Flow electrification characteristics of oil-pressboard insulation under polarity reversal electric field

Converter transformer plays an important role in the HVDC transmission system. The valve side winding insulation in converter transformer withstands polarity reversal voltage during operation. In order to investigate the effect of polarity reversal electric field on flow electrification in converter transformer, an oil/pressboard tube model with concentric cylindrical electrode geometry was built and the flow electrification under the polarity reversal electric field was investigated in laboratory. The polarity of applied voltage reversed twice (positive changed to negative and then positive) and the streaming current reached a pulse value right after reversal and then decreased exponentially with time. The decreasing speed of streaming current changed with different temperature and flow velocity. Both the pulse value and the steady state value of streaming current showed a peak effect as the temperature increased. With the increase of applied voltage amplitude, the streaming current was enhanced and the changing tendency after the first reversal turned from decreasing to increasing. Flow velocity is another important factor closely related to the flow electrification characteristic. Based on the results, it is considered that space charge due to applied voltage can greatly affect the electrification characteristic. The pulse value of streaming current after reversal can be attributed to the release of charges from the interface to the oil flow. The influencing factors of flow electrification such as amplitude of applied voltage, temperature, and flow velocity and insulation configuration can influence the oil streaming current by affecting the charge migration process and the space charge density.