Research On Control Strategy For Grid-connected Inverter With LCL Filter

With the wide application of photovoltaic grid-connected power generation system and the increasing demand for grid-connected power quality,the control technology of photovoltaic grid-connected inverter is becoming a hot area of research.Grid-connected inverter is the core device in the whole photovoltaic power generation system.Its performance is the guarantee of high quality electric power and efficient grid generation.Therefore,the research on photovoltaic grid-connected inverter control technology has high theoretical significance and application value.In this thesis,T-type three-level inverter topology is selected as the main research object,LCL filter is the output filter.Aiming at the problem of unbalanced midpoint potential in T-type three-level topology,resonance caused by LCL filter and deficiency caused by traditional PI controller,the mathematical model is established to study the SVPWM modulation algorithm,the active damping control principle of LCL filter and the control technology of the photovoltaic grid-connected inverter.Firstly,based on the comparison and analysis of the system structure and topological cost,the T-type three-level inverter is selected as the main circuit topology from four typical three-level inverter topologies of midpoint clamp,flying capacitor,unit series and T–type.The mathematical model is established in different coordinate systems and its working principle and characteristics are analyzed.The calculation method of LCL filter is given.The simulation results show that LCL filter has good harmonic filtering effect and the obtained grid-connected current is of good quality.Secondly,in order to solve the problem of unbalanced midpoint potential of T-type three-level inverter,an improved SVPWM midpoint potential balance modulation algorithm is adopted.According to the complementary property of the positive and negative small vectors to the midpoint potential,when the composite vector is selected for the reference voltage,the positive and negative small vectors appear in pairs.The distribution factor was introduced in the modulation cycle and the time of the positive and negative small vector was adjusted and controlled by using the principle of the volt-second equilibrium,so as to achieve the balance of the midpoint potential.Simulation results show that the proposed method can effectively control the neutral point potential balance.Finally,aiming at the phase and amplitude error of the traditional grid-connected control strategy when PI controller is used to track the grid-connected current,the fuzzy PR controller is proposed as the main controller of the grid-connected current feedback loop,and the grid voltage feed-forward is introduced to improve the response speed and dynamic performance of the system.The capacitance current proportional feedback loop is used to suppress the resonance caused by LCL filter.Therefore,this thesis adopts the compound control strategy of capacitive current proportional feedback control and fuzzy PR controller and the control strategy of the power grid voltage feed-forward.At the same time,the parameters of the control system are designed according to the Routh criterion,and the system performance of is verified by the amplitude margin and phase margin of the system open-loop bode diagram.The simulation control system is built in MATLAB/Simulink.The simulation results show that the system can run stably and has no error in tracking the grid-connected current,which improves the quality of the grid-connected current.The system has good dynamic response ability and stability margin,and meets the control requirements of photovoltaic grid-connected system.