Simulation Study On Coupled Machine-grid Subsynchronous Oscillation Characteristics And Mitigation Of Large Turbine Generators

With the development of the long distant and large capacity transmission technology,the application of series capacitor compensation lines and DC transmission lines is becoming more and more extensive,which relieve power utility pressure in some areas as well as threaten the safe and stable operation of grid.One of the safe problems is Subsynchronous Oscillation,which attracts a lot of attention.Hulun Buir League power system is complicated for there are series capacitor lines as well as DC transmission lines,therefor Hulun Buir power plant and Ewenke power plant is on the tremendous risk of SSO.So the following aspects about SSO will be studied in this paper:Time-domain simulation method is selected to analysis SSO in this paper.According to the mathematical models of the generator,shaft and speed regulator,the MATLAB/SIMULINK models of Hulun Buir League power system are built and simplified when Ewenke power plant is selected as the main study object.SSO risks and SSO characteristics in different mode of Ewenke power plant are analyzed when the simulation is conducted at different working conditions.Based on the operation principle of reactive power compensation,the SSO mitigation mechanism is deducted.According to the mathematical model of STATCOM,the decoupling control strategies of the STATCOM system used for reactive power compensation and SSO mitigation are proposed separately.When STATCOM is installed at power side or grid side to mitigate SSO,the extraction method of reactive current component is different correspondently.The advantages and disadvantages are discussed.SVPWM based on 120` coordinate system is adopted as the modulation algorithm for STATCOM.Because the sector judging and evolution are omits,it has advantages in simple calculation compared to traditional SVPWM.As for the neutral point voltage unbalance of neutral point clamped three-level STATCOM,a balance strategy that reallocated the action time of positive and negative small vector is proposed.A neutral point clamped three-level STATCOM model is built in MATLAB/SIMULINK to verify the modulation algorithm and neutral point voltage balance.Aiming at Ewenke power plant,a mitigation system based on neutral point clamped three-level STATCOM is designed,and the model is built in MATLAB/SIMULINK as well.Several simulations are conducted and the results demonstrate that the mitigation system is valid under different working condition.