Online partial discharge measurement of a high-voltage direct current converter wall-bushing

An online Partial Discharge (PD) measurement performed on a High Voltage Direct Current (HVDC) wall bushing successfully identified the presence of internal discharges. The wall bushing is a sulfur hexafluoride gas-insulated bushing, rated for 500kVdc and terminated on a thyristor controlled HVDC converter bridge. The measurement of PD within the HVDC station environment is particularly challenging due to the high levels of electromagnetic noise caused by thyristor switching events, and external air-corona from the neighboring high-voltage equipment. An additional challenge is the “mixed“ voltage stress on the bushing insulation, which has both ac and dc high-voltage components. There are also fast transients during the firing of thyristors in the HVDC conversion process that cause added stress to the insulation. As a result, the analysis and interpretation of PD data for HVDC equipment is more complex; PD pulses may occur in response to the ac, dc, or switching transient voltage stresses. In this paper, an online PD measurement strategy for noise filtering and isolation of PD sources within the bushing are discussed. The PD measurement data is plotted on a phase-resolved diagram where the line cord voltage was used as a reference. The phase-resolved diagram appears to suggest that the fast transients caused during switching, trigger some PD events. The findings from the online PD measurements are verified with physical evidence, found after the bushing was removed from service.