Research On Control Strategy Of Three-level Active Power Filter

Due to the widespread application of non-linear loads in China’s low-voltage power distribution system,power quality problems such as harmonic and three-phase unbalance in the power grid are becoming more and more serious.Active Power Filter(APF)is able to compensate for harmonic and unbalanced loads effectively.In this paper,the threelevel NPC structure parallel APF is taken as the research object,the harmonic current detection method and current tracking control strategy are studied separately.An ip-iq harmonic detection method based on improved virtual flux orientation is studied.In order to ensure that the system can work effectively under the harsh conditions of the power grid,the detection error caused by the traditional ip-iq harmonic detection algorithm due to grid voltage asymmetry,waveform distortion and DC offset component injection is eliminated.The virtual flux orientation based on composite second order generalized integrator is proposed.The orientation strategy enables the detection methods to ensures the accuracy of identification under severe conditions.An improved model predictive current control strategy for three-level APF multiobjective optimization is proposed.Estabish the mathematical model of the three-level NPC structure APF.The influence of each switch vector on the midpoint potential of the DC side is analyzed.The sector segmentation method is used to divide the vectors into intervals to reduce the number of optimal switching vector rolling times.The midpoint potential and the switching frequency constraint are introduced into the value function to achieve multi-objective optimal control.The compensation effect under the constraint condition is compared and analyzed.Finally,the dynamic response performance of the algorithm is verified by the sudden load simulation.The simulation results show the feasibility and superiority of the simplified multi-objective model predictive current control strategy.Finally,a model predictive voltage control strategy based on sector classification judgment is proposed.Aiming at the problem of multiple rolling in model predictive current control strategy,the idea of space vector pulse width modulation(SVPWM)based on αβ coordinate system can be used to determine the optimal from 27 vectors by using the judgment of three judgements of the sectors.By converting the three-level reference voltage vector into a two-level reference voltage vector,then using the midpoint potential constraint to select the redundant small vector to perform the inverse correction of the two-level vector,the optimal switching vector can be determined without scrolling optimization.The process improves the efficiency of optimization.The simulation results verify the effectiveness of the improved model predictive voltage control strategy under steady and dynamic states.This paper has 51 pictures,8 tables and 74 references.