Research On LCL Active Power Filter

With the advancement of industrial technology,more and more non-linear loads and power electronic devices are connected to the power grid,which has caused a serious impact on the power quality of the power grid.The Active Power Filter(APF)can compensate the harmonic current generated by the nonlinear load,which can effectively reduce the harmonic content in the power grid.Compared with the L active power filter,the LCL active power filter has the advantages of low cost and good effect on high frequency harmonic attenuation,but it has resonance problem in the high frequency band.How to effectively solve the resonance problem has become one of the hotspots of current research.At present,there are two methods for solving the LCL resonance problem: passive damping and active damping.In this paper,the active damping method with the introduction of the filter capacitor voltage correction is studied.The effect of suppressing the resonance after adding different controllers is compared.It is found that the compensation effect based on the proportional resonance(PR)controller is the most prominent.The specific research contents are as follows:Firstly,the circuit topology of the three-level active power filter of Neutral Point Clamped(NPC)is analyzed,and its basic working principle is introduced.The advantages and disadvantages of the classification of the external filter circuit are compared,and the mathematical model of the active power filter is established for analysis.Several different methods for extracting harmonic current are listed,and the theory of instantaneous reactive power is expounded.The A detection method based on instantaneous reactive power theory is analyzed.Secondly,aiming at the midpoint unbalance problem of DC side voltage,the mathematical model of each switch state is obtained,and the direction of phase current in different states is analyzed to find its influence on the midpoint voltage fluctuation.The method of controlling the total voltage and midpoint voltage of the DC side of the three-phase four-wire APF under Sine Pulse Width Modulation(SPWM)is analyzed.Thirdly,several methods of adding resistance to the passive damping method are analyzed and compared,and the active damping method control strategy thatintroduces the voltage correction of the filter capacitor is introduced.In order to observe the difference of suppressing the resonance effect,by adding different controllers,draw the control block diagram,find the transfer function and get the corresponding Bode diagram for analysis,and build the simulation platform with Matlab/Simulink software to carry out the simulation experiment.Fourthly,a 50 KVA rated-capacity three-level active power filter device was built,and the active damping control strategy proposed in this paper was tested on the experimental prototype.The experimental results show that the proposed method can effectively suppress the resonance problem of LCL active power filter.