Partial Discharge calibration using frequency domain measurement in power cables

Partial Discharge (PD) calibration is always a challenge in power cable diagnosis, which is particularly difficult in online PD testing. The conventional calibration procedure requires injecting a calibration pulse into the tested cable, which indicates only offline PD testing system can be calibrated. The calibration procedure assumes the measured PD pulse magnitude is linear to the charge value, which is true only if the measuring bandwidth is close to the DC component of the PD signal spectrum. However, the PD pulse bandwidth in a power cable varies with the distance between the testing location and the discharge source due to the frequency dependant attenuation in the power cables. Therefore, even in offline PD test, depending on the bandwidth of the PD testing device, the measured PD pulse magnitude is not always linear with the charge value, and significant error would be expected if the PD test is calibrated with the conventional procedure. A patented "calibration" method is introduced to obtain the apparent discharge value from the measured PD spectrum in frequency domain, which overcomes the drawbacks of the current calibration procedure. The proposed "calibration" does not require injecting a calibration pulse into the tested cables. This method reconstructs the time domain PD current pulse waveform from the measured PD spectrum with a reasonable assumption. Since the reconstructed PD current pulse does not contain interferences and has ideal waveform, the PD charge can be obtained by integrating the reconstructed PD current pulse without many difficulties. Moreover, this method is possible to be applied in online PD testing, as no calibration pulse is required to be injected in the cables with this process.