ALBERT

Description: The critical pulse width ( τ c ) is a pulse width at which the surface flashover threshold ( E f ) is equal to the bulk breakdown threshold ( E BD ) for liquid-polymer composite insulation systems, which is discovered by Zhao et al. [ Annual Report Conference on Electrical Insulation and Dielectric Phenomena (IEEE Dielectrics and Electrical Insulation Society, Shenzhen, China, 2013), Vol. 2, pp. 854–857]. In this paper, the mechanism of τ c is interpreted in perspective of the threshold and the time delay ( t d ) of surface flashover and bulk breakdown, respectively. It is found that two changes appear as the pulse width decreases which are responsible for the existence of τ c : (1) E BD is lower than E f ; (2) t d of bulk breakdown is shorter than t d of surface flashover. In addition, factors which have influences on τ c are investigated, such as the dielectric type, the insulation length, the dielectric thickness, the dielectrics configuration, the pulse number, and the liquid purity. These influences of factors are generalized as three types if τ c is expected to increase: (1) factors causing E BD to decrease, such as increasing the pulse number or employing a dielectric of lower E BD ; (2) factors causing E f to increase, such as complicating the insulator's configuration or increasing the liquid purity; (3) factors causing E BD and E f to increase together, but E f increases faster than E BD , such as decreasing the dielectric thickness or the insulation length. With the data in references, all the three cases are verified experimentally. In the end, a general method based on τ c for solid insulation design is presented and the significance of τ c on solid insulation design and on solid demolition are discussed.