Study On The Propagation Characteristics Of UHF Signals Induced By Partial Discharges In Actual Power Transformers And The Simulation Method

Ultra-high frequency(UHF)detection method is an effective method for online monitoring of partial discharges(PD)in power transformers.The UHF detection,due to the advantages such as noiseless measurement,high sensitivity and on-line monitoring capacity,has become a very essential method for PD measurements.However,there are few studies on the propagation characteristics of UHF signals in the actual power transformers.The simulation method of electromagnetic(EM)waves propagation is relatively simple without verification.The installation arrangement of UHF detection antennas lacks reference basis.These problems restrict the application of UHF PD detection technology on the power transformers.For solving problems mentioned above,this thesis investigated the propagation characteristics of EM waves in the actual power transformers,the simulation modeling technology of EM waves propagation and the optimal installation arrangement of UHF detection antennas.The main innovative achievement of this thesis are as following:(1)Based on the 110 kV and 380 kV actual power transformers,the experimental platform of UHF detection were performed.The typical partial discharge models and monopole antenna were used as excitation sources of EM waves respectively.The amplitude,cumulative energy,frequency band,time-frequency characteristic and propagation time of UHF signals at various measuring positions in the actual power transformers were analysed and investigated.The results show that the amplitude of EM waves decreases nonlinearly,and the attenuation rate slows down with increasing distance from the PD source.The cumulative energy of EM waves has the similar change rules with the amplitude,whereas,the cumulative energy of EM waves has a relatively wider variation range.During the propagation process,higher frequency of UHF signals suffers greater attenuation,leading to the main energy of UHF signals in the 300 MHz-600 MHz frequency band.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.(2)Based on the actual power transformer,the attenuation characteristics of UHF signals induced by the corona discharge,surface discharge,floating discharge and air gap discharge were analysed respectively,and the differences in attenuation characteristics of different PD defects were investigated.The attenuation characteristics of different PD types show the nonlinearly downward trend and decreasing attenuation rate with the increasing propagation distance of EM waves.However,their attenuation ranges are distinct.The corona discharge shows a relatively larger attenuation range,followed by the surface discharge,floating discharge and air-gap discharge sequentially.The propagation characteristics of frequency show that,higher frequency suffers more attenuation.Consequently,the PD-induced EM waves including more proportion of high-frequency components will suffer more attenuation,leading to a larger attenuation range.(3)A simulation modeling method for electromagnetic(EM)wave propagation in power transformers based on time domain finite integration algorithm(FIT)was proposed,which can accurately solve the PD-induced UHF signals of power transformers.The modeling method of the monopole antenna,dielectric window antenna and oil valve antenna was proposed based on the gap-feeding method.The simulation models were built which have the similar structure size with the test tank and the actual power transformer platforms.By comparing the waveform,amplitude,cumulative energy and frequency distribution of the simulated and measured UHF signals,the simulation results show reasonable agreement with the experimental ones.Therefore,the new simulation modeling method and results for the EM wave propagation in transformers were successfully verified.This simulation technology can provide guidance and assistance for evaluating the propagation characteristics of UHF signals in transformers,verifying UHF detection sensitivity,and optimizing the installation arrangement of UHF antennas.(4)Based on the FIT simulation method,the simulation model of the 110 kV actual power transformer for optimizing installation of UHF detection antennas was established.The simulation analysis method for optimizing installation arrangements of UHF detection antennas was proposed.The coverage and detection sensitivity of detection antennas at different positions were analysed,and the installation scheme of UHF detection antennas in the power transformer was obtained.The results show that the single detection antenna installed on the medium height tank between two-phase windings has relatively better coverages and detection sensitivity.For higher sensitivity partial discharge detection,two antennas are recommended to install diagonally at different heights on the front and back of the tank.The research results will provide scientific guidance and reference for the installation arrangement of UHF detection antennas.