The main contribution of this paper is to investigate the correlation between Tan ?? 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 #x003B4;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 #x003B4;testing and assessment of the ultimate performance of the cables. The Tan ?? diagnostic measurements are represented by the Tan #x003B4;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 #x003B4;measurements for a particular test voltage level.
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