HVDC XLPE (Cross-linked Polyethylene)

Favourable installation, operation and testing parameters have led to the development of extruded insulation cables for HVDC application. Extruded cables are insulated using cross-linked polyethylene as an alternative to oil or MI. Thermal properties of cross-linked polyethylene (XLPE) allow continuous maximum conductor temperature of 90°C and a maximum short circuit temperature of 250°C and therefore higher transmission capacity per cable comparing to other technologies.


Technology Types

The following technology types of XLPE cables can be differentiated:

  • Lying method o Onshore o Submarine
  • Voltage level
  • Conductor type and its diameter o Aluminium o Cooper o Insulation of single wires
  • Insulation material (depending on manufacturer different additives)
  • Metallic screen
  • Outer sheath

Components & enablers

Not defined.


Advantages & field of application

Polymeric HVDC cables are used mainly with voltage source converters VSC that enable power flow to reverse without polarity reversal. Today, this technology has been implemented for voltages up to ±320 kV with a capacity of 1000 MW for a symmetrical monopole. For voltages up to ±525 kV cable systems are now under prequalification, but not pilot project exists. The basics for HVDC submarine cables are the same as those of HVDC land cables except for mechanical features. Their advantages are related to their weight (20-35 kg/m) and diameter (90-120 mm), which make them very competitive with the other insulated DC cable types.


Technology Readiness Level

Onshore:

  • TRL 7 (2019) TRL Extruded HVDC, 320 kV
  • TRL 5 (2020) Extruded HVDC, 525 kV
  • TRL 3 (2020) Extruded HVDC, 600 kV

Research & Development

Current fields of research:

  • Increase in voltages up to ±600 kV
  • Increase in conductor size from 2500 to 3000 mm²
  • Development of new extruded materials other than XLPE
  • For subsea cables, the laying depth should reach more than 2.5 km by 2050

Other: There are issues with uneven distribution of charges inside the insulation which in the case of rapid polarity reversals can cause localized high stress which results in accelerated ageing of the insulation.


Best practice performance

  • Maximum capacity: about 1000-1200 MW today, expected to increase to 1900 MW in 2020 with increased conductor size
  • Current rating: 1.9 kA
  • Voltage rating: ± 525 kV
  • Longest distance: no limitation for the cable itself
  • Maximum sea depth: 1000 m

Best practice application

UK (Kent), Belgium (Zeebrugge)

2019

Description
A cable system with 130 km subsea route and 11.5 km on land installed and taken into commercial operation early 2019.

Design
DC ± 400 kV, transmission capacity 1 GW.

Results
Compared to conventional cables, higher temperatures can be reached. In addition, oil is no longer needed as an insulation making it more environmental-friendly.

Ireland, Great Britain

2013

Description
"East-West Interconnector" 500 MW Sea Link between Ireland and Great Britain, installed in 2013.

Design
Two parallel submarine cables were installed. The DC voltage is rated at 200kV. The total length is 262 km with a submarine cable share of 186 km.

Results
Better insulation allowing higher temperatures. Submarine construction enabled.

France, Great Britain

2020

Description
"IFA 2" 1 GW, ±320 kV DC Sea Link between France and Great Britain, to be installed in 2019-2020.

Design
The total length is 230 km with a submarine cable share of 202 km. Energization should occur in 2020.

Results
Water crossing possible through submarine construction with the help of XLPE insulation.


References

[1] ENTSO-E. Technologies for transmission System. [Link]

[2] A. Setyowibowo, Suwarno, A. Cavallini and G. C. Montanari, "Partial discharge measurements in XLPE cables with misplaced grading system under different applied voltage frequencies," 2017 International Conference on High Voltage Engineering and Power Systems (ICHVEPS), Bali, 2017, pp. 460-465. [Link]

[3] Joint Research Center Technical reports. HVDC Submarine Power Cables in the World. [Link]

[4] Nexans. Cross-Linked Polyethylene (XLPE) Cables. [Link]

[5] Prysmian Group. Prysmian solutions for the expansion of energy grids. [Link]

[6] National Grid. UK Electricity Interconnection: Driving competition and innovation in the HVDC supply chain. [Link]

[7] Siemens. Siemens and Sumitomo Electric win order for HVDC link and agree on cooperation. [Link]

[8] Sumitomo Electric. J-power Systems wins contract with NEMO LINK for HVDC Subsea Interconnector Cable System between UK and Belgium. [Link]

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

[10] ENTSO-E. TYNDP 2018 Project list. [Link]