Adaptation options for electricity transmission and distribution networks and infrastructure

Bar the case of optimizing software, all options in this class involve installing or modifying infrastructures on the ground, in urban, industrial, rural and natural areas. Stakeholder interaction at the local level (with land owners, local authorities and the general public) along the routes of the installed/upgraded grids is thus crucial in order to ensure social acceptability and the timely and cost-effective deployment of the infrastructures. For underground cables, coordination with other cabling entities can reduce economic costs and minimize the nuisance to local communities by limiting the duration of digging activities to the bare minimum.

Underground cabling is dependent on the availability of the correct technology and know-how as to installation, monitoring and management. Co-operation with other underground cabling entities, such as telecommunications companies helps minimize disturbance to populations through digging activities, and cost sharing of digging operation reduces the costs borne by each entity. Although underground cabling could be exposed to new climate hazards, in particular from flooding and soil movements related to landslides, so far these risks remain hypothetical. Excavation due to other construction or maintenance activity represents a key risk of damage to installed underground cables. This risk can be reduced by applying digitalization and GIS technology to underground cables, to inform excavators about the location of underground cables.

A major difference between underground and overhead cables is the way electrical insulation is provided. Overhead cables are insulated by the air that surrounds them, the cheapest and simplest insulation solution available. Underground cables need to be insulated to avoid power losses and risks of electrocution through direct contact with the soil. The electric resistance generated by insulation generates heat and hence transmission losses. This calls for larger and/or multiple cables in order to compensate for the losses and, a cooling system (forced ventilation, water or gasses) to dissipate heat. Underground cables need to be buried in trenches, to be protected from accidental damages and to be accessed with ease when maintenance is needed. Overall, this results in a larger use of land by underground cables compared to overhead cables during installation, although once buried, the land use and visual impacts they generate is considerably lower.

Maintenance of underground cables is much more complex and costly than that of overhead cables: “if a fault occurs on a 400 kV underground cable, it is on average out of service for a period 25 times longer than 400 kV overhead lines. This is due principally to the long-time taken to locate, excavate and undertake technically involved repairs. These maintenance and repairs also cost significantly more” (National Grid, 2015).

Finally, there are technical limitations to land use in the vicinity of cables specific to underground lines. Beside the need to reserve some land to secure access to the lines for maintenance purposes, there are also restrictions on the planting of trees and hedges over the cables or within 3 m of the cable trench to prevent encroachment by vegetation. Tree roots may penetrate the cable backfill surround which in turn may affect the cable rating or even result in physical damage to the cable. Similarly for overhead lines, tree growth is discouraged and controlled beneath the overhead line conductors or within distances where trees could fall onto the lines. There will also be height restrictions for machinery or especially high vehicles, such as agricultural equipment, near overhead lines for safety reasons. In urban areas, the land surface used for buried cables far exceeds the one required for an equivalently rated overhead line. Cables have historically been routed under roads to avoid subtracting land from alternative uses; however traffic disruption during fault investigation and repairs can be significant. Where cables are installed by direct burial in rural areas, there are restrictions on the use of deep cultivating agricultural equipment to avoid the risk of damage. The burying of high voltage cables is also more complicated than the laying of gas and water pipes. In addition underground joint bays, which are concrete lined and wider than the trenches themselves, have to be built every 500–1,000m.

For the climate-proofing of overhead cables, a detailed knowledge of future local climate conditions at high resolution is crucial in order to plan the necessary interventions. A clear advantage of gaining the most accurate scenarios for overhead cables is related to understanding to what extent they can continue to be a valid option. If extreme events are projected to affect significantly the areas in which overhead cable networks are installed or planned, a switch to underground cabling may eventually be taken in consideration. Even in less extreme circumstances, identifying the routes that are going to be the least exposed in the future to the aforementioned threats to overhead cabling can help planning future network development.

Besides direct future climate impacts, for both underground and overhead networks, it is important to get insights on the future market conditions in which transmission system operators (TSOs) and distribution system operators (DSOs) will operate.

In general, the operation of underground cable costs roughly the same as that of overhead cables (National Grid, 2015). However, the capital costs related to building underground lines are much higher than those for overhead cables. Alonso and Greenwell (2013) report 4 to 14 times higher building costs for underground cables based on a 2011 study of the Public Service Commission of Wisconsin. The actual costs, however, depend on the geological and geographical characteristics of the cables’ route, the installation method (tunnel installation costs more than direct burial), the transmission capacity of the line and the options chosen for insulating and cooling underground cables.

Raising pole height is relatively inexpensive: a case study on overhead lines in UK reports that the costs of procuring wooden overhead poles 0.5 meters taller depend on the height of the original pole, but they can be as little as around £10 (€11) per pole.

For overhead cables, specific national norms in each EU country regulate the maximum height of the poles and the minimum clearance from the ground.

Building overhead or underground power lines is subordinated to national permitting regulations, as any other major infrastructure. There are a number of specific environmental drawbacks to be taken into account in the permitting process. In rural areas, disturbance to flora and fauna, land use and archaeological sites must be assessed. In this respect, overhead lines are normally less disruptive than underground cables and cause fewer disturbances. However, in specific cases underground cables can have a significant positive impact for some endangered species; for instance they can reduce mortality due to power line collisions in migrant or resident birds’ populations (Bernardino et al., 2018). In both urban and rural environments, land disruption is greater when laying underground cables than when erecting overhead line towers. The volume of soil excavated for an underground cable, where two cables per phase are installed, is some 14 times more than for an equivalent overhead line route. Vegetation has to be cleared along and to the side of trenches to allow for construction and associated access for vehicles.

The implementation time varies according to local geographical and geological conditions and the installation method used. However, it is considerably longer for underground cables compared to overhead cables.

Cables, whether overhead or underground, are usually designed to be in operation for 60 years. A UK case study reports that the expected lifetime of wooden poles that support overhead lines is comparable: 40-60 years.