Case Studies

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• Punctured insulators• Hollow core insulators• Wooden pole fire • Switchgears• Bird chewing

Punctured Insulators

HQ Laboratory TestHQ Laboratory Test

Field ExperienceField Experience

Punctured Insulator

Hollow Core Insulators

Hollow Core - Sealing DefectsHollow Core - Sealing Defects

Indication The problem

The core should be sealed at the bottom with a cork/rubber plug.The cork/rubber plug was cracked compromising the seal. Without the seal, moister was introduced into the hollow core.

The dielectric properties of the insulator were changed affecting the electric field. The condition would eventually lead to insulator failure.

Wooden Pole Fires

ExamplesExamples

Discharge on dead end

Arcing to a metal bar supporting the pole

Arcing to a bolt holding cross arm

Discharge in the middle of insulator

The ProcessThe Process Contamination that accumulates on surfaces, in cracks of insulators and on cross armsmay become conductive in the presence of moisture such as dew, fog & high humidity.

This conductive material may provide a path for leakage currents to flow from the energized conductors to earth.

Such leakage currents may flow on the surface within a cross arm and will collect and concentrate on the metal bolt that attaches the cross arm to the pole.

If the bolt is not grounded, the bolt will get hot. This heat can lead to a chemical reaction with thepreservatives in the wooden cross arm which can lead to wood pole fire.

CatalystsCatalysts Old wood

Allowing higher amount of leakage current as compared to the new wooden structures [=mental fatigue] Old=20 to 30 years

Wet wood

Transmitting more leakage current than dry wood The rain is enough to reduce the electrical resistance of wood

Moisture

Reduces the wood’s electrical resistivity

Switchgears

Switchgear ExperienceSwitchgear Experience

Once corona becomes active, it leaves behind a conductive “tracking” path on surfaces and also creates a very conductive cloud of air around itself. A flash-over can occur once a tracking pathway is completed from phase to phase or phase to ground. It can also occur from the conductive cloud of surrounding air once it finds a path to ground.

[Testing Distribution Switchgear for Partial Discharge in theLaboratory and the Filed”, IEEE 2008]

What Causes Corona in Switchgears?What Causes Corona in Switchgears?

Geometric factors such as sharp edges on conductors, connections and switchgear cabinet components.

Spatial factors such as small air gap spaces between conductors and switchgear cabinet components.

•Conductors being tie-wrapped together•Conductors touching insulators, cabinet's edges •Non-shielded cables in contact with grounded surfaces•Bus bars in close proximity to insulation board

Environmental conditions

• Contamination in the form of dust, oil/fluid and other particulates on conductors and insulators will create corona.

• Switchgear rooms that are hot because of poor air circulation and cabinet enclosures that are subject to wet/humid conditions are more likely to have corona and tracking activity in comparison to those cabinets that are cool and dry.

What Causes Corona in Switchgears?What Causes Corona in Switchgears?

UV Signs in SwitchgearUV Signs in Switchgear

Bird Chewing

Bird Chewing – E FieldBird Chewing – E Field

An 110kV post insulator with bird damage on sheds.The core is exposed due to sheath damage.

Chewing damage caused by parrots in highly polluted area is another reason to perform corona inspection as it changes the E field distribution over the insulator.

Parrots sometimes position themselves on the grounded terminals of HV post insulators and chew their path along the sheds of the insulator towards the energized terminal.

Once the electric field approaches levels that are irritable to the birds, they flee the insulator leaving only a small portion of its sheds operational.