Research On Three-phase Four-leg Three-level Active Power Filter

With the rapid development of science and technology,power electronics and non-linear loads are widely present in power systems.The problem of harmonic pollution has become more and more serious.Because of its large capacity and flexible compensation advantages,Active power filters(APF)has become a research hotspot in the field of controlling power quality.This paper takes the three-phase four-leg three-level APF as the research object,and studies the harmonic detection algorithm,the harmonic tracking control technology and the modulation strategy respectively.The modulation strategy is the focus of this paper.First of all,this paper briefly introduces the basic working principle of APF and its important components,and establishes the mathematical model under the abc and ??0 coordinate systems respectively.For the diversity of grid load,an improved mobile window integral harmonic detection algorithm based on the feedback principle is proposed,which improves the dynamic response speed of the harmonic detection algorithm.At the same time,in order to solve the problem that the resonance controller has a narrow bandwidth near the resonance point,an improved proportional resonance controller is used in this paper to solve the problem of harmonic tracking.The 3D-SVPWM modulation strategy of the four-leg two-level converter in the ??0 coordinate system is analyzed in detail.Based on this,the three-level is decomposed into the two-level format and the 3D-SVPWM of the four-leg three-level APF is realized.The analysis of the vector state before and after the decomposition is based on the principle of "Volt-second balance",which fully proves the equivalence of the 3D-SVPWM modulation strategy proposed in this paper and the traditional algorithm.Aiming at the inherent neutral point potential imbalance in NPC topology,the influence of the base vector on the neutral point potential is analyzed.A neutral point potential control strategy based on charge conservation is proposed.By adjusting the vector action time,the purpose of maintaining the balance of upper and lower capacitor voltages is achieved.Aiming at the complexity of 3D-SVPWM modulation strategy,a simplified PWM modulation strategy is proposed.The simplified algorithm directly determines the state and action time of the base vector according to the magnitude and positive or negative of the three-phase reference voltage.Aiming at the problem of neutral point potential imbalance,taking the single-phase bridge arm as the research object,the mechanism of the neutral point potential imbalance is analyzed,and a neutral point potential control algorithm based on the simplified PWM modulation strategy is proposed.By changing the action time of the o state,ultimately maintains on the capacitor voltage balance.Finally,the two modulation strategies are compared by simulation and experiment,and the conclusion that the simplified PWM modulation strategy is better than the 3D-SVPWM modulation strategy is obtained.The hardware and software of the three-phase four-leg three-level APF are designed in detail,and the experimental platform is built.The algorithms proposed in this paper are verified under steady-state and dynamic environments respectively,which proves the correctness of its application in APF.