Ultra-low frequency oscillation analysis and robust fixed order control design

Yunnan Grid, a hydro-dominated system, has experienced many times ultra-low frequency oscillations (ULFO) since the grid is connected to the China Southern Grid via DC ties in 2016. The root cause of the oscillation phenomena is investigated in this work. The vector margin (VM) method, which has a closed-form stability margin expression, is utilized to analyze the phenomena, the analysis reveals that the large phase lags of hydro-power turbines at the oscillation frequency are the culprit. It is proved analytically that a hydro-dominated system model always includes a right-half-plane zero, and this zero introduces the large phase lag afore-mentioned and places a fundamental conflict between the stability and tracking performance. A frequency-domain robust fixed-order controller design method is proposed to maximize the tracking performance with specified stability margin in multiple operating points. Case studies of the four-machine two-area system and Yunnan Grid validate the introduced results.