HVDC Mass Impregnated (MI) cables

HVDC Mass Impregnated (MI) cables are a very consolidated and traditional technology mainly deployed for subsea applications. Initially, MI cables where used with Line Commutated Converters. Currently, this technology is mainly deployed for Extra High Voltage (EHV) DC subsea applications. MI cables are composed of a very high viscosity impregnating compound which does not cause leakage in case of cable damage or failure. The latest innovation allows voltages up to 600 kV.


Technology Types

There are three main types of cables based on the solution used to insulate the conductor: Self-contained fluid-filled cables; Paper insulated (lapped insulated) cables; Extruded cables.

MI cables belong to the mass paper insulated cables types, which comprise:

  • MI (mass-impregnated) or PILC (paper-insulated lead-covered); the mass impregnated insulation consists of a mass impregnated paper with high-viscosity insulating compound.
  • PPL (paper polypropylene laminated) cables, where the insulation comprises an extruded sheet of polypropylene on either side of which are bonded two layers of thin paper.

Components & enablers

The main components of MI HVDC cables are: • the copper or aluminum (Al or Cu) Conductor • the mass impregnated paper insulation • the lead alloy sheath • the overall protective plastic sheath (Polyethylene) • and the steel armor for submarine cables in order to improve the mechanical performances.

Mass impregnated cables are compatible for the HVDC conversion systems available today, i.e. CSC (current source converter) and VSC (voltage source converter).


Advantages & field of application

MI HVDC cables are currently the most used cables for HVDC submarine applications. Compared to oil filled cables, the compact design makes them particularly suitable for deep water applications. On the contrary their use for land applications is limited compared to Extruded Cross Linked Polyethylene (XLPE) cables. Benefiting for about 50 years of experience in service, with a proven high reliability, they can be provided by European manufacturers at voltages up to ± 600 kV and 1800 A, which makes about 2200 MW per bi-pole. Cable industry experts expect improvement of that mature technology (underground and submarine) towards several directions: increase of power transmission level above 2300 MW for a dipole, reduced level of losses (consequence of the upgrading of operating voltage).


Technology Readiness Level

TRL 9 – System ready for full scale deployment [3]

For on-shore (underground – corresponding to 5% of applications above 110 kV) and off-shore applications (submarine), a TRL 9 has been reached (System ready for full scale deployment at the current performance).


Supply Chain

Commercial availability: Yes

Production capacity: High

Exemplary vendors: ABB, Nexans, Prysmian, NKT, JPower/Sumitomo

Market development: MI-HVDC cables have been implemented for more than 40 years [1]

submarine: expected growth from 2300 km (2013) and 3000 km (2020) to 4500 km± 15% (2050) [3]

underground: 1000 km (2013), 1500 km (2020) to 2500 km±20% (2050) [3]

Reliability

Standard definition: submarine cable Cigré Electra no. 189

Rate of failure: Negligible [3]

Time of interruption: 200 h (underground) [3]

Non-availability: Negligible [3]

Expected lifetime: > 40 years [3]


Research & Development

Current fields of research: Increase in power, reduction of losses, increase in water depth for submarine installations; alternatives for use of lead in sheath; ageing of mass impregnated insulation systems; increase the speed and manufacturing quality of the on-shore cable joints.

Innovation Priority: Voltage (up to 800 kV), maximum sea depth (over 2500 m by 2050).


Best practice performance

For on-shore (underground) and off shore (submarine):

Maximum Installed Rating: 2200 MW bi-pole (±600 kV and 1800 A)

Longest distance: no limitation, example of North Sea Link of 720 km (submarine, planned completion 2021)

Maximum sea depth (off-shore only): 1000 m


Best practice application

Norway, Netherlands

2008

Description
The NorNed HVDC MI sea cable interconnection between Norway and the Netherlands was completed in 2008. The rated DC voltage is ± 450kV DC and the transmission capacity is 700 MW. The entire link has a length of 580km.

Design
Impregnated non-draining, paper insulated HVDC cable. Two different designs are used: twin-core and single-core cable. Both cables consist of copper conductor(s) and a layer of semi-conducting carbon paper. [6]

Results

Denmark-Norway

as of end 2014

Description
The Denmark-Norway Interconnector (SKAGERRAK 4) is a joint project between Statnett and Energinet.dk. It is the first VSC connection at a voltage of ± 525 kV with a monopole MI cable of 700 MW. The transmission capacity is 715 MW and the water depth is 550 m.

Design
The submarine part of the cable is 137 km long and the underground directly buried portions in Denmark and Norway are 92 and 13 km respectively.

Results
The interconnector facilitates more renewable energy production in both countries and benefits consumers and the industry amongst others through more stable power prices and increased security of power supply.

North Sea - between Norway and UK

2021

Description
North Sea Link will be operational in 2021 and will be the longest subsea interconnector in the world. Capacity is 1.4 GW, length 720 km at a voltage of ± 515 kV.

Design
The major part will consist of two parallel HVDC submarine cables, buried 1 to 3 m into the seabed. On both sides a short distance of underground cable will also be constructed from the landfall to the converter station.

Results
Linking the two countries will provide opportunities to share renewable energy to meet new targets, will increase supply security and provide additional capacity.


References

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

[2] Nexans. Submarine HVDC MI Cables by Nexans. [Link]

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

[4] Statnett, nationalgrid. NorthSeaLink. [Link]

[5] O’Rourke P, Egan J, Sellick R, Tomlinson P, Johnson B, Svensson S. Overview of the 500MW EirGrid East-West Interconnector, considering System Design and Execution-Phase Issues. [Link]

[6] ABB. The NorNed HVDC Connection, Norway – Netherlands. [Link]

[7] 4C offshore. Skagerrak 4 Interconnector. [Link]

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

[9] Prysmian Power Link, 'Synthetic description of performances and benefits of undergrounding transmission’. [Link]

[10] Europacable, e-HIGHWAY 2050 project, Deliverable D3.1 ‘Technology assessment from 2030 to 2050’, Annex ‘Transmission Technologies - Cables’.[Link]

[11] Nexans, ‘Case Study - Skagerrak 4’. [Link]

[12] Prysmian Power Link, ‘High Voltage DC and submarine cable system- Practical considerations’. [Link]

[13] SINTEF Energy Research, ‘ Chemical ageing of subsea mass impregnated insulation cable paper.

[14] ENTSOE, Offshore Transmission Technology, 24/11/2011 (update 16/10/2012). [Link]