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 XLPE as an alternative to oil or MI. Thermal properties of XLPE allow a continuous maximum conductor temperature of 90 °C and a maximum short circuit temperature of 250 °C and, therefore, higher transmission capacity per cable compared 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

N/A

Advantages & field of application

Polymeric HVDC cables are used mainly with VSCs 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 1,000 MW for a symmetrical monopole. For voltages up to ± 525 kV, cable systems are now under prequalification, but no 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 compared with the other insulated DC cable types.

Technology Readiness Level

Onshore:

• TRL 9 (2019) 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 2,500 to 3,000 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 localised high stress which results in the accelerated ageing of the insulation.

Best practice performance

• Maximum capacity: Approximately 1000 – 1200 MW today, expected to increase to 1,900 MW in 2020 with an increased conductor size
• Current rating: 1.9 kA
• Voltage rating: ± 525 kV
• Longest distance: No limitation for the cable itself
• Maximum sea depth: 1,000 m

Best practice application

References

[1] ENTSO-E, TYNDP 2018, Technologies for Transmission System, October 2019. [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] Stefan Habild, Geschäftsbereichsleiter Grid Systems, ABB AG Erdverkabelung im Bereich der HGÜ-Leitungen, 2015.

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