Study on the Aggregation Structure and DC Conductivity and Breakdown Characteristics of Polypropylene Insulation for HVDC Cables

ABSTRACT This paper investigates elastomer‐toughened polypropylene (PP) insulation to meet the application requirements for green noncrosslinked PP cables in high‐voltage direct current (HVDC) transmission. It focuses on the formation of β‐crystals in isotactic polypropylene (iPP) by adding a β‐nucleator. It examines how varying concentrations of β‐nucleator and elastomer (POE) impact the aggregation structure of PP insulation and its conductivity and breakdown characteristics in the DC field. The results indicated that at a β‐nucleator agent content of 0.1 wt%, the samples with various POE contents achieved the highest crystallinity, the maximum proportion of β‐crystals and the most uniform elastomer distribution. The nucleating agent facilitates the formation of β‐crystals in PP and enhances the order degree of the elastomer molecular chains, thereby improving their crystallization capabilities. Evaluations of DC performances and trap characteristics reveal that when the amount of the β‐nucleator is set at 0.1 wt%, the sample demonstrates the lowest trap density, an exceptional and lower electric field coefficient of conductivity at elevated electric fields and a superior DC field breakdown strength at 90°C. Compared to samples without β‐nucleator, the reduction of DC field breakdown strength for PPB x ‐0.1 from 25°C to 90°C is approximately 4.86% lower. This improvement is attributed to the ability of the β‐nucleator to improve the aggregation structure between PP and POE while optimising the stability of the two‐phase interface. Thus, although DC electrical characteristics are maintained at normal temperatures, the DC characteristics are significantly improved at elevated temperatures.