Rating methodology of high voltage mass impregnated DC cable circuits

Abstract

With the continuing growth in energy consumption worldwide, the move towards a European wide super grid will result in significant changes in how modern transmission and distribution networks are operated. Fundamental to this is the need to accurately know or determine the available ampacity of high voltage cable circuits, because huge bulk power volumes need to be transmitted between maritime nations through dc power cables. Therefore, an accurate cable rating becomes paramount towards an efficient and safe operation of transmission networks, while the finance for large scale network construction schemes is limited.

Although the standardised thermal-limited rating has been successfully implemented for traditional ac cable networks for over 50 years, the move towards dc cable transmission imposes extra physical constraints on the cable rating, which are not considered by standard rating approaches. The two main concerns are the potential dielectric electrical breakdown prior to a normal thermal runaway and the development of dielectric cavities during cable cooling. In addition, the thermal-limited rating of submarine dc cable crossings, within a complex marine environment, requires an advanced numerical modelling method, where the traditional IEC thermal-limited rating method does not apply. Besides the technical value, significant interest exists both within the electrical power industry and organizations such as Cigré and IEC, because this work will inform future international standards for rating high voltage dc cables.

Considering the dielectric electrical stress constraint as the limiting factor for cable ratings, an analytical electrical stress-limited rating method has been developed and successfully benchmarked by numerical simulations for a practical cable design. This method allows ratings to be calculated against a criterion of maximum dielectric electrical strength.
Considering the dielectric cavity creation threshold as the limiting factor for cable ratings, a comprehensive study has been conducted, including thermal dynamics, theory of elasticity and electrical circuit theory. Subsequently, the analytical calculation of the cable internal pressure has been originally developed, together with a concept of the mechanical pressure-limited rating. The
method has been successfully demonstrated for a practical cable design, yielding a rating which prevents the creation of cavity due to potential plastic deformations of the cable sheath.
When crossings are inevitably installed, cables are pushed towards their thermal limit, as a result of the mutual heating. In order to accurately rate these circuits under various ambient conditions; Finite Element Analysis (FEA) methods have been developed. Compared to the traditional IEC calculation, FEA modelling provides a more reasonable and accurate solution, by releasing idealistic assumptions in the IEC method. In addition, a systematic cable rating strategy has been suggested and successfully demonstrated through rating submarine high voltage dc cable crossings, which considers highly coupled physics: thermal, electrical and mechanical. In summary, this thesis contributes towards the modern rating methodology development for hvdc mass impregnated cable circuits, under a purpose of efficient and reliable long-term operation.

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ORCID for Paul Lewin: orcid.org/0000-0002-3299-2556

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