For large-scale wind power transmitted by direct current（DC） system， three solutions are discussed including wind power island system transmitted by DC system， wind-coal-bundle island system transmitted by DC system， and wind-coal-bundle interconnected with AC system transmitted by DC system. The security and stability of these solutions are studied， especially the frequency and voltage stability problems， and the influence of wind power fluctuation and line-to-ground fault on the systems are discussed. The most appropriate solution of large-scale wind power transmitted by DC system is presented. Based on the results， system security and stability is studied for the Jiuquan wind-coal-bundle transmitted by ultra high voltage direct current（UHVDC） system， and the control strategy based on the DC modulation control method is proposed to mitigate the impact of wind power fluctuations on system frequency and improve the system stability. The validity of control strategy is verified by simulation.