| With the initial formation of large scale AC/DC national interconnected power grid and the proposal of "global energy internet",the scale of interconnected power grid will extend from nationwide further to global wide.The large scale interconnected power grid improves the economy and reliability of power generation and transmission,but it also increases the number of power disturbances rapidly,such as the failure of speed governor,the pressure fluctuations of steam turbine,the mechanical power fluctuations of hydro turbine and the wind farms,photovoltaic power plants and electric vehicles with the characteristics of intermittence and volatility.The continuous periodic power disturbance will lead to the oscillations of power grid,which are called forced oscillations.Moreover,when the frequency of the power disturbance is close to the frequency of low frequency oscillation mode,the phenomena of resonance will occur and cause even larger amplitude of oscillations.Forced oscillations features fast starting,large amplitude and widespread,threatening the safe and stable operation of power grid seriously.This paper focuses on power system forced oscillations and studies the monitoring(analysis of power disturbances and the distinguishment of oscillations)and suppression of it.The main work is as follows:(1)Different disturbances in the power turbine and its governing system are studied and the influence of different disturbances in different types of power turbines and governing system to forced oscillations is analyzed.Firstly,the model of power system considering the disturbances is constructed and the principle of forced oscillations is analyzed.On this basis,the models of power turbine and it governing system are constructed.Further,the possible disturbances in power turbine and its governing system and characteristics of forced oscillations caused by different disturbances are studied.Theoretical analysis and simulation are carried out to analyze and compare the forced oscillations caused by different disturbances in steam turbines and hydro turbines and the conclusions have important reference value to the understanding of the mechanism and the suppression of forced oscillations.(2)An identification method of forced oscillations based on zoom fast fourier transform(Zoom-FFT)and inverse distance weighted(IDW)are proposed.Through the spectrum analysis of low frequency oscillation of negative damping mechanism and forced oscillations,the characteristics of the spectrums of the two kinds of oscillations are generalized and the criteria for the identification of oscillation type is extracted.The Zoom-FFT method is adopted to obtain the spectrum of oscillations from the data,which has the advantages of low computational complexity and high resolution and can distinguish the difference between the spectrums of the two kinds of oscillations.Then,IDW interpolation is adopted to fit the spectrums and calculate the identification index.Combined with energy threshold and weight threshold,the type of oscillations can be identified.Finally,simulation is carried out in the four-machine and two-area system and East China Power Grid.The results verify the effectiveness,robustness and good anti-noise ability of the proposed method.(3)Considering the diversity of disturbances and the complexity of oscillation modes in power system,the influence of the characteristics of disturbances and the oscillation modes to forced oscillations are studied and their mathematical relationship is also deduced.With time domain analysis and frequency domain analysis,the influence of the form,frequency and amplitude of disturbances as well as the frequency and damping ratio of oscillation modes to forced oscillations are studied.The general laws of the characteristics of disturbances and oscillation modes influencing forced oscillations are concluded.Numerical calculations and simulation are conducted to verify the conclusions.The obtained results provide a theoretical foundation for the suppression of forced oscillations.(4)Considering the characteristic of forced oscillations and multiple oscillation modes,a power system stabilizer(PSS)design method based on particle swarm optimization(PSO)for suppressing forced oscillations are proposed.Traditional PSS design methods mainly aims at low frequency oscillations of negative damping mechanism and ignores the forced oscillations.Analyzing the mechanism of PSS for suppressing forced oscillations,the objective function aimed at forced oscillations is designed and the weight factor is assigned according to the mode characteristics of forced oscillations.PSO is used to obtained the parameters of PSS.Simulation is carried out in the four-machine and two-area system and the results demonstrate that the performance of PSS designed by the proposed method is better than PSS designed by traditional method.Moreover,the good performance of suppressing low frequency oscillations of negative damping mechanism is maintained.(5)Based on the mechanism of forced oscillations,the mechanism of static synchronous compensator with energy storage(E-STATCOM)suppressing forced oscillations are studied and the superiority of its ability to suppress forced oscillations are demonstrated theoretically.The structure and principle of E-STATCOM is introduced.Through constructing the model of power system with E-STATCOM,the influence of the active and reactive power of E-STTACOM on forced oscillations is analyzed.The theoretical analysis demonstrates that E-STATCOM is feasible to suppress forced oscillations.Moreover,compared with STATCOM,which can only provide reactive power,E-STATCOM can provide the active and reactive power at the same time and has stronger ability to suppress forced oscillations.(6)Considering the continuing energy injection of forced oscillations,the suppression method based on E-STATCOM with resonant controller is proposed from the aspect of energy control.Resonant controller is adopted to perform a closed-loop control on the active and reactive power flowing from the disturbed area to other areas of power system and the forced oscillations propagating to other areas are suppressed.Based on the model of power system with E-STATCOM,the control method of E-STATCOM based on resonant controller is proposed and the suppression mechanism is analyzed.The placement and capacity of E-STATCOM is studied in detail.Simulation results in single machine and infinite bus system and four-machine and two-area system demonstrate the effectiveness of this method. |
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