When excessive power supply is caused by large amount of renewable energy sources, existing synchronous generators (SG) are supposed to adjust the outputs to balance supply and demand. Transient stability may deteriorate depending on the magnitude of curtailment for SGs. In this paper, transient stability have been evaluated from the viewpoint of critical clearing time (CCT), assuming the case that SG output is curtailed when wind farm (WF) output increases. As the result, it is clarified that SG is difficult to step out and transient stability improves in the case of calculation condition where the difference of acceleration among SGs during fault becomes small. As a method for improving the transient stability utilizing this, we proposed the adjustment method of SG outputs that makes acceleration among SGs during fault coincide.