Research Of Photovoltaic Grid-connected Inverter Based On LCL Filter

In a photovoltaic grid-connected inverter based on an LCL filter,the parameters of the LCL filter are designed according to the rated power of the grid-connected inverter.The power generation of photovoltaic modules is closely related to the intensity of solar radiation.Since the intensity of solar radiation is constantly changing with time and season,the grid-connected inverter will not keep operating at rated power,so that the filtering performance of the LCL filter cannot be maintained.In the best condition.Based on this,this paper studies the resonant peak suppression strategy of LCL filter and the optimal working state of LCL filter to improve the filtering performance of LCL filter under various solar radiation intensities.The specific content includes:1.According to the project requirements,the two-stage photovoltaic grid-connected structure is taken as the research object,and the mathematical model of three-phase grid-connected inverter based on LCL filtering is derived.At the same time,the current control loop and voltage control loop are designed separately.The control parameters of the current loop are designed according to the Ziegler-Nichols frequency domain PI parameter tuning method,and the PI parameters designed by the method are verified by simulation to have the advantages of widening the system frequency band and stabilizing the passband gain.2.Based on the discussion of the LCL filter parameter design method,the frequency characteristics are analyzed in combination with the Bode diagram.According to the transfer function of the LCL filter,the suppression strategy of the resonance peak of the LCL filter is derived.The advantages and disadvantages of the passive damping method and the active damping method of the LCL resonant peak are compared and verified in MATLAB.Focusing on the filtering performance of the LCL filter under non-rated power,combined with the LCL filter parameter design method,a control algorithm that can effectively improve the grid-connected power quality of the grid-connected inverter under non-rated power is given.Based on the grid voltage oriented vector control(VOC),a frequency control loop is established to determine the optimal switching frequency of the switching tube in the current power state,and the power evaluator,frequency evaluator and frequency hysteresis in the frequency control loop are analyzed and designed.The comparator,in MATLAB,verifies that the proposed algorithm has the advantage of improving the grid-connected power quality of the grid-connected inverter under non-rated power conditions compared to the conventional algorithm.3.According to the technical index of 5kW photovoltaic grid-connected inverter,the specifications of each power device are deduced and the device selection is carried out,and the detailed main circuit design drawing is obtained.For the control requirements,the main circuit is designed with signal detection circuit,drive circuit and other control circuits to complete the DSP-based software program design.4.According to the circuit diagram,a 5kW photovoltaic grid-connected inverter experimental platform was built in the laboratory.The experimental verification of the adaptive switching frequency control algorithm can adjust the switching frequency according to the output power of the grid-connected inverter,so that the LCL filter can adapt to various output powers.Achieve better filtering effect and effectively reduce the harmonic content of the grid-connected current.