The Research Of Three-Phase Three-Wire Shunt Active Power Filter Based On LCL Filter

The nonlinear load represented by the power electronic device has become an indispensable part of people’s production and life,and its application has made the harmonic pollution of the power grid become more and more serious.Active power filter(APF)has the advantages of high compensation precision,fast response,flexible function and less susceptibility to power grid.Under the requirement of market demand and the standard of power quality,using APF to compensate harmonic will become one of the important development directions to improve power quality.In this paper,three-phase three-wire shunt APF based on LCL filter is taken as the research object,and its main circuit parameters and control strategy are studied from the following aspects:Firstly,the resonant principle of LCL filter is studied,and it is pointed out that leading the damping term into the transfer function of LCL filter is an ideal resonant damping method.The passive damping method of series resistance and parallel resistance on filter capacitor is studied from the perspective of transfer function.It is revealed that the essence of passive damping is the introduction of damping term,and the parallel resistance method is an ideal damping form because of only introducing the damping term.From the perspective of the damping tenn allocation,the feasibility of the active damping by feedback of four state variables is studied.It is pointed out that the feedback of the capacitor current is the best method.Secondly,the mathematical model of APF in three-phase stationary abc coordinate system,two-phase stationary αβ coordinate system and two-phase rotating dq coordinate system is established.The relationship between energy exchange and power flow in steady state of APF is analyzed,the design method of DC bus capacitance is given based on the energy fluctuation.The compensation target of APF and the frequency characteristics of LCL filter are studied,and a design method of LCL filter for APF is proposed.Then,two most commonly used harmonic detection methods are described and compared from the perspective of engineering application,which include i_p-i_q method based on the time domain and sliding-window iterative DFT method based on the frequency domain.It is pointed out that the practicality of sliding-window iterative DFT method is better.The DC bus voltage controller and the soft start strategy are designed.Based on the three-phase stationary coordinate system,the mathematical model of the outer loop about compensation current and the inner loop about capacitor current is deduced.The current controller,the feedback parameters of capacitor current and the feedforward strategy of grid voltage are designed.Finally,the simulation model based on Matlab and the experimental platform based on TMS320F28335DSP are set up.The main circuit parameters and control strategy designed in this paper are validated by simulation and experiment.The experimental results are consistent with the simulation results,which verify the validity of control strategy and the design of main circuit parameters,and also verify the feasibility of the LCL filter in the application of APF and the superiority of LCL filter relative to L filter.