Dynamic Behavior and Residence Characteristics of Space Charge in Oil-Paper Insulation Under Polarity–Reversal Electric Field

Abstract

As one of the core components of the direct current (dc) transmission system, the converter transformer bears not only the alternate current (ac) electric field but also complex electric fields, such as polarity reversal. In the process of polarity reversal, the phenomenon of space charge residence occurs in the oil-paper insulation inside the converter transformer, which distorts the internal electric field, leading to insulation deterioration or even failure. However, studies on the dynamic characteristics of space charge during the reversal process and the mechanism of insulation degradation are not comprehensive. Therefore, in this study, a test pulse matching circuit was developed to accurately match the test pulse with the polarization voltage of polarity reversal and perform the multi-point rapid test of the space charge transient characteristics during polarity reversal. The space charge tests (of the unaged specimen) were performed at the polarity–reversal time of 60, 30, and 10 s, and the influence of polarity–reversal time on the charge residence characteristics of oil-paper insulation was compared and analyzed. Additionally, space charge tests were performed on oil-paper insulation samples with accelerated electro–thermal–mechanical aging treatment for 0, 2.5, 5, 10, and 15 days to compare and analyze the influence of oil-paper insulation aging degree on charge residence characteristics under the action of multi-physical field coupling. Research results indicate that the shorter the reversal time, the shorter the time required for the charge to dissipate, and the more the charge resides, the stronger its enhancement effect on the electric field. As the aging degree of the sample increases, the dissipation rate of the resident charge becomes slower during the reversal process, and the resident charge increases, so that the maximum electric field intensity inside the sample increases under the same polarization electric field. The study of the charge residence characteristics and the dynamic charge behavior during the polarity inversion process has important guiding significance for clarifying the deterioration mechanism of the converter transformer oil-paper insulation under polarity–reversal electric field.