Eaton SEFELEC 5 Series User Manual

User manual SEFELEC 5x Series 187 v1.06ENVIRONMENTAL INFLUENCESTemperature, pressure and humidity conditions have an influence on the dielectric tests results.19.3.1. TEMPERATUREThe temperature having an influence on the gas density, this one is altering the performancesof the liquid or gaseous insulating material. The oils, often used as insulation are never pure,the dissolved quantity of gas increases with the temperature and is decreasing the insulationquality of the oil. The large variety of materials used as solid insulation does not allow to de-duce a general rule on their behaviour with the temperature (the insulation specifications hav-ing a tendency to dissipate when the temperature increases).19.3.2. PRESSUREThe withstand voltage in gas changes with the pressure following the Paschen’s law. This lawshows a minimum of the breakdown voltage for a particular value of the pressure by distanceproduct, otherwise the more the pressure increases, the more the breakdown voltage is high.The liquids used as dielectric insulation are influenced by the pressure, the dielectric strengthincreasing with the pressure. In theory, the solid insulations are a little influenced by the pres-sure because this one does not modify a lot their thickness and their internal composition.19.3.3. HUMIDITYThe withstand voltage of gas changes with humidity. In example for the air and for values ofrelative humidity < 80%, the dielectric strength increases a little with the humidity increasing (thewater molecules, more dense than the gas, slows down the avalanche phenomena). The waterpresence in an insulating liquid such as oil degrades the dielectric strength by water electrolysis,development of gas producing partial discharges bringing to breakdown. Under combined effectof the humidity (> 95%) and the temperature (> 100°C) most of the polymers dissipate. The wa-ter can produce inflating in the insulation and create cracks which will facilitate the electric arcsadvance.