Characterization of Ageing for MV Power Cables Using Low Frequency Tan δ Diagnostic Measurements

Abstract

This paper describes very low frequency (VLF) tan delta experiments performed on field-aged and non-aged distribution medium voltage (MV) cable samples. The field aged samples constitute a uniform set of Cross-linked Polyethylene (XLPE) of 15 kV unjacketed cables removed from the same service area having experienced similar operating and ageing conditions. The non-aged samples are a diverse set of crosslinked polyethylene (XLPE) and water tree retardant cross-linked polyethylene (WTRXLPE) cables of 15 kV and 25 kV and Ethylene Propylene Rubber (EPR) cable of 25 kV. The experiments are designed to contribute in understanding, time, voltage and discharge time dependence of Tan delta diagnostic measurements at VLF of 0.1 Hz. Results help in clarifying issues that arise when characterizing MV cable insulation by Tan delta diagnostic measurements. The issues include time-on-test, voltage level as a diagnostic tool, diagnostic features, and reproducibility and repeatability of the measurements. The paper shows that higher insulation losses, non-linearity, hyteresis, and variation in voltage and time of Tan delta diagnostic measurements at VLF are indicators that can be used to properly characterize the insulation and enhance the diagnosis.