Experience with high potential testing hydro generator multi turn stator coils using 60 Hz AC, DC, and VLF (0.1 Hz)

There are many different methods of employing a high potential test on a stator winding. Three such methods that this paper will explore with reference to one another are the AC (50-60 Hz), DC, and very low frequency (VLF) (0.1 Hz). Some users choose the AC high potential test knowing that this test best simulates the voltage stress on the winding while in service. Other users prefer the DC high potential test largely due to ease in performing the test. However, the DC voltage does not stress the stator coils the same way as when they are in service and may result in overly pessimistic results due to the influence of surface contaminants in the end windings. Finally, the VLF test, due to recent advances in technology, is becoming more practical for use in field conditions. However, the present standard governing the test is almost 40 years old and there is significant interest in what VLF voltage level best correlates with the AC and DC high potential tests. This paper reports preliminary test results on three generator windings that were destructively tested using the AC, DC, and VLF methods as part of an ongoing effort to provide a database upon which to set the appropriate VLF hipot level for modern synthetic resin-based stator insulation systems.