Electric spring (ES) can suppress the voltage fluctuation caused by large-scale grid-connected renewable energy sources. However, ES can only work within a certain range of gird voltage fluctuations. This means, if the grid voltage fluctuates greatly, the ES will expose to the risk of off-limit failure. Focusing on this problem, the steady-state operating range and the failure mechanism of ES are analyzed. Then the influence of changes in critical load target voltage on the operating range of ES is analyzed. After that, a calculation method for critical load target voltage based on a two-port impedance model of ES is proposed, which can expand the operation range of the ES through optimal adjustment of the critical load target voltage effectively. Finally, a simulation model of the three-phase ES is built, and the detailed simulation results verify the validity of the proposed method.