ACSR||ACCC Lisbon 325|
|Diameter||22.4 mm (0.88 inches)||21.78 mm (0.86 inches)|
|Nominal phase current||662 A||1380 A|
|Number of circuits||Two|
|Number of conductors per phase||One|
|Type of towers||Double-circuit vertical|
|Configuration of the circuits||Super bundle||Low reactance|
|Maximum operating temperature||75°C (167°F)||180°C (356°F)|
|Nominal voltage||150 kV|
|Circuit length||20 km (12.4 miles)|
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