Study On Harmonic Suppression Of Three-phase Grid-connected Inverter Based On FFT

In recent years,three-phase photovoltaic inverters have been widely used as important interfaces for grid connection of solar power generation.Due to the access of a large number of non-linear loads,there are more odd harmonics in the power grid while the photovoltaic inverter is transmitting active power to the power grid,which distort the voltage and current,affect the power quality of the power grid seriously.Therefore,how to improve the harmonic suppression performance of grid-connected inverters has become a current research hotspot.In the actual field,the DC-side capacitor voltage imbalance,phase-locked loop accuracy and the real-time nature of harmonic sampling all have a large impact on the effect of harmonic suppression.Therefore,this paper proposes a three-phase grid connection based on Fast Fourier Transformation(FFT)inverter harmonic suppression strategy.First of all,this paper proposes a DC inverter control system for photovoltaic inverters.By designing Boost circuits,DC-Side capacitor voltage equalization circuits,active and reactive decoupling controllers,the topology and control are optimized to achieve the photovoltaic inverter DC-Side voltage regulation control.Then this paper performs harmonic detection,uses the instantaneous reactive power detection method to detect the load current and extract the total harmonic current;uses FFT algorithm based on Field Programmable Gate Array(FPGA)to detect the 25 th odd harmonics and extract the amplitude and phase.Harmonic compensation is performed after the harmonic detection is completed.In order to address the problem of harmonic tracking performance of the current inner loop of the Active Power Filter(APF),a parallel quasi-proportional resonance controller is used to perform no-static control of each harmonic.In order to improve the adaptation to current fluctuations ability to reduce the amount of mutual interference output from the parallel controller.This paper proposes an adaptive APF control strategy based on FFT,which improves the accuracy of harmonic compensation.Secondly,in order to solve the “out-of-lock” phenomenon of traditional phase-locked loops in complex power grid environments,this paper designs a three-phase phase-locked loop based on a double second-order generalized integrator.The CORDIC algorithm is used to perform trigonometric function calculations instead of the look-up table and implement on the FPGA.The above implementations improve the phase lock accuracy and speed.The on/off of the inverter power device affects the harmonic suppression effect directly.This article uses an Space Vector Pulse Width Modulation(SVPWM)modulator to control the switching tube to improve the voltage utilization rate and reduce the harmonic components of the inverter output current.Finally,Simulink is used to simulate the phase-locked loop,detection algorithm,SVPWM controller and other modules to verify the module function.The simulation results of the full odd harmonic suppression and the harmonic suppression of harmonic HRU exceeding 5% are compared.The current THD value was reduced from 21.24%before suppression to 1.87%(full suppression)and 1.83%(adaptive),which verified the effectiveness of the adaptive APF control strategy.Build a three-phase photovoltaic inverter experimental platform and verify the feasibility of the harmonic suppression strategy by observing the current waveform of the oscilloscope.This project study adapts to the development needs of China’s green grid,it is of great significance to further increase the performance of three-phase photovoltaic grid-connected inverters and improve their practical engineering application capabilities.