Analysis of Thermo-Mechanical Stresses in Joints and Cables

Abstract

The landscape of the power sector is undergoing a significant transformation. The growing demand for energy, the depletion of world’s fossil energy resources and the greenhouse effect are well-recognized challenges for the power sector. Through the integration of renewable resources to the energy mix, the power sector is becoming cleaner and more affordable. A fundamental requirement to handle these new developments is to have an efficient transmission and distribution network. Effective transmission and distribution of electricity depends on the condition of underground power cables and joints. Any malfunction in these elements can affect the reliability of the entire grid and this could have an economic impact on the system. Undoubtedly, having an efficient and reliable transmission network implies the need to know and determine the electrical, thermal and mechanical limits of cable systems (cable + accessory). The stresses affecting the service performance of cable system may be classified as thermal, electrical, mechanical and chemical or a combination of the aforementioned. The contributory effects of thermo-mechanical stresses on the degradation of cable joints have gained great attention. Correct expectation of the thermo-mechanical behaviour of all the components in the cable system allows optimisation of both the performance and reliability of these components while maintaining the component life. This research explores some aspects of the thermo-mechanical stresses inside the cable system components in more depth. This work investigates thermomechanical stresses in silicone rubber pre-moulded cable joints and in three core submarine export cables.

It is known that the electrical breakdown strength of insulation interfaces in cable joints depends on the mechanical pressure. Based on the literature, it has been shown that increasing the mechanical pressure in cable joints at the interface is important for ensuring high electrical breakdown strength. However, there is no clear benchmark for interface pressure, and little knowledge about how the thermal behaviour of the cable system will affect it. Furthermore, there is no correlation between interface pressure and different parameters such as material strain, temperature, thickness and elastic modulus. In this research, the mechanical design of solid-solid interface in cable joints is studied to evaluate the main effects of the relevant parameters. More specifically, a model is developed using the finite element method to calculate mechanical stresses based on a non-linear elastic model. In addition, an improved analytical approach is developed to determine the changes in interface pressure during operation. This proposed analytical approach takes proper account of material properties and their changes with temperature. The analytical algorithm is validated using finite element method. The use of a more realistic interface pressure model increases the integrity of the cable joint design and better enables industry professionals and system operators to determine the limits of their system during operation.

In three-core cables, the cores are subjected to higher internal thermo-mechanical stresses than single core cables since each core is expected to be more restricted. The effect of internal thermo-mechanical stresses on the electrical performance of the three-core cable should be examined properly, but is often neglected. During electrical testing of AC submarine cables, only one core of a three-core cable could be tested. In this thesis, the internal thermo-mechanical stresses generated inside a single core and a three core cable are examined. In particular, a 2D thermo-mechanical coupled model is developed using finite element modelling. The model is analysed through thermo-mechanical analysis, to investigate how the thermally induced deformations could affect the electrical performance of the cable. This study demonstrates the significance of testing the full three core submarine cable with armour rather than testing only one single core.

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Identifiers

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

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