On-line calibration of partial discharge monitoring for power cable by HFCT method

Abstract

Partial discharge (PD) detection is an effective method for finding insulation defects in HV and EHV power cables. PD apparent charge is typically expressed in picocoulombs (pC) when the calibration procedure defined in IEC 60270 is applied during off-line tests. During on-line PD detection, measured signals are usually denoted in mV or dB without transforming the measured signal into a charge quantity. For AC XLPE power cable systems, on-line PD detection is conducted primarily with the use of high frequency current transformer (HFCT). The HFCT is clamped around the cross-bonding link of the joint or the grounding wire of termination. In on-line occasion, PD calibration is impossible from the termination. A novel on-line calibration method using HFCT is introduced in this paper. To eliminate the influence of cross-bonding links, the interrupted cable sheath at the joint was reconnected via the high-pass C-arm connector. The calibration signal was injected into the cable system via inductive coupling through the cable sheath. The distributed transmission line equivalent circuit of the cable was used in consideration of the signal attenuation. Both the conventional terminal calibration method and the proposed on-line calibration method were performed on the coaxial cable model loop for experimental verification. The amplitude and polarity of signals that propagate in the cable sheath and the conductor were evaluated. The results indicate that the proposed method can calibrate the measured signal during power cable on-line PD detection.