Accurate Calculation of Induced Voltages and Currents in Fibre Optic Units Integrated in Three-Core Armoured Cables

Cables used in offshore wind farms are usually three-core (3C) with metallic armour, including one or more integrated Fibre Optic Cable (FOC) units for data transmission. Voltages and currents induced in the metallic part of FOC are crucial for the integrity, not only of the FOC unit itself, but also of the entire 3C armoured cable. To compute them, the cable series impedance at power frequency is necessary. The various metallic components of the cable, i.e., the power cores, FOC and armour, are mutually coupled and this is expected to affect the derived figures. The existing methods do not account for the actual three-dimensional (3D) effects, stemming from the complicated cable geometry. Twisting effects may be included in a two-and-a-half-dimensional (2.5D) fashion, which, however, does not consider the magnetic field component driven along the armour wires. This paper proposes a 3D method to fully consider the actual coupling between the various cable components and compute induced voltages and currents at the FOC, along the cable route. The proposed method is evaluated against existing approaches and, finally, is used to study several operating scenarios.