On Distinguishing The Types Of Power System Low Frequency Oscillation

With the increasing scale of interconnected power system in china,low frequencyoscillation has become one of the main problem severely threatening the secure operationof power systems.The common mechanisms of low frequency oscillation are negativedamping mechanism and forced power mechanism,with a large number of power systemstabilizers putting into use,the negative damping low frequency oscillation has beenreduced significantly,but the forced low frequency oscillation occurs more and more.It hasdifferent measures to restrain the different kinds of low frequency oscillations,so it canguide the power grid staff suppress the oscillation with the right measures bydistinguishing the mechanism of the oscillation immediately,it’s very important to the gridsecurity.For this reason,this paper presents three methods to differentiate the mechanismafter consider the differences of two kinds of the low frequency oscillation. The mainachievements are as follows:1) The two kinds of low frequency oscillation mechanisms are deduced.And thedifferences at damping torque coefficient and damping ratio has been found,this is thefoundation of the follow-up study.2) It introduce the theory of decomposition of electrical torque based onone-machine infinite bus system,and build its simulation model by Matlab/Simulink.Use itto get the oscillation data of different mechanisms,and the data is analyzed by the methodof decomposition of electrical torque.The results prove the effectiveness of this method.3) The natural excitation technique and Hilbert-Huang transform are studied.Use thenatural excitation technique to obtain the impulse responses from the white noiseresponses,then the impulse response is analyzed with HHT method to obtain modalparameters.Finally the mechanism of low frequency oscillation is determined by the size ofthe damping ratio.The data of the simulink modal of four-machine two-area system andPMU data prove this method is feasible.4) The theory of eigensystem realization algorithm is introduced.There are twocomponents during the oscillation starting phase,the forced component which only exist inforced low frequency oscillation can be found by eigensystem realization algorithm,andthis can help us to distinguish the mechanisms.The feasible of this method is proved by hedata of the simulink modal of four-machine two-area system and PMU data.