Electrical treeing characteristic under combined DC-pulse voltage in LN2

It is reported that epoxy resin has been used as insulation material of high temperature superconducting DC cable terminals due to excellent electrical and mechanical properties. Lighting, switching over-voltage and commutation failure will produce pulse voltage, which addresses threats to power transmission system. In this work, electrical tree initiation and growth characteristics in epoxy resin with application of combined DC-pulse voltage in liquid nitrogen (LN2) are studied. With the change of voltage frequency and polarity, three different combined waveforms of DC and pulse voltage are used as the applied voltage: (1) positive pulse and positive DC combined voltages; (2) negative pulse and positive DC combined voltages; (3) negative pulse and negative DC combined voltages. Result shows that the electrical tree grows longer and tenser when the DC voltage and the pulse voltage are the same polarity. Under the same positive DC voltage, the tree length and accumulated damage with negative pulse voltage are lower than that with positive pulse voltage. The pulse frequency is a key factor for the tree initiation and propagation. The tree initiation probability increases with the frequency changing from 50 to 200 Hz in epoxy resin. The charge transportation processes were discussed mainly from three different combined waveforms. With different voltage waveforms, the charge transportation processes are different.