Current Tracking Robust Control Of Grid-connected Inverter Based On State Space Approach

The grid-connected technology and output current tracking of three-phase grid-connected inverter are among the hot research topics at present,with a wide range of applications in efficient power equipment,electric car charging stations,distributed generation,active power filter(APF)and other fields.The circuit topology structure and control strategy of these applications are almost the same,and the essential problem here is the robust control of grid-connected inverter current tracking.Therefore,this thesis makes an effort to develop a current tracking robust control scheme for grid-connected inverter based on state space approach,selecting the SAPF as the target system.A thorough research has been conducted,focusing on current tracking control strategy,aiming to develop a new SAPF control strategy for fast tracking of compensation current with higher accuracy,while effectively suppressing harmonics and compensating reactive power.Firstly,this thesis analyzes the key problem in suppressing noise harmonics of inverter power switching device,i.e.,the selection of a suitable output filter.An LCL type filter has a better attenuation effect than an L type filter,but it is more likely to produce resonance.On the other hand,the traditional passive damping scheme suffers a big system loss.So in this thesis,a full state feedback active damping scheme is adopted to improve the system dynamic response and stability via pole placement,achieve resonance suppression and avoid the damping loss as well.Secondly,in view of the need for an accurate mathematical model in the development of SAPF control strategy based on state space,the thesis analyzes the state space models of the traditional ABC coordinate system,alpha-beta coordinate system and dq coordinate system respectively.Next,the concept of plural matrix is introduced.The complex form of state-space model is first established in the synchronous rotating d q coordinate,in which the active and reactive power components can be decoupled,and at the same time the harmonic current can be directly extracted.Thus,harmonic detection and compensation control in the APF system are unified in the same realization,and their discretization control are then carried out.Next,the thesis elaborates on the core control part.The detection of harmonic current is achieved via an improved dq method based on the instantaneous reactive power theory.The current tracking control strategy is formulated in the full state feedback framework.The current controller is enhanced with bus voltage stability control and power grid voltage feed-forward compensation in the design.With the introduction of many state variables,the cost of system would increase significantly if each variable is measured via a sensor.So,state observers are designed to provide estimations of state variables.Finally,the discretized controller for SAPF with IGBT high frequency switches is established and run in MATLAB/Simulink platform.Through the analysis of the output compensation waveforms,it is shown that the new control strategy can follow the target current quickly in the transient process,and has high compensation accuracy at the steady state.As for the serious distortion caused by nonlinear rectification load,the total harmonic distortin can be reduced to less than 5%,indicating a great improvement on the quality of power grid,so the correctness and validity of the control scheme is verified.The integration of detection and control,removal of damping loss and the robustness in current control will bring new insight to engineering applications.