Experience in VLF Testing of Stator Winding Insulation

There are limited diagnostic testing techniques for the stator winding insulation of large hydro-generators. Some popular AC insulation tests cannot be applied to a large hydrogenerator since a large AC power source is required to energize the stator winding with a high capacitance. This challenge brings difficulty and a high cost for AC diagnostic tests for a large hydro generator. A 0.1Hz test has an AC component in its test voltage waveform that can be used to perform AC diagnostic tests on the stator winding, such as dissipation factor testing. Since VLF (Very Low Frequency) test equipment is portable, it can be easily taken to a site to perform testing. In this paper, VLF diagnostic testing was applied to a hydro-generator stator winding, such as VLF dissipation factor testing and VLF withstand testing. A comparison test between 0.1Hz and 60Hz tests was also conducted to check if a VLF test result can provide a similar one to a 60Hz test result. The paper presents the 0.1Hz and 60Hz test data, VLF withstanding test data, and condition assessment of a hydro-generator stator winding.