The DC line voltage derivative protection identifies ground faults on the pole bus of the converter station or on the DC line by detecting the polarity of the DC line current. This strategy can effectively detect direct ground faults on DC transmission line, but its reliability is insufficient when dealing with ground faults caused by same-polarity lightning strikes that lead to line insulation breakdown. To solve this problem, this paper conducts a deep research on the behavior of variation power direction during same-polarity lightning strikes and ground faults. Then, it reveals that the variation power direction of the faulty DC line remains constant throughout the process of the same-polarity lightning strikes which cause the line insulation breakdown. In addition, a novel voltage derivative protection is proposed for UHVDC transmission lines, utilizing variation power direction to achieve high immunity to lightning-induced disturbances. Finally, a three-terminal flexible UHVDC transmission model is constructed by using real-time digital simulation system. Simulation results verify the feasibility of the proposed DC line voltage derivative protection.