| As the core equipment of DC transmission system,the safe operation of converter transformer is of great significance to power grid.Compared with the common AC transformer,the working environment of converter transformer is worse.Its valve side winding not only bears the mixed voltage of DC superimposed AC,but also bears a lot of harmonic voltage.The effect of harmonic voltage makes the partial discharge characteristics of oil paper insulation change greatly,which seriously threatens the operation characteristics of the insulation materials of the converter transformer.Therefore,the fault rate of the converter transformer is higher than that of the common AC transformer.At present,most of the researches are limited to the study of partial discharge of oil paper insulation materials by using AC-DC superimposed voltage or partial harmonic voltage.However,there are few reports on the actual partial discharge test under the valve side voltage waveform and the related quantitative analysis and research.However,the research on the partial discharge characteristics of oil paper insulation under the actual valve side voltage waveform for the insulation design of converter transformer and Safe operation is of great significance.In this paper,a ± 800 kV UHVDC transmission system simulation is built by combining simulation and test.The harmonic components in the valve side voltage waveform of converter transformer are analyzed,and the simulation results are verified by measuring the actual DC transmission system with the same voltage level.A three-dimensional converter transformer model is built to simulate the internal power of converter transformer under the actual working voltage.The field distribution is calculated and the relevant electric field characteristics are analyzed;a partial discharge test platform which can generate any waveform is built;the partial discharge test of typical defects of oil paper insulation under the actual working voltage waveform of the converter transformer is carried out;the maximum discharge,average discharge,total discharge and other relevant parameters of the partial discharge are analyzed;the The limitation of the equipment,the partial discharge simulation model is built to realize the mutual verification and further extrapolation with the test.The main innovative achievements of this paper are as follows:(1)A three-dimensional model of the main insulation of the converter transformer is built,and the nonuniformity of the internal electric field distribution of the converter transformer is calculated by COMSOL multiphysics.Under the AC voltage,the difference of the electric field strength of the same insulating part at different positions along the winding circumference is nearly twice.The calculation of the three-dimensional model can provide a more accurate reference for the insulation margin design.The distribution of electric field strength under the working voltage of Y / a connection and Y / Y connection is calculated.Under the voltage of Y / ? connection mode,the maximum change rate of voltage in the insulation structure can reach 1.4 kV / ? s;under the voltage waveform of Y / Y connection mode,the maximum change rate of voltage in the insulation structure can reach 3.20 kV / ? s,while under the same voltage level,the maximum change rate of power frequency sine voltage is only 0.05 kV / ? s.According to the three-dimensional model and its electric field simulation results,it can provide the corresponding reference for the actual insulation layout and material selection of converter transformer in practice.Through the simulation results of the model,it also provides the reference for the voltage related settings in the subsequent partial discharge test.(2)The partial discharge characteristics of typical cavity defects of oil paper insulation under the action of harmonic voltage on the valve side of converter transformer are studied and compared with the discharge characteristics under power frequency voltage waveform.Compared with power frequency sinusoidal voltage,the phase distribution of partial discharge of oil paper insulation with cavity defect under valve side voltage waveform is narrower and the discharge amplitude is larger.The maximum discharge quantity Qmax and average discharge quantity qmean under y / ? connection and Y / Y connection voltage can reach 2 and 3.5 times of sinusoidal voltage respectively.The results show that the total discharge energy in the partial discharge process is more than that in the power frequency AC voltage.(3)Aiming at the limitation of the test,considering the physical process of the occurrence and development of partial discharge,the simulation of partial discharge characteristics of oil paper insulation with solid cavity defect is constructed,and the partial discharge simulation calculation of the defect model under the action of valve side voltage waveform is realized by the mixed programming of COMSOL multiphysics and MATLAB.The simulation results are in good agreement with the test results.The total partial discharge of harmonic voltage is higher than that of sinusoidal voltage.The simulation model not only makes up for the limitations of the actual test and avoids the influence of waveform distortion,but also makes a better prediction of the partial discharge characteristics of oil paper insulation under higher voltage level.At the same time,it avoids the non discharge signal and noise interference caused by harmonic voltage wave in the test,which makes the analysis results more accurate.In this paper,the simulation of DC transmission system generated from harmonic voltage,the simulation of main insulation electric field of converter transformer under harmonic valve side voltage,the analysis system of partial discharge test and electric field simulation of typical defects of oil paper insulation under the action of valve side voltage are constructed,and the influence of valve side voltage containing harmonic wave on partial discharge of oil paper insulation cavity defect is studied,.It is the first time to carry out the partial discharge test under the actual working voltage waveform of the converter transformer,and realize the quantitative comparison with the partial discharge under the power frequency AC voltage.It provides a basis for further analysis of partial discharge of oil paper insulation defects of converter transformer,and also provides a reference for relevant design and operation of converter transformer.Some suggestions are put forward for the potential deficiency of the converter transformer test in the current regulation. |
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