Abstract
In recent years, the ultra-low frequency oscillation of power system is one of the most concerned frequency stability problem. The governor deadband are considered as one of the main influences. However, the existing methods can only deal with single deadband segment. As for a multi-machine system with multiple deadband segments, it lacks the analysis method for frequency oscillation. To solve this problem, this paper first define the extended description function applied to nonlinear composite system according to the principle of traditional description function method. The extended description function can quantify the relationship of input and output of a system with multiple nonlinear segments. Then, based on the uniform frequency model of multiple governors, the turbines and governors of hydro and thermal power units with different deadband widths are equivalent to one extended description function. The locus of minus reciprocal of the extended description function with oscillation amplitude and the Nyquist curve of the generator are plotted on the complex plane for the analysis of system stability. Next, the critical oscillation amplitude of disturbance to trigger the system frequency oscillation is calculated according to the point of intersection of both curves. Finally, the simulation results show that the critical oscillation amplitude can reflect the stability of system. The change of deadband width is positively correlated with the critical oscillation amplitude. The appropriate setting of deadband and configuration of hydro-thermal generation are effective measures to suppress the ultra-low frequency oscillation while retain the ability of frequency regulation.
Highlights
The development of direct current (DC) transmission project has promoted the transition from synchronous interconnection to asynchronous interconnection of regional power grids in China
This paper proposes a novel extended describing function method, in which the nonlinear composite system of multi-deadband is equivalent to a structure of an extended describing function and a linear transfer function
If the negative reciprocal curve of the extended describing function intersects with the Nyquist curve of the generator, the critical amplitude corresponding to the intersection point is the minimum value of disturbance magnitude to cause frequency instability
Summary
The development of direct current (DC) transmission project has promoted the transition from synchronous interconnection to asynchronous interconnection of regional power grids in China. By calculation the eigenvalue of the equivalent model, the influence of hydro-thermal power ratio and hydro governor parameters on the ultra-low frequency oscillation is analyzed. In order to derive the analytical solution of oscillation frequency and damping of a single-machine system with load model, Reference [8] simplifies the governor to a regulation coefficient It focuses on how the turbine parameters and the power-frequency characteristic of load affect the damping ratio. Reference [11] proves that the parameters of hydro governor leading to system negative damping are the main cause of ultra-low frequency oscillation. The ultra-low frequency oscillation occurs when the system is subjected to a certain magnitude of power disturbance according to the Nyquist criterion This method is only applicable to a system with one single deadband due to the restriction of description function method. The influence of multiple governor deadbands on frequency oscillation is quantitatively evaluated
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