1 Introduction This note has been prepared by Europacable. Its purpose is to provide a high level overview of current developments in Extra High Voltage (EHV) Alternating Current (AC) and Direct Current (DC) underground cables across Europe. 2 Underground cable technologies When considering the technology for the connection of a new EHV transmission line, the most important aspect is ensuring the security of electricity supply. Other objectives, such as environmental impacts or cost implications resulting from the choice of deploying certain transmission technologies are highly relevant, but are a secondary consideration. The parameters affecting the choice of transmission technology depend on a number of technical aspects, such as: the power to be transmitted; the length of the connection; the characteristics and the accessibility of the route; environmental constraints. Underground and submarine cables, as an alternative to overhead lines, have been in use since the early stages of electricity transmission: EHV AC underground cable transmission technology finds its application in urban and suburban densely populated areas, in areas of outstanding natural beauty and, in general, where the implementation of overhead lines is difficult or impossible. EHV AC underground cables are the core technology for partial undergrounding, which can complement overhead lines in sensitive areas. EHV AC cables are also used extensively on submarine connections. EHV DC cable transmission technology has so far mainly been used in submarine applications, either connecting offshore wind farms to land or transmitting high electrical power over long distance through the sea where overhead lines cannot be used. Increasingly, EHV DC cables are also being used for large on-shore transmission projects or power highways. The advantages of underground cables are a reduced visual impact on the environment and a limited right of way. Cables are installed out of sight, underground, in tunnels or under water. Terminal ends are often the only visible evidence of an underground cable s presence. Transmission System Operators (TSO) have traditionally given preference to overhead lines for economic reasons, risk management and for higher power ratings. However, due to ever increasing environmental concerns and public acceptance, cable technology mainly featuring the XLPE (Cross Linked Polyethylene) insulation has received more and more attention in recent years, leading to an increased deployment in Europe and around the World.
The principal differences between AC and DC electric power transmission are: In general, AC transmission is suitable for relatively short connections, while DC is suitable for the transmission of high power for long connections Another major difference between AC and DC is the ability to realize meshed AC transmission networks due to the simplicity of the commercially available switchgear systems. Today, DC connections are typically used to transmit or exchange power point-to-point. The possibility to design and build meshed or multi-terminal transmission networks depends on the availability of the HVDC switchgear, which is still at the early stage of commercial development. Error! Reference source not found. 2.1 Extruded XLPE HVAC Cables Today, extruded XLPE insulated AC cables are the most commonly used cables for the transmission of electric power. This cable type is available up to voltages of 550 kv. The transmittable power depends on the installation method. The concept of partial undergrounding Increasingly across Europe, public acceptance is proving to be the most critical bottleneck delaying the permitting procedures of new infrastructure projects. Several TSOs, such as National Grid (UK), TenneT (Germany and the Netherlands), Elia (Belgium and Germany) and Amprion (Germany), have now embraced the concept of partial undergrounding to complement overhead lines in sensitive areas with undergrounded sections. Doing so allows them to implement transmission projects in areas where public acceptance for overhead transmission lines has not been achievable. EHV XLPE AC power cables are the core technology to be used in partial undergrounding solutions. The availability and feasibility of partial undergrounding was officially confirmed in the Joint Paper Feasibility and technical aspects of partial undergrounding of extra high voltage power transmission lines published by the European TSO body ENTSO-E and Europacable, in January 2011. 2.1.1 On-going extruded HVAC projects With the increasing deployment of partially undergrounded EHV transmission projects, the reference list of XLPE transmission cables installed in Europe is constantly increasing. Projects completed between 2010 and 2015, or currently in the permitting stage include: Randstad (the Netherlands) This 85 km 380kV double circuit that connects Rotterdam, The Hague and Amsterdam has a capacity of 5200 MVA and includes two 10km underground sections. The southern section came into commercial operation in 2013 and the northern section is due for completion in 2017.
Enlag (Germany) The Power Grid Expansion Act (EnLAG) was introduced in 2009 with the intent to accelerate the expansion of Germany s transmission grids. It covers an initial 23 EHV projects (1,800km in length) and regulates the use of underground cables when proposed lines pass close to occupied residences. On 4 pilot projects, over 40km of underground cables are planned. A proposed revision to the Act in 2015 envisages an additional 12 projects will be included under the legislation. Stevin (Belgium) This 47 km 380kV double circuit with capacity of 3000 MVA between Zomergem and Zeebrugge aims to secure Belgium s future electricity supplies, especially in the coastal region. It incorporates a 10km underground section between Van Maerlant and Gezelle in the Bruges area and is due for completion in 2017. Hinkley Point connection (UK) National Grid is currently applying for consent to build a new 400kV connection between Bridgwater in Somerset and Seabank in Avonmouth. The proposal is for a 49km overhead line and 8km of underground cable passing through the Mendip Hills Area of Outstanding Natural Beauty. A final decision on the proposal is due to be made in early 2016, with completion expected around 2019. 2.2 HVDC Underground Cables Mass Impregnated (MI) cables are currently the most used cables for HVDC applications. Benefiting from more than 40 years of experience in service, with a proven high reliability, they can be provided by European manufacturers at voltages up to ±600 kv, with a capacity of around 2200 MW per bipole. Polymeric HVDC cables are used mainly with VSC converters that enable power flow to reverse without polarity reversal. Until recently this technology has been implemented with voltages of ±320 kv and power ranging from 800 MW to 1000 MW per bipole but projects are now being built with voltages of ±525 kv. 2.2.1 On-going HVDC projects As shown in the figure below, there has been a significant worldwide increase in projects deploying XLPE HVDC underground or submarine cables over recent years. It should be noted that this graph aims at showing the trend of XLPE HVDC projects. Power ratings depend on the specific link. The tendency is to increase power ratings at the maximum available levels.
Specific projects include: France-Spain Interconnector (INELFE) This 65 km link came into commercial operation in 2015 and consists of four XLPE cables for two bipoles each carrying 1000 MW of power at ± 320 kv with VSC technology. For the majority of the route, the cables were installed in a duct bank with a dedicated tunnel where the route crosses the Pyrenees. Denmark-Norway Interconnector (Skagerrak 4) This project came into commercial operation at the end of 2014 and is the first VSC connection at a voltage of ± 525 kv with a monopole MI cable of 700 MW. The submarine part of the cable is 137 km long and the underground directly buried portions in Denmark and Norway are 92 km and 13 km respectively. Sweden: Barkeryd-Hurva connection (South-West Link) The project is for the reinforcement of power grids in southern Sweden and the Oslo area of Norway. It consists of two bipoles carrying 660 MW each. AC/DC conversion is VSC technology operating at ± 300 kv. The connection is roughly 250 km long, 190 km underground and 60 km overhead. Completion is due at the end of 2015. Western Link (UK) Western Link will be the most powerful HVDC cable connection ever realized and will connect Scotland (Hunterston) to Wales (in Deeside). The connection will be a bipole with
two MI PPL cables ± 600 kv for the transmission of 2200 MW of power. The length of the submarine cable route is 370 km and the length of the land underground route is 37 km. The system is due to be commissioned in 2016. Suedlink, Germany Suedlink is an 800 km 4GW 380kV line that will transmit power from north to southern Germany. The latest proposals envisage that 90% of Suedlink will be put underground. Construction is due to commence in 2019 with completion by 2022. Two other long projects (West and East corridor) will also be subject to the same criteria for priority undergrounding. Planned HVDC projects that are included in the European Commission s 2015 list of Projects of Common Interest (PCI) are: France - Italy (Piemonte Savoie) This 1200MW project comprises ± 320 kv extruded HVDC underground cables comprising two 600 MW bipoles along a 190 km land route between Piossasco (near Turin) and Grand Ile in Savoy (France). Commissioning is due for 2019. Belgium - Germany (Aachen Liege Electric Grid Overlay ALEGrO) This 1000MW project is currently in the tendering stage and will be the first HVDC cable to be integrated into the meshed AC grid of Central Europe. The cable route will be around 100 km and is due for completion in 2019. Italy Slovenia This is a 1000MW project connecting Salgareda to Divaca, a distance of around 150 km (submarine and land cable). The project is currently under study (having received financial assistance under the EU CEF-Energy programme in 2014) and is planned for completion in 2022. Ireland UK There are several projects that have been proposed by merchant operators to strengthen the electricity connections between Ireland and the UK. The projects include Energy Bridge, Greenlink, Marex and Greenwire. DC cables are planned and the land cable sections would most likely be put underground. 3 Environmental impacts during installation Civil works required to install high voltage AC or DC underground cables have a considerable albeit temporary impact on the environment during installation. Heavy machinery is required for trenching as well as for delivery of cable drums. During the
construction period, access tracks and haul roads are required. These are removed following completion of the works but there is a need to consider on-going requirements for operational access. Waterways or particularly sensitive areas can be crossed (up to 1 km) by applying modern drilling techniques to install the cable. These sections often determine the rating of the line or the size of the cable. In most cases, the cable system is directly buried and as a result, 70% to 80% of the soil can be re-filled into the trench. During the period of installation, the soil can be stored alongside the trench. Up to 30% of backfill material has to be transported to the trench and the equivalent soil must be transported away from the site. Depending on the type of vegetation, the landscape is usually reinstated within 18 to 24 months. The surface vegetation above the installation is managed to ensure no route encroachment for the life time of the cable system. 3.1 Environmental impacts during operation Underground cables are generally laid directly in trenches. The dimensions of the trenches depend on a number of factors. In particular, the required width depends on the number of cables, which in turn depends mainly on one or more of the following specifications: Type of transmission technology, DC or AC; Desired transmission capacity; General geology of the soil; Existing surrounding structures (undercrossing roads, highways, rails roads, rivers or waterways); Thermal resistivity of the refilled soil material in the trench, and soil drying behaviour; Presence of other cable systems adjacent to the new ones; Available space; Thermal mutual influence of other adjacent heat sources. Each circuit is installed in a trench of approximately 1 to 1.5 meters (m) deep and 1 to 2 m wide. If two systems are to be installed in two separate trenches spaced 5 m apart, the total space or right of way would be less than 10 m. If three trenches are required, the total space would be less than 15 m. If four trenches are required, the total space would be around 20 to 25 m. The illustrations hereafter provide an indicative example of a 400 kv HVAC XLPE cable system, directly buried, with a current rating per circuit of 3600 A (2500 MVA). As it can be shown by more precise calculations, the recommended spacing between trenches will depend on the thermal resistivity of the soil, laying configuration, depth of laying, and, for the AC cables only, the type of metallic sheath connection. The figure below illustrates the dimension of a single trench with a single circuit of 400 kv AC cables capable of carrying 1250 MVA (corresponding to approximately 1000 MW).
Figure 1: Typical Example of a single AC 400kV system carrying 1250 MVA (space depends on soil thermal resistivity) Figure 2: Example of a double AC 400kV circuit carrying 2500 MVA (space depends on soil thermal resistivity) For the transmission of a higher amount of power, a 4 circuit arrangement in parallel should be necessary and the trench may have the dimension given in the Figure below. In case of 4 parallel circuits, a working area can be managed between circuits in order to facilitate repairs. Figure 3: Example of two double AC 400kV circuits carrying 5000 MVA in total (space depends on soil thermal resistivity)
When adopting the DC transmission technology for the transmission of the same active power as indicated in the previous figures (1000 MW or multiple), the total number of cables is lower and consequently the occupied space (trench dimension) is lower as indicated in the examples of the following figures: Figure 4: Example of one DC bi-pole circuit at the voltage of 320 kv carrying 1000 MW (space depends on soil thermal resistivity) Figure 5: Example of two DC bi-poles circuit at the voltage of 320 kv carrying 2000 MW in total (space depends on soil thermal resistivity) About Europacable Europacable represents approximately 85% of the European wire and cable manufacturers. Founded in 1991, our member companies include global technology leaders as well SME s highly specialized in the production of energy, telecommunication and data cables. Europacable is registered with the EU Institutions at 453103789-92.