Effect of thermal ageing on electrical performance of polypropylene-based cable insulation

Abstract

To meet the requirements of sustainability, polypropylene (PP) is a promising candidate for cable insulation applications in high voltage direct current transmission (HVDC) systems due to its low overall cost and eco-friendly properties. However, cables are subjected to various stresses during operation, which trigger ageing reactions and ultimately lead to failure. Among these factors, thermal-oxidative ageing is one of the prominent causes of physicochemical degradation and subsequent electrical deterioration. The aim of present research is to investigate this relationship throughout the lifecycle of PP-based materials, which is crucial to predicting the insulation lifetime, and optimizing cable design and operation. Specifically, antioxidants (AO) and propylene-based elastomers (V2200) are introduced to enhance the thermal stability and structural flexibility, respectively. Four groups of PP-based material are involved in this research, including neat PP, PP-AO, PP-V2200 and PP-V2200-AO.
The ageing experiment was conducted in a fan-assisted oven at a temperature of 150 °C to achieve an accelerated ageing in the laboratory work. Thin-film specimens with a thickness of ~ 100 μm were exposed to heat and oxygen from both sides. Samples were collected before reaching the stage of failure with severe defects, after maximum ageing duration of 21, 42, 5 and 28 days, respectively, for neat PP, PP-AO, PP-V2200, and PP-V2200-AO samples. The corresponding estimated lifetimes achieved at the operating temperature of cable, 90 °C, are approximately 23, 50, 5, and 30 years based on Arrhenius calculations. Ageing characteristics were analysed using Fourier Transform Infrared (FTIR) spectroscopy, focusing on surface chemistry, Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) to assess crystal structure, and Thermogravimetric Analysis (TGA) to check thermal stability. Electrical properties were evaluated through the pulsed electro-acoustic (PEA) method, DC conductivity and breakdown strength measurements, studying the charge trapping and transport dynamics.
It has been observed that the addition of elastomer can address the mechanical limitation of PP but will reduce its lifetime. This weakness can be compensated by the introduction of AO. The presence of AO inhibits the auto-oxidation and deterioration of thermal stability, allowing a gradual increase of crystal perfection throughout the ageing period. The chemical oxidation products, notably carbonyls, introduce traps in the polymer. Due to the diffusion-limited oxidation (DLO) effect under elevated temperature, the formation of traps initiated from the sample surface, then progress to deeper positions, dominate the impact on space charge distribution and charge transport. If the AO involves in, the consumption of AO, similarly, originate form the sample surface and then move towards the bulk of samples. Under applied ageing conditions, the presence of AO, the second structure of AO (AO*) and the oxidation products introduce multi-scale traps which impact on the charge dynamics.

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Identifiers

ORCID for Xiwen Wu: orcid.org/0009-0009-9535-9676

ORCID for Thomas Andritsch: orcid.org/0000-0002-3462-022X

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