Abstract: The functions and necessity of the 100% capacity bypass system for combined cycle power units are discussed in this paper. The effectiveness of the 100% capacity bypass system in different operation statuses, such as, starting up, shutting down, load shedding and unit tripping, is analyzed. The data acquired with new method improve the bypass control performance .and the effectiveness is verified through tests. Thereby, it is concluded that the automation level of the bypass system can be increased by virtue of 100% bypass capacity, and further, by better performance of the bypass system itself and related auxiliary equipment. Additionally, the potential risks in the 100% bypass system operation are not only analyzed from the bypass system itself and the related auxiliary equipment, but also verified through field operation. The risk analysis helps to create the check list which should be done in the design stage.

Key words: combined cycle unit, bypass system, capacity test