Breakdown Due to Treeing and Tracking

Breakdown Due to Treeing and Tracking

• Tracking is the formation of a permanent conducting path across the surface of an insulation.
• The spreading of spark channels during tracking in the form of the branches of tree is called treeing.
• The conducting film is usually moisture from the atmosphere absorbed by some form of contamination such as carbon cells, dust, industrial deposits, cellulose fibers etc. Conducting path can also arrive from metal dust.
• Consider a system of solid dielectrics having conducting film and two electrode units surface. On application of voltage, the film starts conducting resulting in generation of dust and surface starts becoming dry the conducting film becomes separate due to drying and sparks and drawn which damage the dielectric surface.
• With organic insulating materials such as paper and Bakelite, the dielectric carborizes at the sparking regions. The carborized portion acts as a permanent conducting region resulting in an increase of stress over the rest of the portion between electrodes.
• This process repeats if the environment is such as to from a conducting layer again (like a humid atmosphere). Thus due to repatriation insulation failure occurs when carborized tracks bridge the distance between the electrodes.
• This phenomenon is common between layers of Bakelite paper.
• Tracking can occur at as low voltages as 100 V and mainly caused by high current density at spark tips. In high voltage tracking, the conducting path begins by the impinging of a discharge on the dielectric surface.
• Slow deterioration and ultimate failure may be caused by several process slow erosion by ionic decomposition, chemical degeneration by discharge and loss of mechanical strength and thermal instability.

Therefore to prevent tracking

1. Insulation should be kept at clean and dry surface
2. Clean and dry environment
3. Use to track resistance material sometimes moisture replacements gases are used.

Treeing

• This occurs due to erosion of material at the tip of the spark.
• Erosion results in the roughening of the surface and hence becomes a surface of dust accumulation.
• This causes increased conductivity resulting either on the formation of a conducting path bridging the electrodes as in mechanical failure of the dielectric.
• When a dielectric material lies between two electrodes there is possibility for two different dielectric media the air and dielectric to come in series voltage across the two media are shown in Figure.

V₁ = V d₁/d₁+( ∈₁/
∈₂)d₂

• Hence more voltage is concentrated on across d i.e. across air gap. The sparking will occur in air-gap and charge accumulation takes place on the surface the insulation sometimes spark erodes the surface of the insulation.
• As time passes breakdown channels spread through the insulation in an irregular “tree” like fashion leading to formation of conducting channels. This is observed in cables.
• Treeing requires high voltage
• To avoid treeing care must be taken to see that surface is clean, moisture free, dry.