Ultra-low frequency oscillations caused by the governor of power system occur in actual power systems recent years. In addition,Governor’s dead band also introduces nonlinear link to the system and make it more difficult to analyzing the frequency stability of power system. In this paper, the different influences of non-step dead band and step dead band on frequency oscillation are analyzed with the describing function method in the single-machine system. The locus of minus reciprocal of the describing function of dead band with oscillation amplitude, and the locus of open-loop transform function with oscillation frequency are plotted on the complex plane. According to the Nyquist stability criterion, The relative location of the two loci determines system stability. When the minus reciprocal locus entirely locates on the left of the open-loop frequency characteristic locus, the system is stable. Furthermore, The non-step dead band and step dead band can change the stability of system when the system is unstable without dead band. Results of the time-domain simulations validate the conclusion.

ultra-low oscillation,frequency stability,descrbing function,dead band