Return voltage measurements on paper insulation of windings and cables

The RVM method is capable of detecting moisture in paper insulation, but its sensitivity is highly dependent on the geometry of the test object. The sensitivity depends on the electrode distance through oil and seems most sensitive when both electrodes are touching the paper insulation. In a practical transformer geometry, detection of moist winding insulation between windings with dry insulation, may be very difficult. The time of the maximum of the RVM spectrum is not affected by a variation of the input impedance of the measuring system, within reasonable bounds, of course. Because of extensive measuring times, the RVM method may be unpractical for paper insulated cable, as far as RVM spectra are concerned. Attention should then be focused on single or small sets of RV curves, instead of RVM spectra.