Research On Robustness Of Grid-connected LCL-filtered Inverters For Distributed Power Generation Systems

As the interface between the renewable energy power generation and the grid in the distributed power generation system(DPGS),the grid-connected inverter plays an important role in improving the power quality and reliability of DPGS.With an LCL filter,the grid-connected inverter performs better in suppressing switching frequency harmonics,reducing the filter volume and weight while meeting the grid current standard.Thus,such filter has a high practical value and is widely researched and applied in grid-connected inverters.However,the inherent resonance of the LCL filter has a serious impact on the robustness of the grid-connected inverter as well as the DPGS.Especially when a large proportion or large capacity distributed power is connected to the grid,the non-ideal factors at the PCC,including the grid impedance,voltage fluctuation and flicker,harmonics,three-phase imbalance as well as series and parallel resonance can affect the steady-state performance and robustness of the LCL-type grid-connected inverter.Therefore,this paper studies the key technologies of the grid-connected LCL-filtered inverter for high performance and high robustness.In view of the lack of existing research on the mechanisms of LCL resonance suppression,this paper presents a systematical study of the active damping of LCL filter.The control mechanisms of the LCL-filter active damping(AD)are revealed,and then the current control for damping the LCL resonance is divided into four categories: the feedback compensation,cascade(series)compensation,zero-placement and pole-placement based methods.Based on the systematical study of the AD and its mechanism,the existing AD control methods are all included,and it is readily to propose novel AD control methods.Considering the grid voltage harmonics impact,the aliasing in the discrete sampling,the digital control delay and the implementation cost,the current control methods for the LCL-type inverter are deeply evaluated,and the merits and demerits of each method are clearly identified.With the considerations of good economy and high performance,this paper deeply researches some novel,low-cost,high-performance control methods for LCL-type grid-connected inverter.For the dual-loop current control with the capacitor current and grid current feedbacks,the theoretical derivation proves that improving the inner-loop stability is conducive to simplify the control design as well as to enhance the robustness.Based on improving the phase margin,the improved strategies featured by the phase leading are systematically proposed and studied.By establishing the model for analyzing the grid current low-frequency harmonic suppression,the demerits of conventional grid voltage feedforward and harmonic resonance control are revealed,and the combination of the two low-frequency harmonic suppression strategies is proposed to achieve a high quality grid current.In addition,on the basis of the AD control mechanism,not only the AD control with the second-order high-pass filter feedback of the inverter-side current but also the AD based on the feedback of the grid current through a high-pass plus phase-shifting filter is proposed and deeply analyzed.It has been proved that the proposed AD methods own the advantages of low cost,good performance and high robustness under the disturbance of the LCL-filter parameters themselves.The LCL-filter parameter optimization design is also critical.On the one hand,different current control methods can perform quite different or even fail in damping the resonance for different LCL resonance frequencies;on the other hand,it has been demonstrated that the LCL-filter parameters can affect the ability of the grid-connected inverter in suppressing the low-frequency grid current harmonics.On the basis of above analysis results,with the considerations of the control performance,low-frequency current harmonics suppression and the switching-frequency harmonics suppression,this paper proposes an improved parameter design method integrating with the filter and controller parameters optimizations.Without the need for additional passive damping or active damping,the effective cooperation of the single-current feedback control and the LCL parameters can achieve the stable operation and high robustness of grid-connected inverter while meeting the low-frequency and switching-frequency grid current harmonics standard.When the inverter is connected to the weak grid,the grid impedance can have a serious impact on the system performance.At first,a current control model considering the grid impedance is built,which reveals the reason for the oscillation of the grid-connected inverter in the weak grid.That is,the voltage signal used for the feedforward compensation contains the voltage drop across the grid impedance so that the PCC voltage feedforward introduces an extra grid current positive feedback loop that endangers the stability margin.In addition,the inappropriate harmonic resonant control and AD of the LCL filter can also cause system instability.On the basis of the above foundings,from the aspects of robust control and adaptive control separately,the design and control methods of high performance grid-connected inverter under the weak grid are studied in depth.On the one hand,based on the equivalent cascade model of the inverter-grid system,for improving the robustness(including not only the stability but also the current harmonic suppression)in the weak grid case,a robust design criterion is proposed,and several robust control methods based on the current regulator optimization,the PCC voltage feedforward optimization and the combined optimization are proposed.On the other hand,starting from the root of system instability(i.e.,the additional positive feedback loop introduced by the PCC voltage feedforward),an adaptive grid current feedback control based on the grid impedance estimation and an adaptive PCC voltage feedforward compensation method without additional grid impedance estimation are proposed,with which the current harmonics suppression and the stability in the case of grid impedance varying and voltage distortion are well achieved.As a result,the adaptability of the grid-connected inverter in the weak grid is much enhanced.The grid-connected inverter performance in the case of fluctuation and flicker,harmonics and three-phase imbalance of the grid voltage is analyzed in detail.Under the abnormal change of the grid voltage,the impact of each system parameter on the transient response of current and voltage in the grid-connected inverter is discussed based on the inverter output impedance model.In order to ease the adverse impact of the transient shock on the safety operation of the inverter,the method for improving the reliability by selecting filter and controller parameters is analyzed.In addition,the three-phase grid imbalance and harmonics lead to the existence of harmonics in the phase-locked loop,affecting the synchronization and control of grid-connected inverter.In order to assure that the inverter can still accurately track the current reference,the control scheme based on the variable sampling period phase-locked loop and the negative-sequence grid voltage feedforward is proved to be able to achieve the high performance under non-ideal grid voltage conditions.Finally,the robustness of the grid-connected system with multi-inverter modules in parallel is deeply studied.An analytical model of the paralleled grid-connected inverter system for analyzing the resonance and current harmonic suppression is established.It is revealed that the stability of the paralleled grid-connected inverter system under a specific grid impedance is equivalent to that of a single inverter under the N-times grid impedance(named the equivalent grid impedance).Because of the large equivalent grid impedance,the resonance of the parallel system can move to the low frequency region so that the impact of the phase-locked loop cannot be neglected in the system modeling.When considering the phase-locked loop,the low-frequency amplitude and phase of the inverter output impedance are greatly reduced,which aggravate the system resonance phenomenon.Besides,the expression of the phase-locked loop appears in the denominator of the inverter output impedance,resulting in the limitation of the output impedance amplitude at the low frequency and the suppression of low-frequency current harmonics.Moreover,the large grid impedance can easily cause the inverter over modulation and arouse plenty of harmonics including the integer and inter harmonics.Based on the above analysis of the resonance mechanisms,a series of robust control and adaptive control means for improving the robustness of the paralleled grid-connected inverter system are proposed,including the optimized design of current regulator parameters,the optimized capability of the phase-locked loop in filtering the harmonics and the adaptive phase-locked loop and feedforward strategy based on the grid impedance estimation.