Nyhet | 2016-05-09 | 14:33

Rating of power cables for dynamic load situations

Master's thesis by Andrew Kitimobo, Electric power engineering at The Royal Institute of Technology (KTH).

Supervisor at Vattenfall: Urban Axelsson.

Summary

The overall objective of the project was to evaluate the viability of applying dynamic rating to the dimensioning of offshore wind farm power export cables, usually three core armoured cables.

Following various publications in the cable industry indicating that the armour loss estimation methods proposed by IEC for three core armoured cables are overly conservative, the project aimed to develop models for more accurate estimation of the armour losses of three core armoured cables so as to realise less conservative cable rating calculations.

Models were developed for offshore wind farm export cables for the typical installation conditions in which these cables are applied. The models were then validated with measurements of cable temperature evolution with loading taken for the power export cable of Eegmond aan Zee offshore wind farm located in the North Sea.

The validated models for power cables in the different installation environments relevant for offshore wind farm power export cables were then used to perform cable rating calculations for the power export cable circuits of one of the proposed East Anglia offshore wind farms. The power profile with time as expected to be generated from the wind farm was applied to the models for the cable rating analyses.

As part of the modelling process, the capabilities of Cymcap, a commercially available software tool for the rating of power cables were compared with those of Comsol, a finite element tool, for steady state and dynamic cable rating calculations.

For purposes of achieving less conservative cable rating results than would have been obtained with IEC methods, a good understanding of the behaviour of the armour losses in submarine cables was obtained from various publications.

Two concepts understood to be important in the estimation of the armour losses were that the losses were due to the magnetic field component parallel to the armour wires and that their magnitude was influenced by the crossing pitch between the power cores and the armour wires of the cable.

For accurate estimation of the armour losses, 3 D FEM models are required for the entire complex cable geometry which was not possible with the available computing resources.

Approximate expressions for the magnetic field parallel to the armour wires were obtained and applied to 3 D and 2 D FEM models of individual armour wires to compute the armour losses. The results with 3 D FEM were similar to those obtained with 2 D FEM.

However, a comparison of the armour loss estimates obtained with two different expressions for the magnetic field gave varying results for the armour losses and their dependence on the crossing pitch making it difficult to draw any conclusions on the exact relationship between the armour losses and the crossing pitch.

Secondly, the unavailability of any information on the crossing pitch between the power cores and the armour wires of the cables used in the study also made it difficult to draw conclusions on the armour losses of these cables from the models. The armour losses eventually used in the cable rating calculations were estimates based on the armour losses obtained from published results of 3 D FEM simulations of a cable of similar dimensions.

In the validation of the models, the results from the models of buried cables showed close agreement with the measurements. However, for cables in J tubes, the inability of 2 D models to capture convective heat transfer resulted in large deviations between the results from the models and the measurements.

From the cable rating analyses, the conductor temperature attained with dynamic loading was observed to be less than that from steady state loading for the same cable dimensions. This indicated that it was possible to use smaller conductor cross sections when using dynamic rating than with steady state rating of offshore wind farm power export cables.