10. Underground Cables

This chapter on insulated cables comprises 13 sections. Section 10.1 is an introduction to this topic that highlights the role of CIGRE in the development of insulated cables, while Sect. 10.2 gives the current state of cable development, describes modern high-voltage () alternating current () systems and the various types of cables used in them, and explores emerging trends in cable design.

Some basics of the theory of insulated conductors are given in Sect. 10.3 to prepare the reader for subsequent sections, including discussions of electrical fields, electrical characteristics of cable systems. In Sect. 10.4, the basic design aspects of insulated cable systems are introduced, such as insulation coordination, ageing mechanisms, special bonding issues, calculation of cable characteristics and rating.

In Sect. 10.5, various applications of insulated cables are described. The main configurations of cable systems in networks are described and the technical issues associated with each configuration are discussed: matching Cable and OHL ratings, Harmonic resonance, Ferranti effect, magnetic field. Offshore generation cables are explored in depth.

10.6 covers cable design and manufacturing. Major steps in the manufacturing processes of lapped and extruded cables are described. Various options for extrusion lines are compared. Submarine cable armoring is discussed. The manufacture of accessory components is also described, including the various molding and casting processes, which depend on the materials used.

10.7 deals with the construction, laying, and installation of cable systems. Traditional techniques are reviewed and innovative techniques are introduced, such as trenchless technologies: mechanical laying, pipe jacking, microtunneling and horizontal drilling. Installation in tunnels or shared structures is detailed. The main installation configurations are illustrated using examples.

In Sect. 10.8, issues relating to testing are addressed. As indicated in the introduction to the chapter, testing has been a major issue since insulated cables were first invented and used. The introduction of extruded HV and (extra-high voltage) cables would have been impossible without stringent testing procedures.

Various aspects of the operation of insulated cable systems are covered in Sects.10.4–10.8. Fluid-filled and extruded cable systems operation have been presented. Also briefly described are monitoring techniques that can optimize equipment usage: management of the overload capacity of underground systems, partial discharge () monitoring, and sheath condition monitoring. Strategies for protecting against short circuits are also addressed. To complete this information, Sect. 10.9 is covering the maintenance of land and submarine cable systems and the various diagnostic techniques that are available. Repair and methodologies that are used to limit repair times, such as fault location, are also included in this section. Main failure causes of cable systems are inventoried. Appropriate maintenance strategies are proposed, and tools for asset management are overviewed. Examples of maintenance guidelines, fault-location techniques, and rapid-response repair are also briefly presented.

Appropriate maintenance strategies are proposed, and tools for asset management are overviewed. Examples of maintenance guidelines, fault-location techniques, and rapid-response repair options are given for land and submarine cables.

10.10 introduces the various options for upgrading and uprating existing cable systems. The life expectancy of an underground cable system is usually more than thirty years, which makes it difficult to predict the changes that will occur in the cable environment or how the operating conditions will evolve during the expected lifetime of a cable system. Addressing the need for a cable system to meet a higher demand can be highly problematic. Replacing a power link or installing an additional circuit requires significant financial investment. It may even be impossible for the operator to extend the system in some congested areas. The difficulties involved in obtaining planning permission for new sites also favor extending the lifespans of existing facilities, often with the goal of transmitting higher power with higher reliability in order to minimize the duration of asset unavailability.

Life Cycle Assessment is discussed in Sect. 10.11. A new technology—superconducting cables—offers interesting options for both AC and cable systems, as described in Sect. 10.12. Finally, other undergrounding options (gas insulated lines, s) are briefly described in Sect. 10.13.