Very low frequency methods to detect discharge activity in large machines

This paper describes the outcome of a review of technical publications and other information concerning the development of very low frequency (VLF) test methods for HV insulation systems, and the use of those methods to investigate discharging in HV rotating electrical machines. The majority of papers concerned the development of VLF power supplies and their application to cable testing. ERA has several years of experience of applying VLF techniques to cables. There were very few papers concerning VLF tests on large machines. This paper is an attempt to draw together the development of this technology