On-line Small Disturbance Security Assessment Based On Signal Excitation

With the interconnection of Chinese power system and the development of power market, the risk of small disturbance instability becomes bigger and bigger, which threatens the power system secure operation. The accurate assessment of small disturbance security is quite important to the power system. However, most methods of disturbance security assessment are based on models and parameters, which are difficult to obtain accurately, so results from them may not accord with the real power systems. In this dissertation a method of on-line small disturbance security assessment based on signal excitation was proposed: by injecting periodic disturbance power signals with special waveform generated by a disturbance source into the power systems to excite low-frequency oscillation modes under this operating condition, and measuring and analyzing frequency response curves, the damping and frequency of modes could be obtained and the margin of the small disturbance security currently could be assessed. This method avoided the influences of inaccuracy of models and parameters.Systematical research on the method was done. Firstly, the disturbance power signal was designed, which had the same frequency band with the power system low frequency oscillation and could exert the same excitation on every frequency point. The disturbance power signal was easy to transmit in power systems and the responses excited by it varied with the operating points of power systems. The state-space representation of power systems with the disturbance signal was deduced. Based on it, the relationships of frequency response excited by disturbance signal with operating states and oscillation modes were explained. Partitioning the power system into subsystems by sensitivities of eigenvalues to loads, the weak small signal security area of power systems and the location of disturbance source could be determined. Through the observation index set of all branches, the suitable monitoring site could be chosen. Based on modified Levy curve fitting method, a local third-order transfer function fitting model was built, and then the damping and the frequency of modes were obtained by fitting the curves near the maximum values of frequency responses. Monitoring the variations of the damping and the frequency of modes, dynamic small signal security of power systems could be assessed. The simulation results of China-EPRI 36 buses and Chuan Yu Power System showed that the method was feasible and effective. Because SMES was able to control active and reactive power flexibly and quickly and output a large amount of power with a little energy, SMES was chosen as the disturbance source. To demonstrate the feasibility of this method, experiments were carried out with a grid model SMES system. The experimental results showed that the assessment results under the disturbance could reflect the trend of the small disturbance security of the power system.