A novel calibration method for PD measurements in power cables and joints using capacitive couplers

Partial discharge (PD) measurements are universally accepted as a technique giving some indication of the state of the insulation in high-voltage apparatus. Cable end users are keen to adopt online PD monitoring during commissioning of systems. However, because of noisy environments and the problems of interference the conventional methods are difficult to implement. As a consequence, ultra-high frequency (UHF)/very high frequency (VHF) techniques of on-site detection of partial discharges have been developed. A technique based on capacitive coupling has received much attention. It has been demonstrated that it is possible to use the technique to monitor the partial discharge in cables, particularly in joints/terminations. However, in order to obtain quantitative information about PD, calibration is required for this technique. Existing calibration methods are difficult to implement on-site. In this paper, a novel method is proposed and compared with the conventional method on a short piece of cable. It has been shown that an individual capacitive coupler can also be accurately calibrated on-site and online using the new method, therefore it provides quantitative information about the amount of apparent discharge. In practice, this is important for electricity utilities as the quantitative information about PD can be used to determine the quality of the cable system and to decide whether the system needs to be repaired or replaced.