Dielectric strength of oil-paper-insulation under transient voltage stress

The dielectric stress in the insulation of power transformers during test and operation may become complex. Due to oscillations in the active part and due to reflections after switching operations in the power system, the relevant transient stresses during the lifetime of a transformer significantly differ from the standard lightning impulse voltage. For an efficient insulation design, it is important to know not only the dielectric strength under standard lightning and AC voltage, but also to know the dielectric strength during "non-standard" impulse voltages. This paper describes the investigation of insulation strength as a function of test voltages in the lower frequency range. With the breakdown model shown in this paper, reference measurements with 1.2/50 mus standard lightning impulse were performed. First results are shown hereafter. In future investigations, the model will be used, to compare the strength of the model under oscillating stresses with the standard lightning impulse stress