Heat is the prime cause which leads to electrical insulation deterioration. Some possible causes of heat generation in the electrical insulation are as follows:
- Heat is generated in the electrically stressed insulation, due to dielectric loss and conduction currents.
- Heat is imparted to insulation by the neighboring current carrying parts.
Now if we look at the dissipation of heat from the insulation, then heat is lost in insulator by conduction, convection and radiation. Special cooling facility is provided wherever necessary. As the resistivity of some dielectric materials decreases with increase in temperature whereas the loss angle increases. Thermal equilibrium implies a stable temperature at which the heat lost is equal to heat gained. If thermal stability is lost (say, due to inadequate cooling), the temperature rises indefinitely, leading to insulation failure.
Every 10°C rise in temperature above the limit, reduces the insulation resistance as well as its expected life by around half. At prolonged high temperature, the organic insulating materials start to dry and become brittle, which leads to build up of tracks.
Following are the main causes of electrical insulation deterioration:
Formation of Track
What is Track in Electrical Insulation?
Tracking is formation of permanent conducting path along the surface of insulator. Following are the causes of track formation in electrical insulation:
- Degradation of surface due to sparking.
- Presence of conduction film along the surface of insulator due moisture, dirt, salt layer, etc.
- Mechanism of sparking along the surface arising due to breaking of leakage currents along the surface.
Presence of Oxygen and Humidity
Some organic and inorganic insulation material oxidise in presence of oxygen, ozone particularly when exposed to light. Polyethylene oxidises when exposed to bright sun-light. Rubber oxidises and cracks when exposed to light.
Some materials absorb moisture and lose electrical and mechanical strength under humid conditions due to hydrolysis, polythelene, cellulose esters, other polyesters are typical examples of such materials. Thus oxygen and moisture lead to insulation deterioration.
Excess Electrical Stress
Electrical Stress is one of the most important reason for insulation deterioration. The discharge occurs on the surface or within the insulator wherever the stresses exceed breakdown value. The surface discharge is caused by higher electrical stresses than permitted value.
Chemical Deterioration
The stability of organic insulation material increases with time and temperature. Paper losses its mechanical strength within a few days at temperature of the order of 150°C, even if it is protected from moisture and air. Oxygen and moisture cause rapid deterioration at such temperatures.
Moisture causes deterioration of transformer oil and other insulation materials whether solid / liquid / gaseous. Moisture creates special problems in enclosed equipment. The moisture gets condensed on insulator surface and cause flashover.
At temperature of 400-500°C, mica products slowly deteriorates, both electrically and mechanically.
Mechanical Stress
Some components are weak in tensile load, some are weak on impact load, and therefore components may fail due to excessive mechanical stresses. Vibration are unavoidable at many places in industries and this is one of the main reason for insulation failure.
Incompatibility of Dielectric Material
Incompatibility means not suitable to be used together. Some dielectric materials are not suitable in particular assemblies because of their incompatibility with surrounding materials. For example, some synthetic materials deteriorate rapidly when placed adjacent to current carrying copper at temperature of about 80° C. Thus compatibility is one thing which must be taken care while selecting the material for insulation.