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High Voltage Direct Current (HVDC) systems are increasingly being adopted around the world to integrate renewable resources, increase power transfer, and allow flexible grid operation. HVDC overhead lines play a critical role in this, and focus is needed on the research of composite outdoor insulators failure. In the case of a pole to ground fault of a bipolar scheme, the healthy pole experiences a slow transient overvoltage similar to a Switching Impulse (SI) superimposed on the operational DC voltage. However, there is no standard test for such voltage superimposition. This paper investigates the performance of a composite insulator stressed with such superimposition under dry and rain conditions. To determine the direct voltage influence on the flashover voltage, the results have been compared with the SI only case. Four different rain conductivities have been used in the test program because these insulators can be exposed to different acid conductive rain types, typical of industrialized environments. Both vertical and horizontal orientations have been studied. It was possible to conclude that, in most cases, the direct voltage pre-energization has a negative impact on the flashover performance. The increase in rain conductivity leads to lower flashover values, and the horizontal orientation outperforms the vertical configuration. Under these test conditions, the flashover voltages were all larger than a typical DC operational switching surge. Thus, in the case of pole to ground fault, the healthy pole failure risk appears to be low. However, further research is needed on higher voltages and polluted conditions.

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