Abstract: Based on the small fluctuation stability analysis theory, this paper constructs an analytical model for a hydro-turbine governing system with or without a surge tank by using state space method, examines its instability features in the regulating zone I of governor parameters, and analyzes the influence of phase frequency characteristics on its stability. Results support the conclusion that this instability is essentially originated from the inertia of the water volume in the penstock that results in a phase lag of variation in penstock discharge behind guide vane opening, and consequently it generates a regulation error to the extent of instability. And integral gain can cause a phase lag of variation in guide vane opening behind generator speed. The system’s total phase lag, or the sum of these two, can be altered by proportional gain . At a relative small this total lag decreases with an increase in , while at a large it increases with . Generally, the stability of a closed-loop system requires a total phase lag to be less than 90°.

Key words: hydropower station, small fluctuation, modern control theory, stability, phase-frequency characteristics