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Undergrounding High Voltage Electricity Transmission Lines

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Undergrounding High Voltage Electricity Transmission Lines Undergrounding high voltage electricity transmission lines The technical issues Contents Introduction 03 National Grid electricity transmission – 03 an overview Section 1 Insulating underground cables 04 2 Capital costs for installation of 05 underground cables 3 Underground cable technologies 06 4 Planning and environmental issues for 08 underground cables 5 Land use restrictions over cable routes 09 6 Operation, maintenance, refurbishment 10 and up-rating cables 7 Cable installation methods 11 Direct buried cables 11 Direct buried cable installation 12 Ducted method 13 Surface troughs 13 Deep bore tunnels 14 Cut and cover tunnels 15 8 Components of underground 16 cable systems Cable sealing end compounds 16 Joint bays 16 Stop joints 17 Water cooling 17 Reactive compensation 17 9 Electric and Magnetic Fields (EMFs) 18 from underground cables Front cover photo: National Grid works with Arup to deliver London Power Tunnels Page 03 Introduction The purpose of this document is to provide circumstances in which underground cables are information about the technical merits and challenges a more preferable option than overhead cables. associated with undergrounding high voltage electricity lines, compared with installing overhead This document explains the cost and potential impact lines. Overall, there are a number of issues that on the environment of cable installations. It explains make the undergrounding option more technically the cable types available and the various installation challenging and expensive. However, despite the methods, as well as the separate components that costs and technical challenges, there are make up an underground installation. National Grid electricity transmission – an overview National Grid owns the high voltage electricity National Grid has a statutory obligation under the transmission system in England and Wales and Electricity Act 1989 to develop and maintain an efficient, operates the system throughout Great Britain at coordinated and economical system of electricity 275,000 and 400,000 volts (275kV and 400kV). This transmission, and to facilitate competition in the supply transmission system is made up of approximately and generation of that electricity. The company also has 7,200 kilometres (4,470 miles) of overhead line, a statutory obligation to have regard to the preservation 1,400 kilometres (870 miles) of underground cable of amenity when developing electricity lines or and about 330 substations. undertaking other works in connection with electricity transmission infrastructure. In doing so National Grid has Government planning policy relating to electricity to make careful and informed judgements on the relative infrastructure can be found in two National Policy merits of both overhead lines and underground cables. Statements (NPS) – the Overarching NPS for Energy (EN-1) and, more specifically, the NPS for Electricity In terms of reliability, capability, cost, construction Networks Infrastructure (EN-5). These are the main impacts and land use, overhead lines do have documents that inform decision making on major advantages when compared to underground cables. infrastructure projects. Therefore, they provide However, a significant benefit of undergrounding National Grid with important guidance when considering cables is the reduction in visual impact. In certain whether to use overhead lines or underground cables. areas, such as protected landscapes, this benefit could be a primary consideration and outweigh disadvantages of undergrounding such as restrictions on land use and the impact on ecological and archaeological sites. National Grid considers the views and requirements of its customers and undertakes extensive stakeholder consultation before deciding which solution to take forward. Page 04 1. Insulating underground cables Conductors that transmit electricity When designing a cable system, there are a need to be electrically insulated. number of factors to consider relating to the One major difference between physical environment of the cable, which will overhead lines and underground help optimise their electrical performance. These include ensuring: cables is the way they are insulated. Adequate heat dissipation to prevent Overhead lines are insulated by air, while overheating and subsequent reduction in its underground cable conductors are wrapped in capacity for carrying current (cable rating) layers of insulating material. Air is the simplest Adequate physical protection to protect the and cheapest insulation and the heat produced cable from damage and to ensure it is not by the electricity flowing through the bare a potential danger to people while in service overhead conductors is removed by the flow of Proper access for maintenance crews to air over the conductors. When conductors are the cable to inspect, repair or replace it. buried underground, robust insulation is needed to withstand the very high voltage. The insulation Cables are laid in method depends upon the type of cable used a trench in groups (see section on ‘Underground cable technologies’). of three. Each cable trench is a metre across and Underground cables, because of the insulation over a metre deep and surrounding environment, tend to retain the Fill-in must be well heat produced in the copper/aluminium conductor. compacted to aid the removal of heat This heat then has to be dissipated to the surrounding environment. Concrete cable covers prevent accidental damage To compensate for this, underground cables to cables and electrocution are generally bigger to reduce their electrical resistance and heat produced. How the heat Cement-bound sand produced is dissipated will depend upon the cable installation method. For direct buried cables each cable needs to be The conductor is much thicker well-spaced from others for good heat dissipation. than for an To match overhead line thermal performance for Tough plastic overhead cable cover protects a 400kV double circuit, as many as 12 separate against corrosion cables in four separate trenches may be Aluminium or lead sheath needed, resulting in a work area up to 65m wide. to retain insulating fluid In addition, water cooling may be used (see The conductor is insulated by a thick section on ‘Components of underground cable wrapping of fluid-impregnated paper. systems’). For cables installed in deep bore The fluid is kept under pressure by tunnels, cable cooling is provided by forced fluid tanks along the cable route air ventilation or water cooling. A typical cable installation method Page 05 2. Capital costs for installation of underground cables The costs of underground cable to properly insulate, while at the same time systems vary and have been maintaining the cable’s rating, means that special the subject of much debate and insulation is needed. Generally, tunnel installation discussion. In January 2012 the costs more than direct burial; however, civil engineering costs for all methods of cable Institute of Engineering and Technology installation are considerable compared to those (IET) and Parsons Brinkerhoff published of an overhead line. the findings of an independent study into the costs of new electricity A wide range of values are quoted for apparently transmission infrastructure. The similar circuits due to factors such as the local report analysed the costs of installing ground conditions. It is the added cost of undergrounding (in both monetary terms and and maintaining new high voltage other factors, such as threats to sensitive habitat transmission circuits under the and damage to archaeological heritage) that is ground, under the sea and overhead. important and must be weighed against the benefits (largely visual) that it brings. The actual The report found that, excluding build costs, costs will depend on the type of cable used, the cost of operation, maintenance and energy method of installation, local environmental losses over the life of the connection was broadly conditions and, in particular, the rating required the same for undergrounding and overhead from the circuit, as this will dictate the number lines. However, the report also concluded of cables installed. Underground cable systems that the capital build costs on their own vary need to be tailored to meet local conditions and greatly, depending on terrain, route length and the same solutions may not be applicable in power capacity. other locations. A copy of the report can be found on the IET Visual intrusion impacts and threats to sensitive website (http://www.theiet.org/factfiles/ habitats will vary along the route corridor as the transmission-report.cfm). While the study went landscape and species mix changes. To help into a great deal of detail, the report recognised reduce some of these impacts undergrounding that costs and cost differentials between different may be considered in some cases. However, technologies will vary depending on length, power, this can create further difficulties as the transition voltage and ground conditions. It concluded that from overhead lines to underground cables underground cables are always more expensive requires termination points, known as sealing when compared to equivalent overhead lines. end compounds (see page 16). These are often large structures that require sensitive siting. For A major element of this cost differential is this reason numerous adjacent short sections of accounted for by the cable itself. The underground undergrounding are unlikely to be desirable due conductor has to be bigger than its overhead to the requirement
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