Robust Current Control Of LCL Filterbased Grid-connected Inverters

Voltage source inverters play an important role in connecting energy sources with the grid utility.Such inverters are exponentially increasing as a consequence of rapidly growing distributed power generation systems(DPGS)based on renewable energy sources in the last few years.Grid-connected inverters are typically controlled by a current control loop to regulate the current injected into the grid based on the grid demands.An output filter is commonly placed between the inverter and the grid to attenuate the high frequency switching harmonics of the inverter.Regarding the output filter,inductive-capacitive-inductive(LCL)topology has gained much attention with clear advantage in terms of high harmonic attenuation,reduced component size and minimal cost compared with an L filter topology.However,due to the resonance hazard of such filter,damping methods are needed to ensure system stability.To avoid the power losses associated with passive damping,active damping is recommended for better efficiency.Active damping control has significant drawbacks.First,an accurate knowledge of the resonance frequency is required during the controller design process but unfortunately,this frequency depends on the grid impedance value at the point of common coupling(PCC)and such parameter is uncertain or unknown to the manufacturer.Additionally,high disturbance rejection of grid voltage disturbances is required.In fact,small distortion of the grid voltage considerably increases the total harmonic distortion(THD)in the injected current which may lead to system instability.Therefore,robust active damping control that can well accommodate both disturbances and uncertainties is necessary.In this sense,robust control techniques become of interest.This thesis focuses on the current control of voltage source inverters connected to the grid through LCL filters.The main objective is to propose new robust active damping control to improve the stability performance of grid-tied inverters under parameters uncertainties and grid disturbances.Recent advances in robust control theory are a key challenge in this research.Presented in this work are three control strategies used in this purpose.First,a robust observer-based state feedback controller is proposed to suppress the resonance that is caused by the LCL filter-based grid-tied inverter.The proposed control is straightforward to design and uses the available grid current measurements to estimate the state feedback by eliminating the need for additional sensors.The control approach is novel,in the sense that both parameter uncertainties and grid disturbances are considered in the observer design where new sufficient linear matrix inequality(LMI)conditions are derived to determine the control parameters.Compared with classical observers,the proposed approach achieves better stability performance under grid impedance variations.Capacitor current feedback control based on uncertainty and disturbance estimator(UDE)is developed in the second part of this thesis.The state feedback controller is designed to damp the LCL filter resonance,while the UDE is introduced to achieve the desired control performance and robust stability against the effects of parameter uncertainties and grid disturbances.A simple tuning algorithm for the proposed control design is also provided.It is shown that the UDE-based control offers a new method for designing implementable robust controllers for LCL-filtered grid-connected inverters.During the last part of this research work,a novel robust control for grid-connected inverters with LCL filters is proposed based on active disturbance rejection control(ADRC).Following the stability analysis,the control scheme is greatly simplified based on low-order approximation of the LCL filter.The control strategy is also applied to grid-connected photovoltaic systems.All the theoretical analysis is supported by simulation results to illustrate the absolute stability of the control system.Finally,the proposed control method is implemented on a laboratory scaled LCL filtered grid-connected inverter in order to validate the effectiveness and practicality of the ADRC based control method.