Studies On Characteristic And Suppressing Strategies Of Subsynchronous Oscillation With DFIG-based Wind Farms

With the large-scale wind farms integration into grid, on one hand, the high voltage series compensation technology has been widely used to achieve the long-distance transmission capability, but the use of series compensation may cause sub-synchronous oscillations(SSO). On the other hand, Wind & Fire baling transporting modes was utilized to improve the wind energy utilization rate and grid stability, but the integration of wind farm will change the SSO damping characteristics of synchronous generator system. Regarding the above SSO problem caused by integration of large-scale wind farms, this dissertation focusing on doubly fed induction generator(DFIG) based wind farm, aim to deeply study the SSO characteristics, reveal the rules, and explore the control ability of DFIG to suppress SSO.In order to study the SSO characteristics in series compensated DFIG based wind farms, the small-signal models of double-fed wind turbine connected to power grid via series compensation are established, the eigenvalues and modal participation factors are computed, the dominant state variables that effect SSO mode are determined, and the impact the relevant parameters on the SSO are analyzed, particularly the impact of converter control parameters and the phase-locked loop dynamic characteristic on SSO are studied.Considering the existing damping control strategies rarelyinvolvedthe impact of converter control features, based on impedance analysis, start from circuit model of PI controller of rotor-side converter, the equivalent circuit model of double-fed wind turbine connected to power grid via series compensation are established, then by introducing a virtual resistance in the rotor circuit, a SSO damping control strategy based on virtual resistance is developed, the depressing effects are verified and compared with conventional control strategies.In order to research the impact of large-scale DFIG-based wind farms integration on SSO of turbo generator, by utilizing modal analysis and parameter sensitivity analysis, the impact of wind farms integration on the changes of the system SSO modes and the sensitivity of SSO damping ratio to wind farm parameter are analyzed, the impact rules of wind turbine active power and reactive power output variation on SSO damping are derived.In order to explore the control capability of large-capacity wind farms depressing power system SSO, by introducing the transfer functions between turbo-generator speed and wind farms output active/reactive power, the expression of the DFIG-based wind farms active/reactive power for the damping ratio and damping effects on the power system is derived. Based on the condition of the positive damping, the phase compensation parameters of the PID controller are optimized by genetic algorithm, an additional damping control strategies based on the active/reactive power loop of DFIG is proposed to considering power output constraints.The research achievements have important reference value to ensure the stability of the power system as well as to improve the wind power output level. Furthermore, it can provide theoretical support and technical means for suppressing the SSO of wind power systems.