Influence of suspension patterns of UHVDC line composite insulator on its impulse flashover characteristics

V-string insulators are widely used in transmission lines, but the difference of impulse flashover characteristics between V-string insulators and I-string insulators remains unclear. To this end, we carried out discharge tests of ±800 kV V-string and I-string composite insulators under lightning and switching impulses at the (Kunming) National Engineering Laboratory for UHVDC Engineering Technology. The electric field distribution around the tested insulators was calculated and analyzed using the finite element simulation software as well. The results show that, compared with I-string insulators with the same effective insulation distance, the 50% impulse discharge voltage of V-string insulators is higher under negative impulses, but is lower under positive impulse voltages. The electric field distribution of the air gap between grading ring and cross arm in a V-string is similar to that of a rod-plate air gap, while the electric field distribution of the air gap between two grading rings in an I-string insulator is similar to that of a rod-rod gap. It is concluded that the difference of impulse flashover characteristics between V-string and I-string insulators is caused by the polarity effect of different electrode structures. ©, 2015, Science Press. All right reserved.