A sustained ultra-low frequency oscillation occurs in Yunnan Power Grid after asynchronous interconnecting to China Southern Power Grid. The mechanism of ultra-low frequency oscillation is analyzed theoretically based on the Nyquist vector margin. The result shows that the large phase lag of hydro-power unit at the oscillation frequency

which is brought by the right half plane zero in turbine transfer function

is the key cause of ultra-low frequency oscillation. To avoid the reduction of tracking performance due to the disabling of governors

an optimization method of parameter in frequency domain for governor is proposed to maximize the tracking performance under robust stability constraints

which guarantees the stability margin in various operation points and optimizes several governors simultaneously. Case studies of Yunnan Power Grid validate the effectiveness of the analysis and optimization method.