| The point of common coupling(PCC)of large-scale wind farm is the first place where the bus or node on the high-voltage side of the booster station in the wind farm connects with the grid,and it is an important node for evaluating the power quality.Affected by wind shear and tower shadow effect,there are forced cyclical closely-frequency in narrow-band low-frequency oscillation powers/voltage fluctuations(FCCNL-OPs/FCCNL-VFs)in PCC.With the increase of permeability and single unit capacity,FCCNL-OPs and FCCNL-VFs have become important factors affecting the power quality of wind farm.In this paper,the high permeability wind power intergrated system is taken as the research object,and the following works and innovations are made around FCCNL-OPs suppression,flicker detection error reduction,and joint monitoring,suppression and detection of FCCNL-OPs and FCCNL-VFs:(1)In order to maximize the potential of wind farm to restrain FCCNL-OPs,taking the direct-drive wind power intergrated system as the research object,and using the low frequency oscillation transfer mechanism analysis methods based on the shafting transfer function and the system state space expression respectively,the transfer process of the components such as 3P,6P,etc.in FCCNL-OPs in the system is analyzed.Aiming at the problem that the 3P component can be fully transferred or even amplified,a method of adjusting PI parameters is proposed to improve the system damping,reduce the resonance peak and enhance the ability of passive suppression of the 3P component.(2)In order to realize the active and effective suppression of FCCNL-OPs,a method of FCCNL-OPs suppression based on virtual inertia control is proposed.After analyzing the influences of the proportional and differential coefficients on the FCCNL-OPs suppression effect,the system damping limitation and the shafting torsional vibration limitation,the feed-forward compensation channel of the wind turbine speed is established basing adding the given output power of wind turbine and the second-order band-pass filter into the compensation amount.This method improves the active suppression ability of FCCNL-OPs without reducing the system damping and increasing the torsional vibration of shafting.(3)Aiming at the problem of larger flicker detection error of PCC in wind farm using the traditional digital implementation method of IEC flicker detection,an improved hybrid time-frequency domain digitization method based on image data expansion is proposed.Using the linear convolution and cyclic convolution theory,the FFT,weighted in frequency domain,and inverse FFT processe is transformed into time domain,which avoids the analysis difficulties caused by repeated conversion between time domain and frequency domain.Based on the analysis of the characteristics of two time domain sequences involved in convolution and their influences on the runaway,the flicker detection error of PCC in wind farm is reduced and satisfied the limitation requirement of IEC61000-4-15 design specification for detection error by using the characteristics of mirror data expansion,which can change the phase equivalently and reduce the values of FCCNL-VFs in both ends.(4)Aiming at the problem that the current equipments used for low frequency oscillation monitoring,suppression and detection are independent,self-contained and can not be used for each other,a digital implementation method of IEC flicker detection based on analytic mode decomposition is proposed.This method utilizes the causal relationship among FCCNL-OPs,FCCNL-VFs and flicker to realize the joint monitoring,suppression and detection of FCCNL-OPs and FCCNL-VFs.In addition,the image data expansion is proposed to improve the runaway of Hilbert transform in both ends caused by lower frequency resolution.According to the characteristics of FCCNL-OPs and FCCNL-VFs in wind farm,the bisection frequencies are selected appropriately to meet the requirements of real time and error limit. |
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