With widely use of power electronics in medium voltage level, there is increasing concern about behaviour of insulation material under arbitrary voltage stress. Solid-gas insulation is a general choice in medium voltage level. Therefore, it's unavoidable to encounter surface discharge problem. In this paper, the surface discharge on epoxy-air interface is investigated, which is a commonly used insulation material. Point to plate electrode was used to test the surface flashover voltage and partial discharge (PD) behaviour at 50Hz, 500Hz, 1kHz, 2kHz and 5kHz. As the result, with increasing frequency, PDIV keeps roughly constant and flashover voltage first increases and then decrease, and the frequency obtaining highest flashover voltage is related to gap length.

In this work, breakdown strength and lifetime curve of oil-impregnated paper (OIP) are compared at 50Hz and 1500 Hz. For the ramp breakdown tests, sinusoidal voltage ramp of / is used. The ramp breakdown experiments is conducted on OIP samples made of single and double sheets of OIP. Weibull plots and interpretation of the results are presented. In order to obtain lifetime curves, a step sinusoidal voltage waveform whose peak value is close to the minimum OIP ramp breakdown voltage is applied on single sheet of OIP samples. Time to breakdown is measured, and each experiment is repeated between 20-30 times to obtain good statistical results. The experiments were focused on four different voltage levels and the lifetime curves are plotted for those two target frequencies. Parameters of the lifetime curves are extracted and the interpretation is given.

Nonlinear voltage distribution along the transformer winding during a lightning overvoltage can cause severe electric stress specially to the area of the winding close to the high voltage terminals. For high voltage transformers different methods such as interleaved winding, electrostatic shield wires, and wound-in shield wires are implemented on the first few disks to improve such nonlinear voltage distribution.Through application of those methods, the series capacitance of the first few disks is increased which results in reduction of electric stress on the highest stress region close to the high voltage terminal. In this paper disk series capacitances are calculated using the energy summation method and a simplified analytical formula is derived for different configuration of disk winding construction.