Renewable energy sources have continued to grow considerable interests. Therefore, implementing large scale installations in a bid to mitigate the issues related to non-programmable sources necessitates versatile as well as compact interconnection systems. Therefore, submarine three-core cables are at the heart of linking off-shore wind farms with the electrical grid along with energy storage facilities. Three-core cables are an interesting application when considering multi-purpose structures, which host, in the same facility, power transmission systems as well as other services such as water and gas pipes.
Total power losses in a.c. three-core armoured submarine cables is a critical aspect in order to precisely calculate the current rating. It has been known for time now that the cores and the armour wires are normally stranded with varying laying pitches for mechanical reasons. IEC does not take into consideration these layings and provides an overestimation of screen as well as armour power losses. This is majorly because it disregards the fact that the helically lay of the armour wires and the core conductors have a cancelling effect of the induced current in the armour. This overestimation is also the case even when 2-D model is considered.
The concept of implementing finite element analysis core laying pitches that are shorter than the real ones in a move to reduce the CPU time as well as model complexity does not allow representing the real electromagnetic interaction between the conductive components of the cable. It is for that reason not possible to precisely compute the cable power losses and it becomes hard to identify an optimal three-core cable layout in terms of laying pitch length as well as stranding direction.
Researchers led by Professor Roberto Benato at the University of Padova in Italy presented a complete three-meter-long 3-Dimensional finite element method model of a submarine armoured three-core cables. In this model, they took into consideration the varying core laying pitches as well as armour steel wires. Their research work is published in journal, Electric Power Systems Research.
The authors observed that a ratio of 2/3 between the core laying pitches and the armour was efficient to realize lessened induced current losses. Therefore, unilay and contralay strandings provide for possibilities of reducing the circulating currents in the armour.
The choice of a three-meter long cable was important in trying to understand the interaction between screen induced currents and armour ones. This length however does not represent the actual phenomena that occur in practical cases. In this 3-meter long cable, complete periodicity of core layings and the amour is not always performed.
In a bid to realize periodicity with armour laying pitch equal to 2m, a 6-meter long cable would be necessary. Although this model length would be impossible to solve with the existing finite element resources, Roberto Benato and co-workers study provides two cases with varying armour laying pitches which represent a periodicity.
Roberto Benato, Sebastian Dambone Sessa, Michele Forzan, Marco Marelli, Davide Pietribiasi. Core laying pitch-long 3D finite element model of an AC three-core armoured submarine cable with a length of 3 meters. Electric Power Systems Research, volume 150 (2017), pages 137–143.
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