Carrying High Power Over Long Distances

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Writer: Ernesto Zaccone, Chairman Europacable High Voltage Systems Group

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High Voltage Direct Current (HVDC) cable installations have seen a significant increase over the past years. HV XLPE DC cables will be a key technology moving into the future.

They allow for long distance, point-to-point transmission of high power: lower power connections are in use to connect islands to mainland or feed offshore platforms; and more powerful connections are used for interconnection and connection of offshore wind generation. Over recent years, a continuous increase in transmittable power has been achieved.

Most recent projects have reached voltages of ±320 kV with a capacity of 800-1,000 MW. This trend will continue in the years to come. With switchgear technology becoming available, meshed HVDC networks will be gradually introduced, and Europe will see the creation of an additional HVDC overlay network. With current networks operating in AC, any DC connection needs to be converted back into AC. The required converter technology is fully available today and will certainly see further evolution in the future.

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HVDC Subsea Cables

High Voltage DC subsea cable systems are a key technology for the development of future European electricity transmission networks. They serve two functions:

1. to interconnect countries or islands separated by sea; and

2. o connect remote offshore platforms to t main transmission grids.

For interconnections today, HVDC subsea transmission technology is largely applied in single point-to-point connections. Among the latest best practices in Europe is the Western Link UK, connecting Scotland to England via a 425 km submarine cable link deploying a bi-pole with two mass-impregnated cables of ± 600 kV for the transmission of 2,200 MW. The Western Link UK will be the most powerful HVDC connection ever realized with a maximum water depth of 350 meters.

Looking into the future, meshed HVDC subsea systems will become available, thereby interconnecting remote parts of the European Union to one single electricity network. In addition to serving as an interconnector, HVDC subsea cable systems can also connect offshore substations on platforms to mainland grids. Offshore platforms can export electricity generated from one or several offshore wind parks to land or they can serve as a power distribution hub foroffshore oil and gas operations. Current best practices include the Borwin 2 Project, a large scale offshore wind installation in the North Sea located some 120 km north of the German coast. When completed, two XLPE DC cables ± 300 kV 800 MW will connect the Borwin beta platform to the transmission grid over a distance of 75 km.

The installation of HVDC XLPE subsea cable systems is a considerable challenge: the maritime environment adds a significant complexity to the cable installation, operation and maintenances. A distinction needs to be made dependent on the laying depth: in shallow waters up to a maximum depth of 500 meters, burial is mandatory to protect the cable against the risk of damage from fishing gear and anchors; in waters of 500 to a maximum of 2,000 meters deep, potential threats from fishing gear and anchors are non-existent and consequently the cable can be laid directly on the seabed.

As access to subsea cable systems is difficult once installed a cautious and precise installation by highly-skilled experts is key to ensure long-term reliability. Installation vessels and submarine robots are at the center of the installation operation. Depending on the cable weight and diameter, typically up to 100 km cable length can be carried on a rotating turntable on the laying ship.

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HVDC Underground Cables

Generally speaking, HVDC power transmission on land can be reliably installed either as overhead line or as an underground cable. HVDC mass-impregnated underground cables have a long operational track record of over 40 years and are a proven and reliable transmission component. HVDC XLPE AC underground cable technology has been in service in voltages up to ± 200 kV since 2002. Europacable expects XLPE to be the key DC cable technology of the future with voltages up to ± 550 kV. In principle the installation of HVDC XLPE underground cables is similar to that of XLPE underground cables, yet the environmental footprint is smaller as less cables are required.

This allows for comparatively narrower trenches as can be seen in Figure 3. Individual cable sections are linked by so called joint bays which are typically directly buried into the ground. If required, joint bays may be placed into an underground structure which has no or only very little visibility above ground. As for AC power transmission projects, partial undergrounding is also technically available for DC land interconnectors today. For DC projects, partial undergrounding sections may cover distances from 50 km onwards. Hence, the concept of partial undergrounding can also be applied as an intrinsic characteristic for Europe´s future HVDC overlay grids.

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 This article appeared in Revolve’s special Cable Power report, read it in full here.

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