The low excitation limit of a synchronous generator set needs to operate before the loss of excitation protection. However, the criteria for the low excitation limit and the loss of excitation protection are completely different. Therefore their setting and coordination have always been a difficulty in network source coordination. Using the power equation of the static stability boundary of a round pole synchronous generator, the parabolic expression of the static stability boundary eliminating the terminal voltage on the power plane is derived. The criteria of loss of excitation protection and low excitation limitation are unified into the parabolic static stability boundary of the power plane, and a new scheme combining these criteria is proposed. The scheme has clear physical meaning and is easy to set. This can ensure the strict cooperation between loss of excitation protection and low excitation limit. The effectiveness of the new scheme is verified by the analysis of and calculation for a 600 MW thermal power generation unit. It ensures the safe and stable operation of the unit and the power grid while improving the leading phase operation capacity of the synchronous generator unit. This work is supported by the Science and Technology Project of the Headquarters of State Grid Corporation of China (No. 5700-202055332A-0-0-00).