Research On Harmonic Detection And Control Strategy Of Three-level Active Power Filter With LCL Filter

With the widespread application of nonlinear loads,harmonic pollution is becoming more and more serious.Active power filter(APF)has been a research hotspot in recent years due to its good effect of harmonic suppression,fast response speed and flexible control mode.This thesis focuses on harmonic current detection method,parameter design,damping strategy and current tracking control strategy of three-level shunt APF with three-phase three-wine NPC structure and LCL filter.Firstly,aiming at the error and delay caused by the phase-locked loop in the traditional FBD detection method,an improved FBD harmonic detection method based on sinusoidal amplitude integrator is presented.In the three-phase three wire system,this method takes use of sinusoidal amplitude integrator to replace phaselocked loop to integrate amplitude of the grid voltage under the ab coordinate system,which obtains the positive sequence component of the fundamental voltage,and then the harmonic component is detected.At the same time,in order to avoid the influence of power grid frequency fluctuation on the harmonic detection accuracy,a frequency adaptive link is structured by introducing the frequency locked loop.Compared with the traditional FBD method,the method owns simple structure and fast dynamic response,the improved FBD detection method based on sinusoidal amplitude integrator has simple structure and fast dynamic response.The simulation results show that the method can detect harmonic components accurately and quickly when the grid voltage is asymmetrical or frequency fluctuates.Secondly,aiming at the problems of traditional parameter design method,such as complex process and large amount of computation,a parameter design of FOA with adaptive step length is proposed based on the analysis of the principle and performance parameters of LCL filter.In order to avoid the problem that traditional FOA is easily trapped in the local optimum,the iterative step length adaptive link is introduced.The method automatically adjusts the FOA iteration step according to the previous generation optimal concentration judgment value and the current iteration number,and uses the harmonic attenuation rate as fitness function.Compared with traditional FOA,the optimization result of this method is not easy to fall into local optimum.The simulation results verify the correctness of the design of the parameters.Thirdly,the damping of LCL filter and current tracking control strategy isstudied.AS for the LCL damping strategy,a compound damping strategy is proposed based on the analysis of the influence of grid side inductance on passive and active damping.The mathematical model of three level APF with LCL type is established in abc??? and dq coordinates,and the neutral point potential balance of the three-level inverter DC side is analyzed.The MPCC strategy of LCL threelevel APF is studied.Aiming at the problem of strong dependence of MPCC control on system parameters,a compound MPCC strategy based on repetitive control is proposed,and the method has good stability and adaptability to parameter changes.Simulation results verify the effectiveness of the proposed damping and compound MPCC strategy.Finally,the experimental platform of the three-level shunt APF with threephase three-wire NPC structure and LCL filter is built,and the main and control circuit of the system are designed.The improved FBD harmonic detection method based on SAI,the parameter design of LCL filter based on FOA with adaptive step length and compound MPCC strategy are verified on experimental platform.The experimental results show that the research content of this paper is correct.Aiming at the problem of strong dependence on system parameters of MPCC control,a compound MPCC strategy based on repetitive control is proposed.