Seasonal changes in partial discharge activity on hydraulic generators

Six hydraulic generators in two separate power stations were equipped with partial discharge cable type couplers for long term PD monitoring. The stator windings used four different insulation systems from three different manufacturers. Partial discharge activity was monitored monthly on each generator for periods of time ranging from 2 to 9 years. PD data was acquired in the frequency range of 40 to 350 MHz. External noise pulses were filtered out using the differential method with two cable couplers per phase. The present paper is a case study showing how PD activity varies dramatically depending on the time of the year the online testing was performed. A ratio higher than 100 was recorded between winter and summer readings. Winter readings, (January-March) were always higher than summer readings (July-September). A strong correlation was found between temperature, atmospheric absolute humidity levels (water partial pressure in mm Hg) and the well-known PD basic quantities such as Q/sub max/ and NQN values. An interpretation of the monthly-acquired data is offered based on both external and internal PD activity.