Research On Control Strategy Of Three Level LCL APF Based On FPGA

In recent years,nonlinear load has been widely used,resulting in a large number of harmonic current and reactive current injected into the power grid,causing pollution to the power grid,and the active power filter(Active Power Filter,APF)can effectively solve the harmonic pollution problem.Because of its unique topology,the three level APF has the advantages of high voltage resistance and small switching stress.It has great research value and wide application prospect in high voltage and high power applications.Based on the three-phase three wire diode clamped three level active power filter as the research object,the current tracking control strategy,parameter optimization design for LCL filter and active damping control strategy and other key techniques are analyzed.The operation principle and mathematical model of three level APF are the foundation of its control strategy.This paper firstly introduces the working principle of three level APF,a mathematical model is established based on the three level APF in ABC coordinates,in order to facilitate the design of the controller,to continue in the DQ coordinates of the mathematical model and the small signal model is derived,the basis for the design of controller.Current tracking control strategy is the key factor to determine the performance of three level APF.Because the control system is a digital control system,it is inevitable to bring a beat delay for the system control.The traditional predictive current control strategy only makes predictions based on instruction current,this paper presents an improved predictive current tracking control strategy considering the output compensation current,using three level APF discrete mathematical models to predict the output compensation current,which can effectively eliminate the digital system delay,significantly improve the three level APF and dynamic compensation effect response speed.The parameters of the LCL filter have important influence on the three level APF compensation effect and the high frequency filtering performance.For this reason,this paper adopts a graphic based method to design the parameters of LCL filter.The mathematical model of single-phase LCL filter is established,and the parameters,design indexes and steps of LCL filter are explained.LCL filter has resonance problem.To solve this inherent problem,an active damping control strategy based on state feedback pole placement is proposed in this paper.The resonant spike of the LCL filter is suppressed by configuring the closed-loop poles of the system.In order to facilitate the debugging of three level APF,a general debugging system of power electronic converter has been developed,which can realize the status monitoring,data display,modification and system control of three level APF equipment.The system includes two parts of the upper and lower machine,the upper computer is responsible for equipment condition monitoring,data display,modify and.control system,the lower machine is responsible for data collection and storage;the upper and lower computer to complete data transmission through the Modbus communication protocol.To verify the control strategy proposed in this paper,the experimental platformof three level LCL active power filter is designed.The design of the design process of the RC filter parameters of the control circuit are described in detail.Finally,the improved predictive current tracking control strategy and the active damping control strategy are completed on the experimental platform,and the effectiveness of the control strategy is verified.