Research On Control Strategy Of LCL Grid Connected Inverter In Weak Grid

Grid connected inverters are the key devices of new energy generation units such as photovoltaic and wind power to transmit power to the grid.Their control performance is directly related to the safe,stable and high-quality operation of grid connected inverter systems.With the large-scale interconnection of new energy generating units,the power grid gradually presents weak grid characteristics,such as the grid impedance can not be ignored,and changes in a wide range,unbalanced grid voltage,etc.In weak grid,how to improve the control performance of grid connected inverters needs to be researched in depth.In this thesis,three phase LCL grid connected inverters are taken as the object of research.The adverse effects of grid impedance and unbalanced grid voltage on grid connected inverters are mainly researched,and corresponding improved control strategies are proposed.A common coupling point voltage feedforward compensation strategy is proposed to solve the problem of grid-connected current distortion and system instability caused by the strong coupling between grid impedance and phase-locked loop.A small signal model of LCL grid-connected inverters with phase-locked loop in dual dq coordinate system is established by using small disturbance modeling method based on impedance model.The coupling mechanism between the grid impedance and phase-locked loop in weak grid and its adverse effects on output impedance and system stability of grid connected inverter systems are analyzed.The output phase offset of PLL caused by strong coupling is compensated by high frequency voltage feedforward,which changes the output impedance of grid connected inverters,improves the phase margin of the system,and effectively restrains the grid connected current distortion.The simulation results are carried out in different grid impedance conditions.Aiming at the problem that the traditional grid current feedback active damping strategy may fail when the grid impedance changes in a wide range,an improved grid current feedback active damping strategy based on phase advance compensation is proposed.By analyzing its active damping characteristics and failure mechanism,it is clear that the phase lag caused by control delay is the main cause of active damping failure.The phase advance compensation method is introduced into the grid-connected current feedback control,which effectively reduces the influence of control delay in active damping,enlarges the effective damping range of the active damping,and improves the robustness of the system to the grid impedance.The damping effect of the two active damping strategies is simulated and compared in the wide range variation of grid impedance,and the superiority of the proposed strategy is verified.Aiming at the problem that the current control and power control of grid-connected inverters are difficult to coordinate when the voltage is unbalanced,a current/power coordinated control strategy of grid connected inverters in static coordinate system is proposed.The mathematical models of the system in two control modes are established,and the generation mechanism of grid connected current distortion and unbalance,output power fluctuation of grid connected inverters is analyzed.A cascaded low-pass filter is designed to filter the harmonic component in the instruction current signal.The harmonic content in the instruction current is adjusted by weighting method to coordinate control the grid current quality and output power to meet the operation requirements.It is easy to realize because it does not need phase-locked loop,voltage/current positive and negative sequence separation,etc.The simulation results verify the effectiveness of the proposed control strategy.A weak grid experimental platform of three phase LCL grid connected inverter is designed and built.Experiments verify the validity of the proposed common coupling point voltage feedforward compensation control strategy and the improved grid current feedback active damping strategy when the grid impedance varies in a wide range,and the grid current/power coordination control strategy when the grid voltage is unbalanced.The experimental errors are also analyzed.