Breakdown mechanisms in solids under working conditions

Major breakdown mechanisms in solids

Breakdown of a solid dielectric is the permanent loss of its insulating ability. The major mechanisms are:

1. Intrinsic (electronic) and avalanche breakdown – very fast ($\sim10^{-8}$ s), at very high stress
2. Electromechanical breakdown – mechanical collapse under electrostatic compression
3. Thermal breakdown
4. Electrochemical (chemical) breakdown
5. Breakdown due to treeing and tracking
6. Breakdown due to internal (partial) discharges

The first two occur mainly in laboratory conditions with pure specimens; 3–6 are the mechanisms that occur under actual working conditions.

Breakdown mechanisms in actual working conditions

In practical equipment the working stress is far below the intrinsic strength of the insulation, yet solids fail over months or years. These failures are caused by slow processes produced by the actual working conditions: heat, chemical attack, moisture, surface contamination and partial discharges.

1. Thermal breakdown

• Conduction loss (DC, $W = \sigma E^2$) and dielectric loss (AC, $W = \frac{E^2 f\varepsilon_r\tan\delta}{1.8\times10^{12}}$ W/cm³) heat the insulation.
• Conductivity and $\tan\delta$ rise with temperature, so heating feeds on itself.
• If heat generated exceeds heat conducted away, temperature rises without limit (thermal runaway) and the material melts or chars.
• Worse under AC than DC, with high ambient temperature, poor cooling and thick insulation.

2. Electrochemical (chemical) breakdown

• In the presence of air, moisture and heat, insulating materials slowly undergo chemical change:
  • Oxidation (e.g. rubber and polythene become brittle and crack)
  • Hydrolysis (moisture breaks down paper and some polymers, raising loss)
  • Chemical reaction with ozone and nitric acid produced by discharges
• Under DC, electrolysis of ionic impurities creates conducting paths.
• Rate roughly doubles for every 8–10 °C rise in temperature (Montsinger rule), so life depends strongly on operating temperature.

3. Breakdown due to internal (partial) discharges

• Voids or gas bubbles left in the solid during manufacture have lower permittivity and strength.
• Stress in a void is $\varepsilon_r$ times the stress in the solid, so the void discharges at a voltage well below working voltage.
• Each discharge (pC level) erodes the void walls by ion bombardment, heat and chemical action.
• The void enlarges and develops into channels (trees) until the remaining solid fails.

4. Breakdown due to treeing

• At sharp points, protrusions, contaminants or voids, the local field is very high.
• Small discharges cut fine branched channels into the solid, looking like a tree.
• Electrical trees grow by repeated discharges; water trees grow in polymer cables with moisture at much lower stress.
• The tree progresses step by step and when it bridges the electrodes the insulation breaks down.

5. Breakdown due to tracking

• A surface film of moisture and dirt on an insulator (bushing, cable termination) carries leakage current.
• The current dries parts of the film unevenly; small arcs (dry-band arcs) form across the dry bands.
• The arcs carbonize the surface, leaving a conducting track that grows until it bridges the electrodes, causing surface flashover.
• Organic materials (resins, polymers) are most prone; porcelain and glass are resistant.

6. Erosion and ageing

Surface discharges and corona in air gradually erode the surface; combined with mechanical and thermal cycling, cracks form and lead to failure.