| In recent years,wind power generation is getting great progress.Due to the places with abundant wind resources are away from the load center in our country,in large-scale long-distance transmission projects,series compensation is usually used for enhancing the transmission capacity of transmission lines,the sub-synchronous interaction(SSI)may appear under small perturbation while doubly fed wind induction generator(DFIG)is connected to power grid via series compensation when series compensation degree is inappropriate or the system operation mode changes.SSI results in fan off the net,which makes the system instability.In addition,wind power has the characteristics of the randomness and fluctuations,this paper studies the SSI and its mitigation measures based on probabilistic method in the system with wind power integrated,the primary contents and original contributions are listed as follows:(1)The plug-in modeling technology(PMT)is employed for building the complete mathematical model of the small-signal stability analysis model of the whole power system,including the doubly-fed induction generator,the direct-drive permanent magnet synchronous generator(PMSG),synchronous generators,load,static VAR compensator(SVC)and additional damping control system.Probabilitic method combined with Lyapunov first method is utilized to calculate the probabilistic attributes of the sub-synchronous modes for analyzing theirs characteristics.(2)The variation of the system operating conditions would affect the SSCI.Considering the random uncertainties of wind farm output,synchronous generators output and stochastic fluctuations of loads in the power system,probabilistic eigenvalue method is proposed to obtain the the statistical attribute of the oscillation modes to analyze the probabilistic stability under multi-operating conditions.Simulations on the test systems illustrate that new sub-synchronous modes are introduced by the system with wind farm integrated through series compensated lines.The participation factor are obtained to identify the oscillation modes,and the SSCI strong-related state variables are analyzed,which lays a foundation for the design of the mitigation measures.(3)The action mechanism of the SSCI is analyzed,and the influence factors including the degree of series compensation and the type of wind turbine are discussed on the sub-synchronous probabilistic stability.Probabilistic sensitivity index(PSI)is proposed to analyze the impacts of the parameters in the converters on the sub-synchronous mode,and on that basis,wind turbine generator is used to SSCI mitigation by application of a power system stabilizer(PSS)added to the converters.PSI is then employed for accurate positioning of the DFIG-PSS.The damping perfermances of PSS installed on the converter controller are compared under four input signals to choose the optimal one,and PSS parameters are optimized by probabilistic sensitivity matrix to improve the effect of SSCI mitigation.(4)Combined with the probabilistic approach,static var compensator(SVC)with an additional damping controller is proposed to suppress the SSCI.The simulation of a multimachine text system shows that this suppression measure can improve the probabilistic small-signal stability of the power system with large-scale wind generation,the probabilistic density curves of damping constant and damping ratio are compared with and without the mitigation measure.The effectiveness of the sub-synchronous suppression measure is also verified by the results of the time-domain simulation. |
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