Letsi hydropower Sweden
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Letsi hydropower plant

Turnkey cable replacement in Northern Sweden

Key facts

Client

Vattenfall Hydro

Location of the project

Vuollerim (Northern Sweden)

Quantity of order

Turnkey cable replacement of a 400 kV AC 1-core XLPE cable with a 486 MW power rating

Duration

2019 - 2021

Scope

Hydropower has, for many decades, played a major role in fulfilling Sweden’s electricity requirements. The Letsi hydropower plant in Northern Sweden is one of the oldest of its kind in the region. When renovation and replacement works had become necessary, NKT provided a turnkey replacement solution.
NKT's solution consisted of:
  • Turnkey cable replacement on Letsi hydropower plant
  • Project management and planning
  • Engineering, production, testing, and delivery of new 400 kV XLPE cable and terminations
  • Removal of old oil-filled cables
  • Installation of cable and cable accessories
Letsi hydropower Sweden
Letsi hydropower Sweden
Letsi hydropower Sweden

The project

Replacing a hydropower transmission cable in a challenging physical environment

Sweden is a giant in renewable energy, with hydropower accounting for nearly half of its total electricity output. The country’s most important region for hydropower generation is the Luleälv river, in the far north. Letsi power plant was the first step in the development of this vital hydropower region. Its construction began in 1960 with the plant being commissioned in 1967.
After five decades of power generation, Letsi’s operating company, Vattenfall, assessed that the plant would benefit from a program of hardware renewal. That included replacing a 400 kV AC single-core XLPE cable with a 486 MW power rating.
In October 2019, NKT was awarded the cable replacement element of this sizeable project and provided a complete turnkey solution consisting of planning, management and execution of replacement works.
The cable replacement work came with some major challenges due to the nature of the physical environment. The route started with a 60-meter horizontal section but then turned upwards into a 95-meter vertical shaft. That was followed by a further 80-meter horizontal section before the cable reached its outdoor termination.
Despite these challenging circumstances, NKT was able to offer and complete a turnkey replacement solution.
Letsi map

The solution

NKT provided a complete solution, including planning, management, and a four-stage execution process.

With full responsibility for every aspect of the project, NKT specialists were able to carry out a careful and thorough planning process in late 2019.
The project execution began in February 2020 with production of the new cable and accessories. These items were subjected to rigorous sample and FAT testing. Vattenfall – who participated fully in the testing process – were particularly impressed with the comprehensive documentation NKT made available to them.
In September 2020, the old cables were drained of oil, removed along with their terminations, and recycled. The major installation tasks could then begin. Between October and November 2020, NKT staff pulled in the new cable and clamped it, sealed the cable ends, installed outdoor terminations, and prepared for later installation work at the transformer.
By December 2020, the new transformer was in place and ready to receive the GIS terminations. This phase was completed in January 2021. To conclude the technical delivery, a sheath test was carried out at the beginning of February 2021 followed by a “walk-through” with Vattenfall on site. The new system was then approved, and connection to overhead lines was completed in April 2021.

Throughout the project, NKT has been extremely professional, taking every precaution to ensure the quality of their delivery. We have been very satisfied with NKT’s work and their open and responsive approach.

— Robert Falk, Project Manager at Vattenfall

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