By M36, several key technical and/or scientific achievements have been obtained in NEWGEN.
1. Optimized polypropylene (PP)- copolymer based thermoplastic base insulation blends with suitable dielectric and thermomechanical properties to be applied in HVDC cable insulation have been developed. Moreover, novel polymeric additives with grafted functional groups have been developed and tested as well-dispersing space charge mitigating additives in the PP insulation blends. The new PP insulation compounds with and without additives have been shown to exhibit improved dielectric and thermomechanical properties over state-of-the-art XLPE insulation, particularly at elevated temperatures where XLPE shows reduced performance. For instance, higher short-term breakdown strength and significantly reduced space charge accumulation in comparison to XLPE has been demonstrated with the best-performing PP insulation compounds.
2. New design for the insulation distributor and screw was developed for thermoplastic PP based compounds and used in conjunction with several different material compounds to produce prototype HVDC cables. Thermoplastics dedicated extrusion crosshead has been type-tested by producing HVDC cable prototypes. Three different insulation distributor designs have been tested and modelled. Simulations have been found to match color test results relatively well, which gives validity to simulation approach in further optimization of the flow distributors. In total four full-size HVDC cable prototypes (with different processing conditions and base insulation blends) and six reels of model “A” cable have been produced between M19-M36.
3. The measurement approach for Partial Discharges under DC voltage stresses has been consolidated in terms of measurement chain, acquisition configuration and long term monitoring, and the ability to measure PDs well below 100 pC threshold has been established. Artificial defects have been designed and manufactured to characterize the short term physical behaviour and the long term degradation rate of the materials in the presence of various typologies of defects (internal, surface and corona). A large number of PD acquisitions has been carried out, contributing to the creation of a database with more than 2000 acquisitions. Regarding the development of the leakage current sensor based on a magneto-optical method, a working prototype and its calibration procedures were developed. Performance tests were performed; 1 mA currents were measured in a controlled environment.
4. The development of the life model for HVDC cable systems is complete. The novel developed model is now capable to estimate the life and reliability of HVDC cables under the following working stresses: rated (design) electrical and thermal stresses, load cycles, voltage polarity reversals (VPRs), Voltage transients during disturbances and contingencies. Moreover, optimization of Bipolar Charge Transport model parameters at various temperatures and electric fields, and accelerated life tests on SoA DC-XLPE and the novel PP ternary blends with the life model fitting and statistical post processing have been performed.
