Electrochemical Chemical Breakdown
- These are much related to ‘thermal’ breakdown.
- Temperature rise in an insulating material (due to losses etc.) accelerates the chemical deterioration.
- Electrochemical deterioration is due to the presence and mobility of ions is the insulation which are responsible for leakage current and energy loss in the material.
- In the presence of air and gases some dielectric materials undergo chemical changes when subjected to continuous electrical stresses.
The reasons are :
- Some materials oxidize under the presence of air or ozone. The rubber is the one which oxidizes and cracks in the presence of ozone. Polythene oxidizes in strong day-light.
- Some of materials, mostly organic in nature, show chemical instability or they are operated under high temperature.
- Different insulating materials come into contact with each other in any practical apparatus. Chemical reaction occur between these various materials leading to reduction in electrical and mechanical strengths resulting in failure.
When moisture or water vapour is present or surface of a solid dielectric hydrolysis occurs and the materials lose their electrical and mechanical properties.
- Cellulose ethers and other polyesters hydrolyse and lose their electrical and mechanical properties when they are moist and hot.
- Electrical properties of materials such as paper, cotton tape, cellulose materials deteriorate very rapidly due to hydrolysis.
- Plastics like polyethylene undergo changes and their service life considerably reduces.
Some of the insulting materials get contaminated when they come in contact with chemical substances in the electric equipment.
- Rubber and polypropylene age rapidly at elevated temperature when in contact with copper.
- Celle lose degraded rapidly in the presence of traces of acidic substances.
Glass fabrics made from glass of high sodium content lose their mechanical strength under warm and damp conditions, sodium being very mobile reaches to the surface of the fibres and the strong alkali formed with the moisture present, leads to deterioration.
- It is important that the insulating formers, filmic or wire wound resistors be free from sodium.
- Almost all insulating materials have free ions which are responsibleh for leakage currents in the presence of electric field. Such ions after reaching the electrodes reduce their charge and may also attack the electrode metal evolving some gas or some other substance may be deposited on the electrodes. Such type of activities of the ions are chemically or electrically harmful and many times lead to rapid failure of insulation.
- The rate of electrochemical deterioration is determined by the magnitude of leakage current and other factors like concentration of ions into the insulation, temperature and whether the material is polar one or not.
To avoid electrochemical deterioration
- The impurities should be avoided in the insulating materials.
- It should not be operated at elevated temperatures.
- Care should be taken to avoid contamination in polar materials which otherwise will result in high leakage currents.