The ultra-high frequency（UHF） detection is one of the important methods for online monitoring of partial discharges in power transformers. Due to the complex internal structure of power transformers

the propagation characteristics of electromagnetic（EM） waves generated by partial discharges in power transformers are not clear

which limits the application of the UHF method to power transformers. In this paper

based on a 110 k V three-phase three-winding oil-immersed power transformer

an experimental platform of UHF detection was established and the propagation characteristics of UHF signals in the power transformer were investigated. The amplitude

cumulative energy

frequency and propagation time of UHF signals at various measuring positions were analyzed. The results show that the amplitude of UHF signals decreases nonlinearly and the attenuation rate slows down with the increasing propagation distance. The cumulative energy of UHF signals has the similar change rules and wider variation range compared with the amplitude. During the propagation process

the higher the frequency of EM waves is

the more attenuation and the weaker diffraction ability will be

thus the main energy of UHF signals will distributes in the 300～600 MHz. When the detection position is blocked by the windings and iron core

EM waves reach antennas by the diffraction. The first wave of the UHF signals is attenuated seriously

which may affect the location accuracy. The research results in this paper can provide reference and theoretical basis for the selection of UHF antenna frequency band

the installation positions of antennas and the improvement of UHF detection sensitivity.