Real-life case study

A flashover between the insulator and the pole caused the line to trip out at the zone substation and when power was restored, a visual inspection of the line failed to show up any defects.

what tripped the 22kV line?
".. We were inspecting a 22kv power line that at the start of the power line the cable comes out from the substation underground and then up a concrete and steel pole to become an overhead line. When this line was constructed, the centre phase stand off insulator was installed too close to the metal upright section of the pole. Either inclement weather or a bird has caused a flashover between the insulator and the pole, causing the line to trip out at the zone substation. Once power was restored, a visual inspection of the line failed to show up any defects. The line had tripped a few times since and then we were asked to inspect the feeder. Thermal Testing found no thermal faults, however Thermal Testing found what appeared to be a very minor corona discharge on an insulator. On closer inspection, Thermal Testing found the insulator that had been the cause of the intermittent outages. A close-up of the insulator using the zoom feature of the corona camera.		The corona we detected The weather on this day was a clear fine day with low humidity and about 18 degrees Celsius. A day where you would not expect to find any corona on 22kv equipment. Paul Landman Thermal Testing Services Pty Ltd. December 2004 Paul Landman Thermal Testing Services Pty Ltd. December 2004