Study On UHF Electromagnetic Waves Propagation Characteristics And Monitoring Antennas Of Partial Discharges In Converter Transformers

Partial discharge (PD) is not only the major factor resulting in the invalidation ofinsulation in power transformer, but also is an indicator of insulation integritydegeneration. So it is meaningful to study on the PD online monitoring for powertransformer. Valve-side windings of converter transformer withstand complex stressescombined with AC, DC and pulse voltages. On the one hand, the insulating strength ofwindings would reduced under the action of pulse voltage and PD will be happened orworsen in weak insulation, ultimately causing operation accidents of convertertransformer. On the other hand, the pulse voltage could be detected by pulse currentsensor, which would reduce the sensitivity of impulse current method.Ultra-high frequency (UHF) detection technology has the advantage of stronganti-interference, which makes it immune to the pulse interference. However, there arestill unsolved problems on the application of UHF detection technology for PD onlinemonitoring in converter transformer, such as UHF electromagnetic wave propagation,UHF sensor, UHF signal acquisition method and so on. Therefore, this paper hasresearched the approach on UHF Hilbert fractal antennas and propagation characteristicof PD in converter transformers based on UHF method, electromagnetic simulation,microwave and antenna theory, signal processing technology. The main contents are asfollows:①Simulation model of a500kV single-phase converter transformer is set up withFinite Difference Time Domain method. The influences of transformer parts tomagnitude, arrival time, frequency spectrum and waveform feature of UHFelectromagnetic wave are investigated. And changes in electric-field distribution ontransformer walls under different positions of PD source are contrasted. Then changes inmagnitude, frequency spectrum and waveform feature of UHF electromagnetic waveare analysis after spread to outside via net-side bushing and valve-side bushing.②The third order Hilbert fractal patch is improved by adding two parasiticconductors. And stacked structure is applied to stacked multiple-arm Hilbert fractalantenna. The influences of parasitic conductor width, substrate thickness and otherparameters to the performance of stacked multiple-arm Hilbert fractal antenna havebeen analysis in this paper. And then the stacked multiple-arm Hilbert fractal antennawas designed as antenna array involving2×2elements and fed by the Wilkinson’s four-way power divider. UHF PD signal acquisition system is designed based onfrequency mixing technique. The system can reduced the frequency of UHF signals bymixing filter.③Then actual PD experiments had been carried out for for typically artificialinsulation defect models while the proposed antenna was used for the PD measurement.Single UHF PD pulse signal and PD statistical pattern were captured. PD experimentalresults show that the proposed antennas is found to be superior in PD detection and.theUHF PD signal acquisition system can gather PD signal whose frequency from300to1000MHz and lower the frequency to selectable band, that is10MHz,20MHz,40MHz and80MHz, and keep the phase characteristics. The PD phase-resolveddistributions of different insulation defect models based on these PD signals wereconstructed.