| With the large scale development and utilization of wind energy in our country,wind power transmission with long distance and large capacity has became necessary.Meanwhile,some severe problems are caused inevitably.Series compensation capacitor technology can improving transmission capacity of transmission line and enhance system stability,which is widely used in large scale wind power transmission with long distance.However,wind turbines connecting to a series compensated transmission system will introduce serious Subsynchronous Oscillation(SSO).In this paper,firstly,the research background and research meaning,the damage,the current research status of SSO,including the induced mechanism,research method,and suppression measures of SSO are elaborated.Different types of wind turbines' structure and their possibility of introducing SSO are analyzed and compared.According to IEEE first benchmark model,a model of doubly fed induction generator(DFIG)based wind turbine connected to grid via series compensated transmission line is established.Next,frequency scanning and time-domain simulation are performed to study how the transmission line length,series compensation degree,the capacity and the number of DFIGs,and the grid voltage level impact on SSO.And the reasons are analyzed from the aspect of mechanism.Then,a large-scale wind farm is divided into two sub wind farms,represented by two aggregated DFIGs at different locations.Impedance models of two sub wind farms and the transmission line are established and Nyquist criterion is applied to analyze the effect of DFIGs at different locations on SSR when wind speed,series compensation degrees and the parameters of converters vary.Finally,droop control and PSS additional damping control are respectively applied in the inverter of DFIG-based wind turbine to suppress SSO and improve the output power of DFIG.The model for simulation is built on MATLAB/SIMULINK platform,which verifies the effectiveness of the control strategy. |
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