To understand the concentration distribution of rejuvenation fluid along the vertical direction of XLPE cables and its influence on rejuvenation effect

we investigated the propagation characteristics of water trees

and the changes of micro morphologies of insulation layers on the upper and lower sides of prior rejuvenated（inject the rejuvenation fluid into the cables before aging） cable samples. Two groups of prior rejuvenated cable samples were subjected to an accelerated water tree aging for different times. The morphologies and sizes of water trees on the upper and lower sides of the cable samples which were aged for different times were observed by a microscope. Meantime

the changes of the micro morphologies and the rejuvenation fluid concentration in water tree regions on the upper and lower sides of the samples were observed（detected） by a Scanning Electron Microscope（SEM） and an Infrared Spectroscopy（IR）

respectively. The test results show that after 30 days of the accelerated aging

the water tree size on the upper side of the samples reaches 310.5 μm

which is greater than that（251.2 μm） on the lower side of the samples. Furthermore

the absorbance intensity of C—Si bond（in a direct proportion to the rejuvenation fluid concentration） in water tree regions on the lower side of the samples is 0.83%

which is higher than that（0.72%） on the upper side of the samples

showing that the rejuvenation fluid concentration in water tree regions on the lower side of the samples is higher than that on the upper side of the samples. However

after 40 days of the accelerated aging

the water tree size on the lower side of the samples reached 371.8 μm

which is higher than that on the upper side of the samples（350.3 μm） instead. Moreover

the absorbance intensity of C—Si bond in water tree regions on the lower side of the samples decreases to 0.48%

which is smaller than that（0.63%） on the upper side of the samples

showing that the rejuvenation fluid concentration in water tree regions on the lower side of the samples is lower than that on the upper side of the samples instead. The research shows that gravity has an important influence on the rejuvenation fluid concentration distribution and its changes along vertical direction of the cable samples. Under the influence of gravity

the rejuvenation fluid is easy to diffuse downward though the cables. For this reason

the rejuvenation fluid concentration can decrease on the lower side of the samples

which can further result in the increase of the water tree propagation rate and the aging extent on the lower side of the samples.