Improved Active Disturbance Rejection Control On LCL-Type Grid-Tied Inverter Systems

Disturbances and uncertainties widely exist in control systems,and how to suppress these unexpected factors is a promising research area in control engineering.Active Disturbance Rejection Control(ADRC)provides a simple but effective method to handle non-idealities in real industrial systems.As a key interface between distributed power generation system and the grid,grid-tied inverter is confronted with many non-idealities likewise need to be suppressed in control,such as dead-time in PWM,switch on time and on-voltage of switching devices,perturbation of parameters and background harmonics from grid voltages.Due to these negative factors,performance of the system is deteriated and the output of inverters could results in high total harmonic distortion(THD)and high tracking error.In this thesis,LCL-type grid-tied inverter is the object of the study,and the introduction of ADRC and its application to the inverter is presented in detail.The parameters design of extended state observer(ESO)is also improved to enhance the performance of ESO.Besides,based on the essence of ADRC as well as internal model principle,a novel disturbance rejection scheme,ESO based resonant internal model molding strategy(RIM),is proposed in this thesis for LCL-type grid-tied inverter.The investigation of schemes mentioned above is conducted through theoretical analysis,simulation and experiments,and the primary work as well as some innovative results is summarized as follow:1)The disturbance in LCL-type grid-tied inverter system and conventional disturbance rejection schemes are presented,and the core idea and fundamental princle of ADRC is presented in detail by a example of n-order system.2)The design procedure of ADRC on the LCL-type grid-tied inverter is studied in detail.To improve the performance of ADRC controller by utilizing known information of the model,the model compensation-based ADRC is introduced.The conventional ESO designed by bandwidth parameterization needs a large observer bandwidthω_o to achieve an accurate estimation of disturbance,however theω_ocan not be set too large in engineering implementation.An improved design of parameters of ESO is proposed and it provides a more accurate estimation of low-frequency disturbance without increasing the observer bandwidth.Besides,ADRC combined with the feedforward of grid voltages is discussed,and some problems resulting from feedforward of weak grid voltages and corresponding solution are also presented.The effectiveness of proposed schemes is verified by simulations.3)The reference signal as well as the disturbance of an inverter system is sinusoidal signal,while the high tracking accuracy of the reference can be achieved by implanting a resonant internal model in the controller according to internal model principle.Considering the idea of ADRC in which the total disturbance can be compensated so that the actual real performs as a standard form does,a noval disturbance rejection scheme,RIM,is proposed in this thesis.The RIM scheme compensates a real plant into a standard form containing resnant internal model in the inner loop,deploys a PI controller in the outer loop,offers parameters-tuning methods on the basis of stability margin,and ensures a closed-loop system a good performance as well as an ability of disturbance rejection.Considering power loss of passive damping,RIM with active damping is also studied.Finally,the comparision between RIM and conventional ADRC is conducted via simulations and experiments,and the effectiveness of RIM is verified.