In order to make a comprehensive assessment of operating state about UHV convert transformer bushing, partial discharge characteristics of resin impregnated paper bushing with metal particles defect was tested under combined AC-DC voltages with proportion of 1:3, 1:5 and 1:7. High-frequency current-transformer sensor were used to detect the partial discharge characteristics. The results showed that compared with the mere AC voltage, the AC-DC voltage can significantly reduce the discharge pulse number in the negative half cycle of partial discharge. The discharge characteristics can be seen obviously when the effective value of the AC-DC superposition voltage is about twice that of the mere AC voltage. Interference signal can be picked out quickly by the method of three-dimensional time-frequency separation, which provides technical support for solving the problem of separation difficulty of interference signal in complex electromagnetic environment of UHV convert transformer.

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