Unexplained Flashovers on High Voltage Direct Current Transmission Lines under Negative Polarity Voltages

In this paper, evidence of unexplained flashovers on the negative pole of a bipolar HVDC transmission line system is provided, both historical and present, with prevailing environmental conditions. The unexplained flashovers occur in under high temperature, low humidity, and low wind conditions. Finite element modelling is utilized to demonstrate the impact of conductor to ground clearance and conductor configuration on the mid-span conductor surface electric field intensity. Hypotheses are developed to understand the cause of the unexplained flashovers. This paper also presents short-term mitigation recommendations and directions for future research.