The operating modes of hydro-turbines have a significant influence on the stability of hydropower plants. In this study, a linking method of multiple hydropower plants and the frequency equation of the interconnected system are proposed. The state space modeling of the hydro-turbine governing system (HTGS), including the pipe system and surge tanks, is established under interconnected operating modes. The proposed model is used to simulate the small fluctuation process under isolated and interconnected operations based on the parameters of two simulated hydropower plants. The stability of the systems under different operating conditions is compared, and the effect of surge tank area on the stable region is analyzed. The results indicate greater stability under interconnected operating conditions because of the change in regulating modes of the turbines. A hydropower plant with a smaller proportion of the total output has an insignificant effect on the system when the interconnected system is properly stable. In addition, the difference in the surge tank periods should be maintained outside the adverse range. (C) 2018 Elsevier Ltd. All rights reserved.