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

I. Chaleplidis, D. Chatzipetros, A. Chrysochos, K. Bitsi, C. Traianos (Hellenic Cables, Greece)

The wind farms being installed offshore have been rapidly expanding during the last decades. The power cables, typically used to transmit the offshore power to the mainland, integrate one or more fibre optic cables (FOCs) for data transmission. FOC is crucial for the integrity of the entire export cable and many failures are attributed to issues related to the FOC design, as discussed in [1]. The high induced voltages and currents are responsible for the failure mechanism due to potential breakdown and overheating, respectively: hence, the accurate calculation of voltage and current profiles is necessary to reduce the risk of failure and adopt safe design practices. There is currently no Standardised calculation method for this purpose. The Cigré working group B1.70 deals with this topic and is expected to provide recommendations. The existing calculating methods involve simplifying assumptions on the geometry and physics considered, such as the pipe-type approach suggested by [2] and the analytical methods proposed by [3] and [4]. Other, more sophisticated methods, such as MoM-SO [5] account for the twisting effects of the armour wires, although still ignoring the solenoid effects and the longitudinal magnetic field driven in the armour wires. The latter is accounted for by the recently published work [6], although does not consider the FOC unit. The present paper proposes a hybrid method combining state-of-the-art 3D FEM models with the Js-method [7] to compute induced voltage and current profiles in the FOC. A special consideration for the inclusion of the earth return term is also made. The results derived are compared against the existing methods under both normal and fault operating conditions. Interesting conclusions regarding the achieved accuracy and the computational efficiency of the methods are drawn. [1] Offshore Wind Programme Board, “Export cable reliability - Description of concerns”, Transmission Excellence Ltd., 2017. [2] A. Ametani, “A general formulation of impedance and admittance of cables,” IEEE Trans. Power App. Syst., vol. PAS-99, no. 3, pages 902-910, 1980. [3] Karlstrand, J. et al., “Electromagnetic coupling in HV and EHV three-core submarine cables during test and operation,” in Proc. Jicable 2019. [4] Kvarts, T. et al., “Inherently safe designs of fibre optic cables integrated in three-core submarine power cables,” in Proc. Jicable 2019. [5] B. Gustavsen, et al., “Inclusion of wire twisting effects in cable impedance calculations,” IEEE Trans. Power Del., vol. 31, no. 6, pp. 2520-2529, 2016. [6] A. I. Chrysochos et al., “Impact of Solenoid Effects on Series Impedance of Three-Core Armoured Cables”, accepted for publication in Electric Power System Research, Elsevier, March 2023. [7] Y. Yin and H. W. Dommel, “Calculation of frequency-dependent impedances of underground power cables with finite element method,” IEEE Trans. Magn., vol. 25, no. 4, pp. 3025-3027, 1989.