Partial discharges in high voltage direct current mass-impregnated cables

A HVDC MIND (mass impregnated nondraining) cable shows a quite typical and repeatable partial discharge behavior during the current-off stage in a type test cycle. After an initial increase in the level of the repetition rate n at current off, it decreases after a certain time. The time to reach the maximum in the repetition rate is larger for a `warm´ cable than that for a `cold´ cable. A simple model has been made to explain this behavior under type test conditions. Although several assumptions and approximations have been made, the model predicts the general behavior of the repetition rate of the discharges and the time to reach the maximum value of the repetition rate after switching off the heating current. This means that the underlying principle assumptions are most probably valid. These are: the number of defects is increasing after switching off the heating current based on the fact that the temperature of the cable is decreasing; and the time constant of recharging the defects is increasing after switching off the heating current, also based on the fact that the temperature of the cable is decreasing