Basic study on applying a new partial discharge diagnosis method to capacitor voltage transformer

Summary

Partial discharge (PD) characteristics were studied using a simulated capacitor to develop a new degradation diagnostic technique for a capacitor voltage transformer (CVT). The technique can determine the presence of PD in CVT by focusing on the frequency components of electromagnetic (EM) waves associated with PD because our group discovered that frequency components vary according to the insulators. Therefore, we expected that characteristic frequency components will appear in radiated EM waves if a PD occurs in a CVT. As a result, we found that frequency bands around 300 MHz appear at the rising part of the EM waves associated with a PD in a capacitor, although they do not appear in the creeping discharge. These analyses were performed using wavelet transformation. In addition, we found that the PD charge and EM wave strength increase with time. From these results, we showed that the occurrence location of a PD and the degradation state of a CVT can be diagnosed by measuring the EM waves. On the other hand, we were apprehensive that the EM waves in a CVT will not propagate outside due to shielding by foil electrodes. Therefore, the EM wave propagation in a CVT was investigated using numerical simulation. As a result, we found that EM waves can easily propagate outside. EM waves propagate among the foil electrodes without loss because the foil electrodes function similar to a waveguide. Therefore, if a PD occurs in the same gap in the electrodes, the EM wave strength is not different whether the PD occurs at either the edge or at the center. Furthermore, dissolved-gas analysis of the insulating oil was performed for comparison. In the dissolved-gas analysis of the insulating oil, C2H2 is generally used as an indicator of the abnormality determination caused by a PD. However, C2H2 was not detected in our test. Moreover, in the other gas, the change in the PD did not also significantly appear compared with that in the EM waves. The amount of charge in the PD that occurred in the capacitor in our test was small. However, we considered that each capacitor used in our test had broken down with time. Our test results show that the EM wave measurement is more sensitive than the dissolved-gas analysis for PD diagnosis.

Additional informations

Authors

SCHURCH, ORELLANA

Keywords

Capacitor voltage transformer, Partial discharge, Electromagnetic waves, Degradation diagnosis