Breakdown behavior of liquid-solid systems - A comprehensive model

Composite Liquid-Solid Insulation systems have been used in transformers ever since. Optical investigations of the breakdown behavior were carried out over the last years using lightning impulse voltage. Gaps with Solid-liquid barrier systems show specific behavior in the breakdown process. Gap insulation ability increases with barriers and the breakdown process is influenced by the appearance of predischarges in the liquid gap with subsequent micro-discharges at the board surface. Micro-discharges lead to surface-charges on the board creating Lichtenberg-figures with charge-accumulation on the board surface. These charges in turn reinfluence streamer inception and propagation in the liquid. The breakdown process itself is due to degradation processes of the board and depends strongly on prestressing phenomena. In order to get towards a breakdown model for the composite insulation system, liquid-breakdown and solid-breakdown have to be modelled according to the chemico-physcal properties of the liquid and the impregnated pressboard. Attention has to be paied to interdependences between the respective processes. This contribution will present a model for the breakdown-behavior of the composite system based on experimental findings. Influence of the investigated parameters will be discussed.