Abstract: Installing distributed synchronous condensers (SC) in renewable energy stations can improve the new energy delivery capacity. However, the coupling between the SC and the new energy unit during the fault easily causes transient power angle stability problems. Improving the SC's excitation control strategy is an economical and effective means of solving this problem. Firstly, the energy function of the SC is constructed, and the power adjustment requirements conducive to transient stability under the four-quadrant fluctuation of the active power and speed of the SC are analyzed by Lyapunov's second method. Secondly, the influence mechanism of excitation control on electromagnetic power is explained. An additional excitation control strategy is proposed based on active power zero crossing and speed overrated points. The transient stability of the SC is realized by superimposing the additional reference value of the terminal voltage. Finally, a mainstream type of excitation controller is developed and applied in China, and the effectiveness of the proposed excitation additional control method is verified by a hardware-in-the-loop simulation test. The results show that the proposed method can significantly improve the transient power angle stability during both deceleration and acceleration of the rotor.