New approach for air humidity correction factor under positive switching impulses for indoor applications

In high voltage equipment, bus terminals and interconnectors, it is common the use of rounded electrodes to control high electric fields, improve the air insulation strength and be able to provide compact designs indoors. The dielectric strength of external insulation depends on several parameters such as type of overvoltage stresses, geometry, location and surroundings, and the effect of the combination of atmospheric conditions such as humidity, pressure, and temperature, among others.Usually, the equipment site location has conditions different from standard or normal service design conditions, e.g., maximum ambient temperature and altitude not exceeding 1000 m.a.s.l. For the sake of achieving proper testing of high voltage apparatus, atmospheric correction procedures are recommended in the international standards to translate site or laboratory conditions to standard atmosphere conditions. These corrections are performed to temperature, humidity, and atmospheric pressure. The complex process behind the physical phenomena of the atmosphere under different conditions like transient overvoltages, and electric field conditions, is still not fully understood. Different empirical or semi-empirical approaches are recommended in the international standards, to achieve such corrections. Investigations have shown that the recommended correction procedures in the standards need to be improved and they are not applicable for all kind of electrode geometries.The present paper is dedicated to study the atmospheric corrections used in external air insulation for different ranges of absolute humidity and various electrode configurations under positive voltage switching impulses. Mathematical fitted approximations to achieve atmospheric corrections for electrode arrangements type sphere-plane and toroid-plane are here analyzed. The new recommended equations provide higher accuracy to bring site conditions to standard atmospheric conditions for the tested geometries than the recommendation given in IEC standards.