The demand to install more cable systems continues to increase as easements for overhead lines become more difficult to obtain. In addition existing cable systems globally are aging, often at the same time as the loads are increasing. In this context it is appropriate to consider whether the assumptions about the thermal environment implicit in the cable ratings, remain valid. Working Group B1.41 has examined the long-term performance of soil and backfill of cable systems and has recently produced Technical Brochure 714. A useful source of information for the work included two of the most significant cable failures over recent decades. The Australian member on the Working Group was Jeff Cairns. Jim Lyall, who is a member of Australian Panel B1, produced this article.
The working group report is a valuable source of current knowledge and practice of the often-neglected subject of the technical performance of backfill for cable systems. It emphasises that soil/backfill thermal resistance is a critical component of the rating of cable systems and that over the life of the cable, conditions can change. If not accounted for, this could result in cable damage and failure.
Useful information from a literature review is presented as well as results of a worldwide survey of practices and reports of experiences from a number of countries, including Australia and New Zealand. In particular there is a detailed description of the deterioration of backfill materials that occurred over time at a Transgrid installation and lessons from the 1998 blackout of Auckland.
Soil types and backfill together with terminology are described in an introductory chapter. It is noted that, ever since the failure of cables in London in 1962 due to the soil drying out from cable heating and the consequent high values of thermal resistivity, the critical conditions for dry out due to cable heating have been researched and conservative practices adopted. These include using special backfills and assuming fully dried out values of thermal resistivity.
Moisture content is also dependent on rainfall and for the effect of rainfall on backfill thermal resistivity and seasonal correlations reference is made to a paper on an Australian study. Daily automated measurements of thermal resistivity carried out at a site in Brisbane showed that its value can double in a period of one month of dry weather. Elsewhere in the document it states “The pattern of annual cycles of ambient temperature and moisture content fluctuations normally stays unchanged for decades …”. However this may reflect the fact that most of the research on this topic has been carried out in temperate climates in the northern hemisphere and is not necessarily the case in Australia which has a warmer climate and experiences long periods of dry weather.
Installation procedures and standards commonly used for thermal resistivity measurements are considered in the report. In addition, mechanisms by which deterioration of the installed backfill might occur, methods of assessment of such changes and potential consequences of these changes are examined. How the deterioration might be mitigated or avoided is then discussed.
A chapter is devoted to a User’s Guide to assessing the rating of a circuit and then guidance for a field survey if this was considered necessary. The guide directs users to relevant sections of the document.