Control Studies Of LCL-Type Grid-Connected Inverter Based On Cascade ADRC

With the rapid development of society and the rapid expansion of the population,traditional energy sources are facing exhaustion,and new energy sources such as photovoltaic and wind energy have been vigorously developed due to their environmental protection and inexhaustible advantages.As an important part of energy conversion-inverter,it has important research value.Because of its small size and good filtering effect,LCL-type filters are widely used in the filtering process of grid-connected inverters,that is,LCL-type grid-connected inverters.However,under the traditional control strategy,the LCL grid-connected inverter has problems such as coupling,resonance,and poor disturbance rejection performance,and it is difficult to achieve satisfactory results.Based on this,this paper applies cascade ADRC to the control of LCL-type grid-connected inverter to solve the problems of coupling,resonance and poor disturbance rejection performance.And the virtual harmonic resistance function is added to the LCL-type grid-connected inverter,which can suppress voltage harmonic of the power grid to improve the power quality of the power system.The main work and innovations of this paper include the following aspects:1.The state equation of the LCL-type grid-connected inverter in the dq coordinate system is derived,and the frequency characteristics of the inverter system under open-loop and traditional closed-loop PI control are analyzed.To analyze the mechanism of resonance,coupling and disturbance,three-dimensional amplitude-frequency characteristic diagram with various parameters are drawn.2.A three-stage LADRC current controller based on full state feedback is designed,which feeds back the three state variables of the inverter system and sequentially applies first-order LADRC control to make the output current track the reference value.The controller can eliminate resonance and coupling,while enhancing the disturbance rejection performance and robust performance of the system.Based on this,one state variable feedback is cancelled,and a two-stage LADRC current controller based on two state feedback is designed.The second-order LADRC and first-order LADRC controls are applied in sequence,eventually achieving the same result as the three-stage LADRC controller.Taking into account the large number of parameters,the second-order LADRC of the two-stage LADRC controller is changed to the first-order LADRC,that is,reduced-order two-stage LADRC current controller.It uses two state variables and two first-order LADRC controls to achieve output tracks input with no error.Then,the parameters of the three controllers designed: three-stage LADRC,two-stage LADRC and reduced-order two-stage LADRC are tuned,and the three-dimensional amplitude-frequency characteristic diagrams of the system changing with the controller parameters are drawn,so as to analyze how controllers' s parameters affect the system.Finally,a set of better parameter values are set,and the effectiveness of the three controllers was verified by MATLAB simulation: suppression of system resonance,current decoupling control of the dq axis,enhancement of the system 's robust performance and disturbance rejection performance.3.Using the remaining capacity of the grid-connected inverter,the function of virtual harmonic resistance is added to the grid-connected inverter.The grid-connected inverter can suppress the voltage harmonic of the PCC in progress of new energy power generation.It can absorb the power harmonic from power grid,and convert it to the fundamental power back to the power grid.When determining the virtual resistance value of each harmonic,the perturbation and observation method is used to automatically adjust the reciprocal value of the virtual harmonic resistance,so that the power of each harmonic absorbed from the power grid is maximized.Finally,the effectiveness of the proposed control strategy is verified by simulation.4.The experimental platform of LCL-type grid-connected inverter with FPGA + dual DSP as the control architecture is built,and the hardware and software are designed.The final experimental results verify the effectiveness of the control strategy proposed in this paper.