Experimental investigation and signal processing techniques for degradation assessment of XLPE and PVC/B materials under thermal aging

Operating conditions subject power cables insulation to a variety of stresses. Among them, the thermal constraint is considered as one of the most severe factors causing irreversible damage to the electrical insulation. In light of this, accelerated thermal aging experiment was performed on cross-linked polyethylene (XLPE) and Polyvinyl chloride class B (PVC/B) samples. This paper deals with an experimental study based on the measurement of the AC current through the insulation bulk. First, the waveform in addition to the RMS value of the current were analyzed at different aging stages and for five AC applied voltages ranging from 3 to 12 kV. Fast Fourier Transform (FFT) spectral analysis is employed to assess harmonic contents of the current signal and evaluate its total harmonic distortion (THD). Further in-depth analysis was proposed by decomposing the current into several time-frequency bands using the Discrete Wavelet Transform (DWT) method, then Standard deviation-Multi Resolution Analysis (STD-MRA) representation is adopted for the sake of determining the details that correlate with the degradation level of the both studied materials. The obtained results show that our experimental-signal processing techniques methodology could be an efficient diagnostic tool for monitoring the degradation process of insulation materials used in extruded power cables.