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The negative onset corona voltage of high-voltage direct current bare overhead transmission conductors has been studied by numerical calculations and gas discharge experiments. First, the charge simulation method, an accurate numerical method, is employed to calculate the distribution of the electrical fields in the vicinity of a bare conductor. Then, the electrical field values calculated and a criterion from the gas discharge theory are utilised to evaluate the onset voltage of the negative corona on the bare conductor. Furthermore, a formula for a high altitude correction factor is used to calculate the negative onset corona voltage of a bare conductor located at a high altitude. Finally, it is found that the calculated onset-voltage values agree satisfactorily with the data measured on laboratory models and full-scale test lines.