The Influence of Space Charges on the Life Exponent derived from Accelerated Life Tests on PP-based HVDC Cable Insulation. Part 1: Ternary Blends

Abstract

The goal of this investigation is to validate the Inverse Power Model (IPM) – Arrhenius electro-thermal life model via the results of Accelerated Life Tests carried out at various voltages and temperatures on polypropylene-based blends developed in the NEWGEN project, to assess the endurance of this new insulation for HVDC cables to electro-thermal stress. The final aim is to select the best PP blends and space charge suppressing additives in the perspective of manufacturing prototypes of full-size HVDC cables. The present first part of the investigation focuses on a ternary PP blend put forward in a first stage. The second part of the investigation – a next paper – is devoted to a binary PP blend developed later, as such binary blend performed better from the manufacturing viewpoint. The values of Arrhenius parameter B obtained for the ternary blend are quite large, thus proving very good thermal endurance of the blend. Moreover, for both ternary and binary blends the values of life exponent of the IPM found in the first and second part of the investigation are also quite large; this result is a proof of the good voltage endurance of the PP blends. Failure times have exhibited a large scatter and a strong dependence on electric field and temperature - as expected - as well as a puzzling behavior of the life exponent versus temperature. As explained in Part 2, such behavior stems from the increase of the local field enhancement factor due to space charges as electric field and temperature rise.