Study On The Injected Current Control And Active Damping Control Of Gird-Connected Single Photovoltaic Inverter With LCL Filter

In exploitation of new energy resource, grid-connected inverter is an interface between the new energy resource and grid or local users. Therefore, the performance of the grid-connected inverter affects directly the efficiency of the new energy. Digitalized high frequency pulse width modulation(PWM) technique makes the control of gridconnected system more flexible, however, it leads to a higher performance requirement for the inverter. Compared with the traditional filter with single L, the LCL filter holds more excellent harmonic suppression capability. Moreover, with the same filter performance, the volume of LCL filter is much smaller than the L filter, therefore, the application of LCL inverters will result in lighter weight and cheaper price. However, the inherent resonance of the LCL filter may make the system unstable, we have to be very careful to design control system. In order to achieve the stability of the system, this thesis presents the damping control for the LCL filter, and also considers the current control and the system stability analysis. The current controller and damping controller are designed, robustness on system parameters variation and adaption on grid voltage fluctuation are analyzed. It also deals with modeling and control in discrete domain, as well as the way and features of current feedback. According to the analysis results, from the point of active damping control, a filter parameter design method based on the optimizing control factors is proposed.In this thesis, the damping control of LCL filter and injected current control are treated as a system. A continuous mathematic model of the system is established. The modified proportional-resonant(PR) controller is adopted as a current controller, and the feedback of the capacitor current is adopted as active damping control of LCL filter. The thesis also deals with the grid impact and proposes a feed-forward control method to eliminate the grid effect on the output currents of the inverter. With the target of stable margins, a systematic parameter design scheme is proposed, and the robustness on the system parameters variation and grid voltage fluctuation are analyzed. Therefore, the system can be stable within certain limits.Because practical application is in the digital form, the system model is transformed into the discrete-time domain. For three cased, i.e., no delay, half period delay and one period delay, the effects of system of performance resulted by delay of sampling and control are analyzed quantitatively respectively. Based on the mathematic model, the range of the active damping control parameter under which system stability maintains is derived. The proportion gain, resonant factor and active damping factor influence the system stability margins, these functions are studied by Nyquist plots.In the grid-connected system, LCL filter provides two possible ways for current feedback, which are discussed particularly in discrete domain. The relation of sampling frequency and resonant frequencies is also considered. The ranges of damping factors are derived for different sampling frequency and different current feedback ways, and then the range of the current controller parameters is also derived. The variation of the damping factor influences the current controller parameters. We study qualitatively these influences. The features of two possible current feedback ways are compared in different sampling frequencies from gain margins, phase margins and vector margins, respectively.Based on the magnitude-frequency and phase-frequency characteristics of the system, an additional digital filter is designed by using damping method to maintain system stability. The mathematic models of the grid current feedback control and inverter current feedback control with additional digital filters are established in discrete domain. To improve the phase-frequency characteristic of the system, a lowpass filter and a lead filter are proposed separately, and the method for designing filter parameters is proposed. The features of grid current feedback control and inverter current feedback control with different filters in different sampling frequencies are compared. To improve the magnitude-frequency characteristic of the system, a notch filter is proposed. The resonant sharp of LCL filter can be compensated by the notch filter. And the notch filter parameter is designed. The features of grid current feedback control and inverter current feedback control with notch filter in different sampling frequency are compared.Considering the designing LCL filter parameters from the point of the active damping control. Taking optimizing the range of the proportional factor as the orientation. To guarantee the system stable, when the capacitor current feedback active damping control is adopted, the ratio of the filter resonant frequency and the control frequency meets different proportional relation, the optimizing method of inductor ratio and the corresponding damping factor is discussed in detail.Based on the theoretical study, the analysis results are verified using MATLAB, and the related experiments are tested in a 3kW laboratory prototype. The simulation and experimental results verified the validity and feasibility of the theortical analysis and parameters design sheme.