Study On The Process And Mechanism Of Harmonic Interaction In Large Scale Wind Power Grid-Connected Systems

With the transformation of the energy consumption and the energy structure,renewable energy such as wind power has become an important component of the new energy system.The power system is characterized as multi-objective and strong interaction coupling due to the centralized access of the new energy,and the high power-electronized trend of the source-network-load.As a result,wide-frequencydomain harmonic oscillation problem is easily caused when it is disturbed,which poses a potential threat to the safe and stable operation of regional power grids.Aimed at harmonic oscillation problems in large-scale wind power grid-connected systems,this dissertation investigates the harmonic resonance,the interaction influence,the harmonic current propagation characteristics,and the harmonic power responsibility division of the interacted wind power grid-connected systems.Certain reference significance can be provided by this dissertation to suppress the harmonic oscillations and improve the power quality of the new energy grid-connected systems.The main contributions of this dissertation are from the following five-folds.1)A harmonic model of the frequency domain is established for the wind farm grid-connected systems through the problem analysis and the status analysis of harmonic oscillation,and the harmonic dynamic characteristics of main devices including double-fed wind farm units,transformers,and static var generator are analyzed.The harmonics generated by wind turbines can satisfy the grid-connection requirements through the comparative analysis of the system harmonic currents injected into wind farms and wind turbines.Therefore,the wind farm is selected as our research object.A wind farm aggregation equivalence approach of harmonic power loss is proposed to model and calculate the aggregation equivalence of double-fed wind farms.The broadband dynamic characteristics of wind farms can be reflected with the proposed aggregation equivalence model,and the equivalent calculation complexity of the wind farm is reduced.In addition,the actual harmonic current emitted by the wind farm is calculated with the harmonic current at the grid connection point of the wind farm,and the calculation result is closer to the theoretical value of the harmonic current.Finally,the harmonic interaction between the wind farm and the power grid,and the characteristics of harmonic propagation are investigated to provide the theoretical support for the propagation mechanism study of the harmonic interaction in large-scale wind power grid-connected system.2)Aimed at the harmonic resonance problem of wind power grid-connected systems,the mechanism of series-parallel resonance in wind farm grid-connected systems is first explained to address the issue of harmonic resonance.Based on this point,a harmonic resonance analysis approach is proposed,where the frequency scanning and modal sensitivity are simultaneously combined.Specifically,the frequency points at which the resonance is occurred in the wind power grid connected system,can be quickly determined with the frequency scanning,and potential series-parallel resonance frequencies of the system can also be observed.Further,the resonant mode of the system is theoretically derived and calculated by means of the combination of modal and sensitivity analysis.Through theoretical calculations and experimental verification,the proposed approach can determine the resonance location,the participation factors of the node,and the sensitivity of each component to resonance when the resonance is occurred in the wind farm grid-connected system.Finally,the influence of reactive power compensators,the number of wind turbines,and the line length on harmonic resonance in wind farms is also investigated.3)Aimed at the harmonic interaction problem of the wind farms,the harmonic interaction characteristics of wind farm grid-connected systems are investigated quantitatively and qualitatively.On the one hand,the principle of harmonic interaction within the wind farms is explained with the principles of primary-and-secondary emissions,and the main influencing factors of the primary-and-secondary emissions in wind farms are analyzed.On the other hand,the harmonic interaction analysis model of multiple wind farms is established.To this end,a harmonic impedance based quantitative analysis approach is proposed to measure the interaction degree between the primary and the secondary emissions of harmonic currents,and an analytical calculation model of the interaction degree within the wind farms is derived under the principles of primary-and-secondary emissions of wind farms.The interaction factor is defined as quantitative indicators of the interaction degree,and the influence of the connection impedance,the static var generator,and the wind farm equivalent impedance on interaction factors is fully explored.It is shown in the simulation experiment results that the quantitative evaluation of the interaction degree within wind farms can be achieved through the defined quantitative indicators of interaction influence.4)Aimed at harmonic propagation problem under the interaction and cooperation of multiple wind farms,the propagation principle of harmonic current under the same and different voltage levels in wind farms is analyzed.The complex multi-wind farms grid-connected system is simplified into a multi branch radiation network centered on the wind farm through the calculation of the transfer harmonic impedance in multiple wind farms,where the harmonic current problem under the interaction coupling of multiple wind farms is simplified and decoupled.Based on the derivation of the harmonic impedance transferred from wind farm to branch,a harmonic current propagation coefficient is proposed to quantitatively analyze the relationship between wind farm harmonic current and the branch harmonic current,and the specific definition is given.The influence of the transfer impedance changes,the wind farm harmonic impedance changes,and the harmonic frequency on the propagation coefficient are explored.The propagation coefficient of harmonic current can be used to reflect the influence degree of the harmonic current in wind farm on each branch,and the influence range on the system.At the same time,it can be used to distinguish and quantify the source and the influence degree of the branch harmonic current components.To this end,the quantitative analysis of the harmonic currents propagation of wind farms in system branches is achieved.5)Aimed at the accurate evaluation of harmonic responsibility under the interaction of multiple wind farms,a branch harmonic power based wind farm harmonic liability analysis approach is proposed under the consideration of establishing multiple harmonic liability assessment models for wind farms.Firstly,the harmonic contribution coefficients of wind farms to the branch harmonic currents and the node harmonic voltages are constructed and quantitatively analyzed when the harmonic currents are decoupled.Based on this,the definition and calculation approach of the branch harmonic power responsibility coefficient are given by the combination of the contribution coefficients for harmonic current components and harmonic voltage components,where the direct calculation complexity of the harmonic power responsibility is simplified.The proposed approach comprehensively considers the influence of harmonic current and voltage,overcomes the limitation when the harmonic current or voltage is used as the quantitative indicators of harmonic responsibility division.In addition,the calculation model of the harmonic power responsibility in wind farms towards the concerned branches and nodes is derived.It is demonstrated that the proposed approach is more reasonable to assess the harmonic power responsibility of wind farms under the harmonic interaction when the proposed approach is compared with the reference approaches,where the quantitative evaluation of the harmonic power responsibility contribution of wind farms to the of system nodes and branches.where the quantitative evaluations of the harmonic power responsibility for the wind farms to the nodes and branches are achieved.