Genetic algorithm for the optimization of rubber insulated high voltage power cables production lines

Abstract

The optimization of vulcanized EPM/EPDM mechanical properties must be a fundamental task for electric cables producers. Insulator mechanical properties depend on the vulcanization degree of the rubber, which is obtained by manufacturers by means of a number of different peroxides acting as reticulation inducers. The industrial production process consists in the extrusion coating of the conductor with elastomeric compounds, and in the successive vulcanization of the insulator rubber through the continuous vulcanization tube, a pressurized high temperature tube filled with nitrogen (the high temperature is a condition necessary to activate the cross-linking agent). Finally, water and/or air are used to cool the insulated cable at ambient temperature. Changes of process variables associated with the vulcanization tube can cause considerable changes in output mechanical properties of the elastomers. In this framework, in the present paper a novel genetic algorithm with zooming and elitist strategy is used for the determination of optimal production line parameters to use in order to maximize rubber output mechanical properties. Nitrogen temperature and rubber exposition time are assumed as production parameters to optimize, whereas EPM/EPDM final tensile strength is considered as objective function. The GA approach proposed exploits a specifically crafted zooming strategy, consisting in the subdivision of the population at each iteration into two sub-groups, depending on individual's grade of fitness (elitist strategy). Different genetic procedures are applied to the sub-groups, consisting of both two typologies of admissible mutations for the elite sub-population and mutation and reproduction for the remaining individuals. In order to improve algorithm convergence, a user-defined population percentage, depending on individual's fitness, is replaced with new phenotypes at the end of each iteration, enforcing in this way the chromosomes renewal. In order to test the reliability of the method proposed, a technical meaningful case study consisting of a high voltage cable used by Italian Railways and cross-linked with a number of different peroxides is discussed. Numerical results show that particular care has to be used by practitioners in the choice of nitrogen temperature and exposition time of production lines to obtain high quality products.