Three-Phase Three-Level Parallel Active Filter Model Predictive Current Control

The active power filter is less affected by variation of system impedance and load,operation mode which can be used to flexibly compensate harmonic current and reactive current in system.Therefore,it is of great practical significance for research on harmonic control and improvement of power quality.In this paper,a Three-level Active Filter with LC output filter and three-phase diode clamped(NPC)topology is taken as the research object,and the model predictive current control of parallel active filter(APF)based on variable sampling frequency modulated sliding window iterative Fourier harmonic extraction algorithm is studied.First,in this paper,the three-phase NPC three-level topology is introduced,of which the basic operating principle is summarized.On this basis,the state equation of APF of the threephase three-level NPC topology in continuous domain of the natural coordinate system is established by defining the switching function.The reason of the unbalance of the DC side voltage of midpoint electric potential of capacitor is analyzed,and the fundamental wave component is taken as an example to deduce the instantaneous fluctuation of the DC side voltage of the midpoint electric potential of equalized capacitors in detail.Then,the mathematical expression of SDFT harmonic extraction algorithm is derived,of which the stability is analyzed meanwhile.Furthermore,the mSDFT harmonic extraction algorithm is derived based on the SDFT algorithm to solve potential instability problem.At the same time,to solve the fixed problem of DFT algorithm of spectrum leakage and hurdle effect caused by cutoff of sliding window and variation of fundamental frequency,the VSFT algorithm with PI correction is proposed,which does not need zero-crossing detection,and realizes the sampling frequency which adapt on actual fundamental frequency while the sampling window length is constant.The whole model of variable sampling frequency mSDFT harmonic extraction algorithm is built in Matlab/Simulink,which is simulinked in mutation of frequency and amplitude.The simulation results show that the proposed algorithm can accurately and quickly extract harmonics and meet real-time of harmonic instruction acquisition of APF.Then,the basic principle of model predictive control is analyzed,and the related cost function is established.On this basis,the optimal modulation voltage vector is obtained by derivation of the modulation voltage by the cost function,which solves the problem that the traditional model predicts the current control switching frequency is not fixed and large amount of rolling optimization calculation.Aiming at the problems of beat difference in harmonic current compensation and delay of digital control system,real-time compensation is realized by multi-step prediction,and the optimal switching vector is converted to the optimal switching sequence by VVSVPWM modulation algorithm embedded with neutral-point potential balance,which realizes APF harmonic current compensation and neutral-point potential balance control.Finally,a three-phase NPC three-level topology APF harmonic current compensation system is built in the Matlab/Simulink simulation platform,based on which the harmonic extraction algorithm and current control algorithm is simulated.The harmonic compensation performance of APF combined with the harmonic current compensation algorithm and the harmonic current extraction algorithm is verified under different load conditions and grid-side voltage fundamental frequency.In order to further verify the control method,the main circuit model of a three-phase NPC three-level topology of APF is built in the RT-LAB hardware-inthe-loop simulation platform.The harmonic extraction and APF harmonic compensation algorithm and modulation algorithm are realized by using DSP+FPGA controller.The hardware-in-the-loop experimental results and simulation results are compared to further verify the correctness and effectiveness of the control method in this paper.