Modern Noise Rejection Methods and their Applicability in Partial Discharge Measurements on HVDC Cables

With an increasing amount of HVDC transmission lines in operation, the length of installed polymer insulated HVDC cable is rapidly growing. While clean production processes and routine testing provide defect free cable segments, the reliability of the complete cable system strongly depends on the quality of workmanship where joints and terminations are inevitable. Just as for HVAC cables, the most sensitive way of detecting localized assembly defects is the measurement of partial discharge (PD) signals during the test after installation and PD monitoring throughout later operation of the cable. The validity of such a measurement is often determined by the ability of rejecting noise signals of higher frequencies, which are especially significant during on-site testing. Since phase correlation, the standard method in AC-PD measurements, is not possible for DC and the overall PD activity is small compared to AC, only a few of the common noise rejection methods are applicable. Out of these, the research presented in this paper shall describe TF-Mapping and wavelet transformation as promising noise rejecting methods which work both under AC and DC voltage and are currently under research by different institutions. Furthermore, HVDC PD measurements, performed on a polymer insulated cable system under laboratory conditions, will be presented and the described methods will be applied. The results can be used to identify important aspects when performing HVDC PD classification and noise rejection.