Translated Abstract
With the growing capacity of AC and DC transmission systems, load current and short-circuit current in the grid keeps rising. Focusing on the optimization and improvement of the interruption capabilities of high-voltage (HV) SF6 circuit breakers (CB), which is a key problem in the field of high-voltage apparatus, the simulation method for the post-arc breakdown properties of SF6 HVCB is put forward in this dissertation. From the perspective of validation and application, the simulation method is applied to the study of post-arc thermal breakdown in 60Hz short-circuit current interruption which is encountered by domestic 50Hz CB, and mechanism of the accidents where post-arc dielectrical breakdown occur in hot standby CB on the condition of continuous lightning.
Since there is a lack of intensive study on post-arc thermal and dielectrical breakdown characteristics, it is hard to evaluate the interruption characteritics quantitatively and accurately. In this dissertation, magneto-hydro-dynamics (MHD) arc model which could consider the interaction between the external circuit and the arc for SF6 HVCB is built at first. Post-arc breakdown model based on current zero model and critical dielectric breakdown properties of hot SF6-PTFE is established. The simulation method for the post-arc breakdown properties of SF6 HVCB is formed eventually, which considers both the pre-zero phase and the post-arc phase. It is determined that quantitative criterion for thermal breakdown is post-arc current, with G200 and G500 as qualitative criterion; basic criterion for dielectrical breakdown is the ratio of real electrical field under recovery voltage and critical breakdown field.
Since there is a lack of micro parameters measurement and analysis of arc interruption characteristics, an experiment system of arc interruption characteristicsis is constructed by integrating the pressure transducer, current-zero measurement system, spectrometer, and a detachable CB prototype. The influence of filling pressure, short-circuit current, and ratiro of mixtures on the pressure variations at nozzle throat and current-zero behaviours is analyzed. The digital delay generator is used to control the operating mechanism of CB, thyristor, and spectrometer. The shielding net is used to attenuate the electromagnetic interference. Heat isolation, electrical insulation, and suppression of pressure wave in the tube are realized through a Pitot tube and the filled damping paticles. MHD arc model is validated by the experimental results. It is found that critical rate of rise of recovery voltage (RRRV) and the pressure increment at the monitoring point is correlated, and critical post-arc currents of SF6-CF4 mixtures are obtained.
Domestic 50Hz HVCB are encountering problems of 60Hz short-circuit current interruption. To deal with the issue, the simulation method for the post-arc breakdown properties is applied to the analysis of interruption characteristics of 60Hz short-circuit current. The simulation method is used to calculate post-arc breakdown characteristics corresponding to several interruption tests of a 252kV SF6 HVCB, where the simulation method is validated. On the basis of the aforementioned study, pre-zero arc voltage and current and post-arc thermal and dielectrical breakdownis characteristics under the operating frequencies of 50Hz and 60Hz with the same arc striking phase angle are analyzed. It is found that the relatively high pre-zero current slope is unfavorable for energy dissipation and results in higher G200 and post-arc current for 60Hz short-circuit current interruption. Based on the calculation results, modification schemes of the arc-quenching chamber by extending the nozzle throat which enhances the pre-zero arc blowing are discussed, where the simulation results show that the critical RRRV of CB is elevated from 1.44kV/us to 2.23kV/us under the same conditions.
Dielectrical breakdown of the hot standby CB have occurred several times in multi thunderstorm area of southern China. To solve this problem, the simulation method is applied to theoretical analysis of restrike phenomenon in an interrupted 126kV auto-expansion CB on the condition of continuous lightning, with the dielectrical recovery strength calculated after the current caused by the first lightning is interrupted. The results show that the calculated critical breakdown voltage is 449.1kV, which is lower than the over-voltage 507.5kV between the contacts as the second lightning strikes. On one hand, the simulation method for post-arc dielectrical breakdown is validated; on the other hand, it can be confirmed that the second lightning caused the thermal state dielectrical breakdown directly. Further study show that the critical breakdown voltage tends to increase as contact rebound decreases, and decrease as arc duration gets longer.
The simulation method for the post-arc breakdown properties of SF6 HVCB proposed in this dissertation could provide effective technical means for the improvement, design, and performance analysis of GCB. It also helps to enrich the arc theories of high-voltage apparatus.
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