Optimized cable systems design which limits soil drying out

Abstract

Underrating of underground cables could result in the drying out of the soil around the cable. This results in inferior cable performance and has potential negative environmental impacts. IEC 60287 is used for cable rating ampacity calculations. One of the parameters used in IEC 60287 is the thermal resistance per unit length between the cable surface and the surrounding medium (T4), or T4 can be defined as the thermal resistance to the outside installation environment of the cable. The soil parameters may be unknown at the initial design phase for final cable rating calculations. Due to the unknown installation conditions; the calculated uncertainty for cable system designs may lead to either under-utilization of the cable system, or underrating of the cable system. With increasing demand for underground cable systems in South Africa; this increases the probability of soil drying out due to unknown installation conditions, hence the need for improved cable rating design and calculations which will eliminate soil drying out. Cable rating solution that prevents soil drying out for South African (native) soil has been investigated. It was found that the following parameters should be taken into consideration when performing current rating that prevents soil drying out: the soil thermal resistivity at zero percent moisture content, sampling rate for measuring the soil thermal resistivity should be less than 60 m and that the soil ambient temperature at cable laying depth should be measured.