The impact of humidity on partial discharge activity of large rotating machines

On-line partial discharge (PD) measuring has become a powerful tool for condition assessment of insulation system of large rotating machines. Monitoring the trend in PD over time is usually used for interpretation of data but since the PD activity could be affected by operational and environmental factors it is strongly recommended that the periodic PD tests should be performed in the same conditions in order to achieve a valid trending. Operational conditions of machines (voltage, load, etc.) could be controlled more effectively than environmental factors (e.g. humidity). This paper shows the result of an investigation on how humidity affects the partial discharge activity on different insulating systems. Laboratory experiments were performed on 3 different bars/coils in two different ambient conditions and the results are compared to reference data gained from tests in standard room conditions.