| To vigorously develop the renewable energy such as wind power is the significant demand of the clean energy transition of China.And the integration of the wind power is the important method for the economic and efficient consumption of the wind power.However,unlike the thermal power unit,the wind power unit is integrated through the power electronic devices which have low inertia,weak anti-interference,and multi-timescale dynamic response,and thus the wind power integration system may face the unclear risk of the multi-frequency oscillation and transient synchronization stability issues caused by the power electronic devices.Therefore,for the stability and secure operation of the wind power integration system,the analysis and control of multi-frequency oscillation and transient synchronization stability need to be researched.This paper focuses on the wind power integration system.First,the multi-frequency oscillation issues of three different types of the wind farms when integrated through a voltage source converter based HVDC(VSC-HVDC)are studied based on the sequence impedance method,and the oscillation mechanism and characteristics are further investigated,as well as the oscillation suppression strategy.In addition,the transient synchronization stability issues of the grid-connected wind farms with virtual synchronous generator(VSG)control under severe fault are studied as well,and the control methods are proposed with or without the fault information acquisition,which take both the transient angle stability and current limitation into account.The main work of this paper and the results obtained are as follows:1.Aiming at the medium frequency oscillation(MFO)in permanent magnet synchronous generator(PMSG)-based wind farm when integrated through a VSC-HVDC,first the single-input-single-output(SISO)equivalent sequence impedance model of the system is derived considering both the model reduction and coupling effect.The mechanism of MFO is explored based on the SISO sequence impedance,which origins from the coupling effect of the dc voltage loop of PMSG.In addition,the influence of parameters from both PMSG and VSC-HVDC on MFO is analysed.Last,a virtual impedance based current feedback control is proposed to suppress MFO of the system.And the correctness of the analysis and the proposed control are validated by simulation.2.Aiming at the subsynchronous oscillation(SSO)observed in double-fed induction generator(DFIG)-based wind farm integrated via a VSC-HVDC,first the sequence impedance model of the multiple DFIGs interfaced with VSC-HVDC is established,and the system simplified equivalent circuit model is furthermore derived based on the reasonable simplification for the intuitive and analytical analysis.The mechanism of SSO is that DFIG behaves as an inductance in series with a negative resistance whose interaction with wind farm side VSC(WFVSC)constitutes an equivalent series resonance circuit with negative resistance.In addition,the impact of factors from DFIG and WFVSC on the SSO characteristics is analyzed.Last,the suggestion and guideline of the system parameters are proposed.And the theoretical analysis is validated by both simulation and hardware-in-loop experiment.3.Aiming at the MFO issue in a DFIG-based wind farm integrated through a VSC-HVDC in combination with local grid,the influence of the “wind-thermal-bundled”method is considered,and first the reduced sequence impedance model of the sending-end converter(SEC)of VSC-HVDC with PQ-control outer loop is derived.Compared with SSO caused by DFIG or the power inverter,this MFO mainly originates from the SEC due to its negative damping effect among the MFO region.Furthermore,the impact of the system controller parameters and operating conditions of the SEC,local grid,and DFIG on the oscillation characteristics is analyzed in detail.Finally,a filter based voltage feedforward control of SEC is proposed which can suppress MFO of the system.And the correctness of the analysis and the proposed control are validated by simulation.4.Aiming at the transient angle instability and overcurrent issues of the wind farms with VSG control during fault with the fault information,first the post-fault large-signal model of VSG is derived via the travelling waves based fast fault information acquisition.Subsequently,with the coordinated use of both active and reactive power loops,a two-stage simultaneous control scheme is proposed to accurately control the transient angle while considering the current limitation during fault state and voltage support during out of service(O/S)state after fault clearance.This method is fulfilled by mode switching and an additional feedback control based on the fault signal.Finally,the effectiveness of the proposed method in both the single machine system and two machine paralleled system are verified.And the application of the proposed method under asymmetrical faults is verified.Besides,the robustness to parameter mismatch and the feasible operating region of the method are discussed as well.5.Aiming at the transient angle instability and overcurrent issues of the wind farms with VSG control during fault without the fault information,where the accurate control is hard to achieve,an non fault information based dual-loop adaptive control is proposed,which takes both the transient stability enhancement and current limitation into account.And the requirement of grid code for the reactive current is considered as well.The feasible region of the control parameters under different fault degrees is analyzed and provides reference for the parameter fitting,which is further applied in the proposed reactive current feedback based dual-loop adaptive control.Therefore,with the proposed method,the control objectives can be achieved since the control parameters are within the feasible region due to the self-adaptive regulation.Finally,the effectiveness of the proposed method in both the single machine system and two machine paralleled system with VSG control and grid-following(GFL)control are validated. |
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