This novel measurement method is an added tool to investigate and gain insight into the PP extrusion process. The theory of modelling it is confirmed and shows the way to optimization.
Manufacturing power cables out of polypropylene-based thermoplastic materials brings new challenges for the manufacturing process optimization. As with all things in life, everything is easy when going slowly and producing small cables. The bigger the insulation thickness, the faster the speed, the process becomes more demanding. Transition from medium voltage cables to higher voltages and insulation thickness that is equal or larger than the conductor diameter gives rise to a new phenomenon in cooling.
First, rapid cooling gradient may affect the geometrical quality of the cable. Thermoplastics are typically extruded at higher temperatures than XLPE insulation, which leads inevitably to greater shrinkage during the cooling process. The phase transition from molten to solid polymer increases the material density which also contributes to the significant shrinkage. These changes in diameter may distort the cable shape if not optimized for in the manufacturing.
Second, even more critical issues come from the risk of void formation during cooling. When the cooling process starts, the outer surface of the polymer solidifies first creating a semi-rigid ring around the cable. This ring is crystallized at a larger-than-final diameter of the cable, since the material is still thermally expanded. When the cooling progresses, the solid ring occupies a larger and larger portion of the polymer layer. The phase transition of the polymer from molten to solid increases its density, which essentially means that if the outer ring does not deform, there will be lack of material on the inside leading to macroscopic voids in the insulation. The tendency for void formation is a function of material properties, cable construction, and process conditions.
We have developed a special measuring apparatus to study this phenomenon in detail for high-voltage cables. The device was inserted between pieces of conductor and underwent the full manufacturing process identical to an HV cable. The device consists of a load cell and a piston measuring pressure towards the conductor during the manufacturing process. This data confirmed our theoretical hypothesis and calculations showing a significant reduction in cable internal pressure during the cooling phase of the manufacturing process.
This revolutionary measurement technique helps us to better understand the thermoplastic power cable manufacturing in depth and to optimize the manufacturing technology. We have already developed a simulation program that can be fine-tuned with this newly acquired data.
The risk of void formation is strongly a material-related property. In the world of thermoplastic PP-based polymers, the material selection is likened to the wild west. There are no common and similar materials with uniform behavior, as is the case with XLPE grades. The differences between thermoplastic insulation materials are huge, making it more difficult to have one-size-fits-all process parameters. Nevertheless, this novel measurement method and research gives us tools to investigate the process more deeply and generate insights into optimization. We have now verified the theory of modelling the process and a way to optimize for the future.
Schematic of the apparatus measuring pressure towards the conductor.
Cooling process curves show external pressure (light green), conductor temperature (green), and cable internal pressure towards the conductor (blue) during the cooling process of a high voltage thermoplastic PP-based cable prototype. Conductor pressure measurement shows a clear reduction during cooling.
Mikko Lahti
Director, R&D
mikko.lahti@maillefer.net