Abstract: As a new type of insulating oil in the power industry，natural gas insulating oil is excepted to solve

the bottleneck problem of high-quality transformer oil's insufficient supply in the future. However，its electrical

parameters including partial discharge withstand strength and sweep frequency spectrum characteristics are not clear yet. A partial discharge test platform for oil-paper insulation from power frequency to ultra-low frequency was built and a ball-plate electrode model was used to carry out surface discharge test with gas-to-oil-paper insulation and traditional mineral oil-paper insulation under an AC voltage when frequency from 50 Hz to 0.1 Hz. The results show that the initial voltage of the surface discharge on oil-paper insulation increases with the decrease of frequency，and the surface discharge of gas-to-oil-paper insulation is higher than traditional mineral oil-paper insulation，and the performance of partial discharge is better. With the decrease of frequency，the maximum discharge，average discharge，and pulse repetition rate of the two kinds of oil-paper insulation surface discharge in positive and negative half-cycles all show a decreasing trend. The overall shape of the phase-resolved partial discharge（PRPD）map and the statistical map（qmax-φ，n-φ and n-q）are consistent at each frequency，but the distribution range is quite different.The results can provide the reference value for pattern recognition of different oil-paper insulation surface dischargeunder different frequency voltages.

      Key words: surface discharge； gas-to-oil-paper insulation； traditional mineral oil-paper insulation；frequency；discharge pattern