Abstract: Frequency stability mechanisms remain unclear in multi-energy complementary systems with wind and photovoltaic integration.An overall system frequency response model considering hydropower governing,wind power frequency support,and photovoltaic frequency support was established to analyze small-signal frequency stability.Based on the Hurwitz criterion,a theoretical stability region was constructed with the proportional and integral gains of the hydropower governor as variables.Pole distribution and time-domain frequency responses were then combined to analyze the variation laws of the stability region and dynamic responses under different operating conditions.The effects of wind and photovoltaic power shares and key frequency support parameters on system stability margins were also analyzed.The results show that,within the studied operating range considering wind and photovoltaic frequency support,increasing wind and photovoltaic power shares does not weaken small-signal frequency stability.Instead,it improves the damping of some oscillation modes and expands the stabilizable range of hydropower governor control parameters.Meanwhile,the slow dynamic process becomes more sensitive to parameter variations,and frequency stability is jointly constrained by oscillation modes and slow dynamic modes.Different types of parameters affect frequency stability through different mechanisms.Parameter tuning should coordinate oscillation suppression,slow dynamic constraints,and stability margins.