| The converter transformer valve-side bushing and DC wall bushing are located in the critical connecting position of UHV AC/DC Hybrid Power Grid.Internal defects and failures may lead to the blocking of the converter station valve groups and the shutdown of both AC and DC power grid system.In recent years,the two types of DC bushing which passed the IEC standard thermal performance test still frequently occurred the current-induced breakdown failures.On the basis of high operating ambient temperature,multi-frequency electric stress characteristic and thermal performance test,the thermal equilibrium process model of DC bushing is established,and the electro-thermal coupling field analysis method of bushing is put forward.This model is of great theoretical significance and practical application value to realize the internal defect assessment,failure diagnosis,current-carrying and insulation combination optimization of UHV DC bushing under high ambient temperature,multi-frequency electric stress.Firstly,the Fourier Spectrum characteristic analysis under multi-frequency current superimposed waveform,heat loss calculation for two types of UHV DC bushing are carried out,and the 6500 A class AC and DC high-current source with steady-current control function is designed.An UHV DC bushing thermal performance test system was also established to imitate the converter transformer oil temperature,valve hall and outdoor limiting temperature.Based on the test results of the most hot spot temperature distribution of the bushing,the thermal equilibrium process characteristics model of the UHV DC bushing is established.The thermal equilibrium process characteristics of three types of UHVDC bushings show that there is a big difference between the steady temperature value of the bushing obtained by using IEC standard criterion and the actual final steady temperature value,and the final steady temperature model derived from the the thermal equilibrium process model can be used as the basis for the electro-thermal coupling field analysis of defective DC bushings under multi-frequency electric stress.Meanwhile,on the basis of the final steady temperature value(for bushings which have passed thermal performance test)or the maximum value of condenser material(for bushings without thermal performance test),a new method of DC bushing thermal field analysis based on the extreme heat loss is proposed.Based on the analysis of AC and DC equivalent condenser model and multi-frequency harmonic voltage excitation model,the multi-frequency electric field analysis method and electro-thermal coupling field analysis method for defective bushing are proposed.The temperature distribution and electric field distortion of the current-carrying defective DC bushing under multi-frequency electric stress are verified through the electro-thermal coupling field analysis under full current and full voltage,and a diagnosis model for the defective DC bushing with current-carrying conductive tube connection is proposed.The results show that the external temperature of the air-side composite hollow insulator can be used to diagnose and evaluate the overheating defect of the connecting part of the current-carrying conducting tube outside the bushing condenser body,by reducing the load current,the development of the defective bushing to final failures can be avoided.At last,in order to solve the problem that it is difficult to diagnose and identify the overheating defects of the current-carrying conducting tube inner the condenser body by external infrared spectrum before the defects evolve into a final breakdown failure,typical penetrating failure diagnosis and analysis methods for UHV DC bushings are proposed.Through the analysis of the electro-thermal coupling field of a failed wall bushing with both radial puncture and axial flashover,it is verified that the unreasonable design of current-carrying and insulation combination is an important cause for the electro-thermal coupling failure.In addition,by using the extreme heat loss and multi-frequency electro-thermal field analysis method,current-carrying and insulation structure optimization and checking are carried out respectively on ± 1100 k V valve-side bushing of converter transformer and DC DC wall bushing.And the insulation performance of these two types of±1100 k V UHVDC bushings were verified with a method of stratified arrangement of low partial discharge background interference in confined electromagnetic shielding test space.The electro-thermal field coupling analysis method proposed in this paper is of great practical value in the aspects of diagnosis for defects and failures,insulation and current-carrying structure combination optimization design of UHVDC bushing. |
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