Location of PDs inside transformer windings using UHF methods

Diffraction of Ultra-High-Frequency (UHF) electromagnetic waves generated by partial discharges (PDs) inside a transformer will decrease the amplitude when passing through the winding and delay the arrival time of initial wave at the UHF sensors installed on the internal wall of the tank. It may be difficult to locate PDs inside the winding based on detection of UHF signals. To study the feasibility of locating PDs inside the winding using UHF methods, the propagation of UHF wave through windings was simulated by the XFDTD software and the cause of arrival time delay was analyzed by a geometric model. Experiments were carried out on a 220 kV oil-immersed transformer to test the measurement sensitivity and the location accuracy of UHF method for PDs inside windings. The theoretical research results indicated that the UHF wave passed through windings along broken lines, and the propagation time from the PD source to UHF sensors was compared to the time of wave travelling along a straight line. It was found that the arrival time delay caused by diffraction was less than 1ns. The experimental results showed that PDs about 500 pC inside the windings could be detected and located. The amplitude of the UHF signal decreased by about 18 dB, the errors of time difference measurements were less than 0.5 ns, and the location errors were less than 0.1 m.