Effects of air humidity and temperature to the activities of external partial discharges of stator windings

According to some unexpected partial discharge (PD) test results during off-line measurements of generators it must be assumed, that a relative high humidity content of the ambient air considerably influences the activities of external partial discharges. This assumption could be verified by means of PD tests performed on models of stator bars exposed to various climatic conditions. These tests have shown, that activities of the external PD are strongly reduced, if the ambient air humidity exceeds 50%. Consequently there is a risk of false diagnosis under such circumstances.