Inter Array Cable Rating – Considerations Beyond IEC standards

IEC 60287-based rating approaches [2].

This paper investigates the physical mechanisms governing these additional losses and develops an analytical model for the cable section near the armour hang-off. The system is described using a reduced multiconductor telegrapher-equation formulation that distinguishes between the bundled three-core array cable below the armour termination and the separated single-core arrangement above the armour termination. Under balanced conditions at power frequency, closed-form expressions for screen currents and associated losses can be derived for the cable section below the armour hang-off, which may constitute the thermal bottleneck of the array cable.

The analysis shows that the additional losses are governed by two key ratios: the ratio between the conductor–screen mutual inductance for the separated power phases and that of the bundled three-core cable, and the ratio between the length of the separated cable section above the armour hang-off and the effective leakage length of the screen, which depends on the conductance provided by the semiconductive layers. These ratios determine whether substantial circulating screen currents can develop near the armour hang-off and to what extent the associated losses are amplified.

The analytical results are verified using finite-element simulations. However, the complexity of the problem renders full 3D finite-element modelling computationally expensive and impractical for routine engineering applications. The results show that increased phase separation and higher effective screen conductance lead to higher circulating currents and pronounced loss concentrations close to the armour termination. The effect is localized to the region below the armour hang-off, typically in the J-tube inside the monopile, and may be relevant for array cables with semiconductive jackets operated close to their thermal rating limits. The findings complement existing IEC approaches by explicitly addressing the impact of geometric transitions and semiconductive coupling in WT tower terminations.

The results are also applicable to other cable types and configurations involving similar geometric transitions, for example at transition joints between submarine and underground cables.