This glossary provides clear, succinct explanations of the main technical and administrative terms used to discuss infrastructure projects.

  • Advance public information meeting

    Legally speaking, an advance public information meeting serves as the first official announcement of a project. At such meetings, a draft project is presented in detail and the public are given the chance to put forward their comments and suggestions, which are recorded then analysed as part of the Environmental Impact Study.

  • Alternating current (AC)
    Alternating current is an electric current that changes direction twice per period and carries equal quantities of electricity alternately in one direction, then the other. The current and voltage alternate 50 times per second (50 hertz).
  • Bird flight diverter
    Bird flight diverters were designed to make overhead lines visible to birds and thus prevent collisions. They are mainly used on bird migration routes and in protected areas.

  • Converter station
    Converter stations convert alternating current (AC) into direct current (DC).
  • Direct current (DC)
    Unlike alternating current, direct current flows continuously in a single direction. This type of electric current is used in battery-operated devices, for instance.
  • Directional drilling
    This drilling technique tends to be used for underground connections when the urban location so requires, e.g. where the cable route crosses tram tracks, train tracks or navigable waterways, or if there are major obstacles on the cable route. The drilling depth depends on the topology, but is generally between 3 m and 20 m. The diameter of the borehole is between 0.4 m and 1 m.
  • Earth wire

    The earth wire is the topmost cable on an overhead power line. Its purpose is to absorb any lightning strikes and route the lightning to the ground through the pylon.

    The earth wire may serve a number of other purposes:

    • evacuating some of the fault current in the event of a fault;
    • transmitting data between substations, if it contains optical fibres.
  • Electromagnetic field
    Electromagnetic fields are characterised by the frequency and wavelength of the radiation generated by their propagation. The frequency and wavelength of electromagnetic radiation are inversely proportional: the higher the frequency, the shorter the wavelength.
  • Étude d’Incidences sur l’Environnement (EIE)
    The EIE is a study carried out before a permit application is submitted. It is used to gauge the probable effects of the project on the environment, justify the option that was selected and describe the measures that will be taken to eliminate, reduce or compensate for any damage that may be caused by the project. The study is submitted along with the permit application.
  • Évaluation Appropriée des Incidences sur l’Environnement (EAI)

    The EAI checks that a project does not harm the biodiversity of Natura 2000 sites or other nature conservation areas and does not jeopardise the conservation targets that were set for these locations. An EAI may also specify which compensatory measures should be taken to offset any damage caused, and may set out guidelines to be observed while work is being carried out.

  • Environmental permit
    An environmental permit is a permit allowing certain types of activities to be carried out and/or certain facilities to be operated. By way of example, Elia needs to apply for an environmental permit whenever it wants to build a high-voltage substation.
  • Federal Development Plan
    The Federal Development Plan is an official document that is binding for Elia and is approved by the Minister of Energy. It sets out the programme of investments that the grid operator has committed to implement in response to the needs identified. It takes account of the need for adequate reserve capacity and factors in projects of common interest designated by the institutions of the European Union in connection with trans-European systems. The FDP focuses on upgrading and expanding the domestic grid, expanding the offshore grid, and upgrading and expanding interconnectors.
  • High-voltage connection
    Just like the transport network, the high-voltage grid is made up of different kinds of ‘roads’ for electricity. Each type is associated with a different voltage level. We draw a distinction between overhead lines and underground cables, which are connected to one another through high-voltage substations. High-voltage lines are also known as conductors, because they carry energy from one point to another.
  • High-voltage grid
    The high-voltage grid can be compared to a network of electricity highways. Elia uses the high-voltage grid to transmit electricity from power producers to large industrial customers and distribution systems. The distribution systems, in turn, transmit it to end consumers (i.e. homes, businesses, and so on).
  • High-voltage substation

    A high-voltage (HV) substation is a central point to which various grid components are connected. These components may be connected to one another through a system of busbars, with the exact arrangement being determined by the company. A grid component may be: a high-voltage line or cable, a transformer (bay), a capacitor bank, or a busbar.

    There are two types of high-voltage substation:

    • Air-insulated substations (AIS): AIS facilities with voltages of 380 kV to 36 kV are generally located in the open air, while substations with lower voltages are usually located inside buildings.
    • Gas-insulated substations (GIS): in this type of substation, all the functional components are placed inside a fully shielded enclosure, which is insulated with gas (usually SF6). Since this gas has a far greater insulating capacity than air, GIS facilities can be much more compact and take up far less space.
  • Junction
    When building an underground connection, junctions between the sections of cable are needed at fairly regular intervals (every 500 to 800 m) along the entire route. Junction equipment takes up far more space than the cables themselves: junction trenches can be up to 15 m long and 3 m wide.
  • Junction trench
    When a cable is laid, it is not all laid at once, but rather in sections measuring around 1 km. The process of joining up these sections of cable is sometimes known as ‘jointing’. The junction trench is the place where two cable sections are joined.
  • Microtunnelling

    This technique is used for underground connections. It is applied when several connections are to be laid in the same location, or when directional drilling cannot be used for technical reasons (e.g. when the cable needs to pass through schist and there is a risk that the borehole will be unstable).

  • Note d’Évaluation des Incidences sur l’Environnement (NEIE)
    The NEIE evaluates how a project may impact the environment in the broadest sense (e.g. soil and subsoil, water, air, energy and climate, biological environment, landscape, urban and spatial planning, mobility and transport, waste, health and safety, socio-economic aspects) and issues recommendations.
  • Open trench

    A standard open trench is the most common form of excavation in the world.

    Trenches are usually dug at a width of around 0.65 m per circuit, depending on the depth. Cables are laid at a depth of around 1.2 m. Once the cables have been laid, the trench is filled in with a special type of backfill material (such as dolomite) so as to maintain the thermal conductivity of the cables by evacuating the heat generated by energy transmission. When laying the cables, work areas measuring 6 to 15 m across must be used, depending on the number of connections and the cable type. There are junctions every 500 to 800 m, depending on the length of the cable sections

  • Overhead connection
    Overhead connections consist of metal three-phase transmission lines, supported by pylons or masts.
  • Phase-shifting transformer
    Unlike transformer substations, phase-shifting transformers do not transform electricity from one voltage level to another. Phase-shifting transformers operate at a consistent voltage level, regulating the flow of electricity on the grid. They can determine the direction and size of energy flows.
  • Plan-MER
    The plan-MER analyses the potential effects of certain activities or procedures on humans and the environment. It is an integral part of a regional land-use plan (“GRUP”). It is drawn up before a plan is finalised or a project is implemented. Its purpose is to assess the impact on humans and the environment at an early stage so that the necessary measures can be taken.
  • Planning permit
    A planning permit is a permit issued by the relevant administrative authority to allow the work required for a project to go ahead. The permit application procedure differs from one region of Belgium to another.
  • Public consultation
    When a permit application is submitted for a project, a public consultation is held. Local residents can review the full planning permit application file and have 30 days to submit their comments to the municipal authorities.
  • Road permit
    A road permit application must be submitted for work that will occupy the public space, be it under ground or in the air. This type of permit allows work to be carried out by the roadside or on public land.
  • Three-phase transmission line
    A three-phase transmission line is the set of three transmission cables making up a three-phase overhead line. Depending on the voltage level, there may be one, two, three or four three-phase transmission lines, each of which may consist of several high-voltage lines or conductors.
  • Transformer substation

    High-voltage transformer substations are the points at which connections are made with the electricity grid. They transform the electricity’s voltage level to enable a connection.

  • Transition substation
    A transition substation is an electrical substation that acts as the transition point between an underground connection and an overhead connection.
  • Underground connection

    Underground connections mainly consist of:

    • the high-voltage cables required to transmit energy (usually three – one per phase);
    • the junctions connecting the sections of cable (usually one every 500 to 800 metres);
    • the terminal boxes (or cable ends) connecting the cable to the equipment in the high-voltage substation.
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