High-frequency Oscillation Analysis And Suppression Of Doubly Fed Wind Farm-connected To Parallel Compensation Grid

With the help of the goals that ‘carbon peak’ and ‘carbon neutral’,wind power has developed rapidly because of mature technology and environmental protection.Wind farms connected to the parallel compensation grid are at risk of high-frequency oscillation,which pose a great threat to the safe and stable operation of the grid.In response to the problem that the high-frequency oscillation can be caused by the interaction between wind farms and the complementary grid,the doubly fed wind farmconnected to parallel compensation grid(DFWF-PCG)system is used as a case to analyze and suppress the high-frequency oscillation by this thesis,while the specific contents and contributions are briefly described as follows.(1)The doubly fed wind farm’s and parallel compensation grid’s impedance models are built through the Thevenin equivalent circuit so that the system operation state is directly analyzed.The Thevenin equivalent circuit of the doubly fed induction generator(DFIG)is built based on the grid-side converter(GSC),rotor-side converter(RSC),induction generator and its control structure,and the impedance model is built at the same time.Then,the Thevenin equivalent circuit belonging to the doubly fed wind farm and the parallel compensation grid are sorted out,the impedances on the both sides of the connected point are also solved.The impedance models are used to judge that the system will be high-frequency oscillation and calculate the oscillation frequencies in case analysis,while it’s verified by simulation that the impedance models can be used for high-frequency oscillation analysis.(2)The gray correlation evaluation method with variable weights is proposed for analyzing the high-frequency oscillation influencing factors and finding the most important factor.The impedance in different parameters on the both sides of the connected point is solved based on the impedance model.After that,the oscillation frequencies with different parameters are solved by the corresponding impedance curves.The gray correlation evaluation method with variable weights,which can analyze the correlation degree between the influencing factors and the oscillation frequencies from the perspective of the variables themselves and the amount of variation,is used to find the main high-frequency oscillation influencing factor.(3)Two strategies,which are RSC rotor current with additional stator voltage feed-forward(RCASVF)control and bidirectional coordinated voltage feed-forward control,are proposed for the impedance reshaping of doubly fed wind farm.And then,the high-frequency oscillation occurrence of the DFWF-PCG system is avoided by the reshaped impedance.The RCASVF control,which reshapes the RSC and induction generator’s impedance by additional stator voltage feedforward,is developed by the rotor current control based on stator voltage orientation;the bidirectional coordinated voltage feedforward control,which reshapes the GSC branch impedance by additional grid voltage feedforward,is developed by improving the GSC control scheme on the basis of the RCASVF control.In the modeling process,it’s proved that the bidirectional coordinated voltage feedforward control has one more damping controller than the RCASVF control,while the DFIG’s impedance reshaping in this way has little dependence on its own parameters;in case analysis,it’s proved that the both strategies can reshape the impedance of doubly fed wind farm and suppress the high-frequency oscillation of the DFWF-PCG system in a wide frequency range.