Breakdown by treeing and tracking

Treeing and tracking are long-term breakdown mechanisms of solid insulation in service. Both progress slowly, step by step, by small discharges at moderate stress, until a conducting or hollow path bridges the electrodes.

Breakdown due to treeing

Treeing is the formation of fine, branched hollow channels inside the solid, which look like a tree or bush.

Cause and process:

1. At a sharp point, protrusion, metal particle, void or contaminant, the local field is several times the average field.
2. Partial discharges start there. Each discharge erodes the material by electron/ion bombardment, heat and chemical attack (ozone, nitric acid).
3. A thin channel forms; its tip again has high stress, so discharges continue and branch out.
4. The channels grow over months or years and form a tree. When a branch reaches the other electrode, the insulation fails.

   needle / protrusion (HV)
            |
           /|\
          / | \     branched channels
         /\ | /\    (electrical tree)
        /  \|/  \
   -----------------------
       earthed electrode

Types of trees:

Water trees grow at low stress and later turn into electrical trees.

Prevention: smooth semiconducting screens, clean and dry materials, void-free extrusion, tree-retardant XLPE.

Breakdown due to tracking

Tracking is the formation of a permanent conducting carbonized path on the surface of an insulator between electrodes.

Process:

1. Dust, salt, industrial pollution and moisture form a conducting film on the surface.
2. Leakage current flows through the film and heats it; the film dries unevenly and dry bands form.
3. Voltage concentrates across the dry bands, and small arcs (scintillations) strike across them.
4. The arcs decompose organic material, leaving carbon. This carbon track is conducting.
5. The track grows a little with each wetting/drying cycle until it bridges the electrodes; surface flashover and failure follow.

  HV electrode ====[ surface film ]==== earth
                  ~~~##~~~~##~~~~
                     dry bands + arcs
                  =====######====== carbon track grows

Features:

• Occurs mostly on organic insulators: resins, epoxies, polymers, bakelite.
• Porcelain and glass do not carbonize, so they resist tracking (but may suffer erosion).
• Measured by the comparative tracking index (CTI) test.

Prevention: track-resistant materials (filled silicone rubber, cycloaliphatic epoxy), longer creepage, hydrophobic coatings, regular cleaning.

Difference