Performance Analysis And Improvement Of Grid-Connected Converter Based On Digital Control

The development of renewable energy is necessary and urgent to alleviate the depletion of fossil energy and its harm to the environment.Distributed generation is an important way to develop and utilize renewable energy.The grid-connected converter is used as the interface between distributed generation system and power grid,which is responsible for the stable and efficient transmission of power generated by distributed generation to power grid.Digital control is widely used in modern grid-connected converter because of its flexible control strategy,but it will make the analysis and design of grid-connected converter more complex and the delay introduced by digital control will also affect the performance of the system.Therefore,taking grid-connected converter based on digital control as the research object,this paper studys the rational approximation method of delay link,the capacitive current feedback active damping and suppressing the grid background harmonics by feedforward control.This paper establishes the continuous domain mathematical models of grid-connected converters based on digital control,and analyzes the influence of digital control delay on the active damping characteristics of capacitor current feedback and the stability of controlled object.Then,the Nyquist stability criterion is used to derive the stability constraints of the system.According to the parameters of grid-connected converter,the control parameters are designed by constraining the cut-off frequency and stability margin of the system.The effectiveness of the designed control parameters is verified by simulation and experiment,which lays a theoretical foundation for the follow-up analysis.When Routh criterion and root locus method are used to analyze the stability of the system,it is necessary to rationally approximate the delay link.The different choice of rational approximation function will affect the results of analysis and design.This paper introduces four commonly used rational approximation functions and their characteristics,including first-order,second-order Taylor approximation and first-order,second-order Pade approximation.Then,for the L and LCL grid-connected converters,the system stability constraints obtained by the four approximation methods and the actual effects of the control parameters designed by the four approximation methods are compared respectively.The research shows that the accuracy of the second-order Pade approximation is the highest.But considering the practicality of the approximation method,the first-order Pade approximation is the most suitable method for L grid-connected converter.For LCL grid-connected converter,due to the large error of other approximation methods,the second-order Pade approximation should be used.Considering the digital control delay,the positive damping region of LCL grid-connected converter with capacitor current feedback active damping is only within 1/6 sampling frequency(f_s/6).Therefore,the controlled object presents stable or unstable characteristics with the different value of capacitor current feedback coefficient H_i,which makes the design of control parameters complex and the poor robustness of system.To overcome this problem,adding lead compensation in the capacitor current feedback channel is used in this paper,which can compensate the digital control delay.By analyzing the relationship between lead compensation parameters,positive damping region and the stability of controlled object,this paper proposes a design method of lead compensation parameters,which can expand the positive damping region and ensure that the controlled object presents stable characteristics within the original H_i value range.Then the stability constraints of the system are unified,the control parameter design becomes simple and the system has good robustness.The background harmonics can be effectively suppressed by using voltage feedforward,but the effect of feedforward will be weakened by digital control delay.In this paper,the influence of digital control on the 5th,7th and 11th harmonic suppression ability of power grid is analyzed for the two voltage feedforward modes of proportional feedforward and full feedforward respectively.The research shows that the weakening effect of control delay on proportional feedforward is more obvious than that of full feedforward.To enhance the suppression ability of proportional feedforward to the background harmonics under digital control,this paper adopts the method of adding lead compensation in the voltage feedforward channel to compensate the digital control delay.By analyzing the influence of lead compensation parameters on the equivalent output impedance,this paper proposes a design method of lead compensation parameters,which can increase the output impedance and improve the ability of the system to suppress the grid background harmonics.To prove the correctness of the theoretical analysis and the effectiveness of the proposed control methods,the three-phase grid-connected inverter is used as the experimental platform.According to the experiment results,the correctness of the system control parameters design,the stability boundary of converter system,the improvement of the robustness and the grid background harmonics suppressing ability of system by lead compensation is verified.