Tangential Electric Breakdown Strength and PD Inception Voltage of Solid-Solid Interface
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- Institutt for elkraftteknikk 
The solid-solid interface is one of the critical region of the electrical insulation systems which consist of a combination of different insulating materials. It is predicted that the microscopic air-filled voids formed at the interfaces are the weak regions due to their lower breakdown strength compared to that of surrounding bulk insulating materials. In the case of an applied electric field in parallel to the interface, there is a high risk of partial discharge (PD) initiation inside the voids which could be led to degradation and consequently electrical breakdown of the solid-solid interface. PD initiation inside the air-filled voids is affected by their size and encapsulated air pressure. Investigating the parameters that influence the size of voids and enclosed air pressure is a key approach for studying the behavior of solid-solid interface under electrical stress. The motivation of this study was to investigate, theoretically and experimentally, the tangential breakdown strength and PD inception voltage of the solid-solid interfaces. It is beneficial to identify the parameters that affect the above two dielectric characteristics of the interfaces. Furthermore, the development of subsea wet mate connectors which consist of solid-solid interfaces assembled under water requires more investigation on dielectric properties of the wet-made interfaces. So, the overall aim of the study presented here was to establish design criteria for the high voltage apparatuses that have the solid-solid interfaces. Using tribology science, a statistical as well as a deterministic contact models were developed in order to estimate the average size of void and the size of the largest void between two solid surfaces, respectively. So, as an initial step in this work, the effect of surface roughness and applied contact pressure on the size of voids formed at the interfaces was studied, theoretically. The estimated void size was used to develop a model for estimating the PD inception voltage using the Paschen law of gas discharge. Furthermore, a model was developed for estimating the difference between the breakdown voltage of dry and wet interfaces. The aforementioned contact models as well as the procedure of estimating PD inception voltage of the interface was one of major contribution of this thesis. Using two different experimental setups, the effect of surface roughness and contact pressure on the measured breakdown voltage and PD inception voltage of the wet and dry interfaces was investigated. The studied interfaces were contact areas between two XLPE insulating materials. In order to investigate the influence of water droplet at the interface, the breakdown voltage and PD inception voltage of the wet interfaces assembled under water were measured, too. It was found that the dimension of voids strongly depend on various parameters, among others surface roughness, applied contact pressure and the properties of material at the interface. So, it was concluded that these parameters influence the breakdown voltage and PD inception voltage of the interfaces, significantly. Additionally, the voids at the interface were found to be ventilated to the surrounding atmosphere and the assumption of a gas pressure of 1 atm inside voids is expected to be correct. Studying the breakdown voltage and PD inception voltage of solid-solid interfaces assembled under water was considered as one of the novelty of the work. The measured breakdown voltage of wet interfaces was found to be around 40% of that of dry interface. The estimated difference value between the breakdown voltage of dry and wet interfaces was around 33% higher than the corresponding measured value. Here, it was assumed that the contact spots of the wet interface was similar to the contact spots of the dry interface. However, in reality, in the case of interfaces assembled under water, the voids are filled by water and conductive wet area is formed at the contact spots. Using tribology in the context of electrical insulation system for estimating the PD inception voltage of the solid-solid interface was one of the contributions in this work. The statistical contact model was found to be immature in the context of studying the breakdown voltage and PD inception voltage of the interface. However, the deterministic contact model can be a potential technique in this context particularly, in the estimation of PD inception voltage of the dry interfaces.