| The implementation of asynchronous interconnection mode can effectively avoid the large-scale shift of power flow induced by the UHVDC double-pole blocking in the AC-DC hybrid power grid.It is helpful to improve the stability of power system and the controllability of the fault range.After asynchronous networking,both actual operating experience and simulation analysis have found that the Ultra Low Frequency Oscillation(ULFO)can easily occur in the power system with high proportion hydropower units,which seriously affects the safety and stability of power system.It is important to research the mechnism,the characteristics and the oscillation parameters of ULFOIn this paper,the Ultra Low Frequency Oscillation of Yunnan Power Grid and Chuanyu Power Grid,that caused by the turbine speed control system is taken as the background.The mechanism and oscillation characteristics of the Ultra Low Frequency Oscillation in the power system of high proportion hydropower units are studied and analyzed.A new method that is used to analyze the Low Frequency Oscillation and Ultra Low Frequency Oscillation of power system is proposed.Firstly,the basic theories and concepts of the small disturbance stability analysis are introduced,and the analysis steps of the small disturbances of the multi-machine system are given.Secondly,the linear state space model of two-machine system with PID governor is established.The torque analysis method is used to analyze the damping characteristics of the turbine and the governor.Based on the linear state space model of two-machine system,the participation factor of each state variable to the ULFO is calculated and the influence of different parameters to the oscillation frequency and damping ratio is discussed.the controllability of different variables and the controllability of different variables is analyzed.The conclusion shows that the ULFO really exists in the power system with high proportion hydropower units.Unlike the Low Frequency Oscillations,all generators in the system have the same direction of oscillation.The turbine and PID governor are deeply involved in the ULFO,but excitation system has little effect on the ULFO.The governor-side power system stabilizer and the DC active power modulation has strong controllability for the ULFO.The conclusion also shows that an optimal value of PID governor proportional coefficient may be exist.Based on the 10-machine 39-node system,the correctness of the conclusion was verified.Finally,the improved STD algorithm,an oscillation parameter identification method based on wide-area measurement data,is proposed.The free oscillation signal of multi-channel signal is used by the improved STD algorithm.Compared with the existing single-channel identification algorithm,it has excellent accuracy and noise resistance,and can simultaneously obtain multiple oscillation mode parameters accurately.Compared with multi-channel ITD algorithm,performance is comparable,but when analyzing noisy signal,there is no need filter.The effectiveness and practicability of the method is verified based on the 16-machine system simulation data and PMU data. |
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