Newsletter/Blog post, March 2024
Ilkka Rytöluoto, VTT
Background
HVDC underground and submarine cables are a vital technology for the electrical power transmission over long distances with low losses, which helps Europe to reduce its carbon emissions and achieve climate neutrality by 2050 following the European Green Deal. Extruded HVDC cables with cross-linked polyethylene (XLPE), and more recently also thermoplastic polypropylene (PP)-based insulation, represent the new generation of HVDC cables. In NEWGEN project, one of the key specific objectives is to develop and demonstrate novel chemical additives for mitigating space charge accumulation in next-generation PP-based thermoplastic and conventional DC-XLPE insulation materials for HVDC cables.
HVDC cable insulation material development in NEWGEN
In NEWGEN project, formulation, processing and characterization of novel PP and XLPE -based HVDC cable insulation materials are carried at VTT’s Polymer Pilot Facility in Tampere, Finland. VTT has a long history of developing and characterizing high voltage insulation materials in several projects focusing e.g. on HV capacitor and cable applications.
The polymeric main insulation layer in a HVDC cable must meet many stringent electrical, mechanical and thermal requirements. Aside from the choice of base materials alone and optimization of material blend composition, proper planning, cleanliness and execution of the compounding process is pivotal in achieving homogeneous, well-mixed and impurity-free thermoplastic insulation blends so that the electrical insulation integrity of the material is not compromised. Impurities are a major concern in electrical insulation materials as they will considerably weaken the electrical properties of the material under high electric field, and can easily lead to accelerated ageing and premature failure, hence compromising the insulation lifetime.
New clean compounding process environment at VTT
Recently, a completely new clean compounding and sample manufacturing environment has been established at VTT, with excellent capabilities for thermoplastic processing of high-quality electrical insulation materials (Figure 1). The clean compounding environment at VTT is based on a new pilot-scale 18 mm twin-screw extruder equipped with precision gravimetric feeders for feeding both granulate and powder form raw materials into the blend. Continuous melt-filtration with a screen changer to remove any impurities present in the base materials, vacuum degassing of the melt, and finally, cooling and pelletizing of the polymer extrudate inside an ISO 6 class mobile softwall clean room environment are standard practices at VTT. With this approach, clean compounding of large (300–500+ kg) thermoplastic insulation material batches can be prepared in NEWGEN, providing pellet form materials for HVDC cable prototypes and characterization.
In addition to standard twin-screw extrusion and compound pelletizing process, VTT is also carrying out twin-screw compounding and direct cast film extrusion using the same extrusion line in NEWGEN, allowing thermoplastic compounding and manufacturing of flat specimens for various dielectric and material characterization in a continuous process. The advantage of the single-stage compounding and film extrusion approach is that it avoids any intermittent cooling, external handling, re-melting and film extrusion steps that are needed in a conventional two-step compounding and single-screw film extrusion-based processes, hence reducing the sources of external contamination and added thermal history.
Finally, the pilot-scale clean compounding process is also complemented by mini-scale twin-screw extrusion and hot-pressing processes at VTT, all with established clean processing practices for electrical insulation materials. This complementary processing allows for screening of developmental-grade materials and formulations in small, grams-scale batches, before up-scaling.
The NEWGEN consortium will continue the development and testing of novel HVDC cable insulation materials and provide further updates and publications on the results later this year.
Figure 1. Left: Photograph of the twin-screw extruder -based clean compounding environment for thermoplastic processing of electrical insulation materials at VTT. Right: Extrusion of 350+ kg batch of base PP insulation blend for HVDC cable prototypes in NEWGEN.
