The Research On The Impulse Flashover Characteristics Of ±800kV DC I-string And V-string At High Altitude

The lighting outage rate accounts for a large proportion of the total outage rate of transmission lines. It is significant to improve the lightning protection performance for the safe and stable operation of the power grid. The flashover criteria of the insulators/gaps are an important part of the lightning protection analysis methods. It is obvious that the lightning flashover criterion of the insulator/gap has great influence on simulation results of the lightning withstanding level. In China several UHVDC power transmission lines across high altitude regions while the altitude affect the external insulation characteristic of insulator. On the other hand, V-string insulators are widely used in transmission lines, but their difference of impulse flashover characteristics from I-string insulators is not clear. As a result, the research on impulse flashover characteristics of insulators with different suspension patterns has important engineering application value and scientific significance.Discharge tests were carried on ±800 k V V-string and I-string composite insulators under lightning and switching impulse voltages at the(Kunming) National Engineering Laboratory for UHVDC Engineering Technology respectively. An improved leader propagation model as the lightning flashover criterion of I-string and V-string is proposed based on the leader propagation model recommended by CIGRE and the analysis of the flashover/discharge process of the insulator/gap. The applicability of the new model was verified. The lightning withstand level for ±800 k V UHVDC transmission lines were calculated using the improved leader propagation model and the influence of the modelling methods of insulator/gap on the lightning withstand level was analyzed. At last, the electric field distribution close to the tested V-string or I-string composite insulators is simulated. The test results show that the 50% disruptive-discharge voltages U50% of the V-string under negative impulse voltages is higher than that of the I-string with the same effective insulation distance, but under positive impulse voltages lower. An improved leader propagation model is proposed based on the analysis of the leader propagation, the average electrical field of the leader El and the height of the final jump hf are considered in the improved leader propagation model. The improved leader propagation model, which can response the influence of high altitude on lightning withstand level, is more proper as flashover criterions for long insulator/gap than the leader propagation model recommended by CIGRE and the volt/time curve recommended by IEEE at high altitude. The simulation results show that the impulse flashover characteristics difference between the V-string and the I-string insulators is caused by the polarity effect of the electrode structure. The polarity effect of V-string is stronger than that of the I-string with the same effective insulation distance.