Modelling the Effects of Voltage Rise Rate on Streamer Initiation in Cyclohexane

Dielectric liquids have been widely used in high voltage equipment and immersion cooled high power density devices. The insulation integrity of these equipment/devices is closely related to the pre-breakdown phenomenon, also called streamer, of the dielectric liquids. Conventional views on streamer initiation mechanisms mainly include “bubble theory” and “direct ionization”. In this paper, localized liquid vaporization is adopted as the main criterion of streamer initiation. Streamer initiation process in cyclohexane is simulated in a needle-plane geometry with needle tip radius of 40 µm under different voltage rise rates. Poisson equation, charge continuity equations, thermal diffusion equation, and Navier-stokes equations are coupled with each other and solved by finite element method in COMSOL Multiphysics. Modelling results showed that direct cavitation is hard to occur during positive streamer initiation in cyclohexane with a voltage rise rate ranging from 75 kV/ µ s to 250 kV/ µ s. When only considering Joule heating as the streamer initiation condition, decreasing voltage rise rate from 250 kV/µs to 75 kv/µs leads to the reduction of streamer initiation length from 0.1057 mm to 0.0611 mm. Correspondingly, the streamer initiation voltage also decreases from 81.25 kV to 61.40 kV. However, the streamer initiation time increases from 325 ns to 912 ns. All these trends are in line with knowledge learned from experimental observations.