B1-312

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andre.stamsaas@nexans.com 
 
 
 
 
 
 
 
The world longest and deepest 420 kV XLPE submarine cable 
 
 
A. STAMSAAS 
Nexans Norway AS 
Norway 
 
H.TOLLEFSEN 
BKK Nett AS 
Norway 
 
H.L.HALVORSON 
SINTEF Energy 
Research 
Norway
 
 
 
 
SUMMARY 
 
The submarine power cable market has high demands for power transmission. The need for longer 
lengths, higher voltage and deeper water depths continues to increase. The Kollsnes – Mongstad 
project that the Manufacturer was awarded by the Transmission Grid Owner in February 2014 is a 
good example of this. The project consists of two cable lengths in separate fjords, approximately 22.6 
km (Hjelteforden) and 9.3 km (Lurefjorden), with the deepest water depth down to 390 m. The fjords 
are connected by overhead lines. 
 
The cable technology used for this cable link is the well proven XLPE technology. Compared to 
earlier 420 kV submarine projects, the challenges were the greater water depth, the new 
recommendations regarding radial water penetration tests on joints, changes in some key materials and 
the manufacturing and testing of the factory joint. A repair joint was also required. 
 
Cigré TB 303 gives recommendations for tests to be performed depending on the changes in design or 
installation conditions. A new type test on a submarine cable with factory joint had to be performed 
for the Kollsnes – Mongstad project. This included mechanical and electrical tests on submarine cable 
with factory joint, longitudinal and radial water penetration tests and non-electrical tests. The 
submarine cable that was qualified was a watertight single core 1200 mm2 copper conductor with 420 
kV XLPE insulation. The insulated conductor had swellable tape, lead sheath, a semi conductive PE 
sheath, two layer of copper armouring with bitumen and PP yarn. 
 
The qualification performed for the Kollsnes – Mongstad project, is an important step for future power 
systems. The ability to manufacture long lengths in combination with factory joints provides great 
opportunities at the 420 kV level. The world longest and deepest 420 kV XLPE submarine cables are 
planned to be installed the summer of 2016 between Kollsnes and Mongstad. Experiences gained in 
the Manufacturers 420 kV projects so far are important for future projects, and form the basis for new 
world records. 
 
 
 
 
 
KEYWORDS 
 
XLPE 420kV – HVAC – type test – power transmission – submarine power cable 
 
 
21, rue d’Artois, F-75008 PARIS B1- 312 
 
 
 
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INTRODUCTION 
 
The submarine power cable market has high demands for power transmission. The need for longer 
lengths, higher voltage and deeper water depths continue to increase. In addition, submarine HVAC 
cables give more opportunities compared to overhead lines. At some locations it is not feasible to 
install overhead lines, for example for long distances over sea. In many cases, submarine cables are 
preferable considering the impact overhead lines may have on nature. 
 
The experience with Extra High Voltage (EHV) submarine cable power transmission systems are 
increasing, in both manufacturing and in operation. The high demands for power transmission 
combined with this increasing experience will provide the market with several XLPE EHV submarine 
projects in the coming years. 
 
 
PROJECT BACKGROUND 
 
The project that the Manufacturer was awarded by the Transmission Grid Owner in February 2014 is a 
good example of demands for longer lengths and deeper water depth. The project scope was to design, 
manufacture and install a 420 kV AC submarine cable system between Kollsnes and Mongstad in 
Bergen (Hordaland), Norway. 
 
The parties agreed to use IEC 62067 [1] and Cigré recommendations [2, 3] as basis to develop and test 
the new submarine cable system. A type test of the cable system was needed to be able to 
manufacture, install and repair submarine cables for this length and depth. 
 
The project consisted of two cable lengths in separate fjords, approximately 22.6 km (Hjelteforden) 
and 9.3 km (Lurefjorden), with the deepest water depth down to 390 m. These fjords were connected 
by overhead lines. 
 
 
 
Figure 1: Location of Bergen. 
 
The connection between Mongstad and Kollsnes was important for the establishment of a robust 
power supply to Kollsnes (natural gas processing plant) and the city of Bergen. The considerable use 
of submarine cables was absolutely necessary in this project in order to protect the unique coastal 
landscape in Hordaland, cultural heritage and a distinctive bird life. 
 
 2 
 
 
 
CABLE TECHNOLOGY 
 
The cable technology used for this cable link is the well proven XLPE technology. The Manufacturer 
performed a Pre-Qualification (PQ) test on a 420 kV AC XLPE submarine cable system in 2006/2007. 
The PQ test included a one-year heat cycling voltage test that was needed to validate the performance 
of the complete EHV cable system. This was necessary because of the limited experience with EHV 
XLPE cable systems [3]. The Ormen Lange project was the world first 420 kV XLPE submarine cable 
system that was installed [4]. In 2013 the Outer Oslofjord Project was finalized with both 420 kV 
SCFF (Self Contained Fluid Filled) and XLPE cables [5], and was also based on the Ormen Lange 
design. This was the first project with installed 420 kV XLPE factory joints. 
 
Compared to earlier 420 kV submarine projects [4,5], the challenges were the greater water depth, the 
new recommendations regarding radial water penetration tests on joints, changes in some key 
materials and the manufacturing and testing of the factory joint. The world longest and deepest 420 kV 
XLPE submarine cables are planned to be installed the summer of 2016 between Kollsnes and 
Mongstad 
 
Cigré TB 303 gives a recommendation for what kind of tests that shall be performed depending on the 
changes in design or installation conditions [3]. A new type test on a submarine cable with factory 
joint was performed for the Kollsnes – Mongstad project. The test performed, will be discussed later in 
this article. 
 
 
CABLE DESIGN 
 
The cross section drawing of the submarine cable is presented below. 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2 : TKZA 420 kV 1x1200 mm2 KQ 
 
 
 
The conductor was made of round stranded compacted copper wires, and was filled with a longitudinal 
water blocking material. 
ID Component Nominal 
thickness 
[mm] 
Nominal 
diameter 
[mm] 
1 Conductor 1200 mm2 43.7 
2 Conductor screen, 
semiconducting XLPE 
 
3 Insulation, XLPE 28 103.7 
4 Insulation screen, 
semiconducting XLPE 
 106.7 
5 Semiconducting swellable 
tape 
 
6 Lead sheath 3.6 
7 Inner semiconducting PE 
sheath 
3.7 
8 Bedding 
9 Armour, flat copper wires 8.5x3.5 mm 
10 PP yarn, outer serving approx 
147 
Table 1 : Cable data 
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The insulation system was made of cross-linked polyethylene (XLPE). XLPE was produced by cross-
linking of thermoplastic polyethylene. The cross-linking of the molecules in polyethylene is a 
chemical process caused by peroxides when subjected to high temperature and pressure. XLPE has 
thermal properties which permit a continuous maximum conductor temperature of 90 °C and a 
maximum short circuit temperature of 250 °C. 
 
A lead alloy sheath was applied as radial water barrier. Semi-conductive water swellable tapes were 
wrapped around the insulated core in the same process. The water swellable tapes will prevent 
longitudinal water penetration if the cable is damaged. 
 
An inner sheath of semiconducting extruded polyethylene was applied over the lead sheath. Then the 
cable armour with two layers of flat copper wires were applied for mechanical protection. 
 
The armour wires were embedded in bitumen, and two layers of polypropylene yarn were applied 
outside the armour as a corrosion protection. 
 
 
FACTORY JOINT 
 
A general drawing of the factory joint is presented below: 
 
 
 
Figure 3 : General arrangementdrawing of a 420 kV XLPE factory joint. 
The conductor was welded with copper. The insulation system for the joint was build up with XLPE 
material. A lead tube was swaged down over the insulation screen, and soldered together with the lead 
sheath on the submarine cable. A PE heat shrink sleeve was applied as the last layer. 
 
Factory joints are generally used to meet the required cable lengths, because of the limitations of the 
lengths from the extrusion lines. The joints are normally manufactured after the XLPE cable phases 
have been AC tested. 
 
Factory joints are also used in case of incidents in the manufacturing (e.g breakdowns). 
 
The first 420 kV factory joints were installed in the Outer Oslofjord in 2013, and are in operation 
today. A new type test was performed on the Kollsnes – Mongstad project because of change of some 
key materials, and because of the new recommendations regarding radial water penetration tests on 
joints [2]. 
 
 
 
 
 
 4 
 
REPAIR JOINT 
 
A general drawing of the repair joint is presented below: 
 
 
Figure 4 : General arrangement drawing of a 420 kV XLPE repair joint. 
 
The conductor is jointed with a copper compression ferrule. The core joint is insulated with a pre 
molded joint (PMJ). In order to maintain the water tightness of the cable core, a lead sleeve covers the 
core joint. The armor wires are clamped in an armor block on each side of the joint. 
 
The repair joint was qualified to 390 m installation depth. The ability to do a repair on the submarine 
cable was an important criterion for this project. 
 
 
TYPE TEST PROGRAM 
 
The tests performed in the type test program followed the requirements that are given in Cigré TB 490 
[2]. This included mechanical and electrical tests on submarine cable with factory joint, longitudinal 
and radial water penetration tests and non-electrical tests. 
 
The type test program also included a repair joint. The test performed on this object was the radial 
water penetration test, according to requirements in Cigré TB 490. Full type test on this object was 
not included in the scope for this project. The electrical type test for this repair joint was performed on 
a previously project [5]. 
 
Figure 5 below shows the test setup for the submarine cable with factory joint and terminations in the 
High Voltage Laboratory. The cable and factory joint was subjected to a tensile bending test before 
this setup. The forces used on the tensile bending test, was calculated to 390 m water depth and a 10 m 
laying wheel. 
 
 5 
 
 
Figure 5 : Type test setup in the High Voltage Laboratory. 
 
When the test setup was mounted, the following test program was performed: 
 
- PD measurement at ambient temperature 
- Heating cycle voltage test (20 cycles) 
- PD measurement at ambient temperature 
- PD measurement at high temperature 
- Switching impulse voltage test (1050 kV) 
- Lightning impulse voltage test (1425 kV) 
- AC voltage test (440 kV) 
 
 
The cable was cut into smaller pieces after the electrical tests were finalized. The factory joint area 
(with cable on each side) was cut out, and the cable ends were sealed, and prepared for the radial water 
penetration test. The object was submerged in a pressurized tank, and tested according to the test 
procedure at 39.5 bar for 48 hours at ambient temperature. 
 
The type test of the submarine cable with factory joint was finalised with an examination. There were 
no signs of electrical degradation, leakage, corrosion or harmful shrinkage. 
 
In addition to the mentioned test setup, the following tests were performed as a part of the type test 
program: 
 
- Conductor water penetration test (390 m water depth) 
- Metal sheath water penetration test (swellable tape, 3 bar) 
- Resistivity of semiconducting screens and polymeric sheaths 
- Non electrical tests (visual examination, dimension check and mechanical properties) 
- Tan δ measurement 
 
 6 
 
These tests were performed on separate samples. All performed tests were passed and demonstrated 
satisfactory performance of the manufacturing line and submarine cable system. 
 
 
MANUFACTURING OF LONG LENGTHS 
 
To be able to produce the required delivery lengths, the manufacturing of the cables for this project 
was planned with factory joints. In addition, the extrusion line was set up for long runs to optimise the 
number of factory joints. 
 
The test equipment for such long lengths at this voltage level needs to be planned carefully, in respect 
to space and test effect. The different combination of lengths and test voltages required different test 
setups and available space in the factory during the manufacturing. The factory joints were X-rayed, 
PD-tested locally and subjected to high voltage AC tests. 
 
 
FUTURE DEVELOPMENTS 
 
The qualification performed for the Kollsnes – Mongstad project, is an important step for future power 
systems. The ability to manufacture long lengths in combination with factory joints provides great 
opportunities at the 420 kV level. The 390 m qualification depth is also an important milestone. With 
this type test, the Manufacturer can produce, install and repair the longest and deepest 420 kV XLPE 
submarine cables in the market today. Future demands will require longer lengths and deeper water 
depths. Experiences gained in the Manufacturers 420 kV projects so far are important for future 
projects, and form the basis for new world records. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 7 
 
BIBLIOGRAPHY 
 
[1] IEC 62067 "Power Cables with Extruded Insulation and their Accessories for Rated Voltage 
above 150 kV (Um = 170 kV) up to 500 kV (Um = 550 kV) – Test Methods and Requirements, 
33 – 55 
[2] Cigré Technical Brochure 490 “Recommendations for Testing of Long AC Submarine Cables 
with Extruded Insulation for System Voltage above 30 (36) to 500 (550) kV” 
[3] Cigré Technical Brochure 303 “Revision of qualification procedures for HV and EHV AC 
extruded underground cable systems” 
[4] Qualification, supply and installation of the world’s first 420 kV XLPE submarine cable system 
in Norway (Paper from Jicable 07) 
[5] The Oslofjord Project - The world's first installed 420 kV submarine cable connection 
combining SCFF cables and XLPE cables with flexible factory joints (Jicable 15)