Series impedance of distribution cables with sector-shaped conductors

Abstract

Models of power distribution networks require accurate cable impedance data. For unbalanced networks, both the self-impedances and the mutual impedances are needed. However, published studies use differing approaches to determine cable impedances, leaving uncertainty over the level of detail required. This paper compares impedances provided by the manufacturer with those from several analytical methods, showing the impact of modelling the non-circular geometry and of including corrections allowing for the AC resistance. The analysis is compared to results from a freely available finite element solver where the current distribution is modelled in detail, taking account of eddy currents and the rotation of the cores relative to the neutral due to the cable lay. At 50 Hz, the analytical methods provide a good approximation but the finite element results show that eddy currents affect the impedance at harmonic frequencies. The results also show the impact of including the ground path in the impedance calculation. The current distribution in the ground has a wide cross-sectional area, suggesting that the assumption of a perfect multi-grounded neutral is inappropriate for LV networks with short cable lengths.