Control Technology Of Direct-driven Permanent Magnet Synchronous Wind Power Generation System For Active Frequency Support

Vigorously developing renewable energy and building a new power system dominated by renewable energy is one of the important ways to help achieve the goal of "double carbon".With more wind power connected into the power system,the safety and stability of power system faces more challenges,especially in the aspect of frequency stability.Therefore,the power system makes higher operation requirements for wind turbine generators(WTGs)and wind power plants(WPPs),especially in the aspect of wind power participating in frequency control.To this end,this dissertation takes the direct-driven permanent magnet synchronous generator(PMSG)-based WTGs and WPPs as the research objects,aiming to study the control technology of wind power system for active frequency support from three aspects of "multi-wind speed scenario","multicontrol level" and "multi-frequency modulation function",so as to improve the ability of wind power participating in frequency control.The main works of this dissertation are drawn as follows:Firstly,the operation characteristic of a direct-driven PMSG-based WTG is analyzed,the application issues of low voltage ride-through(LVRT)criterion are discussed.Based on the modeling of a direct-driven PMSG-based WTG,the operation characteristic of a direct-driven PMSG-based WTG under normal operation and LVRT scenarios is analyzed.Aiming at the LVRT criterion in the grid code for wind power,around two aspects of LVRT curve and dynamic reactive current support,the relevant application issues between WTGs and WPPs are discussed.Considering the boundary condition,a new LVRT curve for WTGs is presented quantitatively,and it is pointed out that the WPP can not meet the requirement only by the dynamic reactive current support from WTGs.Secondly,conventional short-term frequency control(STFC)methods for wind power are analyzed and compared,a disturbance-adaptive STFC method for wind power is proposed.The frequency control system of traditional power system and the evaluation index of frequency control are briefly introduced,the effects of large-scale wind power integration are also analyzed.Conventional STFC methods for wind power are summarized and comparatively analyzed.By the way of tuning the inertia gain and droop gain based on the stages of a frequency disturbance,a disturbance-adaptive STFC method for wind power is proposed,which solve the issues that control gains of STFC methods are hard to tune.The proposed method covers the steps of frequency support and rotor speed recovery,which can improve the frequency nadir and reduce the quasi-steady frequency deviation.Thirdly,the theoretical operation speed range of variable speed WTGs at high wind speeds is analyzed,a control method achieving a wide-speed-range operation for a direct-driven PMSGbased WTG at high wind speeds is proposed.Based on the operation characteristic of variable speed WTGs,the operation speed range of variable speed WTGs at high wind speeds is analyzed in theory.Based on the analysis of operation speed range,a wide-speed-range control method for a direct-driven PMSG-based WTG at high wind speeds is proposed,by considering normal operation,extra active power demand and LVRT scenarios.The proposed method realizes a virtual flywheel system,which can both release and absorb energy,and improve the ability of wind power participating in STFC.Moreover,the discussions on the potential prospect of KE in variable speed WTGs and the transition ways of rotor speed for variable speed WTGs at high wind speeds are also investigated.Finally,a fixed-proportion delta de-loading control method for variable speed WTGs without wind speed measurement is proposed,de-loading control methods for WPPs without wind speed measurement are proposed.Since the de-loading control of a WPP depends on the de-loading control of WTGs,based on the combined ways of variable optimum power coefficient(overspeed)and pitch angle generating additional active reference(variable pitch control),a fixed-proportion delta de-loading control method for variable speed WTGs without wind speed measurement is proposed.Based on the same fixed-proportion delta de-loading control of WTGs,de-loading control methods for WPPs without wind speed measurement are proposed,which solve the problem of scheduling instructions of WPPs and realize two de-loading operation modes of WPP.Besides,the secondary frequency control method for WPPs based on de-loading operation modes is introduced,which improve the ability of WPPs participating in long-term frequency control.