HVDC CABLES – Building Europe’s future electricity backbone

High Voltage Direct Current (HVDC) underground cables have been in commercial use since the 1950’s. Today, HVDC underground cables can carry medium and high power (100 MW up to 1,000 MW) over distances above 50 km.

So far, HVDC transmission has mainly been used in submarine applications, either connecting offshore wind farms to land or transmitting high electrical power over long distance through the sea were overhead lines cannot be used.

Increasingly, HVDC cables are beginning to be used also for on-shore transmission projects. As higher power loads need to be transported over long distances across land, more and more thinking goes into creating HVDC a long distance overlay net, so called “electricity highways”.

HVDC underground cablescan safely transport high power loads over long distances with minimal losses. In addition to this transport efficiency, only a limited number of cables are required, hence allowing narrow trenches. HVDC underground cables are compatible with HVDC overhead technology and can be combined in sensitive areas.

Currently switchgear technology is in the final phase of development and while the cost for the HVDC cable is only 2 – 3 compared to HVDC overhead technology, significant investments are required for the converter stations.

Today, two HVDC cable technologies are installed:

• Single core mass impregnated or self-contained fluid filled (SCFF) cables:

This type of cable is currently the most used. It has been in service for more than 40 years, has proven highly reliable and can be provided by European manufacturers at voltages up to +/- 500kV and 1400A DC which corresponds to a maximum pole rating of 660MW (in service) and bipole rating of 1320 MW. Conductor sizes are typically up to 2500 mm2.

• Polymeric cables, e.g. XLPE:

Polymeric cables are only used in Voltage Source Converters applications that allow to reverse the power flow without reversing the polarity. To date this technology has only been applied at voltages up to +/- 200kV (in service with a power of 400 MW). There are projects at an advanced construction stage at the voltage of 320 kV and 800 MW power and ongoing projects at 1000 MW, but it is expected to increase the voltage and power in the near future.

Due to the fact that the current electricity transmission and distribution networks in Europe are based on AC systems, converter stations are required to convert the DC current and voltage to AC. Converter stations entail significant investment and operational costs which need to be considered when deploying HVDC technology. Two different technologies are available:

• Line Commutated Converter (LCC) technology
  Also called CSC Current Source Converter, has been in existence for several decades. This technology requires to connect two active power points at either side of the link.
• Voltage Sourced Converter (VSC) technology
  This technology has been in commercial use since 1999. Contrary to LCC, it can also be applied for linking isolated networks to the grid, e.g. supply power from generation sources like wind mills or remote islands. Recently developed VSC Multilevel Converters which has low losses appear to be the converter type of the future.

Click here to download the Europacable Introduction to HVDC Underground Cables.