Research On Wideband Oscillation Of HVDC Transmission Connected To Offshore Wind Farms On The Far Coast

Under the background of the carbon peaking and carbon neutrality goals,large-scale and long-distance offshore wind farms have been constructed rapidly.However,with the dramatic penetration of power-electronics-based renewable generation in modern power systems,the wideband oscillation phenomenon appeared on the power electric system.Several Hz to several kHz oscillation frequencies will cause the serious disconnection and shutdown of wind farm connected to MMC-HVDC projects.Thus,the generation mechanism,oscillation suppression methods and characteristics of wideband oscillation in the HVDC transmission connected to wind farms are studied in this paper.The main research contents of this paper are as follows:(1)The complete time state space model of the wind farms connected to MMC-HVDC is established according to the project in rudong,jiangsu,China.Then,the correctness of the mathematical model is verified in MATLAB and PSCAD.After the participant of the whole system is analyzed.The elements which influence the system stability are studied,such as the interaction between the grid converter of PMSG(permanent magnetic synchronous generator,PMSG)and MMC converter,the inner-outer loop parameters of the offshore converter,the interaction between MMC converter and AC grid.Then,the DC impedance of the MMC which adopts the constant DC voltage control is established.Also,the correctness of the DC impedance is then verified.(2)Based on the time-domain state space mathematic model and DC impedance model.The improved virtual impedance control(IVIC)method is designed based on traditional virtual resistance control(TVRC)and phase lag compensation control(PLCC).The structure and design principle of IVIC are studied first.Then,the small-signal stability of the IVIC is studied according to the eigenvalue analysis and the DC impedance analysis in the last chapter.Finally,the oscillation suppression effectiveness of the IVIC is verified in PSCAD/EMTDC.The results in PSCAD verify that the IVIC can suppress the oscillation better than TVRC and PLCC when the parameters of the IVIC,TVRC,PLCC are the same.(3)The IVIC method belongs to the active damping oscillation suppression method.The passive damping oscillation suppression devices such as energy storage can also limit the oscillation phenomenon.Then,Superconducting magnetic energy storage(SMES)device and supercapacitor(SC)device are used to suppress the wideband oscillation phenomenon in this paper.The structure and the installation of the SMES and SC are studied firstly.Then,the small-signal stability of the SMES and SC are studied according to the eigenvalue analysis and the DC impedance analysis in the second chapter.Finally,the oscillation suppression effectiveness of the SMES and SC is verified in PSCAD/EMTDC.The oscillation suppression effectiveness of the SC is influenced by the parameters of the capacitor.However,the SC is better than SMES with considering the safety of the whole system.(4)The fast Fourier transform(FFT)was used in previous studies without considering the time characteristics of the oscillation waveforms.The time-frequency characteristics of the oscillation waveforms are considered in this paper.The short time Fourier transform(STFT)is used to study the characteristic of the wideband oscillation in this chapter.The asynchronous characteristic of the wideband oscillation is presented by using the STFT method.The different oscillation frequencies may appear in the different period.The amplitude of the oscillation frequencies may change with time developing.The asynchronous characteristic of the oscillation may help study the oscillation propagation,location,prediction and suppression.