High voltage DC ramp testing as a diagnostic of stator insulation condition

Machine users have used high voltage DC ramp testing of stator winding insulation for almost 40 years. Traditionally, this test has proved useful in assessing the condition of the groundwall insulation and in finding defects such as tape separation and lack of resin cure. Recently, some organizations have claimed that the high voltage DC ramp test may also be used to detect delamination in the groundwall insulation. This concept has also been incorporated into the latest revision of IEEE standard 95. However, to date, no theoretical basis exists to explain the effect on the V-I characteristic attributed to internal groundwall delamination. Thus, the use of DC ramp to detect internal delamination in groundwall remains somewhat controversial. A recent EPRI study involving high voltage DC ramp testing in the field and under laboratory conditions was performed as part of an investigation into the efficacy of the test as a detector of delamination inside groundwall. This work describes the investigation and the results obtained.