Research On The Current Control For LCL-based Grid-connected Converters

An an energy conversion interfaceconnecting renewable energy power generation system and power grid,,LCL-type grid-connected converter is responsible for converting DC into high-quality AC and injecting it into the power grid..For LCL-type grid-connected converters,current control and resonant damping are two significant issues that count for the reliable,stable and high-efficient operation of renewable energy generation systems.This thesis studies the current control and active damping technology of LCL type grid-connected converter,and the main contents are as follows:1)Based on control mechanism,electric circuit models presented to demonstrate the three kinds of current control strategies for the LCL-type grid-connected converter.In the proposed model,as for the control system,the reference of target control variable can be considered as current source,and regulator and the capacitor current feedback serves as its parallel branch.Through equivalent transformation of the electric circuit,a general electric circuit model is derivated.The capacitor current feedback in parallel branch is equivalent to an impedence seried with converter-side inductor.Based on this model,it reveals that the damping performance of control system depends on the impedence.Moreover,for the LCL-type grid-connected converte with state variables feedback,the generalized electric circuit model also is provided.By carefully dealing with feedback variable selection and their control term,the equivalent impedence,seried with converter-side inductor,can contribute to damping the LCL filter resonance and improving other control performance.The circuit model established in this paper can help to analyze and design different current control strategies and active damping strategies from the perspective of impedance,which provides a new understanding and ideas for the research of converter control.2)In this thesis,the influence of active damping and control delay on the performance of the system is analyzed.The analysis points out that:?1?Considering the control delay,the introduction of grid current feedback is equivalent to a virtual impedance parallelled with the filter inductance in the grid side,and its characteristics are related to frequency.?2?The existence of active damping and the control delay,result in the decline of the low frequency band of the system,and may also introduce the unstable poles into the system loop gain.In order to avoid that,the resonant frequency of the active damped inner loop loop must have a certain phase margin at the resonant frequency of the LCL filter,and at 0and f_cr?crossover frequency of active damping inner loop gain?,and meet the strict amplitude margin requirements,to ensure that the system loop gain does not contain unstable poles.Based on the detailed analysis of the system frequency characteristics,this paper proposes a method to design the controller parameters according to the system phase margin,amplitude margin and cutoff frequency.By this method,the correlation coefficient of the grid current feedback and the value of the parameters of the grid current regulator can be obtained easily and accurately.And the analysis of the system performance and the optimization of the closed-loop control parameters can be facilitated.Finally,the simulation model and the laboratory platform are set up respectively.Through the simulation and experimental results,the correctness of the theoretical analysis is verified,and the rationality of the control parameter design is also tested.