Research On Active Damping And Current Harmonic Suppression Strategy Of LLCL Grid-Tied Inverter Under Weak Power Grid

As an important device in distributed generation system,the control function of gridtied inverter directly affects the quality of grid-tied current.The series resonant branch of the LLCL filter is composed by connecting a small inductor in series to the filter capacitor branch,and the resonant frequency is designed at the switching frequency,so that the harmonic current near the switching frequency flows through the LC series resonant branch to better filter the harmonics at the switching frequency.As the LLCL filter is a high-order filter,there is an inherent resonance peak,which may cause the system to lose stability and should be controlled.Therefore,the research on resonance suppression of LLCL grid-tied inverter is of great significance.In this paper,the suppression strategies of resonance peak and current harmonics are studied for single-phase LLCL grid-tied inverter.In this paper,the topology,working principle and filtering performance of LCL and LLCL single-phase grid-tied inverter are compared and analyzed,and LLCL grid-tied inverter is selected as the research object of the paper.At the same time,the parameter design principle and process of LLCL filter are briefly described.Then,based on Nyquist stability criterion,the stability interval and the causes of instability of LLCL filter are analyzed,and several methods used to suppress resonance are analyzed,and applied and compared in LLCL filter.In view of the fact that the differential link will be introduced into the active damping method based on LC branch current feedback to calculate the feedback coefficient,this paper presents a method to calculate the feedback coefficient indirectly by capacitive current feedback.Finally,on the basis of the idea of "phase reshaping",an active damping method based on biquad filter is proposed,its transfer function and topology in time domain and discrete domain are analyzed,and it is applied to LLCL filter.The feasibility of this control strategy is proved theoretically,and the parameter range of biquad filter is given.Because the grid-tied inverter system contains a large number of low-frequency harmonics in the grid-tied inverter system,it is necessary to select a suitable current controller to suppress.In this paper,the traditional repetitive control based on internal model principle is analyzed,and in view of the poor stability and dynamic performance of the traditional repetitive control,a proportional integral multiple resonant(PIMR)repetitive controller is adopted,and its stability is analyzed,and the parameter design process is given.Because the grid voltage disturbs the grid-tied current and also affects the quality of the grid-tied current,this paper also gives the design process of grid voltage feedforward control strategy and its feedforward formula.By analyzing the cause and influence of the formation of the weak power grid,this paper gives the range of the power grid impedance,and analyzes the stability of the system under the weak power grid.At the same time,the stability criterion is given by combining the biquad filter and the PIMR repetitive controller.In order to verify the above theoretical analysis,the Simulink simulation model of LLCL single-phase grid-tied inverter is built,and the simulation model parameters are given.Then the Fourier analysis of LLCL filter and LCL filter is carried out,and it is proved that LLCL filter can suppress high-frequency harmonics at the switching frequency and has a better harmonic suppression effect than LCL filter.Then,by comparing the Proportional multiple resonant(PMR)current controller with the PIMR repetitive controller,it is proved that the PIMR repetitive controller has better dynamic performance and stability.Then,through the comparative analysis of the current waveforms and Total Harmonic Distortion(THD)of the four damping methods,it is proved that the active damping method based on biquad filter has a relatively good harmonic suppression effect.Finally,the four damping strategies are compared and analyzed in the weak power grid,and it is proved that the damping method proposed in this paper has good robustness even in the weak power grid.