HVAC XLPE (Cross-linked Polyethylene)

HVAC cables are typically used as an alternative transmission technology when overhead lines are not appropriate, e.g. in densely populated and reserved areas, across a river or offshore. Extruded cables are insulated using cross-linked polyethylene (XLPE). With improvements in material science, XLPE reached a high dielectric strength, low dielectric constant, high insulation resistance and good mechanical properties, making it an excellent insulation medium for underground power transmission.


Technology Types

HVAC XLPE cables can be categorized in two types:

  • Onshore cables
  • Submarine cables

Each type can further be distinguished by the number of cores (conductors) laid up together to form the cable. Their configuration may be single or three cores.

HVAC cables are widely used at voltage levels up to a maximum voltage of 550 kV at a global scale. The extruded XLPE insulated cables have been currently applied at 275 kV since year 1995 and at 400 kV since roughly 2000.

HVAC extruded insulation cables are also commonly used for the submarine connections. Today, HVAC cables with extruded insulation cover transmission system voltages up to 550 kV and the maximum power that can be transmitted is in the order of magnitude of 1.5 GVA per circuit, with a 2500 mm² copper conductor.


Components & enablers

Onshore cable:

  • Conductor (Al or Cu)
  • Insulation material (XLPE)
  • Metallic screen
  • Waterproof layer
  • Non-metallic outer sheath

Submarine cable:

  • Conductor (Al or Cu)
  • Insulation (XLPE)
  • Metallic screen (lead sheath)
  • Core sheath (semiconductor)
  • Armouring (galvanized steel wires)
  • Outer serving (bitumen and polypropylene yarn)

Advantages & field of application

Major advantage of XLPE as insulation for medium and high voltage cables is their low dielectric loss. The dielectric loss factor is about a tenth of that of paper insulated cables and about a hundred times lower than that of PVC-insulated cables.

In addition, improvements in manufacturing and installation process has led towards a dramatic increase in the utilization of this type.

XLPE cables have a lifetime of at least 40 years. HVAC XLPE permits a temperature of 90°C and a maximum short circuit temperature of around 250°C. When implementing HVAC XLPE cables, some challenges need to be tackled, e.g. long outage times after damage or failure, transient behavior and reactive power produced by cables. Specific risks for network operation have to be carefully analyzed case by case.


Technology Readiness Level

  • TRL 9 for offshore HVAC cables and onshore HVAC cables with voltage rating equal or less than 245 kV.
  • TRL 7 for onshore HVAC cables with voltage rating equal or higher than 420 kV.

Research & Development

Current fields of research: System integration margins due to reactive power compensation, partial discharge detection, fault localisation, new environmentally friendly lying methods, bedding materials, 4000 mm² conductors, long term thermal stability of insulating material.


Best practice performance

On-shore:

  • Maximum capacity: 1250 MVA (3-phase cable system)
  • Current rating: 1.8 kA
  • Voltage rating: 420 kV

Off-shore:

  • Maximum sea depth: 100 m
  • Maximum length: 162 km*

*This cable length corresponds to a cable connected to 145 kV with a capacity of 55 MVA. It is the cable installed at the Martin Linge offshore gas field and is the longest AC submarine cable of the world.


Best practice application

Berlin, Germany

1998

Description
A 400 kV underground XLPE transmission link was built in 1998 in the centre of Berlin. The cable is located in a tunnel of 6.3 km length, 25 m below the surface.

Design
400 kV XLPE AC, 1100 MVA power transmission.

Results
Long-term reliable performance of high power transmission is promised through the XLPE cable as compared to the former low-pressure oil-filled cable.

Croatia, Brac / Adriatic Sea

1995

Description
Two XLPE cable types were supplied to connect islands of Croatia to the mainland in 1995.

Design
Types of XLPE cables are 1) three-core 35 kV AC cable 23 MVA, 21 kg/m with 3 x 150 mm² Cu, 100 km length and 2) 100 kV AC cable, 100 MVA, 1x300/400 mm² Cu, 14 kg/m, length 100km. The cores are twisted together with a filling material in a trefoil. Swelling powder and tapes are used under the lead sheath to protect from water.

Results
Reliable supply of electricity to the Croatian island economy.

France, West Brittany, St-Brieuc - Lorient

2018

Description
A major 225 kV underground cable to enhance the reliability of energy supply to West Brittany customers. Energization was done in 12/2018. It was the longest cable ever put in service in France.

Design
76 km in total length (three unipolar cables and two fiber optic cables in PEHD pipes ) in two sections 46 km and 30 km with an intermediate substation. 83% is 2500 mm² Al and 17% is 2000 mm² Cu, 58 joint chambers, 56 water flows to cross. 26 months of field work.

Results
Reliable supply of electricity to West Brittany.


References

[1] ABB. XLPE Land Cable Systems User’s Guide [Link]

[2] Nexans Submarine XLPE Cables. [Link]

[3] ENTSOE. Technologies for Transmission System. [Link]

[4] Z. Guiping, D. Xiaowei and Z. Chen. Optimisation of reactive power compensation of HVAC cable in off-shore wind power plant. [Link]

[5] Vrana T K, Mo O. Optimal Operation Voltage for Maximal Power Transfer Capability on Very Long HVAC Cables. [Link]

[6] S. M. Gubanski. Insulating materials for next generations of HVAC and HVDC cables. [Link]

[7] ABB. Erdverkabelung im Bereich der HGÜ-Leitungen. [Link]

[8] Modern Power Systems. Installation of the first 400kV XLPE cable in Berlin. [Link]

[9] ABB. Electrifying the Croatian Archipelago. [Link]

[10] Pedrazzoli, G & Rinzo, Giovanni. Longest HVAC Cable Systems: A Review. [Link]

[11] UCable. XLPE Insulated Power Cables. [Link]