Research On Harmonic Dynamic Model And Resonance Analysis Of Wind Parks

In recent years,in response to the energy crisis and environmental pollution,wind power is developing rapidly.Compared with thermal and hydroelectric power generation,wind power generation has intermittent and stochastic characteristics of output power.Large-scale access of power electronic devices brings high-frequency and wide-frequency harmonic current,which is easier to produce resonance with the power grid and endangers the safety of equipment.Therefore,the establishment of harmonic domain dynamic model of wind farm grid-connected,the study of harmonic resonance characteristics of grid-connected are conducive to the safe and reliable operation of wind turbinesIn view of the harmonic pollution caused by wind farm interconnection,the harmonic domain dynamic model of wind farm interconnection is established,and the harmonic current of wind farm interconnection point is calculated and its characteristics are analyzed.Based on this dynamic model,the harmonic resonance problem is studied.The main research contents are as follows:(1)Dynamic model of grid-connected harmonic domain for single doubly fed induction generator(DFIG)and harmonic current analysis.Based on the harmonic emission and propagation mechanism of DFlG it is decoupled into grid-side and machine-side grid-connected,two harmonic voltage sources are obtained,the harmonic control and propagation model of converter is established,and the harmonic characteristics of single DFIG grid-connected are analyzed(2)Based on the harmonic model of single DFIG grid-connected,the dynamic effects of wind speed,grid background harmonic,carrier frequency and dead time on Grid-connected harmonic current are evaluated.The background harmonic of power grid affects the low-order harmonic current of grid-connected in a positive proportion;the wind speed mainly affects the high-order harmonic current of grid-connected point.Within the working wind speed,with the increase of wind speed,the amplitude of high-order harmonic current decreases.After reaching the rated wind speed,the amplitude of high-order harmonic current no longer changes;the dead-time affects the low-order harmonic current of grid-connected,and the carrier frequency affects the grid-connected point in a positive proportion.High-order harmonic current is followed by high-frequency harmonic which is odd times of carrier frequency,even times of carrier frequency plus or minus odd times of modulation wave frequency,and even times of carrier frequency plus or minus even times of modulation wave frequency.(3)Establishment of harmonic domain dynamic model of wind farm.Based on the harmonic domain dynamic model of single DFIG grid-connected,combined with the operation characteristics of wind farm itself,the multi-machine equivalent method is used.Firstly,the wind speed state parameters of wind turbines are used as clustering index to calculate the clustering center points and group the wind turbines Then,based on the harmonic model of each component of wind farm,the equivalent parameters of the equivalent wind turbines in the wind farm are calculated,including the equivalent input wind speed,the motor and the equivalent.The harmonic domain dynamic equivalent model of wind farm is established based on the parameters of feeder and transformer connected to the grid.The accuracy of the multi-machine model is verified by comparing the output active power and total harmonic current distortion of the multi-machine model,single-machine model and detailed model with the variation of wind speed.Chinese style(4)Research on resonance of wind farm grid-connected system.Based on the harmonic load model of wind field and modal analysis method,the generation mechanism of resonance problem is analyzed,and the node sensitivity of each resonance point is quantified.The contribution degree of each node to the resonance point is calculated to provide theoretical basis for the suppression of resonance.Based on sensitivity analysis method,the impedance sensitivity and frequency sensitivity of each resonance point are studied,and the influence of system parameters on resonance frequency and amplitude is quantified.According to the analysis results of sensitivity index,the capacitance and inductance parameters of the cable can effectively adjust the resonant amplitude and frequency.The amplitude of resonance decreases with the increase of capacitance to ground and increases with the increase of inductance.The resonant frequency decreases with the increase of capacitance to ground and inductance.At the same time,based on the harmonic domain model of wind power Grid-connected system,the harmonic stability of wind farms connected to the grid is studied by using impedance analysis method and Nyquist criterion.The influence factors of harmonic instability are further analyzed,and the influence of parameters and working conditions of wind farm control system on harmonic stability is studied.Through analysis,it can be concluded that the stability of the system will decrease with the increase of the parameters of the current proportional link and the current integral link on the network side,and the sensitivity of the parameters is higher.The stability of the system decreases with the increase of the rotor side current proportional link parameters and current integration link parameters,and the sensitivity of the parameters is low.Sampling frequency does not affect the stability of the system.