Review of the Performance of High-Voltage Composite Insulators

In this section, a well-known process leading to the breakdown of a polluted polymeric insulator is discussed. Moreover, the impact of a filler, individual or hybrid (containing more than one constituent), in the polymer affecting the process has been elaborated upon.

It has been reported that the dielectric strength of a base polymer can be modified by incorporating different inorganic fillers which generally enhance performance at low filler loading. This may be affected by several factors, including the type of filler, its shape, size and dispersion, as well as the properties of the matrix (base polymer) itself. However, above a certain volume/weight concentration, called the dielectric percolation threshold, the dielectric strength may decrease [ 56 ]. The percolation threshold is defined as the transition behavior of a composite dielectric with increasing filler content. The formation of dielectric percolation is based on the polymer and filler interface, which gives the peak value of dielectric strength of a composite [ 34 ]. Gao et al., for example, found the dielectric percolation threshold in nano silica/epoxy composite is 5 wt% silica [ 34 ]. Similarly, the percolation threshold of carbon black-filled composite is 6.2 wt%, as discussed by Zois and co-workers [ 57 ]. The enhancement of dielectric strength through the addition of a filler has been attributed to the induced shallow traps which retard the accumulation of space charge. However, beyond a certain wt% of filler, the percolation path is formed due to the easy overlapping of charge carriers in a double layer, thus resulting in a reduction in dielectric strength. Moreover, by increasing the filler content, the interparticle distance becomes equal to the diameter of filler particle, which provides an easy path to overlap charge carriers and dielectric strength shows a decreasing trend [ 58 ]. Frechette et al. studied and characterized the interphase and overlap regions by adding large contents of nano filler in epoxy [ 59 ]. They used a 2D computer generated composite model having random particle distribution with no interphase. The interphase volume fraction was reported to saturate at 37 nm of the interphase radius, and the remaining fraction was used as an overlap ratio by the added filler. The reported results also indicate that interphase particle overlap imparts a significant influence on the dielectric response of the composite. The addition of less than 1 wt% nano size ZnO in epoxy resin significantly increased its dielectric strength under AC voltage [ 46 ]. The experimental studies on epoxy/polyhedral oligomeric silsesquioxanes (POSS) were performed to enhance dielectric strength and thermal conductivity behavior by Heid and co-workers [ 60 ]. Andritsch et al. reported an increase in the dielectric strength of epoxy filled with nano BN, which was attributed to the surface modification of the polymer nanocomposite [ 61 ]. The increase in dielectric strength at smaller sized filler loading was attributed to the better suppression of mobile charge carriers at the interface region between the polymer matrix and filler. This restricts the enhancement of electrical stress. It appears that a further decrease in the size of the filler may increase the dielectric performance, since this offers even better penetration charge carriers through the bulk of the composite. The authors in [ 62 63 ] reported the results of dielectric strength of 9000 h-aged samples in a multi-stress environment of unfilled HTV-SiR and its composites filled with different fillers. Among these, HTV-SiR filled with 6 wt% nano SiO+ 20 wt% micro ATH gave the highest AC dielectric strength. Other researchers [ 64 65 ] also found that the dielectric performance of nano and micro/nano filled composites was better as compared to their only micro filled materials. An estimated model offering higher insulation dielectric strength for micro/nano filled epoxy resin was presented in [ 66 ]. It is assumed that the addition of nano filler in micro filled composite enhances its capacity to prevent progression of treeing in the composite.