The effect of DC field on streaming electrification in converter transformer

Converter transformer plays an important role in the HVDC transmission system. The insulation in converter transformer withstands both the AC and DC voltage during operation, which is different from that in normal power transformer. In order to investigate the effect of DC electric field on streaming electrification in converter transformer, an oil/paper tube model with concentric cylindrical electrode geometry was built and the flow electrification under the DC electric field was investigated in laboratory. The results demonstrated that the energization by positive DC voltage can enhance the streaming electrification especially at high temperature. With the increment of temperature, the streaming current first increases and reaches a peak value, and then decreases at higher temperature. The flow velocity has a little effect on the streaming current in the low temperature region. However, in the high temperature region the increase of flow velocity greatly enhances the electrification. Furthermore, ion migration is the main reason that causes the flow electrification under the DC electric field.