Factors on Which Life of Insulation Depends

Factors on Which Life of Insulation Depends

Q. State the factors on which life of Insulation depends.

1. Temperature : The insulation resistance decreases with temperature. The dielectric loss increase with temperature. The resistivity of the insulating material decreases by increse in temperature. The same can be expressed as 

R_t  =  R₀
(1 – α₀t
),                   e_t
 = 
e₀ (1 – α₀t )

R_t = Resistance of material at t⁰c,

R ₀= Resistance of material at 0⁰c

α₀ = Resistance
temperature coefficient at 0⁰c

e₀ = Specific resistance 0⁰c

e_t = Specific resistance t⁰c

1. Absorption of moisture : The absorption of moisture by insulting material decrease its surface resistance. It causes insulation failure or breakdown when they are exposed to atmosphere. The moisture affects the mechanical strength of the insulating material. 
2. Mechanical stresses : Mechanical stresses are developed in the material due electromechanical forces, stresses during running condition , which is true for rotating electrical machines. It develops very small cracks in the insulating material. Hence reduces the life of insulating material.
3. Thermal stresses : The heat is produced in the insulating material due to dielectric loss, copper loss in the conductor. Heat produced by copper loss in the conductor is absorbed by nearby insulating material. If sufficient and proper ventilation and cooling is not provided, then the insulating material looses its thermal stability which reduces the life of insulating material. Thermal stability means, the heat developed in the insulating material should be equal to heat thrown out by the insulating material.
4. Electrical stresses : During lightening heavy impulse surge voltages come across the insulating materials, even switching produces voltage surges. Such transient voltage when greater than the breakdown voltage of insulating material, punctures or breaks insulating material and the insulating material looses insulating property.
5. Deposition of dust, dirt, oil : The surrounding atmosphere contains dust, dirt particles. These are accumulated over the electrical machine and the dirt, dust reduces the insulation resistance. Thus the insulation becomes weak.
6. Impurities : Impurities present in the insulating material reduces the insulation resistance as well as the dielectric strength. All above factors are affecting the life of insulating material.