VLF cable test parameters

During a VLF test an electrical tree at the site of an insulation defect is forced to penetrate the insulation. Inception of an electrical tree and channel growth time are functions of test signal frequency and amplitude. For an electrical tree to completely penetrate the insulation during the test duration, VLF test voltage levels and testing time durations have been established for the two most commonly used test signals, the cosinerectangular and the sinusoidal wave shapes.

The voltage levels (installation and acceptance) are based on most-used practices world-wide of between 2 Uo and 3 Uo, where Uo is the rated voltage for cables rated between 5 kV and 35 kV. The maintenance test level is about 80% of the Acceptance test level. One can reduce the test voltage another 20% if more test cycles are applied (Bach [B2]; Baur, Mohaupt, and Schlick [B5]; Krefter [B16]).  Table 4 and Table 5 list voltage levels for VLF withstand testing of shielded power cable systems using cosine-rectangular and sinusoidal waveforms (Bach [B2]; Eager et al. [B7]; Krefter [B16]; Moh [B17]. For a sinusoidal waveform the rms is 0.707 of the peak value if the distortion is less than 5%.





source : IEEE Std 400.2