Improved current rating methods of subsea HVAC cable systems for renewable energy applications

Abstract

The number and size of renewable projects, such as Offshore Wind Farms (OWFs), has been rapidly growing during the last years, mainly due to the increasingly great environmental concerns. The submarine cables used to transmit the power generated offshore to the mainland are crucial for the entire project’s economic viability. Although cables are manufactured in rather cost-efficient ways and delivered in reasonable timelines thanks to the progress made in insulating material technology, they are presently subjected to a hard compromise: fixed costs are pushed to go down as much as possible, but at the same time the chance of failures is required to be minimised. The golden ratio in this difficult problem can certainly be sought to optimising the cable design.

Three-core (3C), HVAC cables are presently the most cost-effective technical solution for offshore power transmission. They are also expected to be so in the future, at least regarding the interconnection of OWFs located in reasonable distance from shore. To optimise the cable design, the current carrying capacity of the cable, often called as “ampacity”, needs to be determined as accurately as possible. Due to electromagnetic induction, additional induced losses are generated inside the cable, which are dissipated in the form of heat from the cable to its surroundings. In order to investigate any likely optimisation margins, the way these losses are generated needs to be clearly understood. In parallel, the heat paths that enable the dissipation of heat inside the cable must be in depth considered. The existing calculation methods allow for such an analysis and cover, in theory, the larger cable sizes required in modern OWFs. However, empirically derived approximations are often used in these methods instead of rigorously extracted, mathematical solutions and sometimes refer to cable types different from the modern submarine cables. Furthermore, the physical models implied usually rely on simplifying assumptions that are expected to work sufficiently for smaller cables sizes, but need to be benchmarked in larger sizes. Thus, the existing calculation methods have to be reviewed and improved, where necessary.

In order to allow for a quantitative analysis around the accuracy of the presently used methods, models representing more realistically the physical phenomena involved are developed. In 3C cables, the 2-D nature of heat transfer cannot be omitted, due to the physical proximity between the power cores. Traditional methods imply 1-D, radial analysis, which is in principle incapable of capturing the heat transfer occurring in the angular direction. Comparisons between the existing, traditional methods and the models developed demonstrate that this effect can be significant in larger cables. A submarine cable often encounters various conditions, which in some cases may be thermally adverse, forming the so-called “hotspots”. Cables armoured with non-magnetic steel wires are preferred in these points, due to lower induced losses. To avoid any unnecessary increase in conductor size and, thus, any economic impact such an increase would have, an optimum design is sought for. For this purpose, numerical models capable of representing the AC phenomena involved are developed. These are benchmarked against the existing analytical methods and the thermal gain obtained from the more realistic loss generation is assessed.

Cables being armoured with magnetic steel wires are typically preferred in the main subsea section, due to techno-economic reasons. The cable geometry in this case influences the physical model to a great extent. Unfortunately, this is not considered by the traditional methods of calculating the losses, due to its inherent complexity. By applying 3-D electromagnetic analysis, it is possible to study the effect of the cable geometry on the induced losses. Hence, it becomes feasible to evaluate the accuracy level afforded by the traditional methods and, thus, anticipate the potential for design optimisation and further cost reduction.

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ORCID for James Pilgrim: orcid.org/0000-0002-2444-2116

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