Surface electric strength of processed pressboard under composite AC and DC and conventional stresses

In DC transmission systems the converter transformers operate under a superimposed DC bias. A knowledge of the surface strength of processed pressboard in mineral oil under superimposed AC and DC stresses is therefore essential. In this study, the surface strength has been determined experimentally for cylindrical specimens and for three gaps under the influence of superimposed stress in the range of ripple factor from 0.4 to 2.2. The surface strength has also been found under application of AC, DC, lightning and switching impulse voltages. The influence of the interface on the breakdown value of the gap has been found for each of the above types of stresses. It is shown that the surface strength is greatest under lightning impulse voltage, followed by switching impulse, DC and AC voltages. Under DC voltage with ripple the surface exceeds the DC value in the ripple range considered for the 10- and 25-mm gaps. The presence of the interface lowers the breakdown voltage except when either DC or superimposed stresses are applied. For the test configuration considered, the impulse ratio increases with gap length especially under application of lightning impulse voltages