Dielectric liquids have been widely used in high voltage equipment, such as power transformers, for over a century, and have also generated interest in the application of immersion cooled, power dense equipment in recent years. Electrical breakdown and pre-breakdown phenomena in dielectric liquids, known as streamers, are vital characteristics that need studying. In addition to experimental research, modelling studies can provide supplementary descriptions on streamer dynamics and mechanisms, as well as enable parametric investigations by considering a wide range of factors. Therefore, this study will model streamer initiation and propagation phenomena in dielectric liquids. The finite element method based on a set of governing equations including Poisson equation, charge continuity equations, Navier-stokes equations and thermal diffusion equation have been adopted and implemented using COMSOL Multiphysics. Electron saturation velocity was proposed to explain the stable propagation velocity of a 2nd mode positive streamer. Instant streamer velocity shows an initial increasing and then decreasing trend until reaching a stabilised value recognised as the typical 2nd mode streamer propagation velocity. Decreasing ESV greatly decreases streamer propagation velocity. An ESV of 7.5 km/s for cyclohexane is proven suitable, based on comparisons of streamer velocity between simulations in the present work and experiments in  under a wide range of voltage levels. The dynamics of cavity formation induced by electrostrictive force was initially studied in cyclohexane. Electrostrictive force tends to stretch the liquid due to dielectric polarisation. Higher voltage rise rates require lower voltage magnitude to form a cavity. However, when the voltage rise rate is smaller than 30 kV/ns, a cavity is hard to form in cyclohexane. Comparisons of the dynamics of cavity formation in different liquids were then conducted, showing that a cavity is harder to form in cyclohexane and rapeseed oil than in deionised water due to the smaller relative permittivity of cyclohexane and rapeseed oil. The streamer initiation process was investigated over a wide range of voltage rise rates from 0.075 kV/ns to 70 kV/ns. When considering both electrostrictive force and space charge generation, it is hard to form a cavity before the liquid reaches vaporisation threshold during positive streamer initiation. Besides, decreasing the voltage rise rate also reduces the streamer initiation voltage and streamer initiation length, whilst increasing the streamer initiation time. Modelling low-density channel formation during streamer propagation was attempted by combining charge drift model and smooth phase transition model. When the low-density channel is formed due to liquid vaporisation, a secondary electric field peak is formed on the low-density channel tip, which is followed by a positive space charge density peak. The voltage drop along the streamer channel is reduced compared with the case without considering the low-density channel.
|Date of Award||1 Aug 2022|
- The University of Manchester
|Supervisor||Zhongdong Wang (Supervisor) & Qiang Liu (Supervisor)|
- electron saturation velocity