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 study 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 (FE) 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 FE 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 low voltage networks with short cable lengths.
Highlights
Low-carbon technologies such as electric vehicles, heat pumps and solar photovoltaic panels are increasingly being connected to the low voltage (LV) distribution network
Where yS allows for the skin effect, yP allows for the proximity effect and l1 allows for the resistive effect of losses due to eddy currents in the sheath
At 50 Hz, with ground resistivity of 100 Ωm, an equivalent resistance would be provided by a semi-circular conductor with a radius of 1136 m. This suggests that the ground current for short LV cables would be subject to significant end effects
Summary
Low-carbon technologies such as electric vehicles, heat pumps and solar photovoltaic panels are increasingly being connected to the low voltage (LV) distribution network. A hybrid approach was taken in [20] where the current distribution within the cable was solved using a numerical method, combined with corrections from [15] for the ground path These techniques may provide a high degree of accuracy, but tend to be complex to apply and published models for specific cable types cannot be adapted for new applications. The use of this wide range of different approaches suggests that there is some uncertainty over the level of detail needed so that impedances are adequately represented. It may be estimated assuming circular conductors and uniform charge density or, for greater accuracy, FE models using similar concepts to those presented in this paper could be developed
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