Correlation between Tan [delta] diagnostic measurements and breakdown performance at VLF for MV XLPE cables

The main contribution of this paper is to investigate the correlation between Tan [delta] diagnostic measurements at 0.1 Hz (Very Low Frequency-VLF) and VLF breakdown performance for Medium Voltage (MV) cable samples through a laboratory test program. The cable samples used are a set of 15 kV, Cross-linked Polyethylene (XLPE), unjacketed cables removed from the field, the same service area, and having experienced similar operating conditions for almost four decades. The test program includes Tan [delta] measurements at different voltage levels and a subsequent VLF extended step-withstand test. The VLF step test allows the evaluation of risk of failure during VLF Tan [delta] testing and assessment of the ultimate performance of the cables. The Tan [delta] diagnostic measurements are represented by the Tan [delta] value and Tip-up, which are considered the classical metrics. However, the paper also suggests the use of a new additional diagnostic feature that takes into account the scatter in the Tan [delta] measurements for a particular test voltage level. Index Terms--Dissipation factor, Tan [delta], VLF breakdown, power cables.