| As the energy conversion interface between distributed power generation system and the power grid,the safe and efficient operation of grid-connected converter make a great sense to distributed power generation system.Compared with two-level converter,the three-level converter can improve the performance of grid-connected converter due to its attractive features such as low harmonic contents and less passive components.Besides,the power grid may exist some non-ideal factors such as unbalance,harmonic and grid impedance in practical application,those non-ideal grid conditions will directly affect the grid current,even threatening the safe operation of the grid-connected converter.Therefore,this thesis design an experimental prototype of three phase NPC three-level LCL-type grid-connected converter.With this prototype,this thesis is dedicated to the control schemes under non-ideal grid conditions.Firstly,in order to balance the neutral-point(NP)potential,the region where low frequency ripple can be suppressed and the relationship between neutral point current and the zero-sequence voltage are derived in this thesis.With these analysis,three kinds of zero-sequence voltage injection based NP balance algorithm are analyzed and compared through experiments.Besides,the NP potential balance model is built to analysis the NP potential balance with negative and harmonic current for grid-connected application.To suppress the reference error of grid current caused by non-ideal grid voltage in ?? axis,the small-signal model is built to derive the transfer function of SRF-PLL.Moreover,the transfer function from PLL to the grid current reference is also derived,which provides the foundation for the subsequent chapter.For non-ideal grid conditions,the prefilter transfer function in ?? axis of DDSRF?DSOGI?MCCF and MSOGI are derived to analysis the performance and equivalence of each algorithm.Experimental results verify the theoretical analysis.The stability issue of proportional capacitor-current-feedback active damping is investigated using the current loop gain transfer function in z-domain.Analytic result show that the damping critical frequency of proportional capacitor-current-feedback is fs/6,the system achieve low stability margin and bad dynamic performance with fr = fs/6.In order to extend the damping critical frequency,this thesis propose an improved capacitor-current-feedback method,which can extend the damping critical frequency to fb(fs/6 < fb < fs/3)from fs/6 and improve the adaptability of the converter against the grid impedance.Experimental results are presented to verify the proposed method.The output impedance model for LCL-type grid-connected converter is derived to investigate grid current harmonic suppression issue.With this model,the impacts of feedforward schemes of grid voltages are analyzed,and points out its limitations.In order to improve current harmonic suppression ability,the harmonic resonant controller with phase compensation is used.A design method of compensation angle for LCL-type grid-connected converter is proposed.The effectiveness of the proposed method is verified by experimental results.For weak grid condition,the output impedance based stability criterion is derived,where the effect of PLL has been taken into account.It is pointed out that the grid current reference,power factor,bandwidth of PLL and prefilter have effect on the stability of the converter under weak grid.With these analysis,the parameters of PLL under weak grid are designed.Moreover,the stability of converter with grid voltage feedforward is analyzed.Analytic result show that the grid voltage feedforward might worsen the stability of the converter under weak grid due to negative phase shift of output impedance,but a lower feedforward coefficient can improve system stability.A hybrid control strategy is used to improve the stability and current harmonic suppression ability under weak grid.The control strategy is verified by the experimental and simulation results. |
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