| Based on the modular multilevel modular converter(MMC)of high-voltage DC transmission(High voltage DC,HVDC)is currently the only large-scale offshore wind power grid-connected solution with practical engineering experience.The wind power transmission system through the flexible direct transmission system is a typical power electronic equipment interconnection system,when the impedance mismatch between the two caused by the oscillation problem,seriously restrict the offshore wind power outgoing and consumption.The impedance analysis method is suitable for analyzing the interaction stability problems between different subsystems.In this paper,the impedance method is used to investigate the oscillation mechanism for the interconnection system of direct-drive wind farm and MMC offshore rectifier station.The main contents of the paper are as follows:Based on the existing research,a multi-harmonic linearization method is used to derive the detailed positive and negative sequence analytic impedance of the MMC AC side considering the complex dynamics inside the MMC and the complete control link containing AC voltage control,and the correctness of the analytic impedance is verified by frequency sweep.Based on this analytical model,the effects of wind field output,AC voltage controller proportionality factor,integration factor and current control bandwidth on the impedance characteristics of the MMC are investigated.The analysis results show that the proportional coefficient of the voltage controller has a significant effect on the impedance of the whole frequency band,and the impedance characteristics of each frequency band should be coordinated when designing the proportional coefficient;too large an integral coefficient will introduce negative damping near the fundamental frequency.Under the premise of the same voltage control bandwidth,the impedance characteristics of open-loop control,single-voltage loop control and doubleclosed-loop voltage control are compared and analyzed,and the advantages and disadvantages of the small disturbance performance of different voltage control modes are analyzed from the perspective of interconnected system impedance matching to guide the selection of MMC voltage control modes.From the perspectives of steady-state control error and small disturbance performance,the interconnection system with single-loop voltage control mode is analyzed to obtain the best small disturbance stability performance.For the wind power flexible straight interconnection system high frequency oscillation phenomenon,based on the MMC high frequency simplified impedance model analyzes the MMC high frequency band impedance characteristics,clarifies the key influencing factors,and focuses on the analysis of voltage perturbation mechanism;for the wind field equivalent impedance,considering the characteristics of the distribution parameters of the long distance convergence submarine cable,explains the phenomenon of high frequency oscillation still occurs when the actual flexible straight project with no load line.For the case of active damping control on the flexible straight side,the applicability of the existing voltage feedforward link nested low-pass filter is analyzed.Finally,this paper analyzes the voltage disturbance path considering the need to minimize the negative damping region of the high-frequency impedance of the flexible straight,clarifies that both the voltage control loop and the grid voltage feedforward loop should be considered when performing high-frequency damping enhancement,and proposes a high-frequency oscillation suppression strategy based on the grid voltage full feedforward and voltage loop current loop control parameters directed design,and ensures that the negative damping region of the MMC is significantly reduced for a wide delay change.is reduced.Finally,the effectiveness of the proposed high-frequency oscillation suppression strategy is verified by electromagnetic transient model simulations based on the PSCAD/EMTDC platform. |
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