On the mechanism and control for the ultra-low frequency oscillation in NY Power Grid with large-scale hydropower

The power grid with large proportion of hydropower may engage the ultra-low frequency oscillation (ULFO), which threateningthe operation of system. To the NY Power Grid system, whose dimension is over thousands, this paper reveals the cause ofULFO with the phase portrait method, i.e., lies in the saturation of the different limits of the governor system of the generators,in other words, the occurrence of limit cycle is due to the non-smooth bifurcation. And furthermore, it proposes two methodsto suppress ULFO with the philosophy of destroying the condition of non-smooth bifurcation. In detail, first, the phenomenon ofULFO in NY Power Grid, the damping characteristics and the limits in the governor system of the hydropower units are presented.Second, based on the non-smooth bifurcation theory, the mathematical basis of the oscillation in the non-smooth system causedby the saturation of limits is introduced. And then, the non-oscillation and oscillation dynamic of the system corresponding towithout/with limits in the governor system are analyzed. Furthermore, it discusses the influence of the saturation of single limit onthe characteristics of ULFO with the phase portraits. Moreover, the phase portraits of the ULFO under different faults is analyzed,and it shows that the limiting saturation occurs with a large enough disturbance. Thus, the ULFO is resulting from the negativedamping of generation and the saturation of multiple limits under a large enough disturbance. In addition, according to the abovemechanism, two strategies to suppress the ULFO, i.e., the governor parameters tuning and the fault strength relief are designed.The simulation shows the effectiveness of proposed strategies.