Transfer Functions for RF Partial Discharge Signals in Power Cable Joints

Inappropriate manufacturing of cable joints can cause defects in the joint insulation. During operation, partial discharge (PD) may occur at these defects. The development of PD can cause accelerated aging even breakdown of the insulation, resulting in faults in the electric grid. In this paper, for radio frequency (RF) detection of PD in cable joints, a finite-difference time-domain (FDTD) model of a 35 kV XLPE cable joint is established, an excitation source is defined in the model to represent PD of different defects, and then propagation characteristics of RF electromagnetic signals induced by PD in cable joints are studied through the established model. Transfer functions (TF) of RF PD signals obtained with the FDTD model show that both magnitude and shape of the TF are closely related to the location and direction of the excitation source in the model. Two peaks in the magnitude, locating at 380 MHz and 790 MHz, respectively are present in the obtained TF regardless of the excitation source settings. Moreover, the TF magnitude for the excitation source defined in the radial direction of the cable joint model is found to be generally more than one order larger than that of the same source defined in the axial direction. The effectiveness of the FDTD model in representing high frequency properties of the cable joint has been checked against both time domain reflectometry (TDR) and S 21 measurements on a cable joint made in laboratory. The TF results from this work are expected to be helpful for applications of radiometric PD detection to cable joints, e.g., enabling improvement of the detection sensitivity through optimizing frequency responses of RF sensors.