A forced outage incident of the DC system had happened in xingren convertor station, due to a logic defect in valve cooling control and protection system, on December 10, 2010. In order to avoid similar accidents, this paper deeply analyzed all the valve-cooling control protection logic in Yunnan-Guangdong UHVDC transmission project. An improvement for protection logic was proposed, which was to cancel the fault trip logic for two redundant sensors. The result shows that the reliability of Yunnan-Guangdong UHVDC transmission project is greatly increased.