Research On LCL NPC Three-level Active Power Filter

Due to the gradual improvement of industrial science and technology,non-linear loads and the connection of some power electronic equipment to the power grid have caused a serious impact on the power quality of the power grid.Linear load harmonic current has been widely studied to reduce the grid current distortion rate by the Active Power Filter(APF).The LCL type filter,as an active power filter output connected in parallel with the power grid,can effectively attenuate high-frequency ripple current.However,the LCL type filter has resonance spikes in high frequency bands,which easily causes system resonance.The introduction of capacitive current active damping effectively suppresses the system’s resonance spikes,but under digital control,capacitive current active damping will introduce control sampling delay to change the phase characteristics of the system and affect its stability.Therefore,based on the three-phase four-wire NPC three-level active power filter,this paper proposes a capacitor current active damping digital sampling control delay compensation strategy,which effectively reduces the capacitor current active damping control sampling delay.This improves the dynamics and stability of the system.The following are the main directions of this subject research:Firstly,the topology and basic working principle of the Neutral Point Clamped(NPC)three-level active power filter circuit of three-phase four-wire 1-shaped diode are analyzed and introduced.The mathematical model is analyzed in the stationary coordinate system and in the two-phase rotating coordinate system.In addition,several methods of harmonic current detection and extraction are introduced.Finally,the principle of harmonic detection based on the theory of instantaneous reactive power is analyzed in detail.Secondly,some key technologies of NPC three-level active power filter are researched and analyzed in detail.The design of main circuit hardware parameters including LCL filter parameters and the design of DC bus capacitor voltage are introduced.In addition,the voltage control method is studied,including the problem of the neutral point imbalance of the DC bus.A mathematical model of the neutral point potential balance control is established.The DC bus based on the sinusoidal pulse width modulation(Sine Pluse Width Modulation,SPWM)is introduced.Pressure control strategy.Thirdly,based on the analysis of the LCL filter capacitor current active damping method considering the digital situation,the effect of control sampling on the dynamic stability of the system is analyzed,and a capacitor current active damping sampling delay compensation strategy under digital control is proposed.Analyze and compare the stability of the system under three different compensation controllers.Finally,the simulation of NPC three-level active power filter is established by matlab /simulink software platform.Fourthly,the system hardware circuit and the software program design of DSP +CPLD are analyzed.Finally,a 75 KVA three-phase four-wire NPC three-level active power filter experimental prototype platform is built.For this paper,the capacitor current under digital control is proposed The active damping sampling delay compensation strategy was experimentally verified,and the effectiveness of the compensation strategy was finally proved.