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 has achieved a high dielectric strength, low dielectric constant, high insulation resistance and good mechanical properties, making it an excellent insulation medium for underground power transmission.
HVAC XLPE cables can be categorised in two types:
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.
Onshore cable:
Submarine cable:
The major advantage of XLPE as insulation for medium and high voltage cables is its 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 have led to a dramatic increase in the utilisation 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 behaviour and reactive power produced by cables. Specific risks for network operation have to be carefully analysed case by case.
Current fields of research: System integration margins due to reactive power compensation, partial discharge detection, fault localisation, new environmentally friendly lying methods, bedding materials, 4,000 mm2 conductors, long term thermal stability of insulating material.
On-shore:
Off-shore:
*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 in the world.
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.
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.
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.
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[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]