Positive sequence impedance in three-core armoured cables: measurement and modelling aspects

Submarine cables are necessary to interconnect offshore renewable projects with the mainland grid. Their geometry typically involves three twisted power cores and an external steel wire armour, which forms a helix and is used for mechanical protection. The positive sequence impedance, namelyZ1, of such cables is employed in power flow studies and, thus, influences the sizing of the entire project. The Standard-based methods presently used forZ1calculations, such as that suggested by the CIGRE Technical Brochure 531, rely on semi-empirical formulae which omit the cable helical geometry and ignore the true armour material properties, such as its magnetic permeability. Other methods treating the armour as a pipe, although account for its permeability, fail to capture the so-called “twisting effects” stemming from the helical nature of the cable. State-of-the-art models, capable of incorporating the three-dimensional effects of the induced magnetic field, are used in the present paper and compared against in-house experimental measurements. Various cable samples including all steel grades typically used in practice, i.e., magnetic and non-magnetic alloys, are tested. The proposed models provide results ofZ1significantly closer to measured data compared to the existing methods, thus facilitating a detailed parametric analysis in a design base.