Circulating current and hysteresis losses in screens, sheaths and armour of electric power cables-mathematical models and comparison with IEC Standard 287

Computation of the current carrying capacity of a cable involves evaluation of electrical and thermal parameters of the circuit. Formulas for the calculation of these parameters are summarised in Standard No. 287 published by the International Electrotechnical Commission (IEC). The first edition of this Standard was issued in 1969 and the latest revision in 1994/5. The formulas for the computation of the electrical parameters of the cable date to the late 1920s and early 1930s. The computation of the loss factor for cables with magnetic armour wires is based on work performed in the 1960s and is applicable to cables spaced at least 10 m apart. IEC Standard 287 provides loss formulas for simple symmetric cable configurations such as those in flat or trefoil arrangements. Many practical applications deal with more complicated configurations such as less symmetry, multiple circuits in close proximity, and multiple cables per phase within a circuit. The paper introduces accurate equations for the computation of the sheath and armour losses, treating these two components separately. The formula for the sheath skin effect is expanded and the inductance of hollow conductors is derived. The expressions for the sheath and armour loss factors derived in the paper are compared with those given in IEC 287 and the simplifications required to develop standard equations are explained. A numerical example shows the effect of these simplifications on the rating of a typical cable circuit