Digital Filter Based Active Damping And Self-Synchronizing Phase-locked Technology For LCL-type Grid-connected Inverter

As an interface between renewable energy based distributed power generation systems(RE-DPGSs)and power grid,LCL-type grid-connected inverter is widely used due to its better harmonic attenuation ability,smaller size and higher power density.To ensure the safety of the grid-connected system,this thesis devotes to the digital filter based active damping and self-synchronous phase-locked loop(SS-PLL)for the LCL-type grid-connected inverter,and it is organized as follows.First of all,the mathematical model of the LCL-type grid-connected inverter with grid current feedback(GCF)control and digital filter based active damping is established.Based on this model,the nature of the digital filter damping is revealed,i.e.,the frequency response of the loop gain at the LCL resonance frequency f_r is reshaped by the amplitude or phase compensation of the digital filter,which invalidates the negative crossing at f_r.Meanwhile,the quantitative requirement of the system stability on the phase of the digital filter is derived,and the stable phase region is obtained.The stable phase region shows that when f_r is lower than one-sixth of the sampling frequency(f_s/6),only phase-lag compensation is appropriate;while f_r is higher than f_s/6,both phase-lead and phase-lag compensation can be used.In addition,in terms of this stable region,the damping effects of the existing digital filter damping strategies are compared.The result indicates that although these digital filters can extend the stable range of f_r to the frequency band lower than f_s/6,they degrade the inherent damping characteristics more or less,and cannot guarantee the system stability when f_r varies in a wide range.It is known that the variation of LCL filter parameters and grid impedance lead f_r to fluctuate in a wide range.To ensure the system stability under these conditions,four digital filters are recommended,i.e.,first-order phase-lag filter(PLF),second-order phase-lag-lead filter(PLLF),biquad filter(BF)and quasi-notch filter(QNF).Then,an optimization design method of the digital filter parameters is proposed.It shows that the well-designed digital filter can damp the resonance efficiently without degrading the inherent damping characteristics,and the stability range can be extended to the frequency band lower than f_s/6,implying that the high robustness against the variation of f_r can be achieved.To reduce the voltage sensor,the SS-PLL are employed to achieve the synchronization of grid current and grid voltage.The basic principle of SS-PLL is described,and then the phase-locked error of SS-PLL is analyzed.Furthermore,the output impedance of the grid-connected inverter considering SS-PLL is derived,which helps to unveil the influence of SS-PLL on the system stability.It finds that the phase-locked error is highly related to digital control delay,digital filter,current reference amplitude and filter inductances.Besides,it shows that in the stiff grid,the stability of the inverter will not be affected by SS-PLL;in the weak grid,SS-PLL poses great impact on the system stability,and the impact mainly depends on the current reference amplitude rather than the bandwidth of SS-PLL,which is the key difference between self-synchronous phase-locked technology and the traditional phase-locked technology.Finally,experiments are conducted on a 6-k W single-phase LCL-type grid-connected inverter,and the experimental results verify the effectiveness of the concerned digital filters and the proposed parameter design method,as well as the correctness of the theoretical analysis of SS-PLL.