Transient electric field characteristics in oil-pressboard composite insulation under voltage polarity reversal

Polarity reversal generally enhances the electric field intensity in oil-pressboard composite insulation, leading to partial discharge and even insulation flashover or breakdown of the HVDC converter transformer. In an attempt to study the transient electric field in oil at polarity reversal, the present research adopted the Kerr electro-optic effect technique to conduct real-time measurement of the oil electric fields at the polarity reversal time of 10, 60 and 120 s respectively. Dielectric interface conditions were also adopted to capture the dynamic development of interface space charges. Research results indicate that (i) negative charges, under the absorption of insulation pressboard, are more likely to accumulate at the oil-pressboard interface. As a result, the development of electric field in oil at polarity reversal demonstrates prominent polarity effect, with faster electric field attenuation under negative voltage; (ii) the development of electric field in oil shows asymmetric features under double polarity reversals. The first reversal witnesses a high magnitude of electric field in oil that is about 1.47 times of the initial capacitive electric field, whilst the second reversal sees only 85% of the initial capacitive electric field. Such asymmetric development is mainly attributed to different accumulation performances of the positive and negative charges; iii) the shorter the polarity reversal time is, the higher the oil electric field strength will be. The maximum magnitudes of the field strength after polarity reversals with different time, however, are not much different from one another.