High Voltage Alternating Current (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. The extruded cables are insulated using cross-linked polyethylene (XLPE), providing electrical insulation, thermal stability, and protection from harsh environments. This is also the main design difference compared with mass-impregnated (MI) or oil cables, where wrapped paper insulation is used.

HVAC XLPE cables can be categorised into 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 might involve single or three cores. Moreover, the core is made of either aluminium or copper.

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 1995 and 400 kV since approximately 2000.

The benefits of HVAC XLPE cables are listed below:

• The major advantage of XLPE as insulation for medium and high voltage cables is its low dielectric loss. The dielectric loss factor is about one-tenth that of paper-oil insulated cables and around 100 times lower than that of polyvinyl chloride (PVC)-insulated cables.
• In addition, improvements in manufacturing and installation process have led to a dramatic increase in the utilisation of this type. For example, setting a cable joint on land only takes a few days and does not require strong expertise. An MI cable joint requires roughly a week, and any quality deviation might lead to a failure.
• XLPE cables have virtually no environmental risks due to the highly resistant thermoset material used rather than an internal dielectric fluid.
• 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.
• The thermal inertia of XLPE cable systems can be used to temporarily increase the ampacity of the circuit thanks to dynamic line rating (DLR).

The enablers of HVAC XLPE cables are listed below:

• Due to the significant capacitance of the cable, the reactive power compensation needs to be applied. For this reason, there is a natural limitation of cable length, whereby the longest HVAC (145 kV) and extra high voltage AC (220 kV) offshore cables have lengths of 163 km and 94 km, respectively. For 380 kV, the maximal length is about 40-60 km.
• The ampacity of a single land cable with a presently possible diameter of 3,000 mm2 copper is about 2,000 A (depending on the laying principle). Compared with the extra high voltage overhead line that has a typical ampacity of 2,700 A and nowadays 4,000 A, there is a need to use two cables per pole in the extra high voltage grid if the cable should be used as a section of the overhead line.

Several R&D activities listed below can contribute to further improving the technology:

• The sheath of offshore cables (and some land cables in wet environments) is made of lead. The lead sheath is protected from overheating and is not exposed to the environment during normal operation. In case of a fault, negligible emissions can appear. Cable designs with lead are suitable for most submarine applications, except for connection to floating offshore platforms where mechanical bandings are frequent. Therefore, from the technological perspective and considering long-term environmental aspects, there is a need to develop a reliable and sustainable replacement for sheath made of lead [2].
• The integration of real-time thermal rating (RTTR) requires implementing monitoring systems of different manufacturers in a closed information technology (IT) environment of dispatching centres. This process is very time-consuming due to the intensive IT security checks. Virtualisation and standardisation of the interfaces of the monitoring systems would help to speed up this process significantly.
• Developing ageing models for predicding end of life and decision-making on cable renewal.
• 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 must be carefully analysed on a case-by-case basis.

This technology is in line with milestones “Integration of dynamic ratings and AI-based renewable power forecasts” and “Circular economy and environmentally friendly components included in planning and asset management” under Mission 1 and milestone “Development and demonstration of floating platforms and dynamic cable systems” under Mission 2 of the ENTSO-E RDI Roadmap 2024-2034.

Examples

TRL 9 for offshore HVAC cables and onshore HVAC cables with a voltage rating equal to or less than 245 kV.
TRL 8 for onshore HVAC cables with a voltage rating equal to or higher than 420 kV.
TRL 4 for offshore HVAC cables with lead sheath.

1. M. Ardelean and P. Minnebo, “HVDC Submarine Power Cables in the World: State-of-the-Art Knowledge”, EUR 27527. Luxembourg: Publications Office of the European Union, 2015.

2. ENTSO-E, “New ENTSO-E position paper on the use of lead in power cables”, ENTSO-E, 2021

3. IPTO,2021 “Crete-Peloponnese Interconnection: The largest subsea AC cable in the world has been electrified”

4. NKT,2020 “NKT completes upgrade of high-voltage power link connecting Denmark and Sweden,”

5. NKT, “Nordergründe, Germany,”

6. Red Eléctrica.“Majorca-Ibiza”

7. Red Eléctrica,2005 “Partial burying of the 400 kV line San Sebastián de los Reyes-Loeches-Morata – ‘Barajas Plan’”

8. P. Corsaro and R. Gaspari, “The Barajas Airport Project: How a 400 kV cable system can be safely and fully exploited”, 2004 International Conference on Power System Technology – Powercon 2004. Singapore, 21–24 November 2004.