Fundamental aspects of PD patterns of on-line measurements on turbogenerators

This paper discusses partial discharge (PD) measurements performed on turbogenerators using the VHF PD detection technique. With this technique it is possible to perform PD measurements on turbogenerators without interruption of their operation. In addition to PD in any particular phase, the recorded patterns can show crosstalk from other phases and disturbances. The membership of each signal source (phase's own PD pattern, crosstalk, disturbances) to the recorded pattern depends on the selected center frequency. By examination of some typical examples, the relationship between the center frequency and recorded patterns is discussed. Furthermore, PD measurements performed over time on the same generator are studied to discuss possible changes in the spectral responses of particular phases of a turbogenerator. By visual comparison of PD patterns as well as computer-based discrimination tools it was found that several center frequencies are suitable to tune the VHF PD detection technique. However, these center frequencies can change in the course of time for a particular phase of a turbogenerator. As a result, in order to avoid a misjudgment on the insulation condition of turbogenerators, the analysis of on-line recorded PD should not be restricted to just one single center frequency.