179 / 2018-08-26 02:59:17

Nanostructured polypropylene-based materials for HVDC cables: space charge and accelerated life testing

Nanostructured,HVDC,Polypropylene,Cable

The necessity for sustainable materials in the world of HVDC cables insulation is gathering more and more attention. Recyclable insulating materials are becoming more popular, in search for a valid alternative to the traditional cross-linked polyethylene, XLPE. Thermoplastic materials have shown good performances in their nanostructured or blended versions, potentially providing good solutions in the near future. Nanostructured polypropylene (PP)-based materials are currently subject to a thorough screening by an European project, that is, GRIDABLE (Polymer Nanocomposite Insulation Material Enabling Integration of Renewables and DC Storage Technologies in the AC Energy Grid), which was launched under the HORIZON 2020 framework funded by the European Community. Various partners, from universities and research institutions, to cable and capacitor manufactures, are involved. The project aim is to develop a new generation of nanostructured polymeric materials suitable for DC cables and capacitors having enhanced performance with respect to presently used materials, especially in terms of increasing their design electric field and temperature.
This paper presents results regarding the electrical characterization (space charge accumulation, accelerated life tests and dielectric strength) for cable grade polypropylene (PP) materials, in both their neat and filled versions, considering functionalized silica nanofiller.
Pulsed Electro Acoustic (PEA) measurements, performed under high electric field (>40 kV/mm) and temperatures (>60 °C), in order to estimate the amount of space charge near to operating conditions of the cable material, are presented and discussed. They are also put in relation to the results of electric strength (ES) tests (at increasing voltage) and accelerated life tests (at high field, near to ES). In this way the effect of space charge can be associated with the results of electric strength and accelerated life tests, with the aim of deriving a fast procedure which can evaluate the effect of nanostructuration (and blending) of the investigated materials.
The paper shows that nanostructuration may be beneficial, especially at higher temperature, to improve the performance of extruded HVDC cables over the current state of the art (XLPE), allowing for higher operating electric fields and temperatures. Indeed, the tested polypropylene-based blends generally show improved performance with respect to XLPE, depending on the selection of the base PP and the surface treatment of the nanofillers. This motivates further investigations by the authors and other partners of the project in optimizing the composition and manufacturing of those novel thermoplastic insulations.
Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 720858.