Abstract
Recent numerical simulations have shown that in future HVDC power transmission systems faults may lead to transient overvoltages of types not covered by present HVDC cable type tests. Among these are the so-called Temporary Overvoltages (TOVs), characterized by a very slow front and long tail transient superimposed on a dc voltage. To clarify to what extent HVDC cable grade mass-impregnated paper insulation withstands such TOVs, the maximum withstand fields of small-scale samples subjected to same polarity TOVs of different front and decay times are measured. The samples are made by stacking five layers of 90 μm thick paper with an air-filled slit in the third layer simulating a butt gap. As expected, the majority of the breakdowns go through this weak point. The maximum withstand levels are around 150 kV/mm, with a substantial statistical scatter. More importantly, Weibull analyses disclose only modest differences in dielectric strength between samples subjected to the different TOVs, even though the front time spans from 0.25 ms (as in a standard switching impulse) to 10 ms, and the decay times are up to several seconds. This suggests that extending the qualification and type tests to also include long TOVs may not be necessary.