Research On Control Strategy Of Three-phase Grid-connected Inverter Under Unbalanced Grid Voltage

With the energy crisis and environmental pollution becoming more and more serious.The development and utilization of new energy is imminent,such as solar energy,wind energy,and geothermal energy.In recent years,our country is increasing its investment in photovoltaic power generation and wind power generation.A reasonable and efficient control strategy of grid-connected inverter is a key technology for connecting these new energy sources to the grid.And the control strategy will directly affect the quality of the system's power generation.Because of the dynamic nature of the power grid,it cannot be completely balanced and stable.Due to the influence of access load,resonance,and failure,abnormal voltage conditions such as network voltage imbalance and distortion may occur.Especially when the grid voltage is unbalanced,the voltage negative-sequence component will cause double-frequency oscillation of the DC-link voltage of the grid-connected inverter.When the voltage exceeds the maximum value,it will easily cause the system to trip,and the AC side current will superimpose the low frequency harmonics which will cause the distortion and power fluctuations.At the same time,the appearance of the negative sequence voltage component also affects the accuracy of the phase-locked loop,making the frequency estimation or phase detection error larger,and cannot meet the control requirements.Therefore,it is necessary to study the grid-connected inverter control strategy under the grid voltage imbalance condition.The paper will take a three-phase LCL gridconnected inverter with unbalanced grid voltage as the research object,and put forward the scheme of positive and negative sequence segregation based on double second-order generalized integral and the scheme of improving phase locked loop which are able to quickly and accurately separate the positive and negative sequence components from the unbalanced grid voltage,furthermore,it also can extract the angular frequency or phase angle of the positive sequence voltage component,Providing the basic conditions for the research of subsequent grid-connected inverter control strategies.Since the mathematical model of the grid-connected inverters on the ?-axis and ?-axis is strictly symmetrical and mutually independent in the two-phase stationary coordinates,there is no strong coupling term.Based on this characteristic,the current proportional resonance controller is designed,the controller not only can make the current without static error,but also can avoid the synchronous rotation coordinate transformation and decoupling operations in the traditional PI control strategy,and simplifies the controller design.When the voltage is unbalanced,the negative-sequence component causes the inverter power to fluctuate,the DC bus voltage to be unstable,and the quality of the grid-connected current to be poor.In order to effectively solve these problems,the current reference values of the three control schemes that suppress the current negative-sequence component,constant active power,and constant reactive power as control targets are given after analysis and calculation.Combining the characteristics and connections between the three control schemes,a power-current coordinated control strategy is proposed,which can switch back and forth between different control objectives.The control method is flexible and is conducive to the system working in the best state.In order to verify the feasibility and effectiveness of the control strategy,a system simulation model was built on the MATLAB/Simulink simulation component,focusing on the verify of three targets themselves and their collaborative control,such as the current controller's no-difference control,positive and negative sequence separation schemes,phase-locked loops and grid-connected inverters.At the same time,a three-phase LCL filter grid-connected inverter experiment platform was built based on the system simulation model,and the design of main circuit of the system and the scheme of DSP software program design based on TMS320F2812 are given.Based on this,relevant experimental studies were conducted.Finally,the experimental results are obtained and the experimental results are analyzed to verify the feasibility of the proposed control strategy.