The most frequent failure in electrical equipment is the degradation and breakdown (flashover) of the insulation. Hence, it is necessary to mention in this module n basic electrical theory the effect of operating environment on electrical machine insulation.
Electrical insulation can be liquid or solid, organic or inorganic. Organic insulation material consists of enamels, varnishes, resins, or polymers that are applied to the steel surface to provide high inter-laminar (between windings) resistance as found on most air-cooled machinery and some oil-immersed transformers.
Larger transformers are oil-filled with pure mineral oil to provide higher insulation capability and more effective heat dissipation when equipped with external radiators, fans and pumps. Physical insulation inside these transformers is often in the form of oil-impregnated paper wrapped around the conductors.
Inorganic insulation material can include a combination of magnesium oxide, silicates, phosphates, and ceramic powder. This type of insulation is usually heat-treated into the surface of the steel and is less common than organic insulation. No matter what the type of insulation, the two most common contributing factors in insulation failure are moisture and heat.
On air-cooled electrical machinery, the moisture content of the air is very important. With aging of the insulation, small hairline cracks will appear in the insulation. Moisture will seep into these cracks and allow an electrical path to short-circuit between adjacent turns of wire.
Although the voltage between the turns is quite small, when they short together, a closed loop to the magnetic flux is provided, and this causes tremendous currents to flow in the shorted loop. This usually destroys the electrical machine, and it has to be removed and re-wound/replaced.
On oil-cooled machinery (i.e., transformers), moisture can only be detected by regular oil samples. Moisture will be sucked into the oil via the oil expansion air vent, through the continuous process of transformer heating and cooling cycles. Special air dryers (i.e., Drycol) and absorbents can assist in decreasing the rate the moisture is absorbed into the oil.
On air-cooled electrical machinery, prolonged high temperature causes thermal aging. This causes the insulation to become brittle. Eventual failure can occur due to moisture penetration as just discussed, or by physical contact of conductors.
In oil-filled transformers the effect is called insulation aging. Chemical aging occurs more rapidly at high temperatures, with the loss of insulation life being almost exponential with temperature. As an example, for a standard 65 deg C (temperature rise) rated insulation the loss of life increases from 0.001% per hour at 1000C to 0.05% per hour at 140 deg C and 1.0% per hour at 180 deg C.