An approach to the modelling of partial discharges in electrical trees

A partial discharge model has been developed to simulate experimental partial discharge activity within electrical tree structures grown in polymeric based insulation. A pin plane electrode arrangement and tree were defined on a 41 x 44 grid representing an area 2 mm by 2.1 mm. A sinusoidal 50 Hz voltage was applied to the pin tip and 3600 time steps per cycle were used to give a phase resolution of 0.1 degrees. Partial discharges were simulated at each time step by adding one or more dipoles of charge onto the tree structure to reduce the potential difference between adjacent points in the tree from V-on to V-off. Each dipole of charge represents a local electron avalanche occurring over a distance equal to the grid spacing. The model parameters, V-on and V-off, represent the discharge properties of the decomposition gas. Partial discharge current pulses in the external circuit and the induced image charge were calculated as a function of time. The spatial distribution of the emitted light in the tree structure over one cycle of the applied voltage was also calculated. The close agreement between the calculated and experimental data suggests that the underlying assumptions used in the construction of the partial discharge model are appropriate for the case of electrical trees grown in the flexible epoxy resin.