Impact of Twisting on Skin and Proximity Losses in Segmented Underground Cables: A 3D Finite-Element Study

This paper presents a comprehensive three-dimensional (3D) finite-element (FE) study of skin and proximity losses in a five-segment, helically twisted underground power cable. Unlike conventional two-dimensional (2D) analyses—which assume parallel conductors and consequently overestimate eddy current losses—our 3D approach accurately captures the effects of varying lay lengths (λ). Simulations are performed from 0 Hz (DC) to 50 Hz, showing that while the per-unit-length DC resistance remains unaffected by twisting, the AC resistance can increase significantly depending on the pitch. At 50 Hz, the ratio of AC to DC resistance (RAC/RDC) ranges from about 1.32 for very tight twists (λ=0.1m) to nearly 1.72 for gentle pitches (λ=5.0m). Further analysis reveals that short lay lengths enhance magnetic field coupling, improving current distribution and partially mitigating losses. To quantify these findings, an exponential-saturation model is proposed to describe RAC/RDC as a function of lay length, achieving excellent agreement (R2≈0.996) with the 3D FE data. These results underscore the importance of considering full 3D geometry in cable design, offering a practical tool for optimizing both mechanical reliability and electromagnetic performance in high-voltage underground applications.