Effects of space charge on breakdown and creeping discharge of oil-paper insulation

As one of the most important apparatus used in high voltage direct current (HVDC) transmission system, converter transformer has complex structures and operates in a cruel environment. Therefore, more knowledge of the behavior of insulating systems is required. In this paper, experiments are designed according to the operating state of converter transformers, based on which the progress of breakdown and creeping discharge of oil-paper insulation are experimentally studied. Space charge accumulated in oil-paper insulation system is successfully measured. Polarity reversal effects in oil-paper insulation systems are demonstrated by the space charge testing results. In other words, space charge accumulated under the dc electrical stress showed influences on the breakdown of oil-paper insulation are experimentally confirmed. Moreover, the experiments show that space charge could also affect the creeping discharge of oil-paper insulation significantly. Based on these results, it is put forward that traditional transformer design rules may not be suitable for converter transformers. More attentions should be paid on space charge effects in the design and manufacture of converter transformers. Among these effects, the dispersing processes of space charge in oil-paper insulation are stressed to solve the insulation problem induced by polarity reversal in HVDC equipment.