Research On Suppression Of Sub-Synchronous Oscillation In DFIG-Based Wind Farms By Gate-Controlled Series Capacitors

With the rapid development of new energy industries such as wind power,the total installed capacity of wind power in China continues to increase.Sub-synchronous Oscillation(SSO)may occur in large-scale wind power base when the fixed series complement device is applied to improve transmission power of transmission lines.Among them,doubly-fed Induction generator,which is the mainstream type of wind power base,The SSO problem of DFIG is very prominent.In this paper,the SSO suppression method based on Gate-Controlled Series Capacitors(GCSC)for large scale doubly-fed wind farms is studied to solve the SSO problem of the system fed out by fixed series recharging.Conduct the following studies.The mathematical model of doubly-fed wind turbine was deduced,and a detailed doubly-fed wind turbine model based on PSCAD/EMTDC was built.The validity of the model was verified by step response.On the basis of the above model,a grid-connected system model of doubly-fed wind turbine is established,and the influence of system parameters on SSO is analyzed through time domain simulation waveform.The results show that,with the increase of series complement,the decrease of line resistance and the decrease of wind speed,the interaction of sub-synchronous control of the grid-connected system of doubly-fed wind turbines becomes more and more obvious.Aiming at the scenario of doubly-fed wind farm being sent out of the system by GCSC,the composition structure,impedance characteristics and control strategy of GCSC were studied.Based on PSCAD/EMTDC,the GCSC model with constant impedance mode was built,and the double-ended power system was established.The steady-state operation of GCSC was analyzed by time domain simulation method,and the correctness of the GCSC model was verified.Aiming at the problem of the system SSO sent out by fixed series complement in doubly-fed wind farm,a method of SSO suppression based on GCSC was proposed.By using the weak capacitance presented by GCSC at the sub-synchronous frequency and the resistive component it provides,the system not only avoids the resonance point where SSO occurs,but also increases the resistance of the system at the sub-synchronous frequency.Under the combined action of the above two,the system does not meet the conditions for the occurrence of SSO.Impedance scan analysis and time domain simulation verification analysis were used to verify the effectiveness of the inhibition measures.A Hybrid series compensation(HSC)device with GCSC is proposed to suppress SSO.The structure and control strategy of the symmetrical HSC device were studied,and the effectiveness of the HSC device to suppress SSO was verified by time domain simulation.Secondly,an asymmetric HSC device containing GCSC is proposed to suppress SSO.The structure of the asymmetric HSC device is studied,and the electrical damping of the system is scanned by the method of complex torque coefficient.The research results show that the asymmetric structure itself has little effect on alleviating SSO,and the main reason is that the GCSC itself has the ability to suppress SSO.Finally,the time-domain simulation method is used to verify the effectiveness of the device to suppress SSO.