When multiple stable systems are combined into one system, the newly formed hybrid system has a certain possibility to be unstable. Such a natural phenomenon has been verified in the fields of chemistry and biology over the last century. The use of electric springs (ES), which must be implemented in large quantity over the power grids, can also suffer from the same issue if specified design are not carried out. In this paper, a method of tuning Kp and Ki of the PI controllers of the ES to ensure the overall system stability in a weak grid is investigated. The method is based on the concept of relative stability and is achieved through the aid of simulation. Both simulation and experimental results validate that as the number of ES in the weak grid increases, more stringent values of Kp and Ki are required to achieve system stability. Apparently, a corresponding relationship between the optimal tuning of Kp and Ki of the PI controllers and the maximum number of ES installed in the weak grid exists. If the optimal values of Kp and Ki are adopted, a maximum number of ES can be stably installed over the grid.