capacitive bus couplers were installed on four 6 MW global-VPI turbine generators which were mounted on an oil platform. Capacitors were installed in the switchgear as well as within the generator terminal box. The generators were connected to the switchgear by means of about 50 m of shielded power cable. None of the stators had significant PD activity. However there was considerable external noise. Figure 3 shows both the normal pulse height analysis data (2-dimensional plots),which was confirmed by a visual examination of the stator,indicating that the endturn,at least,were clean and dry
Past experience has indicated that it is prudent to plot both the PD and the electrical noise activity. The pulse height analysis plots reveal that the noise is about 5 times higher than the stator PD. If only a single coupler were used to measure the PD on that phase (instead of two), the stator would have been diagnosed as being in poorer condition than is the case. Further observation of the 3-dimensional plots showed that the electrical noise was primarily occurring at the 0 and 180 portion of the ac waveform on A phase. This was the same for the other three generators. This indicates that there are some poor electrical contacts somewhere on the platform on A phase. Platform personnel confirmed that there had been previous problems with bad electrical contacts