Research On Photovoltaic Grid-connected Inverter With Active Filtering Function

With the rapid development of today’s society,all countries are facing energy shortages and environmental degradation.As a new energy source with large reserves and pollution-free,the solar energy has received a extensive attention from scholars at home and abroad.At the same time,the continuous input of non-linear loads has injected a large number of harmonics into the grid,and the power quality has been reduced.In this paper,it is proposes to add active filtering function on the basis of photovoltaic grid-connected inverter,so that the photovoltaic system can control the harmonics in the grid while achieving grid-connected power generation,effectively improve the power quality of the grid,and solve the low utilization problem of photovoltaic grid-connected inverters in poor lighting conditions.This paper focuses on the problems of low utilization of photovoltaic grid-connected inverters and single function of Active Power Filter(APF).Through the analysis of two-stage photovoltaic grid-connected inverters and parallel APF in terms of functions,control methods and The similarity in topological structure and other aspects has been obtained on the same set of photovoltaic grid-connected inverter equipment,while achieving the theoretical basis for photovoltaic grid-connected and active filtering functions.The design puts forward an overall scheme of unified control,and conducts in-depth research on the topological structure and working principle of the system.The two most critical links are harmonic current detection and current tracking control technology.In the harmonic current detection link,the p-q and i_p-i_qdetection methods are analyzed and compared,and the i_p-i_qdetection method,which is more accurate in the case of voltage waveform distortion,is adopted.Aiming at the problem that the phase-locked loop in the i_p-i_qdetection method cannot accurately lock the phase when the grid voltage is asymmetry,the reference voltage extraction method is used to replace the traditional phase-locked loop.This method can still accurately extract the positive sequence under non-ideal grid conditions.Voltage,improve the accuracy of phase lock;considering that the low-pass filter used in the i_p-i_qdetection method causes the system to delay the actual compensation,the current average module is used instead of the low-pass filter to obtain the fundamental component,which improves The dynamic response speed of the system.And analyze the system command current components in different working modes.Through the simulation of the improved i_p-i_qdetection method,it is verified that the proposed harmonic current detection method can accurately lock the phase and has a faster response speed.In the current tracking control,this paper adopts the voltage and current double closed-loop compound control strategy to realize the unified control mode of the system,in which the voltage outer loop adopts the PI controller to ensure that the inverter DC side voltage is stable and the active command current is output at the same time.The single photovoltaic grid-connected mode only needs to use the PI controller to track the active command current.In order to realize the functions of photovoltaic grid-connected and active filtering at the same time,the system mentioned in this article needs to perform the active command current and the harmonic command current at the same time.Tracking control.However,in the dq coordinate system,only a single PI controller cannot track the harmonic current without static error.Therefore,it is proposed to connect multiple quasi-PR controllers in parallel on the basis of PI control,so that the system can control the fundamental component and the main harmonic components.Carry out precise tracking and realize unified control function.The simulation model of the unified control system was established on the MATLAB simulation platform,and the systems running on three different working modes of complete photovoltaic grid-connected,filtering only,and photovoltaic grid-connected and active filter unified control were simulated and verified.The simulation results showed The proposed unified control strategy has a good effect,can meet the requirements of three different working states,and effectively improves the utilization rate of grid-connected inverters.