In this chapter, a range of silicone rubber (SiR), polypropylene (PP) and polyethylene (PE) based composites filled with micro or nano sized boron nitride (BN) particles at different loadings were manufactured to investigate effects of thermal conductivity. The study of SiR attempts to clarify whether the addition of boron nitride (BN) particles can improve the resistance to tracking and erosion of SiR by increasing its thermal conductivity. For PP, in addition to measuring thermal conductivity of various samples, thermal dissipation was also discussed to analyze the relationship between them. Meanwhile, in order to evaluate tracking failure (a kind of surface dielectric breakdown) resistance of the manufactured samples, time to failure, erosion depth and weight loss of the test samples were measured through tracking test. As for PE, Thermal conductivity and relative permittivity were measured to characterize the basic properties of various samples. Obtained results show that several properties of the filled specimens are obviously improved.
TopIntroduction
High voltage direct current (HVDC) technology is increasingly applied in power transmission and distribution lines with many advantages (Cavallini et al., 2015). Glass and porcelain insulators are increasingly replaced with the polymeric insulators around the world because of their lightweight, easy handling and transport, better contamination performance, vandal resistance and reduced cost (Kikuchi et al., 1999; Schneider et al., 1989; Liang et al., 1999). However, reports have suggested that insulators under dc transmission lines are required to deal with significant challenges (Abbasi et al., 2014; Ghunem et al., 2014; Cherney et al., 2015). It has been reported that with the same field conditions the accumulation of pollutants under dc voltage is 1.2 to 1.5 times of that under ac voltage (Cheng et al., 1981).
Given the rapid development in power electronics technology, the trend of electric apparatus is towards high power density, high switching-frequency and miniaturization. This leads to the problem of overheat during operation. However, most insulating materials used in those devices are almost thermal insulating so that the heat generated by overload operation or intensive discharges will accumulate, which results in a rise in the operating temperature of electric apparatus (Ortiz et al., 2009). A small difference in operating temperature has a significant impact on the reliable operation of electric device (Sim et al., 2005). Once operating temperature over the critical temperature, the lifespan of an electric device will be reduced significantly and even the instrument might be damaged (Hong et al., 2012; Takezawa et al., 2001; Lee et al., 2008). In addition, a large number of insulation failures in the power system are related to thermal breakdown (Nagao et al., 1990; Choi et al., 2015). Therefore, the generated heat should be dissipated as quickly as possible to keep operating temperature at a reasonable level.
Recently, several studies have been carried out to study the thermal conductivity of different composites filled with inorganic particles. Weidenfeller et al. (2004) investigated the thermal conductivity of PP composites with various fillers in different contents. It showed that compared with neat PP, the thermal conductivity of PP/talc composites increased from 0.27 to 2.5 W/(m•K) with 30 vol% filler (Weidenfeller et al., 2004). Krupa et al. (2004) pointed out that the thermal conductivity measurements of the composites increased with increasing the graphite content nonlinearly. In addition, the thermal conductivity of filled HDPE was higher than that of filled LDPE because of high crystallization ratio (Krupa et al., 2004).
In this chapter, specimens were prepared by blending micro or nano sized h-BN particles into base matrix at different loadings. Thermal conductivity and thermal dissipation ability of various samples were measured at room temperature to analyze the relationship between them. Influence of the increased thermal conductivity on surface dielectric breakdown was revealed by discussing time to failure, erosion depth and weight loss. Moreover, DC bulk dielectric breakdown characteristic of various samples was investigated using Weibull distribution. The obtained results indicate addition of BN filler can improve the thermal conductivity of composites effectively.