Discharge Characteristics of Insulation Defects in GIS Under AC/LI Superimposed Voltage

Gas-insulated switchgear (GIS) might withstand ac/lightning impulse (LI) superimposed voltage caused by lightning intrusion or switching operation during operation. When there are insulation defects, the breakdown probability is greatly increased, which threatens the safety of GIS and needs to be further studied. In this study, the discharge characteristics of two typical insulation defects, including a metallic protrusion on bus bar and foreign matter attached to insulator, under ac/LI superimposed voltage were studied, and the influence of superimposed phase, polarity, and ratio was investigated. It was found that, under a quasi-uniform electric field, the superimposed breakdown voltage (
${U}_{s}$
) was the same as the LI breakdown voltage (
${U}_{\text {LI}}$
). However, when there were insulation defects,
${U}_{s}$
was significantly affected by the superimposed phase and polarity. For the metallic protrusion, the most critical condition was an −LI superimposed at
$\varphi =90$
, with
${U}_{s}$
decreasing by 11% relative to
${U}_{\text {LI}}$
. For the metallic particle attached to insulator, when an +LI was superimposed at
$\varphi = 0^{\circ }$
or −LI at
$\varphi = 180^{\circ }$
,
${U}_{s}$
decreased most, by as much as 19.6% relative to
${U}_{\text {LI}}$
. Finally, the validation test was performed in a 110-kV GIS with insulation defects and
${U}_{s}$
was 7%–14% lower than
${U}_{\text {LI}}$
. The results showed that, under a proper superimposed phase and polarity,
${U}_{s}$
could be significantly lower than
${U}_{\text {LI}}$
of GIS with insulation defects, indicating the great potential of the ac/LI superimposed voltage test for the high-sensitive detection of GIS insulation defects.