Bypass monitoring for PD interferences of ±800 kV converter transformer

Partial discharge (PD) level under the specified AC/DC test voltages, which is one of the key parameters indicating the insulation reliability of ultra high voltage direct current (UHVDC) converter transformers, cannot be accurately determined due to the impact of the external electromagnetic interference. To solve this problem, we put forward a method that uses the ultra high frequency (UHF) technology to bypass monitor the PD interferences, and applied this method to the factory tests of a ±800 kV converter transformer. The results show that the waveform characteristics of the UHF detection signals can be used for recognizing the interferences caused by communication equipment and the other industrial equipment, as well as the phase distribution characteristics of the UHF detection signals for the floating potential discharge and the point discharge induced by various types of AC/DC test voltages. The results also confirm that the floating potential discharge spots and the point discharge spots can be quickly and accurately located by analyzing the arriving time difference of radio-frequency signals at the different sensors of the UHF antenna array. Based on the monitoring results, the PD interference sources were well treated and then the ±800 kV converter transformer passed the partial discharge measurements. The research results will be further applied to the similar works of the UHVDC converter transformers in the converter stations.